Understanding the local environment of Sm³⁺ in doped SrZrO₃ and energy transfer mechanism using time-resolved luminescence: a combined theoretical and experimental approach

Abstract

A combined experimental and theoretical study on the photoluminescence (PL) properties of strontium zirconate (SZ) and Sm³⁺ doped SZ nanostructures is presented in this work. SZ and Sm³⁺ doped SZ is synthesized by a gel-combustion route and characterized systematically using X-ray diffraction (XRD), transmission electron microscopy (TEM), photoluminescence (PL) spectroscopy, and electron paramagnetic resonance (EPR) experimental techniques. PL studies on nanocrystalline SZ show strong violet-blue and weak orange-red emission under excitation wavelength at 243 nm. An EPR study shows the presence of oxygen vacancy in SZ nanocrystals. Combined emission, EPR studies and theoretical calculations brings out the possible reason for multicolor emission in SZ nanocrystals. The results of the PL spectroscopy measurement imply that the Sm³⁺ emissions, which originated from the ⁴G_5/2 → ⁶H_J (J = 5/2, 7/2, 9/2, and 11/2) intra-4f transitions of Sm³⁺ ions, are due to the indirect excitation of the Sm³⁺ ions through an energy transfer process from electron–hole pairs generated in the SZ hosts. Based on combined experimental and theoretical studies, a possible mechanism for PL of undoped and Sm³⁺-doped SZ is proposed.

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1. Introduction

Among the plethora of inorganic structures, the perovskite structure stands out through its versatility in derived structures and physical properties. Along with related structures, perovskites have been widely studied for their interesting properties and have found many applications in many fields. According to Stolen et al.,¹ these structures have been termed inorganic chameleons due to their great flexibility since the cubic mother structure easily distorts and adapts to the relative size of the ions forming the compound. The source of fascination is diversity of properties and high sensitivity to crystal chemical tuning; that is, a tiny change in chemical composition or/and crystal structure may induce huge changes in chemical and physical properties.

In particular, materials with the ABO₃ perovskite structure are important in the area of materials science and technology owing to their wide variety of potential applications such as solid oxide fuel cells, proton conductor,² magneto-resistors,³ pigments,⁴ ferroelectric and dielectric materials,^5–8 photocatalysts,⁹ and luminescent materials.^10,11 Recently, the 4d⁰ band insulator perovskite, strontium zirconate (SZ), has attracted much attention as a novel electronic material. Because these materials have high melting points (over 2600 °C),¹² they can be used in high-temperature devices, such as electrochemical devices, due to proton conductivity at fairly high temperatures.¹³

Light emission from lanthanide ions plays an important role in solid-state lighting (phosphor-converted light-emitting diodes), display (plasma and field emission displays), and bioimaging (fluorescent markers) technologies. Owing to their high thermal and chemical stability and low environmental toxicity, lanthanide-doped alkaline-earth perovskite oxides of the formula ABO₃ and corresponding solid solutions (A,A′)(B,B′)O₃ (A, A′ = Ca, Sr, Ba; B, B′ = Ti, Zr, Hf) are attractive candidates for nanostructured phosphors in display and bioimaging technologies. Specifically, they have shown potential as phosphors for field emission¹⁴ and electroluminescent¹⁵ displays, and as fluorescent markers for bioimaging.¹⁶ Recently lots of attention has been focused on the photoluminescent (PL) properties of titanates and zirconates with a disordered perovskite structure. The principal reason is the distinct potential of these materials for electro-optic applications.^17,18 The optical properties of disordered semiconductors are characterized by the presence of a broad PL band. This phenomenon is attributed to the electronic states inside the band gap, which are the main defects for an intense PL response. According to Longo et al., the displacement of Zr or Sr atoms in disordered perovskite SrZrO₃ may induce some vacancy defects at the axial and planar oxygen sites of the [ZrO₆] octahedral.¹⁹ It is well known that the vacancies defects may play important roles as luminescence centers and thus it is expected that the perovskite SrZrO₃ may show host emission. Defects-induced violet-blue emission from a strontium zirconate host has been observed by many researchers.^10,19–22

The ABO₃ type of perovskite having various crystalline structures shows interesting physiochemical properties that offer potential hosts for chemical substitution. Substitution at both the A and B sites can lead to changes in symmetry and composition, and thus create various defects via cation or oxygen vacancies that can drastically influence band structures. Substitution then is the main factor in determining the electronic structures. In particular, these materials can accommodate lanthanide ions on the A site or B site, and these doped oxides are not only used as probes to investigate local centers and energy but also to provoke changes in optical behavior. Moreover, doping foreign elements into a semiconductor with a wide band gap to create a new optical absorption edge is known to be one of the primary strategies for developing materials with optical-driven properties. However, the role of rare earth (RE) in the perovskite structure is not really clear and is still being discussed.

Recently lanthanide ion-doped SrZrO₃ materials have been widely investigated due to their significance in fundamental research and high potential for application in optical materials.^23–30 The luminescence efficiency of trivalent rare earth ions doped into inorganic matrices depends on the energy transfer from the host to the ion. It has been shown that the quantum efficiency from rare earth ions doped in nanocrystals increases as crystal size decreases.³¹ The Sm³⁺ ion has a 4f⁵ configuration, and therefore is labeled as a Kramer ion due to its electronic states that are at least doubly degenerate for any crystal field perturbation. Since samarium compounds have a narrow line-emission profile and a long lifetime similar to europium compounds, they can be used as a probe in multianalytical assays.

SrZrO₃ has been prepared by several synthetic routes including solid-state reaction,³² sol–gel,³³ co-precipitation³⁴ and hydrothermal methods.³⁵ The conventional solid-state reaction route requiring temperatures in excess of 1400 K suffers from inhomogeneous and coarse sample formation with non-uniform size distribution. Large and non-uniform phosphor particles are more likely to be prone to poor adhesion to the substrate and loss of coating. For good luminescence characteristics, phosphors should have small size, narrow size distribution, non-aggregation and spherical morphology. Similarly, several disadvantages have also been noted with the other synthetic techniques such as the evaporation of solvents resulting in phase segregation, alteration of the stoichiometry due to incomplete precipitation, expensive chemicals, and time-consuming processes. To overcome these limitations, a facile combustion synthesis route was suggested by Zhang et al., where the combustion reaction takes place at a lower temperature of 300 °C.²⁷ Using this method, a single-phase compound could be synthesized without intermediate grinding or annealing steps.

Gel combustion is a novel method that uses a unique combination of the chemical gel process and combustion. The gel synthesis of ceramic oxides offers advantages such as high purity, good homogeneity, and low processing temperature. Combustion synthesis offers advantages such as low energy requirements, simple equipment, and a short operation time because it uses a sustainable exothermic solid-solid reaction among the raw materials. The gel combustion method is based on the gelling and subsequent combustion of an aqueous solution containing salts of the desired metals and an inorganic fuel such as acetylene black, and it yields a voluminous and fluffy product with a large surface area. This process has the advantages of inexpensive precursors, a simple preparation method, and the ability to yield nanosized powders. Most of the earlier luminescence reports on this ceramic host have been obtained with Eu, Ce, or Dy as the activator ion.

However, when an active dopant is introduced into perovskite or structures with multiple sites (A and B), their optical and magnetic properties are dramatically changed depending on its distribution in the perovskite ceramic. Studies of dopant ion distribution in perovskite have attracted much attention because they may allow better understanding of the correlations between structure and properties such as color, magnetic behavior, catalytic activity, and optical properties, etc., which are strongly dependent on the occupation of these two sites by metals.^36–39 None of the literature available explains the site symmetry of Ln³⁺ in SrZrO₃ nanocrystals. Considering the relatively wide band gaps, high refractive indices and lower phonon energy, SZ is a good candidate that can be used as the host material for lanthanide ion luminescence in order to excite them efficiently and to yield intense luminescence. Metal ions can be conveniently substituted into the SZ lattice if their ionic radii are comparable to that of the Sr²⁺/Zr⁴⁺ cations. Inserting dopants into SZ lattice should affect the photo-absorption behavior of SZ.

In this study, we have employed a combined experimental and first principle calculations-based theoretical approach to investigate in detail the local environment and energy transfer process in Sm³⁺-doped SZ. Time-resolved emission spectroscopy (TRES), EPR, and first principles-based electronic density of state (DOS) calculations were used to explain defect-induced emissions in nanocrystalline SZ. The luminescence efficiency of trivalent rare earth ions doped into inorganic matrices depends on the energy transfer from host to ion. Thus, time-resolved fluorescence spectroscopy is used to identify the local environment around Sm³⁺ ion in SZ which is further validated by first principles based calculations. Finally, combining the experimental observation and theoretically calculated results, the energy transfer process of SZ host-to-Sm³⁺ is analyzed, and the corresponding mechanism is proposed.

2. Experimental

2.1. Sample preparation

All chemicals used in sample preparation were of analytical reagent grade and procured from Sigma Aldrich. Zirconyl oxychloride (ZrOCl₂), strontium nitrate Sr(NO₃)₂, ammonium nitrate (NH₄NO₃), and citric acid (C₆H₈O₇·H₂O) were used as starting materials for synthesis in the molar ratio 1 : 1 : 10 : 1.25. First, Sr (NO₃)₂ and NH₄NO₃ were dissolved in quartz double-distilled (QDD) water with stirring and then ZrOCl₂ solution was added to it. Under vigorous stirring, citric acid solution (2 M) prepared initially was poured into the mixed solution resulting in an opal gel. This gel was dried at 100 °C for 10 h under an IR lamp, and then transferred to a quartz beaker in a muffle furnace and kept at 300 °C for 10 min so as to form an ash-colored fluffy substance. In this step, the actual combustion reaction takes place using citric acid as the fuel. The ash-like product was then calcined at 600 °C for 1 h resulting in a white powder. The obtained white powders were ground and calcined at different temperatures.

For preparation of the samarium-doped sample (1.0 mol%), appropriate quantities of samarium nitrate were added at the initial stage prior to addition of ZrOCl₂ so that the final samarium concentration was 1.0 mol%.

2.2. Instrumentation

The phase purity of the prepared phosphor was confirmed by X-ray diffraction (XRD). The measurements were carried out on a STOE X-ray diffractometer equipped with Ni filter, scintillation counter and graphite monochromator. The diffraction patterns were obtained using monochromatic Cu-K_α radiation (λ = 1.5406 Å), keeping the scan rate at 1 s per step in the scattering angle range (2θ) of 10° to 60°. The K_α2 reflections were removed by a stripping procedure to obtain accurate lattice constants. PL data were recorded on an Edinburgh CD-920 unit equipped with Xe flash lamp with 10–100 Hz frequency as the excitation source. The data acquisition and analysis were performed with F-900 software provided by Edinburgh Analytical Instruments (UK). Time-resolved emission studies (TRES) were carried out on an Edinburgh F-900 unit equipped with M 300 monochromator. The EPR spectra were recorded on a Bruker ESP-300 spectrometer operating at X-band frequency (9.5 GHz) equipped with 100 kHz field modulation and phase-sensitive detection to obtain the first derivative signal. Diphenyl picrylhydrazyl (DPPH) was used for calibration of g-values of paramagnetic species.

2.3. First-principles calculations

Ab initio calculations were performed using the plane wave-based Vienna Ab-initio Simulation Package (VASP).^40,41 VASP is based on density functional theory (DFT), and we used generalized gradient approximation (GGA) for the exchange and correlation potentials as parameterized by Perdew, Burke and Ernzerhof (PBE).⁴² The projector augmented wave (PAW) potentials⁴³ were used for the ion–electron interactions including the valence states of Sr (4s, 4p, 5s – 10 valence electrons), Zr (4s, 4p, 5s, 4d – 12 valence electrons), Sm³⁺ (5s, 5p, 4f, 6s – 11 valence electrons) and O (2s, 2p – 6 valence electrons). For orthorhombic SrZrO₃ (SZ) structure, optimization was carried out with respect to E_cut and k-point meshes to ensure convergence of total energy to within precision of 0.1 meV per atom. The expansion of electronic wave functions in plane waves was set to the optimized kinetic energy cut-off (E_cut) of 500 eV throughout this study. The Brillouin-zone (BZ) integrations were performed on an optimized Monkhorst–Pack⁴⁴ k-point grid of 12 × 12 × 8 for SZ and 4 × 4 × 4 for SZ supercells, respectively. The total energy of SZ was optimized with respect to volume (or lattice parameter), b/a, c/a ratio and atomic positions as permitted by the space group symmetry of the crystal structure. The structural relaxations (b/a, c/a ratio and atomic positions) were performed for each structure using the conjugate gradient algorithm until the residual forces and stress in the equilibrium geometry were of the order of 0.005 eV Å⁻¹ and 0.01 GPa, respectively. The final calculations of total electronic energy and density of states (DOS) were performed using the tetrahedron method with Blöchel corrections.⁴⁵

3. Results and discussion

3.1. Phase purity: PXRD

The XRD patterns of both SrZrO₃- and Sm-doped SrZrO₃ samples annealed at 600 °C are shown in Fig. 1. The patterns matched with orthorhombic phase of SrZrO₃ (ICDD file no. 44-0161). Peaks due to impurities of SrCO₃ observed in the pattern (around 2θ = 25°) are barely avoided at such a low calcination temperature and short time. However, their existence seems to have little influence on the luminescence of SrZrO₃.⁴⁶ The XRD data were indexed on an orthorhombic system with space group Pnma having cell parameters a = 5.817 Å, b = 8.204 Å, and c = 5.797 Å. From the pattern of both undoped and doped samples, it can be inferred that incorporation of Sm in SrZrO₃ did not change the crystal structure.

Fig. 1 XRD patterns of the SrZrO₃ and Sm³⁺ doped SrZrO₃.

3.2. Morphological studies: HRTEM

Fig. 2(a) shows high-resolution transmission electron microscopy (HRTEM) micrographs of Sm³⁺-doped SrZrO₃ nanoparticles. HRTEM was used to examine the morphology of Sm³⁺-doped SrZrO₃ nanoparticles. Qualitatively, the material appears to be comprised of interconnected SrZrO₃:Sm particles that define cavities of mesoporous (20–50 nm) dimensions. The nanostructure is composed of spherical primary particles with features in the 2–5 nm range. These particles are connected to one another to form larger clusters.

Fig. 2 HRTEM micrographs of SrZrO₃:Sm³⁺ annealed at 600 °C (as prepared samples): (a) bright field image, (b) selected area electron diffraction, and (c) high resolution image.

The corresponding selected-area electron diffraction (SAED) pattern (Fig. 2(b)) confirmed sample crystallinity, and indicated that nanocrystals are stabilized in an orthorhombic phase (Pnma), and are randomly oriented. Fig. 2(c) displays a high-resolution lattice image of Sm³⁺-doped SrZrO₃ nanoparticles, revealing the crystalline nature of the sample, further suggesting the absence of any parasite phase. Hence, the TEM studies confirmed the nanocrystalline nature of Sm³⁺-doped SrZrO₃, and ruled out the possibility of any samarium, strontium samarium and zirconium samarium precipitation.

The consecutive lattice fringes were arranged in order even at the corner of a single particle without any crystalline border. The spacing of the observed lattice fringes was deduced to be 0.72 nm, which was associated with the (020) lattice plane of the orthorhombic phase of SrZrO₃.

3.3. Defect-mediated emission in SrZrO₃

It is well-known that large numbers of defects exist in nanomaterials. Fig. 3 shows the room-temperature photoluminescence spectra of SrZrO₃ nanoparticles. When exposed to a 243 nm xenon flash lamp, the sample exhibited visible light emission in a broad range of 400–635 nm. The most intensive photoluminescence peaks centered at 425 nm, implying violet-blue light emission. The PL profile is typical of a multiphonon process, that is, emission that occurs by several paths and involves numerous states within the forbidden band gap. Considerable studies had verified that the photoluminescence properties of perovskite oxides of group IV elements stemmed from oxygen vacancies and structural defects. Kan et al. reported that photoluminescence intensity was directly related to defects in the perovskite SrTiO₃ crystals and films.⁴⁷ Also, Longo et al. confirmed defects-related photoluminescence intensities through controlling the annealing temperature for perovskite SrZrO₃ and BaZrO₃ nanocrystals that were prepared by the combustion method.^19,22,48 The higher the calcining temperature, the more frequent the ZrO₆ conformation and the more ordered the structure. The yellow and red peaks decrease and the violet-blue peaks increase with heat treatment, since yellow-red emission is linked to disordered structure and violet-blue to ordered structure.

Fig. 3 Emission spectra of air sintered SrZrO₃.

Visible spectrum wavelengths are usually between 400 nm and 700 nm. The energy carried by each visible photon is between 3.1 eV and 1.8 eV. Violet-blue color is more energetic than yellow-red-orange color. In our experiments, the intense violet and blue emission was therefore attributed to shallow defects in the band gap and a more ordered structure, while the weak yellow and red emission was linked to defects deeply inserted in the band gap and disorders in the lattice.

Therefore, it can be concluded that in SrZrO₃, each color has a different origin and is linked to a specific structural arrangement. When the sample was exposed to UV light, deep and shallow defects generated localized states in the band gap and non-homogeneous charge distribution formed traps for electrons. Due to energy-dependent localized levels, exciting the trapped electrons requires various energies. After excitation, the blue luminescence originated from the recombination process in which an excited electron of the conductive band (CB) lost its energy and re-occupied the energy levels of an electron hole in the valence band (VB) through localized defect levels.

3.3.1 Theoretical calculations. The calculated lattice parameters for SZ are a = 5.845 Å, b = 8.295 Å, and c = 5.909 Å using GGA-PBE, which agree well with our XRD data of a = 5.797 Å, b = 8.204 Å, and c = 5.817 Å, and other experimentally determined values⁴⁹ within less than 1.5% deviations. Therefore, the GGA approximation is able to provide reliable results for the equilibrium lattice constants of the present system. The GGA-PBE calculated band gap for SZ is 3.7 eV, which is not in agreement with the experimental value of 5.6 eV by optical conductivity analysis of the polycrystalline sample at room temperature.⁵⁰ But our calculated band-gap energy matches very well with previous GGA-PBE calculation by Guo et al.⁵¹ The underestimation of band gap energy is a common problem of DFT calculations in the GGA approximation. Nevertheless, the present calculations properly reproduce the good insulating character of ideal SZ.

Three structural models were built based on the ideal/ordered SZ structure (o-SZ) (Fig. 4) in order to simulate the disordered types and structural complex vacancies associated with them: (i) by displacement of the Zr (f-SZ); (ii) by displacement of Sr (m-SZ); and (iii) by simultaneous displacement Zr/Sr (fm-SZ) as described by Longo et al.¹⁹ The DOS were calculated with the total 0.5 Å vector displacement of the Zr and Sr network in all dislocated models.

Fig. 4 Schematic of SrZrO₃ unit cell. Arrows indicate direction of atomic displacement for the Zr and Sr network. O₁ and O₂ are the axial and planar oxygen atoms, respectively.

The calculated total and orbital angular momentum-resolved DOS for o-SZ, f-SZ, m-SZ, and fm-SZ are shown in Fig. 5, ranging from −4 eV below the top of the VB to 6 eV above, and presenting the principal orbital states that influence the gap states. As seen in Fig. 5(a), the upper valence bands (VB) consist of O 2p states taking equivalent contributions from axial and planar oxygen atoms (shown in Fig. 4) with some additional contributions from Zr 4d states. The lower conduction bands (CB) are mainly Zr 4d states with some additional contributions from O 2p states. This clearly indicates the covalent nature of Zr–O bonds and these DOS characters of SZ are consistent with the previous GGA results by Guo et al.⁵¹ In the case of f-SZ (Fig. 5(b)), the VB is composed of O 2p states, and the upper part of VB, that is, the new states, are composed mainly of axial oxygen 2p states. In the m-SZ case (Fig. 5(c)), the upper part of VB is composed mainly of planner O 2p states. The fm-SZ structure shows a strong axial oxygen contribution in the new DOS present in the upper part of VB, which is analogous to the f-SZ model. Moreover, the band gap energies of o-SZ, f-SZ, m-SZ, and fm-SZ are 3.7, 2.96, 3.10, and 2.76 eV, respectively. Displacement in the network former causes increased disorder in the lattice compared to the network modifier. Greater disorder occurs when both network modifier and former are displaced. This disorder is characterized by reduction in band-gap energy in the disordered model.¹⁹ Even though the numerical values of the GGA-PBE calculated band-gap energies of dislocation model structures are not correct, they follow the same sequence in which degree of disorder is present in these model structures. Therefore, the DOS features of o-SZ, f-SZ, m-SZ and fm-SZ are essentially similar to the DOS calculated by Longo et al.,¹⁹ using DFT-based calculations combined with the B3LYP hybrid functional. Thus the GGA-PBE methodology captures the essential features of structural defects and degree of disorder in SZ resulting from network former and modifier displacements.

Fig. 5 Total and orbital angular momentum projected DOS for: (a) o-SZ, (b) f-SZ, (c) m-SZ and (d) fm-SZ models. The vertical lines represent Fermi level.

The displacement in Zr (former) network promotes an increased disorder in the lattice when compared to the Sr (modifier) network. Greater disorder occurs when both the atoms are displaced. This disorder is characterized by reduction in band-gap energy in the disordered model. The decrease in band gap in structurally disordered powder can be attributed to defects and/or local bond distortion, which yield local electronic levels in the band gap of this material. Increased disorder is linked to deep defects inserted in the band gap and increased order is associated with shallow defects, which disappear when total order is reached. Increased disorder is due to presence of [ZrO₅·] and [ZrO₅·] complex clusters and are deeply inserted in the band gap, leading to orange-red PL emission. [SrO₁₁·] and [SrO₁₁·] complex clusters are linked to shallow defects in the band gap and lead to more energetic PL emissions (violet-blue light).¹⁹ The deep defects linked to the Sr/Zr disorder are associated with the 2p states of axial oxygens and evidently shown in Fig. 5(d). Shallow defects can be ascribed to the 2p states of planar oxygens in the VB as described in our DOS analysis. Increasing the lattice order causes these complex vacancies and the PL emission to disappear. The presence of oxygen vacancy is also confirmed by EPR studies as discussed in the following EPR spectroscopy section.

3.3.2 Excitation spectroscopy. To confirm that emission in SrZrO₃ is mediated by defects and is responsible for the origin multicolor, we recorded excitation spectra (Fig. 6) corresponding to violet, blue, yellow and red emissions. It was observed that excitation spectral feature remains the same in entire wavelength range, dominated by a wide band center at about 246 nm (5.04 eV), which indicates that the UV irradiation energy can be efficiently absorbed by SrZrO₃ host lattices and then is transferred to the emission centers. This band belongs to the host absorption band (HAB) and is generally ascribed to the charge transfer from the oxygen ligands to the central zirconium atom.

Fig. 6 Excitation spectra of air sintered SrZrO₃ under different excitation wavelengths.

3.3.3 EPR spectroscopy. Electron paramagnetic resonance (EPR) shows great potential for studying the local structure and properties of nanoparticles. Note that the influence of external factors on the radio-spectroscopic characteristics of nanosized and large (microns) particles is not identical since the charge state and other characteristics of intrinsic and impurity defects in nanoparticles depend on particle size and surface conditions.

The EPR spectrum of bare SrZrO₃ (Fig. 7), recorded after the final calcinations in air, is not a flat line (as one would expect in the case of the perfect stoichiometry) and shows an intense and asymmetric signal of the Hamiltonian spin at room temperature, which indicates the presence of intrinsic defects in the as-prepared material. In the literature, this signal is related to singly ionized oxygen vacancies and vacancy-related defects,^52–54 where the single asymmetrical peak g presents variations of 1.9560–2.0030. We believe that the changes in g values are related to differences in preparation method, chemical environment, and heat treatment conditions. The broad line width of the signal indicates a certain degree of heterogeneity (several species differ slightly in spectral parameters) typical of disordered environments, such as those found at the surface of nanostructured crystals. Even broader signals are found, for instance, for species formed at the surface of TiO₂.⁵⁵

Fig. 7 X-band EPR spectra of SrZrO₃ sample recorded at RT.

Matta et al.⁵⁶ used EPR measurements to verify the phase transformation from tetragonal to monoclinic zirconia and observed a signal g = 2.0018, which was attributed to trapped single electrons located in oxygen vacancies of ZrO₂. Lin et al.⁵⁷ reported that the EPR band at g = 1.9800 is oxygen vacancy related. Thus, in the disordered structure, these are linked to ZrO₅ clusters, called [ZrO₅·] oxygen complex clusters.⁴⁸

3.3.4 Luminescence decay. Emission lifetimes were recorded using the time-correlated single-photon-counting (TCSPC) technique. Samples were excited with 243 nm laser pulses provided by the frequency-doubled output of the Nd:YAG-pumped, OPO laser-regenerative amplifier operating at a 10 Hz repetition rate.

The decay curves of SrZrO₃ annealed at 600 °C (as prepared sample) are shown in Fig. 8 at excitation wavelengths of 243 nm, monitoring emissions at various wavelengths on a 100 μs scale, and fitted using the following exponential decay equation:

here A_i are scalar quantities, t_i are the times of measurement and τ_i are the decay time values i.e. the time taken for the excited-state population to become 1/e of the original value). The decays measured here were found to be multi-exponential in all cases and could be adequately fit to a sum of two exponentials. The lifetime values and percent occupancy of each species obtained under different emissions are given in Table 1. Lifetime values obtained were ∼2.00 and ∼12.0 μs, which are attributed to diverse types of defects in bare SrZrO₃ nanocrystals. Slower decaying species were attributed to shallow defects in the band gap and a more ordered structure, whereas the faster decaying component is linked to defects deeply inserted in the band gap and disorders in the lattice.

Fig. 8 Decay curves of SrZrO₃ sample. Samples were excited at 243 nm and the emission was monitored at different wavelengths.

Table 1 Lifetime and percent occupancy for lifetime of SrZrO₃ host with selected emission wavelengths

λem (nm)	τ1 in μs (% occupancy for short-lived species)	τ2 in μs (% occupancy for short-lived species)
425 (violet)	1.85 (50)	13.9 (50)
468 (blue)	1.99 (49)	12.2 (51)
593(yellow)	2.00 (48)	11.7 (52)
615 (red)	2.14 (44)	10.4 (50)

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3.4. Excitation and emission spectroscopy of Sm³⁺-doped SrZrO₃

Trivalent samarium ions emit a very bright luminescence in the visible and near infrared region in various kinds of host materials. The luminescence bands observed in this ion emission are due to transitions between the energy levels in the 4f⁵ electron configuration. The excitation spectrum of the system at the 597 nm emission is shown in Fig. 9. The broad band was in the 200–275 nm range, and was assigned to the charge transfer band O²⁻ → Sm³⁺ (CTB) with λ_max at 243 nm. In the wavelength region 320–550 nm, several excitation peaks are observed and located at 346 nm (⁶H_5/2–⁶H_13/2), 365 nm (⁶H_5/2–⁴D_3/2), 379 nm (⁶H_5/2–⁶P_7/2), 407 nm (⁶H_5/2–⁴F_7/2), 417 nm (⁶H_5/2–⁶P_5/2), 438 nm (⁶H_5/2–⁴G_9/2), 462 nm (⁶H_5/2–⁴I_9/2), 469 nm (⁶H_5/2–⁴I_11/2), 473 nm (⁶H_5/2–⁴I_13/2), and 485 nm (⁶H_5/2–⁴I_15/2), which are attributed to f–f transitions of Sm³⁺. From the excitation spectrum, the intensity of f–f transition at 407 nm is high compared with the other transitions, and was chosen for the measurement of emission spectra of SrZrO₃:Sm³⁺ phosphors. The most intense peak at 407 nm clearly indicated that these phosphors are effectively excited by near UV light – emitting diodes.

Fig. 9 Excitation spectra SrZrO₃:Sm³⁺. Inset shows the f–f lines of Sm³⁺.

Emission spectra of Sm³⁺ ion-doped SrZrO₃ with an excitation wavelength of 243 nm (CTB)/407 nm (f–f band) are shown in Fig. 10. Spectral features remained the same on excitation with 250 nm (charge transfer) and 406 nm (f–f band). It was also noted that the intensities of emission bands were found to be low when excited with the characteristic absorption band at 407 nm of the Sm³⁺ ions, compared to that of emission intensities obtained with the LMCT excitation band (250 nm). This may be due to the fact that the Sm³⁺ absorption bands corresponding to the f–f transitions are forbidden and exhibit poor absorptivities in the UV region. The high intensities of emission bands when excited with LMCT are due to the intramolecular energy transfer (IMET) process, which occurs in the UV region.

Fig. 10 Emission spectra of air sintered SrZrO₃:Sm³⁺ at excitation wavelength (a) 243 nm (CTB) and (b) 407 nm (f–f band).

The emission spectra consist of two parts: one is due to host in the region of 400–550 nm, another region comprising three sharp emission lines from 550 to 700 nm are characteristic of the samarium ions. Three peaks are ascribed to the ⁴G_5/2–⁶H_5/2, ⁴G_5/2–⁶H_7/2, and ⁴G_5/2–⁶H_9/2 transitions at 565, 597, and 643 nm of the Sm³⁺ ions, respectively. The peak at 703 nm is due to the ⁴G_5/2–⁶H_11/2 transition. Among these, the transition at 597 nm (⁴G_5/2–⁶H_7/2) had the maximum intensity, which corresponds to red emission of SrZrO₃:Sm³⁺ phosphors. It can be stated that the strong red-emitting transition ⁴G_5/2–⁶H_7/2 at 597 nm (ΔJ = ±1) is an emission band of a partly magnetic dipole (MD) and partly electric dipole (ED) nature. The other transition at 565 nm (⁴G_5/2–⁶H_5/2) is purely MD natured and at 643 nm (⁴G_5/2–⁶H_9/2) is purely ED natured, which is sensitive to the crystal field. Generally, the intensity ratio of the ED and MD transition has been used to measure the symmetry of the local environment of trivalent 4f ions. The greater the intensity of the ED transition, the greater the asymmetrical nature. In our present study, the ⁴G_5/2–⁶H_5/2 (MD) transition of Sm³⁺ ions is less intense than the ⁴G_5/2–⁶H_9/2 (ED) transition, indicating the Sm³⁺-occupied asymmetric site in SrZrO₃.

We know that the coordination number of Sr and Zr ions is 8 and 6, respectively. Since ionic size difference between 8-coordinated Sr²⁺ (126 pm) and 8-coordinated Sm³⁺ (108 pm) is less, Sm³⁺ ions occupying the Sr²⁺ sites will not lead to a large distortion in the lattice, and if the associated defect due to charge difference is at a great distance, the local site will have inversion symmetry. On the other hand, 6-coordinated Sm³⁺ with ionic size 96 pm while occupying 6-coordinated Zr⁴⁺ (ionic size 72 pm) has a larger size differential and can lead to distortion in the octahedra, resulting in the local site without inversion symmetry. Thus, the observed spectra for the ⁴G_5/2–⁶H_5/2 (MD) transition of Sm³⁺ ions that is less intense than ⁴G_5/2–⁶H_9/2 (ED) transition can be attributed to a majority of Sm³⁺ ions occupying the Zr⁴⁺ site without inversion symmetry, although oxygen vacancies are introduced in the vicinity to ensure local charge compensation. In SrZrO₃, the Zr⁴⁺ ion has the local symmetry D_2h within the ZrO₆ octahedron. However, the ionic radius of Sm³⁺ exceeds that of Zr⁴⁺ by about 24 pm (96 vs. 24 pm) and therefore induces a significant lattice distortion. Regarding symmetry considerations, it is known that in a non-cubic environment a^{2S + 1}L_J manifold of RE³⁺ ion containing an odd number of electrons is split to J + 1/2 Stark levels with each level maintaining two-fold Kramer degeneracy.⁵⁸ Indeed, in the case of ⁶H_5/2 and ⁶H_7/2, the corresponding number of spectral lines can be counted. For ⁶H_9/2 and ⁶H_11/2, some of the transitions are probably too weak to be resolved. Splitting in the spectral line of Sm³⁺ further supports the fact that the majority of samarium ions occupy low symmetric Zr⁴⁺ sites.

3.5. Decay time and TRES studies

In the SrZrO₃ perovskite structure, the coordination numbers of Sr and Zr ions are 8 and 6, respectively, and as already discussed, they can be occupied by the Sm³⁺ ions. To get an idea about the nature of dopant ion occupancy in these lattice sites, PL decay time (lifetime) studies were conducted. The decay curves corresponding to the ⁴G_5/2 level of Sm³⁺ ions in 1.0 mol% samarium-doped SrZrO₃ are shown in Fig. 11 at the excitation wavelength 407 nm, monitoring emissions at wavelengths 565, 597, and 643 nm. For SrZrO₃:Sm³⁺, a good fit was found to be biexponential, using an equation similar to eqn (1).

Fig. 11 Luminescence decay time profile of SrZrO₃:Sm³⁺ at λ_ex-407 nm under λ_em-565, 597 and 643.

The percentage occupancy of Eu³⁺ ions exhibiting a specific lifetime is obtained in such case using the following formula:

Broadly, the analysis showed the presence of two components, one short-lived and one long-lived. In all cases, lifetime values were roughly ∼500 μs (short component, T₁, 75%) and 1.6 ms (long component, T₂, 25%), which can be indicative of the presence of two emitting species or states.

Makishim et al.⁵⁹ previously investigated the luminescence of Sm³⁺ in the BaTiO₃ host lattice and found that the spectra consist of two different series with various properties. They also found that certain foreign ions can change the relative strength of emissions in the two series owing to a charge compensation mechanism. On the basis of their results, they concluded that one series of emissions is attributed to Sm³⁺ at the Ti⁴⁺ site, while the other series of emissions is related to the presence of Sm³⁺ at the Ba²⁺ site.

Assuming a given phonon energy (same host for the lanthanide ions), a relatively longer PL decay time should be attributed to a more symmetric site, as the f–f transition becomes more difficult, whereas a shorter decay time is often associated with an asymmetric site due to relaxation in the selection rules. Species T₁ (500 μs), which is the major one that arises because of Sm³⁺ ions occupying the 6-coordinated Zr⁴⁺ site without inversion symmetry, whereas minor species T₂ (1.6 ms) can be ascribed to Sm³⁺ ions occupying 6-coordinated Sr²⁺ with inversion symmetry. These results also corroborate our emission studies where we have observed that the majority of Sm³⁺ ions occupy the Zr⁴⁺ site without inversion symmetry. Such site-selective spectroscopy of lanthanide in host-like silicate, zirconate and pyrophosphate, where multiple sites are available for occupancy, has already been reported by our group.^{24,25,60–63}

In order to identify the environment associated with the species exhibiting different lifetimes, time-resolved emission spectra (TRES) were recorded at various time delays with constant integration time. Fig. 12 shows the spectra recorded with time delays of 600 μs and 3.0 ms, respectively, with integration time of 50 μs. As seen in Fig. 12, after a delay time of 600 μs, the characteristic emission predominated by Sm³⁺ ions in asymmetric environment (Intensity₍₆₄₃₎ > Intensity₍₅₉₇₎) was observed. After a delay time of 3.0 ms, the emission characteristics were similar overall to those observed after the 600 μs delay with a difference in intensity that is usually expected.

Fig. 12 Time-resolved emission spectra of SrZrO₃:Sm³⁺ nanophosphor under the excitation at 243 nm after suitable delay time.

The spectra observed after the 3.0 ms delay is expected from long-lived species (1.6 ms) as the other species would have declined in intensity by a factor of e⁻⁶. The spectra obtained after the 600 μs delay time has contributions from both short-lived and long-lived species. The spectral characteristics of short-lived species were obtained by subtracting the contribution of long-lived species (obtained mathematically using the spectra observed after the 3.0 ms delay) from the observed spectra of the 600 μs delay. Spectra for short-lived and long-lived species obtained after mathematical calculations are shown in Fig. 13. The ⁴G_5/2 → ⁶H_5/2 line is observed at 565, which originates from the magnetic dipole (MD) transition, and does not depend on chemical surroundings of the luminescent centre and its symmetry. However, the hypersensitive ⁴G_5/2 → ⁶H_9/2 transition at 643 nm is magnetic-dipole forbidden and electric-dipole allowed, and its intensity increases as the environmental symmetry declines. The asymmetry ratio was found to be 0.28 and 0.128 for short-lived (μ = 500 μs) and long-lived (τ = 1.6 ms) species, respectively. This is in correspondence with the phonon energy concept where short-lived species will have more asymmetric components than long-lived species.

Fig. 13 Time-resolved emission spectra of for short and long lived Sm³⁺ ion in SrZrO₃.

3.6. Energy transfer from host to rare-earth ions in SrZrO₃:Sm³⁺

3.6.1. Luminescence experiment. An obvious spectral overlap between host emission (oxygen-vacancy) and excitation of Sm³⁺ can be observed, which is shown in Fig. 14. According to Dexter theory,⁶⁴ an effective energy transfer (ET) requires a spectral overlap between the donor emission and the acceptor excitation. Consequently, the effective ET from the host to Sm³⁺ ions is expected. Therefore, the SrZrO₃ and SrZrO₃:Sm³⁺ samples were subjected to oxygen-deficient atmosphere (vacuum) to sinter in an attempt to observe the influence of the oxygen vacancies on the PL of the powders. We adopted vacuum atmosphere rather than reduction atmosphere sintering for preventing the reduction of Zr⁴⁺ in SrZrO₃ and SrZrO₃:Sm³⁺. The results presented in Fig. 15 indicate that the sintering in vacuum is quite effective to improve the violet-blue emission of SrZrO₃ compared to emission intensity of the air-sintered sample, as mentioned in section 3.3. It is safe to say that the vacuum sintering in this study can effectively create oxygen vacancies. Compared with the air-sintered SrZrO₃:Sm³⁺ phosphor (Fig. 10), the red emission intensity increased about 150% when the sample was sintered in vacuum. Based on the above results, we can suggest that the PL intensity enhancement in the vacuum-sintered SrZrO₃:Sm³⁺ phosphor is related to creation of oxygen vacancies. To clarify the relation between the red emission and oxygen vacancies in the vacuum-sintered SrZrO₃:Sm³⁺, we examine the emission (435 nm) of SrZrO₃ and SrZrO₃:Sm³⁺, respectively. The results are shown in Fig. 15; the ET process from host (oxygen-vacancy) to Sm³⁺ can be confirmed, as the violet-blue emission is largely reduced after Sm³⁺ doping into SrZrO₃. Consequently, in vacuum, the ET between the host and luminescence center (Sm³⁺) becomes more effective, which leads to higher red emission intensity.

Fig. 14 Emission spectra SrZrO₃ (λ_ex-243 nm) and excitation spectra of SrZrO3:Sm³⁺ (λ_em-597 nm).

Fig. 15 Energy transfer from host to Sm³⁺ ion (vacuum sintered sample).

3.6.2 Electronic structure. To obtain a clear picture of the effect of samarium doping in ideal/ordered SZ, a 2 × 2 × 1 supercell (with 80 atoms) of o-SZ was modelled and a single Sm atom placed in Sr position (Sr-SZ), which corresponds to a doping level of 6.25 atom%. Another 2 × 2 × 1 supercell with a single Sm atom placed the Zr position was also modelled (Zr-SZ). These two structures were then optimized with respect to volume, b/a, c/a ratio and atomic positions. Our GGA-PBE optimized equilibrium volume shows a reduction of 0.827 ų and increase of 3.0 ų for the Sr-SZ and Zr-SZ compared to o-SZ unit cell, respectively. Regardless of the Sm position, the parent host structure remains orthorhombic, and our XRD data for Sm-doped SZ matches this theoretical prediction.

To identify the localization of Sm ion in the distorted SZ (fm-SZ), two 2 × 2 × 1 supercells (80 atoms) of fm-SZ were modelled. In one case, one Sm atom was placed in the position of a Sr atom (Sr-fm-SZ), and in another case one Sm atom was placed in position of a Zr atom (Zr-fm-SZ), which corresponds to 6.25 atom% doping. Total DOS calculations for these Sm-doped supercells are shown in Fig. 16(a) and (b). In both cases, the calculated band gaps, 2.69 eV for Sr-fm-SZ and 2.77 eV for Zr-fm-SZ (Fig. 16(a) and (b)), are comparable to the 2.76 eV for fm-SZ. A small change in the calculated band gap usually signifies a small change in the degree of order/disorder that prevails in fm-SZ.

PL spectra of the vacuum-sintered SrZrO₃:Sm³⁺ sample (Fig. 15) show an energy transfer from host to Sm³⁺ at a higher wavelength compared to undoped SrZrO₃. In other words, Sm doping increases the disorder that prevails in the host SrZrO₃. Therefore, localization of Sm atoms solely in the Sr position or in the Zr position does not explain the increase of disorder in the host SrZrO₃. In order to further reveal possible localization of Sm ions, a 2 × 2 × 2 supercell (160 atoms) of fm-SZ was modelled, and two Sm atoms were placed, one in the Sr position and the other in the Zr position. Total DOS calculations for this Sm-doped supercell are shown in Fig. 16(c) and the band gap of 2.2 eV can be evaluated from the same. The reduced band gap compared to fm-SZ is a manifestation of the increase in disorder, and thus the energy transfer from the undoped SrZrO₃ to the vacuum-sintered SrZrO₃:Sm³⁺ sample at a higher wavelength. Consequently, localization of Sm ions is most probable in the Sr as well as the Zr position. These theoretical results are in complete agreement with our luminescence lifetime measurements as described in section 3.5. Also, lifetime studies have shown the presence of Sm³⁺ in SZ nanocrystals having a lifetime value of 500 μs and 1.6 ms corresponding to Sm³⁺ at Zr and Sr sites, respectively.

The bonding mechanism could be explained by the partial DOS, and Fig. 16(c) shows partial DOS of Sm³⁺ doped both in the Sr and Zr positions of fm-SZ. Overall DOS for CB and VB of Sm³⁺-doped fm-SZ is similar to that of fm-SZ (as described for Fig. 5(d)). But the presence of additional states in the band gap of Sm-doped fm-SZ makes it different in the band-gap region. The additional states in the upper part of VB (in the energy range −0.3 to 0 eV, scaled by Fermi energy (E_F)) are mainly contributed by the 4f-non-bonding states of Sm³⁺ placed in the position of Zr atoms in fm-SZ. Moreover, the additional states in the lower part of CB (in energy range 2–2.25 eV, scaled by E_F) arise mainly from 4f-non-bonding states of Sm³⁺ localized in the Sr position of fm-SZ. Thus, reduction in the band gap of Sm³⁺-doped fm-SZ can be manifested from the presence of 4f-impurity states of Sm³⁺ in the band gap. Apart from the impurity states, the 4f states of Sm³⁺ (localized at the Zr position) are hybridized with O 2p-states in the energy range −4.2 eV to −0.25 eV. Importantly, the 4f states of Sm³⁺ are concentrated in the energy range −4.2 to −0.25 eV and 2.5 to 5 eV (scaled by E_F); these energy ranges are also the lower VB portion and upper CB portion of the fm-SZ, respectively. Therefore, distribution of 4f states of dopant Sm³⁺ matched well with the Zr(d)–O(p) bonding states, Zr non-bonding d-states as well as defect states of the host fm-SZ. Thus, the overlap of electronic DOS between the host fm-SZ and the Sm³⁺-doped fm-SZ makes the energy transfer process from the host fm-SZ to the Sm³⁺-doped fm-SZ feasible.

Fig. 16 Total DOS for Sm doped fm-SZ in (a) Sr position and (b) Zr position. Total and partial DOS of Sm doped fm-SZ in both Sr and Zr position (c). The vertical lines represent Fermi level (E_F).

3.6.3 Photoluminescence mechanism in Sm³⁺-doped SZ. A combined experimental study and theoretical calculations enabled us to provide further insight into energy transfer and transition mechanisms (shown in Fig. 17). In Sm³⁺-doped fm-SZ, the absorption band is associated with excitation of oxygen-vacancy-trapped electrons from shallow and/or deep defect states (present at the top of the VB) to defect states present in the lower part of the CB. Subsequently, the photo-excited electrons in the defect states of CB may migrate to the Sm³⁺-related multiple excited states through the energy transfer process, due to the energy match between the electronic states of fm-SZ and the energy states of Sm³⁺ (shown in Fig. 16(c)). Finally, the excited photo-electrons at the excited f-states of Sm³⁺ could transfer to the long-lived ⁴G_5/2 of Sm³⁺ via non-radiative relaxation, and then produce strong orange-red emissions (combined by the emission of 565, 579, 643, and 703 nm) via radiative relaxation.

Fig. 17 Schematic of energy transfer mechanism from SrZrO₃ host to Sm³⁺ ions.

3.7. Materials performance: color coordinates

To evaluate the material performance on color-luminescent emission, CIE chromaticity coordinates were evaluated for undoped and 1.0 mol%-doped sample adopting standard procedures. The values of x and y coordinates of the system were calculated to be 0.202 and 0.166, respectively. This is represented by asterisk point (*) in the CIE diagram shown in Fig. 18(a). It is clear from the values that strontium zirconate, gives a violet-blue emission. The color can be tuned to an orangish-red emission (x = 0.550 and y = 0.375) on doping 1.0 mol% Sm³⁺ as shown in Fig. 18(b).

Fig. 18 (a) CIE index diagram of (a) SrZrO₃ and (b) SrZrO₃:Sm³⁺ system showing violet blue white and red emission (point *), respectively.

3.8. Radiative and non-radiative transition in SrZrO₃:Sm³⁺ compared to pure Sm₂O₃ powder-photoacoustic spectroscopy

The principles of photoacoustic (PA) spectroscopy are explained in our earlier work.^65,66 The PA spectra of pure Sm₂O₃ and Sm³⁺-doped SrZrO₃ are shown in Fig. 19. As compared to pure powder, the PA spectra of SrZrO₃:Sm³ is very weak and broad; moreover, most of the transitions are missing. Pure samarium oxide shows very sharp PA spectra with lots of features. The PA spectra of Sm₂O₃ samples can reveal the absorption and relaxation processes of Sm³⁺. The PA intensity spectrum of Sm₂O₃ at room temperature is shown in Fig. 19. It is distinguished by a sharply defined and almost line-like absorption band. Compared with the absorption spectra in solution, the PA spectrum is more complex and more intense.

Fig. 19 PA spectra of SrZrO₃:Sm³⁺.

In Fig. 18, the strongest PA band appears in the region of 400 nm and 471 nm, which is attributed to the transition from the ground state to excited state ⁴P_3/2 and ⁴I_3/2.^67,68 This indicates that the superior relaxation process of ⁴P_3/2 and ⁴I_3/2 is non-radiative relaxation. The weaker PA bands in the region of 556 nm and 525 nm are attributed to the transition from ground state to the excited state ⁴G_5/2 and ⁴F_3/2, respectively. The ⁴G_5/2 level is the first excited state of Sm³⁺, which means the radiative relaxation of ⁴G_5/2 is its prominent process. Since the ⁴F_3/2 level has many similarities with the ⁴G_5/2 level, the energy transferred to this level is easily transferred to the ⁴G_5/2 level. Thus, ⁴G_5/2 and ⁴F_3/2 are two radiative levels of Sm³⁺. As indicated in Fig. 19, the PA intensity corresponding to these two levels is fairly weak.

Among the energy levels of Sm³⁺, the longest-lived is ⁴G_5/2 (about 6.26 ms) and it is also a strong fluorescence energy level.⁶⁹ The electron in the excited level ⁴G_5/2 has a high probability for taking a radiative relaxation process. When the electrons are excited to higher energy levels such as ⁴G_7/2, ⁴F_3/2, they will usually relax to ⁴G_5/2 by a non-radiative process, and then relax by a radiative process (fluorescence), which can be interpreted according to the model discussed by Y. Yang et al.⁷⁰ In Fig. 19, PA signals of energy levels ⁴G_7/2, ⁴F_3/2, and ⁴G_5/2 are so weak that we could barely detect them, whereas excitation spectra show all such transitions.

In the doped sample, PA intensity was very weak compared to pure powder. This study confirms that non-radiative component is negligibly small in SrZrO₃:Sm³⁺, and this can be a good candidate for near ultraviolet (∼400 nm) light-emitting diodes.

4. Conclusions

In summary, SrZrO₃ and Sm³⁺-doped (1 mol%) SrZrO₃ nanoparticles sintered at 600 °C were synthesized by the gel-combustion method as well as characterized systematically using XRD, TEM, and TRFS experimental techniques. The nanocrystalline SZ sample showed defect-induced intense violet-blue and weak orange-red emissions. Based on EPR and theoretical studies, these defects were attributed to the presence of shallow and deep defects, respectively. Their corresponding lifetimes were also calculated using PL decay measurements. On doping Sm³⁺ in SZ, an efficient energy transfer takes place and Sm³⁺ ions are localized both in Sr and Zr positions of SZ. Theoretical calculations also showed that incorporating Sm at individual sites does not change the band gap at all; but incorporating Sm simultaneously at Sr and Zr sites decreases the band gap by 0.7 eV. PL decay time showed the presence of two lifetime values in the case of nanocrystalline SrZrO₃:Sm³⁺: (i) Sm³⁺ at Zr⁴⁺ site (τ = 500 μs) and (ii) Sm³⁺ at Sr²⁺ site (τ = 1.2 ms) in the ratio of 3 : 1. The asymmetry ratio was found to be 0.28 and 0.128 for short-lived (τ = 500 μs) and long-lived (τ = 1.6 ms) species, respectively, which is in accordance with the phonon energy concept. It was also observed that Sm³⁺ doping exhibits strong orange-red emissions (combined by emissions of 565, 595, 643, and 703 nm). The energy match between the electronic structure of SZ and energy levels of Sm³⁺ ions makes energy transfer from the host SZ to Sm³⁺ feasible. The present phosphor is considered to be a novel red light emitting luminescent material with very low non-radiative decay probability.

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