Perovskite SrZrO₃:Ho³⁺ phosphors: synthesis, structure, Judd–Ofelt analysis and photoluminescence properties

Abstract

A series of SrZrO₃:xHo³⁺ (x = 0.01, 0.03, 0.05, 0.07, 0.09, and 0.11 mol) perovskite phosphors have been synthesized by using the sol–gel technique. The structural and optical characteristics of the prepared phosphors have been investigated through powder XRD, FT-IR, UV-visible diffuse reflectance, and photoluminescence analysis. The photoluminescence emission spectra showed a bright characteristic peak at 545 nm (⁵F₄ + ⁵S₂ → ⁵I₈) under the 454 nm excitation, which exhibits emission in the green region of the electromagnetic spectrum. The emission intensity of the phosphors starts decreasing slowly beyond 3 mol% Ho³⁺ ions concentration due to concentration quenching, which is attributed to the dipole–dipole interaction between Ho³⁺ ions. The site symmetry of the Ho³⁺ ions has been studied by estimating the relative Judd–Ofelt intensity parameters (Ω_λ, where λ = 2, 4, 6) from the photoluminescence excitation spectrum of the SrZrO₃:0.03Ho³⁺ phosphor. The obtained findings suggest that the synthesized phosphors will be favorable for their bright green emission and thus, can be widely used for different optoelectronic applications.

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1. Introduction

Perovskite materials with the general formula ABO₃ (where A = Ca, Sr, Ba; and B = Zr, Hf, Ti) have great versatility and outstanding chemical, physical, electrical, and thermo-mechanical properties. These host lattices are getting special attention due to their potential applications as catalysts, photocatalysts, fuel cells, in photovoltaic applications, optoelectronics, and solid oxide fuel cells.^1–3 Perovskite materials possess a fascinating feature in which a slight variation in structure and chemical composition may result in enormous changes in their chemical and physical properties.^4–7 The doping of a foreign element into these ABO₃ type inorganic oxides influences their optical and magnetic properties by creating various defects.^8,9 Especially, lanthanide ions as dopants are suitable candidates since they exhibit unique spectroscopic properties.^10–12 The coordination geometry and oxidation states of uranium ions in the SrZrO₃ perovskite were studied by Gupta et al.,¹³ although Li et al.¹⁴ investigated the spectral characteristics and intrinsic defects of SrZrO₃ perovskite. The earlier report published by Knight et al.¹⁵ reveals the structural and thermoelastic characteristics of SrZrO₃ perovskite.

Perovskites have been reported to be advantageous in high-temperature applications, like hydrogen gas sensors, steam electrolysis, and fuel cells.^16–18 Proton conductivity at elevated temperature enables its usage in typical electrochemical devices. Among different perovskite materials, the SrZrO₃ host material has been suggested for use as a potential substrate due to its large single crystals.¹⁹ In recent years, the structural phase transitions of strontium zirconate perovskites have been studied significantly at room temperature and higher temperatures.^20–22 Initially, the structural investigations revealed that the SrZrO₃ possesses an orthorhombic phase at room temperature, and later Carlsson²³ proposed the presence of additional phases at high temperatures. Mete et al.¹⁷ examined the structural and electronic characteristics of 4d-perovskite: the cubic phase of SrZrO₃. The electronic and structural performances of selected surfaces of SrZrO₃ were investigated by Sambrano et al.²⁴ Singh et al.²⁵ have also published research on the photoluminescence and structural properties of SrZrO₃:Sm³⁺ orange-emitting perovskite phosphors.

Due to long-lived excited states and energy levels, the trivalent lanthanide ions doped luminescent materials (using Tm³⁺, Er³⁺, and Ho³⁺ ions) have been selected as the main subject of interest by several research groups.^26–28 Holmium belongs to the lanthanides and its electronic configuration becomes [Xe]4f¹⁰ when doped into a crystalline host. Several spectroscopic studies revealed that Ho³⁺ is the most desirable ion for mid-infrared lasers and has excellent green emission properties other than Tb³⁺ (green emission only) ions among the rare earth ions due to its various electronic transitions. Ranjan et al.²⁹ reported the enhanced green up-conversion emission of Ho³⁺ doped Gd₂O₃ phosphor by co-doping with Yb³⁺ ions. The luminescence studies of Eu³⁺ and Ho³⁺ doped Sr₂TiO₄ revealed that the Sr₂TiO₄ could be a suitable material in favor of high-pressure mercury vapor lamps or white light-emitting diodes.³⁰

A literature survey confirms a lack of reports available on Ho³⁺ doped ABO₃ type zirconate perovskites. The Ho³⁺ is often used as a structural probe because it can be accommodated at the A-place or B-place of perovskite oxides and in-site changes in the optical behavior and local site surrounding these doped oxide materials. Based on these results, our prepared sample possesses excellent thermal and chemical stability, prompting its practical application. Combining trivalent rare-earth metal ions in zirconate perovskites is considered as a promising approach to developing more useful and stable luminescent materials. Shi et al.³¹ prepared Yb³⁺, Ho³⁺, Li⁺ tri-doped TiO₂ up-conversion materials to enhance the efficiency of perovskite solar cells. First, Hou et al.³² investigated the impacts of Ho³⁺ ion doping over the surface morphology, crystal phase, and magnetic characteristics of BiFeO₃ thin films synthesized by the sol–gel technique. Sharif et al.³³ investigated surface morphology, structural, magnetic, and dielectric properties in the BiFeO₃ with holmium-doped thin films deposited by the pulsed laser deposition technique. Moreover, Hussain et al.³⁴ presented resistive leakage and intrinsic polarization analyses for high-performance piezo/pyroelectric Ho-doped 0.64Pb(Mg_1/3Nb_2/3)O₃-0.36PbTiO₃ binary ceramic materials.

Besides the solid-state reaction process, a conventional synthetic route for preparing phosphor particles, several new synthetic methods have already been developed, for example, co-precipitation, sol–gel, solution combustion, microemulsion, spray pyrolysis, and hydrothermal synthesis. The present work uses the sol–gel method to prepare SrZrO₃ phosphor. Sol–gel is a synthetic route to synthesize ceramic oxides, which provides reasonable control over stoichiometry, high purity, good homogeneity, and reduced sintering temperature. This method may also enable the production of low-temperature phases. A sol–gel method in a liquid includes a polycondensation reaction, which builds the oxide network of a molecular precursor. Although the process consists of several steps, doping concentration positively impacts the luminescence and crystal structural properties of the prepared phosphor. Wurm et al.³⁵ prepared sol–gel SrZrO₃ and SrTiO₃ coatings on C and SiC-fibers. Venkatesh et al.³⁶ prepared a novel strontium zirconate perovskite coating on an Inconel substrate using the sol–gel synthesis method. Liu et al.³⁷ reported sol–gel derived SrZrO₃ memory thin films with resistance switching properties. In this work, the Ho³⁺ doped SrZrO₃ perovskite phosphors were synthesized by using sol–gel synthesis. The prepared phosphors were characterized structurally and optically. To know the spectral characteristics, the measured photoluminescence excitation spectra were used to calculate the Judd–Ofelt intensity parameters, Ω₂, Ω₄ & Ω₆. Detailed photoluminescence properties are discussed herein.

2. Materials preparation and analysis

The SrZrO₃:xHo³⁺ (x = 0.01, 0.03, 0.05, 0.07, 0.09, and 0.11 mol) perovskite phosphors were fabricated by sol–gel procedure. The quantity of the employed starting materials is reported in Table 1. As per the chemical formulae, the stoichiometric quantities of strontium nitrate (Sr(NO₃)₂) (Sigma-Aldrich, purity: 99%), zirconium nitrate oxide dihydrate (ZrO(NO₃)₂·2H₂O) (Kanto chemical, purity: 99%), holmium(III) nitrate pentahydrate (Ho(NO₃)₃·5H₂O) (Sigma-Aldrich, purity: 99.9%), citric acid (C₆H₈O₇) (Junsei, purity: 99.5%) and, a mixture of 6 ml of ethanol and 4 ml of water combine in a 150 ml beaker. The molar ratio was kept at 2 : 1 according to citric acid and total metal ions. Next, the mixture was stirred for 1 h to achieve a clear homogeneous solution; after that, put the resultant in the oven until the solution dried. A hot temperature furnace maintained at 400 °C preheated the acquired gels for 2 h in air. After preheating, the samples were granulated and fired for 4 h at 1050 °C in the ambient condition, giving the fine powder samples. Fig. 1 presents a pictorial view of the synthesis process.

Table 1 Detailed information of sample composition and starting materials

Sample compositions	Weight of starting materials (g)
	Sr(NO3)2	ZrO(NO3)2·2H2O	C6H8O7	Ho(NO3)3·5H2O
SrZrO3:0.01Ho	0.8464	1.0688	3.0738	0.0176
SrZrO3:0.03Ho	0.8464	1.0688	3.0738	0.0592
SrZrO3:0.05Ho	0.8464	1.0688	3.0738	0.0882
SrZrO3:0.07Ho	0.8464	1.0688	3.0738	0.1243
SrZrO3:.0.09Ho	0.8464	1.0688	3.0738	0.1578
SrZrO3:0.11Ho	0.8464	1.0688	3.0738	0.1940

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Fig. 1 Systematic diagram of synthesis process of the sample.

The X-ray diffraction patterns were monitored by a RIGAKU (Miniflex-II) diffractometer attached to an X-ray source (Cu-Kα radiation, λ = 1.5406 Å); the scan rate was set at 5° per minute between 10° −80° for 2θ angle. To identify the functional group present in the prepared samples, a Fourier transform infrared (6700, Thermo Fisher Nicolet) spectrometer was operated in a range of 400–4000 cm⁻¹. A small quantity of prepared SrZrO₃:Ho³⁺ phosphor powder is used to measure diffuse reflectance with A Cary-5000 (UV-VIS-NIR) spectrophotometer coupled to a Praying Mantis diffuse reflectance accessory. Photoluminescence (PL) spectra were analyzed by a Shimadzu (RF-5301PC) spectrofluorophotometer fitted with a Xenon-flash lamp. The emission and excitation spectra were recorded using a spectral slit width of 1.5 nm. The above characterizations were performed at room temperature.

3. Results and discussion

3.1 Crystal structure

The XRD measurement was conducted to study the structural phase and crystallinity of the prepared samples. Fig. 2 displays the XRD patterns of the SrZrO₃:xHo³⁺ powders. For the synthesized phosphors, the major diffraction peaks matched with JCPDS (Joint Committee for Powder Diffraction Standards Card) File No. 76-0167 corresponding to SrZrO₃. The effect of Ho³⁺ ions (at the studied concentrations i.e. 0.01, 0.03, 0.05, 0.07, 0.09, and 0.11 mol) on the structure of the SrZrO₃ lattice seems to be negligible as XRD patterns remain the almost same at different doping concentrations. However, we believe the actual doping level can be clarified by Le Bail method to find the evolution trend of the cell lattice parameters. To calculate the crystalline size, the FWHM (full width at half maximum) of dominant (110) diffraction peaks are considered in leading Scherrer's equation, D = 0.9λ/β cos θ, in which the wavelength of incident X-rays is denoted as λ, the corresponding Bragg's diffraction angle is θ, and the FWHM of the (110) peak is β. The determined crystal sizes were approximately within the range of 24–31 nm. Various oxides with the ABO₃ chemical formula follow the perovskite structure. Fig. 3 showed the simplified crystal lattice of the SrZrO₃ perovskite. The SrZrO₃ has cubic symmetry with a Pm3̄m[221] space group. In the SrZrO₃ perovskite structure, the Sr atoms are situated on the edges of the cubic unit cell, and the 12 closest neighbor O atoms surround the Sr atoms. Similarly, the Zr atom is situated in the centrum of the unit cell and is six-fold integrated with the O closed-neighbor atoms, making an octahedron. The cubic unit cell faces have O atoms, which are two-fold coordinated with Zr neighbor atoms. The Zr-ion and Sr-ion have the coordination numbers 6 and 8, respectively.^1,11

Fig. 2 Powder XRD patterns of SrZrO₃:xHo³⁺ phosphors.

Fig. 3 Crystal structure of the SrZrO₃ perovskite.

3.2 Vibrational analysis

Fig. 4 shows a typical vibrational feature of the SrZrO₃:0.03Ho³⁺ powder sample. An intense absorption band at 558 cm⁻¹ is attributed to the Zr–O stretching vibration. We have also observed a sharp peak at 857 cm⁻¹. Katyayan and Agrawal⁸ studied SrZrO₃:Eu³⁺, Tb³⁺ system and several peaks reported in the range of 509–895 cm⁻¹ were due to the vibrational stretching modes of metal–oxygen bond, i.e., Zr–O bond. However, few peaks lie between 1000 and 1270 cm⁻¹ due to the active modes of asymmetric stretching of impurity ions. Further, sharp peaks were reported within 1302–1588 cm⁻¹ due to the symmetrical stretching of Sr–O bonds. We have also observed bands at 1018 and 1458 cm⁻¹. The observed bands and their assignments can be confirmed with the previous literature.^8,38–41

Fig. 4 FTIR spectrum of SrZrO₃:0.03Ho³⁺ phosphor.

3.3 Diffuse reflectance spectra and optical band gap

The UV-diffuse reflectance spectra were recorded between the wavelength regions of 200–800 nm for the 3 mol% Ho³⁺ doped SrZrO₃ phosphor. Fig. 5 shows the diffuse reflectance spectrum and extracted absorption coefficient with the Kubelka–Munk function. It can be seen that few bands around 365, 421, 454, 468, 489, 542, and 645 nm are related to the 4f–4f configuration of Ho³⁺ transitions: ⁵I₈ → ⁵G₅ + ³H₆, ⁵I₈ → ⁵G₅, ⁵I₈ → ⁵G₆, ⁵I₈ → ⁵F₁, ⁵I₈ → ⁵F₂ + ³K₈, and ⁵I₈ → ⁵F₃, respectively. The extracted optical-absorption coefficient (α cm⁻¹) was calculated by the consecutive expression:⁴²where R stands for sample reflectance. The E_g (optical band gaps) for the Ho³⁺ doped SrZrO₃ phosphor can be estimated with the Tauc relation:⁴³

F(R)ℏω ≈ A(ℏω − E_g)ⁿ (2)

where A is a constant, F(R) is the absorption coefficient with photon energy (ℏω) and n represents the power factor: n = 1/2 allowed direct transitions, and n = 2 allowed an indirect transition. Fig. 6 shows plots of (F(R)ℏω)ⁿ as a function of ℏω (eV). After assuming (F(R)ℏω)² = 0 and (F(R)ℏω)^1/2 = 0 for the linear region within the plot, the energy band gaps of the direct allowed and indirectly allowed transitions were observed to be 5.21 eV and 5.43 eV, correspondingly for the SrZrO₃:0.03Ho³⁺ phosphor.

Fig. 5 UV-visible diffuse reflectance spectrum of SrZrO₃:0.03Ho³⁺ phosphor. Inset of the figure shows extracted absorption spectrum.

Fig. 6 Plots of (F(R)ℏω)ⁿ versus ℏω (eV): (a) direct allowed transition and (b) indirect allowed transition.

3.4 Photoluminescence analysis

Fig. 7(a) demonstrates the photoluminescence excitation (PLE) spectra for the produced Ho³⁺ doped SrZrO₃ phosphors. The PLE spectra were acquired using the emission band at 545 nm. It can be read that the narrow bands owing to the 4f–4f transitions of Ho³⁺ ions and originated from ⁵I₈ to ³K₆ + ³F₄ (334 nm), ³L₉ + ⁵G₃ (346 nm), ³H₆ + ⁵G₅ (361 nm), ³K₇ + ⁵G₄ (387 nm), ⁵G₅ (418 nm), ⁵G₆ (454 nm), ⁵F₂ + ³K₈ (472 nm) and ⁵F₃ (486 nm). The highest PLE intensity was observed at 454 nm; this would be suitable as the excitation wavelength for all prepared phosphors. The PLE band intensity is saturated at 3 mol% of Ho³⁺ ions in the SrZrO₃ phosphor. Usually, the doping of Ho³⁺ ions in place of Sr²⁺ ions will create positive charge defects that would negatively affect luminescence. Therefore, the expected emission intensity is maximum for 3 mol% of Ho³⁺ ions, and then the subsequent decrease of emission intensity is due to the Ho_Sr⁺ defects.⁴⁴

Fig. 7 (a) Photoluminescence excitation spectra of SrZrO₃:xHo³⁺ phosphors (λ_em = 545 nm) and (b) photoluminescence emission spectra of SrZrO₃:xHo³⁺ phosphors (λ_exc = 454 nm).

The emission spectra were monitored using 454 nm as the excitation wavelength for all phosphors and are presented in Fig. 7(b). Strong green and weak red emission bands have been seen around 545 nm and 652 nm. The obtained bands are possibly defined as transitions of Ho³⁺ ions for ⁵F₄ + ⁵S₂ → ⁵I₈ and ⁵F₅ → ⁵I₈, respectively. The intensity of green emission at 545 nm is ∼35 times more substantial than the red emission at 652 nm. Upon excitation of 454 nm, the ions are excited from ⁵I₈ to ⁵G₄, then most of the excited ions decay the lower levels ⁵F₄ + ⁵S₂ and ⁵F₅ levels non-radiatively, and subsequently, radiatively transit to ⁵I₈ level with emission of green (⁵F₄ + ⁵S₂ → ⁵I₈) and red (⁵F₅ → ⁵I₈), respectively. The energy gap difference for the ⁵F₄ and ⁵F₅ to the next lower levels are around 3400 cm⁻¹ and 2400 cm⁻¹, thus the observed difference of green and red emission intensities depends on the population of excited states and multiphonon relaxation rates (W_mpr) since the W_mpr is increasing with the decrease of the energy gap between excited state to the next lower state. Fig. 8 shows a schematic energy level diagram for Ho³⁺ ions with possible radiative and non-radiative relaxation processes.

Fig. 8 Schematic energy level diagram for Ho³⁺ ions with possible radiative and non-radiative transitions.

As seen in Fig. 9, the highest emission intensity was noticed when the Ho³⁺ ions concentration was 3 mol%, and it was decreased according to the increasing concentration of Ho³⁺ ions in the SrZrO₃ phosphor because of concentration quenching. Generally, it is recognized that the Ho–Ho distance reduces with an enhancement of Ho³⁺ ion concentration, leading to fluorescence quenching that comes from an increase in the resonant transfer probability between Ho³⁺ ions. The chances of energy transmission distance between Ho–Ho ions are termed the critical distance (R_c) obtained by the Blasse expression:⁴⁵

where V is the unit cell volume, critical ion concentration is χ_c and N is the number of Zr ions of a unit cell. For our Ho³⁺ doped SrZrO₃ phosphor, V = 552.175 (Å)³,⁴⁶ N = 4, and χ_c = 0.03. The calculated critical transfer distance (R_c) is ∼20 Å, far greater than 5 Å that favors exchange interaction; thus, it can be established that the observed concentration quenching in Ho³⁺ doped SrZrO₃ samples is attributed to the multipole–multipoleelectric interaction.⁴² In addition, the interaction strength can also be calculated using the following equation:⁴⁷where K and β are constants, I is emission intensity, and x is activator ion concentration. The dipole–dipole (d–d), dipole–quadruple (d–q), quadruple–quadruple (q–q) interactions take place with Q = 6, 8, and 10, respectively. Fig. 10 shows the log(I/x) based on log(x) for the SrZrO₃:0.03Ho³⁺ phosphor. It can be observed that −Q/3 = - 2.24, so Q = 6.72. Thus, the quenching in emission intensity of Ho³⁺ doped SrZrO₃ host lattices is due to dipole–dipole interactions. The concentration quenching of Ho³⁺ doped SrZrO₃ phosphor that occurred beyond the optimized concentration (3 mol%) is most useful for emitting green light in optoelectronic devices.

Fig. 9 Variation of emission intensity (I_{545 nm}) as a function of Ho³⁺ ions in the SrZrO₃:Ho³⁺ phosphor.

Fig. 10 Plot of log(I/x) as a function of log(x) of the SrZrO₃:0.03Ho³⁺ phosphor (where I is the green emission peak intensity, and x is the Ho³⁺ ion concentration).

The CIE chromaticity coordinates were determined by the emission spectrum (λ_exc = 454 nm) of optimized Ho³⁺ (3 mol%) doped SrZrO₃ phosphor using 1931 CIE (Commission International de l’Eclairage France) technology, which is an accepted standard for the LED industry in matters related to colors, such as color mixing and color rendering. The chromaticity coordinates were found to be (0.322, 0.671) and this is situated in the green region of the CIE chromaticity diagram (see Fig. 11). The correlated color temperature (CCT) was also estimated by the McCamy experimental equation:⁴⁸

CCT = −437n³ + 3601n² − 6861n + 5514.31 (5)

where with chromaticity epicenter being x_e = 0.3320 and y_e = 0.1858. The obtained CCT is 5654 K for the Ho³⁺ (3 mol%) doped SrZrO₃ phosphor. Thus, this could be useful for w-LEDs because a CCT < 5000 K gives warm-white LEDs for home gadgets.

Fig. 11 CIE chromaticity diagram of SrZrO₃:0.03Ho³⁺ phosphor.

3.5 Judd–Ofelt intensity parameters from PLE and radiative properties

The familiar Judd–Ofelt intensity parameters (Ω_λ with λ = 2, 4, and 6) were adopted to figure out the fluorescence branching ratios, spontaneous emission probabilities, and radiative lifetimes of the excited multiplets to assess the undertaking of lasers and luminescent materials. As per Judd–Ofelt (J–O) theory,^49,50 the J–O intensity parameters (Ω_λ with λ = 2, 4, and 6) can be examined conventionally from absorption spectra by evaluating the measured and computed spectral line strengths of the excited 4f–4f electronic transitions using least-square or chi-square fit methods. However, in recent decades,^42,51,52 a simple approach has been proposed to evaluate J–O intensity parameters from the assessment of excitation spectra. This approach is successfully applied to Nd³⁺, Er³⁺ and Dy³⁺ doped various phosphor powders.

Normally, the excitation and absorption spectral difference lies in the intensity ratio of the fluorescence excitation to the absorption (i.e., relative fluorescence quantum efficiency). Therefore, the excitation and absorption spectra will coincide exactly while relative fluorescence quantum efficiency maintains to be constant at different wavelengths. Once the experimental excitation spectrum is corrected to the corresponding absorption spectrum in which the excited states are followed by a very fast non-radiative relaxation to the monitored level.^53,54 In this work, the excited multiplets of Ho³⁺: ⁵G₅, ⁵F₂(³K₈), ⁵G₆, ⁵F₃ levels are nonradiatively relaxed to ⁵F₄ + ⁵S₂ monitored level may satisfy the above statement, and excited multiplets chosen as ideal ones for the determination Judd–Ofelt parameters. The calculated and measured relative excited line strength for the attended electric-dipole transitions across the aJ and bJ′ levels are determined using the following expressions,^55,56

where Ω_λ is the J–O intensity parameter, which is used in the environmental field effect of intermixing states, i.e., 4f^N−15d and 4f^N−15g. U(λ) is the doubly reduced matrix tensor operator and it is found in the coupling approximations approach, which is taken from the literature.⁵⁶ The term n ∼2.12 is the refractive index of SrZrO₃ material.⁵⁷ The average wavelength of the excitation band is denoted as [small lambda, Greek, macron], N₀ is the ion concentration and [capital Lambda, Greek, macron] is the integrated relative excitation intensity of each band. Ω_λ parameters were predicted by a least square fitting technique.⁴³ Table 2 shows relative spectral line strengths of excited transitions for Ho³⁺ ions (3 mol%) in the SrZrO₃ phosphor.⁵⁵ The root average square deviation (δ_rms) between experimental line strengths is,where q and p are fitting parameters as transition number, and it has been used in our case as q = 8 and p = 3 in the best least square fitting procedure. The observed small δ_rms value (see Table 2) is indicative of the validity and fit quality in J–O theory.

Table 2 Doubly reduced matrix elements ‖U²‖^λ (λ = 2, 4 and 6), relative line strengths (S^ed) (×10⁻²⁰ cm²) for the observed excitation/absorption bands of Ho³⁺ doped various host matrices

Transitions from ^5I8 →	Excitation band wavenumber (ν cm^−1)	‖U^2‖^2	‖U^2‖^4	‖U^2‖^6	^aSrZrO3:0.03Ho^3+		YLiF^55	GdLiF4 (ref. 55)	LuLiF4 (ref. 55)
					S^edmeas	S^edcal	S^edmeas	S^edmeas	S^edmeas
a Present work.
^3K6 + ^3F4	29 922	0.0026	0.1263	0.0073	0.101	0.024	0.309	0.341	0.251
^3L9 + ^5G3	28 902	0.0185	0.0052	0.1169	0.080	0.031	0.394	0.379	0.300
^3H6 + ^3H5	27 685	0.254	0.2337	0.1609	0.059	0.173	1.310	4.377	1.276
^5G4 + ^3K7	25 846	0.0058	0.0361	0.0697	0.012	0.021	0.325	0.345	0.307
^5G5	23 872	0	0.5338	0.0002	0.087	0.085	1.161	1.199	1.120
^5G6	22 020	1.5201	0.8410	0.1535	0.809	0.791	3.893	4.177	4.014
^5F2 + ^3K8	21 186	0.0208	0.0334	0.3576	0.094	0.083	0.392	0.446	0.248
^5F3	20 542	0	0	0.3464	0.084	0.067	0.716	0.811	0.375
δrms					± 0.011		± 0.117	± 0.114	± 0.108

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Table 3 describes the J–O intensity parameters considering various host matrices.^58–62 The Ω₂ parameter indicates ionicity (or covalence) of RE-O bonds and is related to the local structure. Ω₄ and Ω₆ are non-sensitive to the dependence structure and are attributed to the stiffness of the host; however, the various active ions alter the characteristics of the evaluated spontaneous emission transitions. For example, from Table 3, the Ho³⁺ doped SrZrO₃ phosphor shows a lower ionic nature between Ho–O bonds compared with other oxide-based host matrices. On the other hand, when compared with a fluoride-based host, the Ho³⁺ doped SrZrO₃ phosphor shows a higher ionic nature between the Ho–O bonds because of the lesser values of the J–O intensity parameter, Ω₂.

Table 3 Relative Judd–Ofelt intensity parameters, (Ω_λ ×10⁻²⁰ cm², λ = 2, 4 and 6) of various host matrices

Host	Ω2	Ω4	Ω6	Order
a Present work.
^aSrZrO3:0.03Ho^3+	0.41	0.16	0.19	Ω6 > Ω4 > Ω2
Y3Al5O15(YAG)^58	0.04	2.67	1.89	Ω4 > Ω6 > Ω2
Lu3Al5O12 (ref. 59)	0.17	2.08	1.92	Ω4 > Ω6 > Ω2
LaF3 (ref. 60)	1.16	1.38	0.88	Ω2 > Ω4 > Ω6
LiYF4 (ref. 61)	1.01	1.71	1.21	Ω4 > Ω6 > Ω2
LiYF4 (ref. 62)	0.96	2.05	1.43	Ω4 > Ω6 > Ω2

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4. Conclusions

The SrZrO₃:Ho³⁺ phosphors were produced by a sol–gel system and were analyzed by X-ray diffraction, FTIR, UV-visible and photoluminescence spectroscopic techniques. We have identified absorption bands around 365, 421, 454, 468, 489, 542, and 645 nm from the UV-visible diffuse reflectance spectrum of the 4f-4f configuration of Ho³⁺ transitions. The optical band gaps (E_opt) were found to be 5.21 eV (direct transition), 5.43 eV (indirect transition), respectively for the Ho³⁺ doped SrZrO₃ phosphor. Upon 454 nm excitation, the Ho³⁺ doped SrZrO₃ phosphor exhibits high green and low red emission bands that are connected to the respective ⁵F₄ + ⁵S₂ → ⁵I₈ (545 nm) and ⁵F₅ → ⁵I₈ (652 nm) transitions of Ho³⁺ ions. The fluorescence quenching of the studied phosphor samples was evaluated by looking at the critical distance between Ho–Ho ions as well as the strength of dipole–dipole (d–d), dipole–quadruple (d–q) and quadruple–quadruple (q–q) interactions. The obtained CCT was 5654 K for the optimum concentration of Ho³⁺ (3 mol%) doped in the SrZrO₃ phosphor suggesting that it may be useful for w-LEDs. In addition, using excitation spectrum of the optimized phosphor, the Judd–Ofelt intensity parameters (Ω_λ with λ = 2, 4 and 6) for Ho³⁺ were estimated and compared with other hosts. The observed results of photoluminescence properties suggested that the SrZrO₃:0.03Ho³⁺ phosphor may be advantageous for green emitting optoelectronic applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2018M2B2A9065656). This paper was also supported by the KU Research Professor Program of Konkuk University.

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