Evaluation of Li₂SnO₃:Cr³⁺, Mn⁴⁺ as a dual-emitter luminescence sensor for cryogenic temperatures

Abstract

The sensitivity of luminescence properties in materials doped with transition metal (TM) ions to changes of temperature makes them particularly promising for thermometric applications. Designing and optimizing such materials requires a deep understanding of their structure, local environment of emission centres, and luminescence processes. In this work, we investigate the potential of Li₂SnO₃ doped with Cr³⁺ and Mn⁴⁺ as a dual-emitting luminescence temperature sensor. Li₂SnO₃ was chosen as the host material due to it being able to host both Cr³⁺ and Mn⁴⁺ at Sn octahedral positions. As a result, Mn⁴⁺ ions exhibit a distinctive ²E → ⁴A₂ line emission due to a strong crystal field, and Cr³⁺ ions experience an intermediate crystal field strength resulting in both, ²E → ⁴A₂ and ⁴T₂ → ⁴A₂ emissions. Through thorough examination, using powder X-ray diffraction (XRD), electron paramagnetic resonance (EPR) and photoluminescence techniques we identified two distinct types of [SnO₆] octahedral centers that correspond to two types of slightly different Cr³⁺ and Mn⁴⁺ emission centers in the Li₂SnO₃ structure. The high sensitivity of the decay time constant for the ²E → ⁴A₂ emission of Li₂SnO₃–Cr³⁺, Mn⁴⁺ to temperature changes (2.0%/K at 190 K and 5.8%/K at 220 K for Cr³⁺ and Mn⁴⁺, respectively) positions the material as an attractive non-contact temperature sensor. Furthermore, application of such a dual-emitter luminescence material as a temperature sensor expands its sensitivity across a broader temperature range and offers the additional advantage of cross-checking measurements compared to materials solely doped with Cr³⁺ or Mn⁴⁺.

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Introduction

The electronic configuration of excited states of transition metal (TM) ions with an unfilled 3d shell provides the basis for a unified methodology of interpreting their optical properties within the framework of crystal field theory.^1,2 This method has been successfully applied towards understanding the nature of the luminescence of Cr³⁺ and Mn⁴⁺ in different hosts.^3–8 These ions are typically stabilized in octahedral or distorted octahedral environments, coordinated by six oxygen ions, where the energy structure of 3d states and their dependence on crystal field strength are well explained by the classical Tanabe–Sugano diagram.^1,2,7,8 The diagram shows that the energy of most multiplets except ²T₁ and ²E_g are strongly dependent on the crystal field strength, which has a decisive impact on luminescence. Mn⁴⁺ ions doped in various hosts always experience a strong crystal field which gives rise to emission lines from ²E → ⁴A₂ transitions, aided by lattice vibrations.⁵ In contrast, the effect of the surrounding crystal field on Cr³⁺ ions can vary from weak to strong as established through relevant luminescence spectra. Specifically, a broad emission band due to ⁴T₂ → ⁴A₂ transitions and sharp ²E → ⁴A₂ lines are simultaneously observed in intermediate crystal fields, while in weak crystal fields the line emission disappears.⁸ This dependence of the emission properties of Cr³⁺ and Mn⁴⁺ on the strength of the local crystal field enables customization and fine-tuning of the emission characteristics for specific applications.

In this regard, of particular interest is the case when both Cr³⁺ and Mn⁴⁺ can occupy the same crystallographic position. This may occur in the same host despite of the different valence states of the ions. In such a case the needs to compensate a local charge disparity may create additional perturbations of the crystal field surrounding the impurity, resulting in changes of the emission properties. To date only characteristic sharp emission peaks due to spin-forbidden transitions ²E → ⁴A₂ of Cr³⁺ and Mn⁴⁺ have been reported in several hosts, indicating that both ions are experiencing a strong crystal field effect.^9–13 It should be noted that such materials with two types of emission centers are particularly interesting for application as multimodal luminescence sensors of temperature.¹⁴ The dual-emitter sensor allows to extend the temperature range of operation and improves the reliability of the technique. It would be interesting therefore to investigate a material where in addition to the spin-forbidden transitions of Mn⁴⁺ and Cr³⁺ the spin-allowed transitions ⁴T₂ → ⁴A₂, yielding broadband emission, can also occur. Due to the coexistence of two radiative channels, the luminescence characteristics of such a material may exhibit enhanced sensitivity to changes of temperature that can be exploited for temperature monitoring.

To test this idea we turned our attention to Li₂SnO₃, a material currently being investigated for its potential as an electrode in lithium batteries^15–18 and light emitting devices (LED).¹⁹ Li₂SnO₃ has a honeycomb layered monoclinic structure that contains octahedral sites SnO₆ which are suitable for accommodating Mn⁴⁺ and Cr³⁺ ions with smaller ionic radius (r = 0.54 and 0.61 Å, respectively) in place of the larger Sn⁴⁺ ion (r = 0.69 Å). Previous research has shown that Li₂SnO₃ doped with Mn⁴⁺ exhibits strong red luminescence originating from ²E → ⁴A₂ transitions.^20,21 However, to the best of our knowledge no studies have been conducted on Cr-doped Li₂SO₃. We successfully synthesized the material using a hydrothermal method²² and preliminary examinations have indicated that Cr³⁺ ions occupy the octahedrally-coordinated Sn⁴⁺ sites in the Li₂SnO₃ host, experiencing a crystal field of intermediate strength (Dq/B = 2.48). This field is responsible for the characteristic room temperature luminescence of Cr³⁺i.e. a broad luminescence band spanning the range from 650 to 850 nm with a narrow and sharp emission line at 700 nm. These originate from spin-allowed ⁴T₂ → ⁴A₂ and spin-forbidden ²E → ⁴A₂ transitions of the Cr³⁺ ion, respectively. In this study we conducted a detailed characterization of Li₂SnO₃ co-doped with Cr³⁺and Mn⁴⁺ in the temperature range from 4 to 300 K, aiming to establish the potential of this dual-emitter phosphor for application as non-contact luminescence sensor of temperature.

Experimental methods

The Li₂SnO₃–Cr³⁺, Mn⁴⁺ nanocrystalline phosphor was synthesized as described in ref. 22. In particular, the material studied in the present work has been calcined at 1200 °C. The photoluminescence (PL) and the photoluminescence excitation (PLE) spectra were measured using a Horiba/Jobin-Yvon Fluorolog-3 spectrofluorometer with a 450 W continuous spectrum xenon lamp for excitation. The emission was detected by a Hamamatsu R928P photomultiplier operating in a photon counting mode. The PL spectra were corrected for the spectral response of the used system. The luminescence decay kinetics were measured using the same Fluorolog-3 spectrofluorometer with the excitation light modulated by a mechanical chopper. The spectroscopic measurements in the temperature range of 4.2–330 K were carried out in a Janis continuous-flow liquid helium cryostat using a Lake Shore 331 temperature controller. The studies in the temperature range of 300–600 K were done using a Linkam THMS600 temperature stage.

Electron paramagnetic resonance (EPR) spectra were recorded at a magnetic induction in the range of B = 10–1400 mT on a conventional X-band Bruker ELEXSYS E 500 CW-spectrometer, operating at 9.5 GHz with 100 kHz magnetic induction modulation and microwave power of P = 2.002 mW. The temperature dependence of the EPR spectrum was analysed using a nitrogen-flow cryostat (Oxford Instruments ESP) in the nitrogen temperature range. The position of the EPR line is determined by the formula g_eff = 71 44 773 f_ez(GHz)/B_rez(mT), where f_rez is the microwave frequency and B_rez is the position of the resonance line in the magnetic induction. The EPR/NMR program was used to find local symmetry and Spin-Hamiltonian (SH) parameters of the chromium ions.²³ The optimization and normalization of the above parameters was performed using a root-mean-square deviation method.

Results and discussion

Crystal structure

The crystal structure of Li₂SnO₃ doped with Cr³⁺ was studied recently by X-ray powder diffraction and application of full profile Rietveld refinement. Comprehensive details of the refinement procedure along with a complete set of structural data including unit cell dimension, coordinates, and displacement parameters of atoms can be found in ref. 22. The Li₂SnO₃ structure belonging to space group C2/c, Z = 4 can be visualised as stacked slabs comprising interconnected [SnO₆] and [LiO₆] octahedra in a 2 : 1 ratio. These are interleaved with layers consisting of Li⁺ ions (see Fig. 1). A distinctive feature of the Li₂SnO₃ host structure is the presence of three different octahedral positions of Li⁺ ions, located at the 4d, 4e and 8f Wyckoff sites. Furthermore, the structure boasts two non-equivalent four-fold 4e sites, which are occupied by Sn atoms in an octahedral oxygen environment. The octahedral coordination of Sn⁴⁺ ions in both sites provides a conducive environment for accommodating Cr³⁺ and Mn⁴⁺ cations.

Fig. 1 Representation of Li₂SnO₃ structure as stacked slabs composed of interconnected [SnO₆] and [LiO₆] octahedra separated by layers of Li⁺ ions. The drawing shows the shortest Li–O and Sn–O interatomic distances. The right panel shows an enlarged view of two structurally nonequivalent octahedral sites of Sn atoms (Sn1 and Sn2), both of which are suitable for the incorporation of Cr³⁺ and Mn⁴⁺ ions.

An examination of the nearest interatomic distances in the Li₂SnO₃:Cr³⁺ structure, calculated from the atomic coordinates reported in ref. 22, reveals a difference in the configuration of [SnO₆] octahedra around Sn1 and Sn2 atoms. Specifically, the distribution of the six nearest Sn–O distances inside [Sn1O₆] and [Sn2O₆] polyhedra (as listed in Table 1) indicates that the [Sn1O₆] octahedron with a volume of 12.57 ų exhibits significantly greater distortion compared to the [Sn2O₆] octahedron which has a volume of 11.28 ų (see Fig. 1). Given that dopant ions can incorporate into both of these Sn sites, it is anticipated that there will be two different types of Cr³⁺ and Mn⁴⁺centres in the Cr- and Mn-doped Li₂SnO₃ materials. It is worth noting that this peculiarity of the host lattice, namely the existence of two different octahedral Sn sites was not identified in recent publications that studied luminescence properties of Li₂SnO₃:Mn⁴⁺ phosphors.^20,21

Table 1 The shortest intra-octahedral distances in the Li₂SnO₃:Cr³⁺ structure, with corresponding standard deviations given in parentheses. The average distances within different types of [LiO₆] and [SnO₆] octahedra are provided for reference

Polyhedron, atoms	Distances in Å	Polyhedron, atoms	Distances in Å	Polyhedron, atoms	Distances in Å
[Li1O6]		[Li2O6]		[Sn1O6]
Li1–O3	2.02(5)	Li2–O3	2 × 2.02(2)	Sn1–O3	2 × 2.04(2)
Li1–O1	2.17(5)	Li2–O2	2 × 2.16(2)	Sn1–O1	2 × 2.12(2)
Li1–O3	2.25(5)	Li2–O1	2 × 2.28(2)	Sn1–O2	2 × 2.19(2)
Li1–O2	2.32(5)	Li2–Oave	2.15	Sn1–Oave	2.12
Li1–O2	2.32(5)	[Li3O6]		[Sn2O6]
Li1–O1	2.47(5)	Li3–O2	2 × 2.06(2)	Sn2–O2	2 × 2.00(2)
Li1–Oave	2.25	Li3–O1	2 × 2.19(2)	Sn2–O1	2 × 2.03(2)
		Li3–O3	2 × 2.19(2)	Sn2–O3	2 × 2.10(2)
		Li3–Oave	2.15	Sn2–Oave	2.04

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Electron paramagnetic resonance studies

EPR allows detection of even small amounts of paramagnetic ions within the studied materials. It aids in discerning key structural properties such as the local symmetry of paramagnetic ions and their interactions. In the case of Li₂SnO₃ doped with Cr³⁺ ions and Mn⁴⁺ ions, we expect to observe a distinct EPR signal from these dopants. Cr³⁺ ions are paramagnetic ions with an electron configuration of 3d³ and an electron spin S = 3/2. Similarly, Mn⁴⁺ ions are paramagnetic ions with an electron configuration of 3d³ and electron spin S = 3/2. Additionally, for Mn⁴⁺ ions, we expect an EPR signal arising from the ⁵⁵Mn isotope (100% abundance) possessing a nuclear spin S = 5/2. It is worth noting that the position of the signal is strongly influenced by the local environment of the paramagnetic ions. This feature enables us to ascertain the local symmetry of these dopants. Fig. S1 (ESI†) showcases the EPR spectra for the Li₂SnO₃:Cr³⁺, Mn⁴⁺ recorded at temperatures T = 80 K and T = 300 K.

The EPR spectra recorded at room temperature exhibit three distinct lines emerging from two magnetic centres. Specifically, the EPR line with g_eff = 1.999 can be attributed to the Mn⁴⁺ ion while the lines with g_eff = 3.649 and g_eff = 1.38 are characteristic of Cr³⁺ ions within an octahedral environment; notably these two EPR lines corresponds to a single Cr³⁺ centre,²⁴.²⁵ As temperature drops, the components of the EPR lines originating from Cr³⁺ ions become more discernible along with the hyperfine interactions of Mn⁴⁺ ions. To delve further into ion symmetry, we selected the lowest temperature of T = 80 K as this provides a clearer understanding of the structural arrangement of the ions in question.

The spin Hamiltonian (SH) parameters for ions with spin S = 3/2 were determined using the EPR/NMR programme through the following system of equations:

where μ_B – Bohr magneton, B – magnetic induction, g – spectroscopic splitting factor, S – electron spin, D – axial and E – rhombic distortions of octahedral, A – hyperfine interactions matrix, and I – nuclear spin. The spin Hamiltonian for Cr³⁺ ions possessing a spin S = 3/2 comprises a Zeeman term and a zero-field splitting term. In contrast, the spin Hamiltonian for Mn⁴⁺ ions with spin S = 3/2 and I = 5/2 consists of a Zeeman term, a zero-field splitting term and hyperfine interactions. Non-zero values of these parameters offer insight into the symmetry of the paramagnetic centre.

For single Cr³⁺ ions the following values were calculated: g_x = g_y = 1.998(2), g_z = 1.997(2), and D = 0.252(5) cm⁻¹ (E = 0). These parameters suggest an axially symmetric site for the Cr³⁺ ions with insignificant distortions of an octahedra. Similarly, for single Mn⁴⁺ ions the values are: g_x = g_y = 1.985(3), g_z = 1.980(3), D = 0.006(1) cm⁻¹, E = 0, A_x = A_y = 0.0030(2) cm⁻¹, A_z = 0.0029(2) cm⁻¹. These parameters affirm an axially symmetric site for the Mn⁴⁺ ions. The spin Hamiltonian parameters were obtained from the best fit of the experimental EPR spectra to the simulated spectra generated using the spin Hamiltonian. Fig. 2(a) and (b) illustrate the experimental spectra and simulated spectra, derived from the above equations and SH parameters.

Fig. 2 EPR spectrum of a Li₂SnO₃:Cr³⁺, Mn⁴⁺ at temperature T = 80 K. The red (a) and blue (b) curves are graphical representations of the resulting SH parameter fits for Cr³⁺ and Mn⁴⁺ ions, respectively.

It is worth noting that at temperatures below T < 120 K the signal at g_eff ∼ 3.6 apparently consists of two EPR lines (inset of Fig. S1, ESI†) of the same intensity. This may indicate that the signal from Cr³⁺ ions originates from two Cr³⁺ centres (Cr³⁺ → [Sn1O₆] and [Sn2O₆]) with different distortions. EPR studies do not confirm the presence of two different Mn⁴⁺ centres, but it should be borne in mind that the EPR signal from Mn⁴⁺ ions consists of many lines originating from fine and hyperfine interactions, meaning that two Mn⁴⁺ ions with the same symmetry but different distortion cannot be distinguished.

Effect of temperature on the luminesce properties

Previous studies of Li₂SnO₃ doped with Cr³⁺ or Mn^4+ 19–22 provide some insight into the expected emission properties of this material when co-doped with both ions. Given that the concentration of the impurity ions is maintained at low level (<0.1%) and there is no spectral overlap between the emission and excitation spectra of different impurities, no energy transfer is expected between the two emitters. Consequently, the luminescence of Li₂SnO₃:Cr³⁺, Mn⁴⁺ represents a superposition of emissions from individual ions resulting in a combined spectrum as depicted in Fig. 3.

Fig. 3 Luminescence spectrum of Li₂SnO₃:Cr³⁺, Mn⁴⁺ measured at 480 nm excitation (T = 40 K, (a) and excitation spectra monitored at Cr³⁺ and Mn⁴⁺ emission peaks at T = 4.2 K, (b).

The configuration coordinate model offers an innate way towards understanding the emission properties of Cr³⁺ and Mn⁴⁺ ions (see Fig. 4). The vertical blue arrows indicate excitation transitions that promote electrons from the ground state to the ⁴T_1g and ⁴T_2g states via parity-allowed transitions. The luminescence excitation bands due to these transitions are clearly seen in the luminescence excitation spectra displayed in Fig. 3(b). Subsequently, the electrons promptly relax along the parabola to ²E_g and ⁴T_2g states. At this stage the splitting of the subsequent relaxation pathway becomes apparent.

Fig. 4 Energy diagram of Mn⁴⁺ and Cr³⁺ ions showing excitation (blue), emission (red) and thermally activated (black dotted) transitions.

The strong crystal field experienced by Mn ions shifts the ⁴T_2g states above the ²E_g levels, leaving only one channel for radiative recombination through the spin-forbidden transitions ²E → ⁴A₂, giving rise to the sharp emission lines (red arrow). In contrast, the weaker crystal field experienced by Cr³⁺ places the ⁴T_2g states at lower energy, thus opening an additional channel for the radiative recombination through the spin-allowed ⁴T_2g → ⁴A₂ transitions, as shown in Fig. 4. Hence in addition to the line emission due to ²E → ⁴A₂ transitions, broad band emission occurs when the temperature rises, and the ⁴T_2g states are populated by thermally induced transitions from the ²E_g levels (black dotted arrow). The positions of the main excitation bands and emission peaks of Cr³⁺ and Mn⁴⁺ observed at low temperature are listed in Table 2. The subsequent studies of the luminescence of Li₂SnO₃:Cr³⁺, Mn⁴⁺ have revealed the influence of temperature.

Table 2 Position of Mn⁴⁺ and Cr³⁺ emission and excitation peaks in Li₂SnO₃:Cr³⁺, Mn⁴⁺

Ion, (position)	Ē → ^4A2 (R1-line), nm	2Ā → ^4A2 (R2-line), nm	R 1 – R2 gap, meV	^4A2 → ^4T2g, nm	^4A2 → ^4T1g, nm
a The high energy excitation band ^4A2 → ^4T1g of Mn^4+ is not resolved due to overlap with the excitation band, resulting from the charge-transfer transitions O^2− → Mn^4+.
Mn^4+(I)	655.9	654.4	4.0	473	(Not detected)^a
Mn^4+(II)	656.4	655.6	2.4
Cr^3+(I)	699.0	698.5	1.7	617	440
Cr^3+(II)	699.1	698.5	1.6

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Initially, our study focussed on temperature-induced variations of the narrow band emissions of Mn⁴⁺ and Cr³⁺ caused by the ²E → ⁴A₂ transitions in the Li₂SnO₃. Fig. 5(a) shows that with rising temperature the intensity of the emission lines experiences a gradual reduction with increasing temperature. High-resolution low-temperature photoluminescence measurements revealed that the zero-phonon (R₁) line of Cr³⁺ appears as two closely spaced lines which merge into a single line above 100 K while the R₂ line of Cr³⁺ remains unsplit even at the lowest temperatures. In the case of Mn⁴⁺ a pair of R₁ and R₂ lines is clearly visible in low-temperature spectra as illustrated in Fig. 5(b). This dual set of R-lines for both Cr³⁺ and Mn⁴⁺ centers implies the existence of two distinct types of these centers, originating from the substitution of TM ions for two different sites occupied by Sn⁴⁺ cations in the host lattice. It is pertinent to remark that incorporation of TM ions at different crystallographic sites is a common feature observed in complex oxides.^26–28

Fig. 5 Temperature evolution of ZPL emission (²E → ⁴A₂) for Cr³⁺ (a) and Mn⁴⁺ (b) in Li₂SnO₃–Cr³⁺, Mn⁴⁺.

The observed temperature dependencies offer insight into the dynamics governing the population of excited states within the emission centres under consideration. To attain a more comprehensive understanding of this behaviour, we examined the normalized intensity of various emissions, presented as a function of temperature (see Fig. 6). As it is seen, the ²E → ⁴A₂ emission of Cr³⁺ and Mn⁴⁺ is significantly affected by thermal quenching. Our finding suggests that the measured temperature dependence cannot be adequately modelled by a canonical Mott–Seitz type expression which is typically employed to describe thermal quenching as a single step deactivation process.^29,30

Fig. 6 Normalized intensity of the narrow-line (²E → ⁴A₂) emissions of Mn⁴⁺ (blue) and Cr³⁺ black) and broadband (⁴T₂ → ⁴A₂) emission of Cr³⁺ (green) with temperature. Red lines show the fitting of experimental results by eqn (2) and (3).

Aiming to address this behaviour comprehensively, we adopted an approach previously validated in ref. 31 using an equation designed to accommodate the presence of two distinct pathways for thermal deactivation:

In this equation, I₀ is the total emission intensity at zero temperature, C₁ and C₂ are the transition rate constants, E₁ and E₂ are the activation energies for the non-radiative quenching, k is the Boltzmann constant. Using this equation, we achieved a very good agreement of the theoretical curve (the solid red line in Fig. 6) with the experimental data. The parameters of the fit are summarized in Table 3.

Table 3 Parameters of the fit of luminescence intensity as function of temperature, using eqn (2) and (3)

Parameter	Mn^4+ (^2E → ^4A2)	Cr^3+ (^2E → ^4A2)	Cr^3+ (^4T2g → ^4A2)
I 0	1	1	0.084 ± 0.004
A 1	—	—	53.5 ± 3.7
, meV	—	—	24.9 ± 0.8
C 1	1.6 ± 0.6	1.5 ± 0.4	1.10 ± 0.28 × 10^2
E 1, meV	25.9 ± 4.2	14.8 ± 1.7	113 ± 7
C 2	2.8 ± 0.3 × 10^6	8.5 ± 2.1 × 10^2	5.2 ± 0.5 × 10^6
E 2, meV	239 ± 18	93 ± 4	453 ± 31
R ^2	0.999	0.999	0.999

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A prominent feature of the luminescence in Li₂SnO₃ is a broad band emission associated with the ⁴T_2g → ⁴A₂ transitions of Cr³⁺ which becomes prominent with increasing temperature, peaking at ca. 225 K as displayed in Fig. 7. Subsequently, it undergoes a gradual decline, disappearing above 500 K. This type of temperature dependence, referred to as negative thermal quenching, has been observed in the luminescence of some materials,^32,33 including phosphors doped with TM ions.³⁴ The phenomenon is explained by considering two main processes that govern the population of the emitting state (i) thermally activated population of the emitting centres that lead to an increase of the luminescence and (ii) the quenching of the emission intensity due to the non-radiative decay of emission centres. In the framework of the model developed by Shibata³⁵ the observed temperature behaviour of broadband emission can be readily understood, considering that the ⁴T_2g state is being populated through the thermally activated crossover transition from the metastable ²E levels of Cr³⁺ as illustrated in Fig. 4. This process is responsible for the emergence of this band with heating with the decrease of the emission intensity occurring above 225 K, the result of thermal quenching. It should be noted that the increase in intensity of this band coincides with a simultaneous decrease in the intensity of the ²E → ⁴A₂ emission intensity of Cr³⁺, also evident in Fig. 7. To account for this activation process within the model, an additional term is introduced in the numerator of eqn (2) which leads to the following expression for the temperature dependence of the emission intensity:³⁵

Fig. 7 Temperature evolution of the PL spectrum of Cr³⁺ ions in Li₂SnO₃ excited at 610 nm.

In this formula, the additional parameters A₁ and are the transition rate constant and the activation energy necessary to promote the charged electrons from the metastable to the emission state respectively. Consequently, the expression in the numerator is responsible for the rise of emission due to the thermally activated population of the emitting transition ⁴T_2g states. Conversely, the denominator of the equation encompasses processes related to thermally activated non-radiative quenching of the emitting state. The measured temperature dependence of the integrated luminescence intensity of the broad band emission of Cr³⁺ was fitted using this equation over the entire temperature range confirming validity of the model used (see Fig. 6). The best fit was achieved using parameters collated in Table 3. The value for energy obtained from the fit () corresponds to the energy barrier of the thermal activation of ²E→ ⁴T_2g transitions in the Cr³⁺ ion.

Finally, the photoluminescence quantum yield of Cr³⁺ emission in Li₂SnO₃ at room temperature was determined to be 4.5 ± 0.4%. Notably the room-temperature intensity is approximately 0.4 in comparison to that at 4.2 K leading us to deduce an approximate quantum efficiency of 11% at lower temperature that is deemed satisfactory for thermometry measurements.

Temperature-induced shift and intensity ratio of R-lines

Next, we analyze the temperature-induced changes in the spectra of the spin-forbidden ²E → ⁴A₂ transitions as illustrated in Fig. 5, where the changes in the narrow emission line's shape or position are evident. These changes primarily originate from the change in the population of emitting states Ē and 2Ā and from their interaction with lattice vibrations, resulting in a noticeable variation of the intensity ratio of the R-lines and shifts in their position. Therefore, these two spectroscopic characteristics form the basis for two modes of operation which are frequently utilised in non-contact luminescence measurements of temperature. The temperature dependencies of the intensity ratio (F = I_R2/I_R1) for Cr³⁺ and Mn⁴⁺ lines in Li₂SnO₃ are displayed in Fig. 8, and the variations of peak positions of the R-lines with temperature are shown in Fig. 9.

Fig. 8 The intensity ratio (I_R2/I_R1) as a function of temperature for Cr³⁺ (a) and Mn⁴⁺ centers (b) in Li₂SnO₃. The red lines display the best fitting of experimental data (dots) to eqn (4) using the value of the energy gap D as shown in Table 4.

Fig. 9 Variation of the position of R-lines with temperature observed for different emission centers of Cr³⁺ (a) and Mn⁴⁺ (b) in Li₂SnO₃. The red lines show the best fit of experimental results (dots) to eqn (5) using the parameters presented in Table 4.

It is important to note that the presence of two slightly different types of emission centres adds complexity to the interpretation of spectroscopic data. The first issue we encountered was identification of the R-lines of Cr³⁺. The R₁ lines attributed to Cr(I) and Cr(II) centres are distinguishable only below 100 K, as can be seen in Fig. 5, while the R₂ lines are not resolved at all. Therefore, we derived only one temperature dependence of the intensity ratio for the Cr(I) centre displayed in Fig. 6(a). This dependence was fitted using Boltzmann statistics, which describes the population variation of two emitting levels with temperature:

In this equation, A is a constant, D denotes the energy difference between the two emitting levels, k stands for the Boltzmann constant, T is absolute temperature and B accounts for an offset. The fitting resulted in very good agreement with the experimental data as illustrated in Fig. 8(a). It yielded an energy gap value of D = 1.9 meV which aligns well with the energy difference between the R₁ and R₂ lines obtained from the spectroscopic data.

The second complication arose in assigning two pairs of R-lines to the appropriate emission centres of Mn⁴⁺, namely Mn(I) and Mn(II). Fortunately, at low temperatures all the individual lines are fully resolved allowing us to apply eqn (4) for verification of their pairing. The best fit was achieved using the values of energy gaps shown in Table 4 which correlates with the results obtained from the spectroscopic measurements (refer to Table 2). This finding also affirms the suggested assignment of the lines to the specific emission centres of Mn⁴⁺. It is worth noting that an alternative pairing of R-lines results in values of energy gaps that significantly deviate from the experimentally determined ones.

Table 4 Fitting parameters derived from the temperature dependence of the intensity ratio and shift of R-lines in Li₂SnO₃:Cr³⁺, Mn⁴⁺ using eqn (4) and (5), respectively

Emission center	D, meV	α 1, cm^−1	α 2, cm^−1	T D, K
Cr(I)	1.9 ± 0.1	−705	−678	738
Mn(I)	3.9 ± 0.4	−437	−596	510
Mn(II)	1.2 ± 0.3	−215	−257	413

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Following this we examined the temperature-induced changes in the spectral positions of R-lines measured in Li₂SnO₃:Cr³⁺, Mn⁴⁺ (see Fig. 9) using the model developed in ref. 36 In the framework of this model the observed shift of the lines, denoted as Δν, originates from the electron–phonon interaction between the excited states of impurity ions and the lattice vibrations of the host material. This phenomenon can be quantified by the formula:

Here α represents a coupling coefficient for the electron–phonon interactions and T_D stands for the Debye temperature of the material. We performed a correlated fit of the observed temperature dependences of R-lines, enabling us to determine the optimal values of parameter α for each line as well as the single value for Debye temperature T_D. The fitting procedure yielded very good agreement with the measured result and the resulting fitting parameters are summarised in Table 4.

These findings allow us to evaluate the potential use of the spectroscopic characteristics of Li₂SnO₃:Cr³⁺, Mn⁴⁺ for temperature monitoring. Upon inspecting the two temperature dependences displayed in Fig. 8 and 9 a noteworthy shortcoming become apparent: both methods of temperature sensing, whether based on intensity ratio or R-line shift, are effective within a relatively narrow temperature range. The operational range of the intensity ratio method is limited to 80 K, while discernible changes in the position of the Cr³⁺ peak can be detected within the temperature range 100–200 K. This inherent limitation of these two methods has already been noticed in previous studies.¹⁴ However, with Li₂SnO₃:Cr³⁺, Mn⁴⁺ this constraint is more pronounced, limiting the application of two method based on spectroscopic data in non-contact temperature measurements.

Temperature dependence of the luminescence decay time

The change of decay time constant of the R-lines is yet another feature of emission of TM ions that is regularly applied for temperature monitoring.^26,37–39 To verify the merit of these characteristics we measured the variation of luminescence decay time constants of the R-lines in Li₂SnO₃. Fig. 10 shows the temperature dependence of decay time constants for ²E → ⁴A₂ transitions of Cr³⁺ and Mn⁴⁺.

Both measured dependences τ(t) exhibit similar qualitative behaviour. Starting at the lowest temperature, the decay time constant initially increases but after reaching a maximum begins to decrease. The main difference lies in the rate of this reduction: it occurs swiftly in case of Cr³⁺, while in case of Mn⁴⁺ one can see a levelled-off part in the τ(t) graphs followed by a rapid decrease in the decay time constant above 170 K. The observed decrease of the decay time constant at higher temperature is commonly observed in luminescence materials.⁴⁰ It has the same origin as the temperature quenching, which is caused by an increase of the probability for non-radiative de-excitation of the emitting level of TM ions.^30,41,42 The rise of the decay time constant with heating at low temperature is an interesting feature that has been previously documented in a few systems doped with Cr³⁺.^38,43 More recently we observed this phenomenon for both ions in Al₂O₃ co-doped with Cr³⁺ and Mn⁴⁺.¹⁴ This behaviour is explained by considering different radiative transition probabilities from the Ē and 2Ā levels, which together constitute the emitting ²E state. The likelihood of radiative decay from the upper level 2Ā is lower compared to that for Ē, i.e. 1/τ₂ < 1/τ₁. At very low temperature the upper level remains unpopulated, and consequently, the single R₁ line observed in the emission is predominantly a result of radiative transition from the low-lying level Ē which has a shortened decay time τ₁. As the temperature increases the upper level gradually becomes populated, as indicated by the emergence of the R₂ line, and it starts to contribute to the emission process. This leads to the gradual increase of the measured decay time constant of the R-emission with heating.

To analyse the relaxation dynamics of ²E emitting states in Li₂SnO₃–Cr³⁺, Mn⁴⁺, we applied a model developed in ref. 38 that provides a comprehensive interpretation of the observed behavior of the decay time constant of TM ions over a wide temperature range. The model offers a highly accurate quantitative description of all features of τ = f(T) characteristic of Cr³⁺ and Mn⁴⁺ emissions. It takes into consideration the primary processes influencing the dynamics of radiative transitions, which encompass thermalisation between the Ē and 2Ā levels, phonon-assisted relaxation, and depopulation of the levels due to thermally induced transitions from ²E to an upper ⁴T₂ state. It is pertinent to remark that the upper state may subsequently play a different role. For instance, in case of Cr³⁺ it actively participates in luminescence giving rise to broadband emission, whereas in the case of Mn⁴⁺ it undergoes non-radiative decay through the crossover process. Regardless of these nuances the dependence of the decay time constant of the ²E → ⁴A₂ transitions in Li₂SnO₃ on temperature is described by the following relation:

where 1/τ_i (i = 1, 2 and 3) represents the radiative decay rates of the Ē, 2Ā, and ⁴T₂ states respectively. ΔE is the energy difference between the ²E and the upper state, E_p stands for “effective energy” of the phonons responsible for the exchange with the sidebands.

This equation has been used to model the temperature dependence of the luminescence decay time constants of R-lines of both Cr³⁺ and Mn⁴⁺ in Li₂SnO₃–Cr³⁺, Mn⁴⁺. The curves displayed in Fig. 10 exhibit an excellent agreement with the experimental results, as substantiated by a high correlation coefficient (>0.998). The fitting parameters are summarized in Table 5.

Fig. 10 Temperature dependence of the luminescence decay time constant of Cr³⁺ (a) and Mn⁴⁺ (b) R-lines in Li₂SnO₃–Cr³⁺, Mn⁴⁺. The solid lines show the best fit of eqn (6) to the experimental results (dots) using the parameters summarized in Table 5.

Table 5 Fitting parameters obtained from the temperature dependence of the luminescence decay time constants of ²E → ⁴A₂ transitions of Cr³⁺ and Mn⁴⁺ in Li₂SnO₃–Cr³⁺, Mn⁴⁺

Parameter	Cr^3+	Mn^4+
a The value of D is fixed to be equal to the average energy splitting of the ^2E level.
τ 1, ms	1.95 ± 0.01	0.55 ± 0.01
τ 2, ms	2.03 ± 0.01	0.57 ± 0.01
E p, meV	25.4 ± 0.2	35.3 ± 0.1
D , meV	1.7	3.2
τ 3, μs	5.8 ± 0.2	5.5 ± 0.5 × 10^−5
ΔE, meV	103.8 ± 0.7	292 ± 16
R ^2	0.999	0.998

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Upon examining the fitting parameters, it becomes evident that for the ²E → ⁴A₂ transitions of both Cr³⁺ and Mn⁴⁺ the decay time constants of the lower level (τ₁) is shorter than that of the upper level (τ₂), which is in line with the reasoning proposed here for the interpretation of the observed behaviour of the luminescence decay time constant at very low temperatures. It is worth noting that the values of activation energies ΔE obtained here are similar to those derived from fitting the temperature dependence of the emission intensity of R-lines. This correspondence is expected given that temperature changes of both intensity and radiative decay are controlled by the same mechanism of non-radiative quenching.

Thermometric performance

The observed substantial changes in the decay time constant of R-lines of Cr³⁺ and Mn⁴⁺ in Li₂SnO₄ with changing temperature serve as strong indicators that this property can be readily utilized for temperature monitoring. The assessment of efficiency of a measured quantity Q for temperature sensing is done using the quality indicators such as relative sensitivity (S_r) and the temperature uncertainty or resolution (δT) determined using the following equations.^44,45where |dQ/dT| represents the responsivity of the sensor and δQ stands for uncertainty of the measured quantity.

Fig. 11 shows the calculated quality indicator parameters for the thermometer based on the measurements of the decay time constants in Li₂SnO₃–Cr³⁺, Mn⁴⁺ along with pertinent data for the reference sample Ga₂O₃–Cr³⁺.³⁸ Upon close examination of the plots in Fig. 11 it becomes evident that the efficiency of temperature sensing using the Cr³⁺-emission of Li₂SnO₄ is very comparable to that of Ga₂O₃–Cr³⁺. The difference lies in a slight shift of the peak sensitivity from 160 to 190 K and an increase of the relative sensitivity from 1.7 to 2.0%/K. This leads to modest enhancement of the best achievable temperature uncertainty of Li₂SnO₃–Cr³⁺ compared to the reference Ga₂O₃–Cr³⁺, from 0.3 to 0.25 K. Assuming an uncertainty ±1 K as an upper limit for practical application of a temperature sensor, it is evident that this material exhibits resolution ±1 K or better over a broad temperature range (100–350 K) surpassing that of Ga₂O₃–Cr³⁺.

Fig. 11 Relative sensitivity S_r (a) and uncertainty δT of temperature measurements (b) using the decay time constant of ²E → ⁴A₂ transitions of Cr³⁺ and Mn⁴⁺ in Li₂SnO₃–Cr³⁺, Mn⁴⁺. The data for Ga₂O₃–Cr³⁺ are shown for comparison.

Finally, the maximum sensitivity of temperature sensing using Mn⁴⁺ emission of Li₂SnO₃ is nearly three times better, resulting in an impressive resolution δT = 0.08 K at 220 K. While this performance competes with the best uncertainties achieved by non-contact luminescence decay time sensors (refer to Table 6), it is effective merely within a narrow range (180–250 K).

Table 6 Thermometric characteristics of Cr³⁺ and Mn⁴⁺ doped oxides

Material	Temperature	Relative sensitivity, %/K	Temperature uncertainty, K	Ref.
Ga2O3–Cr^3+	165	1.7	0.3	38
Mg2SiO4–Cr^3+	240	0.92	0.2	46
ZnGa2O4–Cr^3+	200	1.9	<2	47
MgTiO4–Mn^4+	277	4.1	0.025	48
Li2TiO3–Mn^4+	332	3.2	0.031	49
LaTiSbO6–Mn^4+	298	3.0	0.05	50
Li2SnO3–Cr^3+	190	2.0	0.25	This work
Li2SnO3–Mn^4+	220	5.8	0.08	This work

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Summary

Due to their high sensitivity to temperature changes, oxides doped with TM ions find extensive use in non-contact luminescence thermometry. In most cases, temperature sensing relies on monitoring the luminescence properties of a single TM ion. However, it has been demonstrated that employing two TM ions can extend the sensor sensitivity to a wider temperature range and add redundancy. Inspired by this concept, our work focuses on exploring the potential of Li₂SnO₃ co-doped with Cr³⁺ and Mn⁴⁺ as a non-contact temperature sensor.

To comprehend the relationship between the structure and luminescence properties of TM ions in this compound, we conducted examinations using X-ray diffraction, electron paramagnetic resonance, and luminescence techniques. Our structural studies revealed two types of distinct [SnO₆] octahedral centres in Li₂SnO₃, both suitable for incorporating TM ions. This finding was further confirmed by EPR and luminescence spectroscopy. Notably, high-resolution, low-temperature photoluminescence measurements distinctly revealed two pairs of R-lines attributed to the ²E → ⁴A₂ transitions in Mn⁴⁺ in Li₂SnO₃. The energy gap between the R-lines of Cr³⁺ remains distinguishable but is significantly smaller, while the separation of R-lines resulting from two different Cr³⁺ centres is very small. All of this indicates a reduced strength of the local crystal field compared to that experienced by Mn⁴⁺ ions.

To evaluate the material suitability for non-contact luminescence thermometry, we investigated the temperature behaviour of the luminescence spectra and decay time constant of Li₂SnO₃ co-doped with Cr³⁺ and Mn⁴⁺. The results were analysed within the framework of relevant theoretical models, which explain the observed temperature dependences arising from various mechanisms affecting the dynamics of exited states due to thermalization, electron–phonon interaction, non-radiative decay, and thermally-activated depopulation. Following this analysis, we found that spectroscopic techniques for temperature monitoring based on measurements of the intensity ratio of R-lines or their spectral shift in Li₂SnO₃–Cr³⁺, Mn⁴⁺ are effective only within a relatively narrow temperature range, limiting their practical application. In contrast, we discovered that thermometry based on measurements of the decay time constant of the ²E → ⁴A₂ transitions of TM ions in Li₂SnO₃ shows great promise, as this luminescence parameter exhibits high sensitivity to temperature changes.

The relative sensitivity of Cr³⁺ emission, equal to 2.0%/K at 190 K, is comparable to that demonstrated by Ga₂O₃–Cr³⁺ taken as reference. This translates to an expected temperature resolution of measurements better than ±1 K over a broad temperature range from 100 to 350 K. The emission of Mn⁴⁺ exhibits even higher sensitivity at 220 K, with a value of 5.8%/K. However, due to steep thermal quenching, practical temperature measurements are only possible within a narrow range of temperatures. Nevertheless, this emission may be very useful for cross-verifying results of temperature measurements obtained from the Cr³⁺ emitter. This added feature enhances the potential of Li₂SnO₃–Cr³⁺, Mn⁴⁺ as a non-contact luminescence sensor for temperature.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The work was supported by the Polish National Science Centre (project no. 2018/31/B/ST8/00774 and 2021/40/Q/ST5/00336) and by the National Research Foundation of Ukraine (grant no. 2020.02/0373 “Crystalline phosphors’ engineering for biomedical applications, energy saving lighting and contactless thermometry”).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3tc03913g

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