Effect of Temperature on Luminescence of LiNbO₃ Crystals Single-Doped with Sm³⁺, Tb³⁺, or Dy³⁺ Ions

Abstract

Crystals of LiNbO₃ single-doped with Sm³⁺, Tb³⁺, or Dy³⁺ and crystal of LiTaO₃ single-doped with Tb³⁺ were grown by the Czochralski method. Luminescence spectra and decay curves for LiNbO₃ samples containing Sm³⁺ or Dy³⁺ ions were recorded at different temperatures between 295 and 775 K, whereas those for samples containing Tb³⁺ ions were recorded at different temperatures between 10 and 300 K. Optical absorption spectra at different temperatures were recorded within the UV-blue region relevant to optical pumping of the samples. It was found that the effect of temperature on experimental luminescence lifetimes consists of the initial temperature-independent stage followed by a steep decrease with the onset at about 700, 600, and 150 K for Sm³⁺, Dy³⁺, and Tb³⁺ ions, respectively. Additionally, comparison of temperature impact on luminescence properties of LiNbO₃:Tb³⁺ and LiTaO₃:Tb³⁺ crystals has been adequately described. Experimental results were interpreted in terms of temperature-dependent charge transfer (CT) transitions within the modified Temperature—Dependent Charge Transfer phenomenological model (TDCT). Disparity of the onset temperatures and their sequence were explained based on the location of familiar zigzag curves connecting the ground state levels of rare earth ions with respect to the band-gap of the host. It was concluded also that LiNbO₃:Sm³⁺ is suitable as an optical sensor within the 500–750 K temperature region whereas LiNbO₃:Dy³⁺ offers the highest sensitivity at lower temperatures between 300 and 400 K.

1. Introduction

Originally proposed in 1964, LiNbO₃ was soon applied in second harmonic generation, electro-optic modulation, acousto-optic, and piezoelectric device application [1]. Subsequent investigation has revealed that it is promising for holographic recording [2,3], optical parametric oscillation [4,5], waveguide lasers [6,7], or self-frequency doubled lasers [8].

However, despite the long-lasting technological success, some fundamental properties of LiNbO₃ remain rather puzzling. One of encountered difficulties refers to the reliability of its band gap. Numerous reported band gap values, all inferred from optical absorption spectra, range from 3.28 [9] to 4.3 eV [10] with the most frequently cited value of 3.78 eV. This is due mainly to the fact that the LiNbO₃ crystals, suitable for optical application, are fabricated commonly by the Czochralski method from a congruent, lithium deficient melt (Li/Nb = 0.94). Hence, they have a high density of point defects which affect the reliability of the measurement. Even with special care near where stoichiometric crystals have grown, recorded optical absorption spectra still suffered from remaining structural defects. Numerous attempts have been made to determine the band gap from ab initio calculations. The calculated band gap values depended markedly on the method applied, ranging from 3.47 to 6.53 eV [11]. Authors of a subsequent paper [12] entitled: “Do we know the band gap of lithium niobate?” have reported the calculated band gap value of 4.7 eV and more recently, the calculated band gap value of 4.9 eV has been reported in [13].

Much attention has been paid in the past to the nature of defects in crystal structures of isostructural LiNbO₃ and LiTaO₃ compounds. In the ideal, stoichiometric composition of the cations Nb⁵⁺, Li⁺, and free sites are located sequentially in distorted octahedra of oxygen ions. The distribution of cations among available sites in congruent LiNbO₃ crystals is not easy, however. In the past it has been interpreted in the framework of the Nb-site vacancy model [14]. Later on, it has been concluded that the Nb split models in which the excess of Nb atoms are distributed among the Li and the vacant cation sites cannot be excluded [15]. The same difficulty relates to the location of incorporated luminescent rare earth ions. Investigation of Eu³⁺ sites in LiNbO₃:Eu³⁺ and LiNbO₃:MgO:Eu³⁺ crystals for different Li/Nb molar ratios has led to the conclusion that Eu³⁺ ions substitute the Li⁺ and Nb⁵⁺ ions [16]. The RBS/channeling spectra of Pr³⁺, Ho³⁺, Yb³⁺ ions in LiNbO₃ revealed that they substitute Li⁺ ions but are slightly shifted from the regular Li⁺ position [17]. In a more recent paper dealing with structural and optical properties of powders of LiTaO₃ doped with Eu³⁺ ions the authors have observed that the Eu³⁺ ions substitute preferentially the Ta⁵⁺ ions [18]. Moreover, it was found that this preference increases with increasing Eu³⁺ concentration (an occupation fraction above 60% for Eu³⁺ concentration of 1 at %). In the present work we deal with LiNbO₃ crystal single-doped with Sm³⁺, Tb³⁺, or Dy³⁺ ions characteristic in that the relaxation of their metastable levels consists essentially of radiative transitions in virtually all inorganic hosts.

In fact, spectroscopic features of Sm³⁺-doped crystals and glasses stem from the energy level scheme of Sm³⁺ ions that consist of low-energy multiplets from the ⁷F and ⁷H terms and a higher-energy group of multiplets from quartet terms. The energy difference between the quartet and septet states is large as compared to the phonon energy encountered in inorganic matrices. Therefore, the relaxation of the ⁴G_5/2 metastable level of Sm³⁺ consists essentially of radiative transitions providing an efficient visible emission. Its spectral distribution within the blue, yellow, and red regions is weakly affected by the change of a host [19,20,21,22,23,24]. Spectroscopic properties of Tb³⁺-doped hosts stem from the energy level scheme of Tb³⁺ ions involving low-energy multiplets from the ⁷F term and higher-energy multiplets from quintet terms. The metastable 5D₄ level of Tb³⁺ is located higher than the next lower energy ⁷F₀ level by about 14,000 cm⁻¹. Therefore, its relaxation consists essentially of radiative transitions providing an efficient visible emission. Literature concerning spectroscopic features of Tb³⁺ doped luminescent materials is rather rich. The mechanism of the ⁵D₃–⁵D₄ cross-relaxation in Y₃Al₅O₁₂:Tb³⁺ was considered in [25]. Several papers were devoted to preparation and spectroscopic features of terbium-doped garnet crystals [26,27,28,29]. Other visible phosphors containing Tb³⁺ ions that have been studied include Y₂LuCaAl₂SiO₁₂:Ln (Ln = Ce³⁺, Eu³⁺, and Tb³⁺) [30], Tb³⁺ doped lithium lead alumino borate glasses [31], Tb³⁺-doped KLu(WO₄)₂ crystal [32], Tb³⁺-doped LiYF₄ [33], Tb³⁺-doped K₃YF₆ [34]. Laser potential and laser performance of Tb³⁺-doped hosts have been considered, too [35]. Spectroscopic properties of Dy³⁺-doped materials stem from the energy level scheme of Dy³⁺ ions that consists of high-energy multiplets from quartet terms well separated from low-energy multiplets belonging to the ⁶H and ⁶F terms. The visible emission of Dy³⁺ results from radiative transitions that originate on the metastable 4F_9/2 level and terminate on the lower-energy levels ⁶H_J (J = 9/2, 11/2, 13/2, and 15/2). Numerous recent papers have been devoted to Phosphor materials single-doped with Dy³⁺ [36], co-doped with Dy+Eu [37] or triple-doped with Dy+Eu+Tb [38] have been investigated and found promising for novel lighting devices. The potential of Dy³⁺-doped crystals as luminescent temperature sensors has been considered in numerous papers [39,40,41]. It has been demonstrated that YAG:Dy³⁺ crystal pumped at 447 nm with a GaN laser diode is able to show yellow laser operation with a slope efficiency of 12% [42]. Optical amplification in Dy³⁺-doped Gd₂SiO₅, Lu₂SiO₅, and YAl₃(BO₃)₄ single crystals has been observed [43] as well. Intention of the present work is to get a closer insight into temperature-dependent processes which are relevant for excited state relaxation of incorporated rare earth ions and to assess the impact of these processes on the potential of system studied for luminescence temperature sensing.

2. Materials and Methods

Single-doped crystals of LiNbO₃ containing nominally 0.65 wt% (1.2 × 10²⁰ ions/cm³) of Sm³⁺, 2.80 wt% (4.8 × 10²⁰ ions/cm³) of Tb³⁺, or 1.94 wt% (3.3 × 10²⁰ ions/cm³) of Dy ³⁺ were grown by the Czochralski method from the congruent melt (Li/Nb = 0.945). By the same method a 3 wt% Tb³-doped LiTaO₃ crystal was grown creating thereby a reference material to help understand the LiNbO₃:Tb³⁺ luminescence. The Czochralski growth of LiNbO₃ and LiTaO₃ crystals has been well known for decades and is described in detail elsewhere, e.g., in [44]. Concentrations given above stem from the trade-off between the intention to achieve required excitation efficiency, on one hand, and to prevent the excessive distortion of the host structure, on the other hand. A Varian 5E UV-VIS-NIR spectrophotometer with instrumental spectral bandwidths of 0.5 nm was employed to record optical absorption spectra in the UV-blue spectral region. To record excitation spectra and luminescence spectra a FLS980 fluorescence spectrophotometer from Edinburg Instruments equipped with a 450 W xenon lamp as an excitation source and a Hamamatsu 928 PMT detector was used. For these measurements, the samples were excited with unpolarized light propagating parallel to the optical axis of the crystal and the luminescence was observed perpendicular to the optical axis of the crystal. Recorded spectra were corrected for the sensitivity and wavelength of the experimental set-up. To record decay curves of luminescence an experimental set-up consisting of a tunable optical parametric oscillator (OPO) pumped by a third harmonic of a Nd:YAG laser, a double grating monochromator with a 1000 mm focal length, a photomultiplier, and a Tektronix MDO 4054B-3 Mixed Domain Oscilloscope was used. For spectroscopic measurement at low temperature the samples were placed in an Oxford Model CF 1204 continuous flow liquid helium cryostat equipped with a temperature controller. A chamber furnace was used for measurements at higher temperature within 295–800 K. The temperature of samples was detected by a copper-constant thermocouple with measurement error less than 1.5 K and controlled by a proportional-integral-derivative (PID) Omron E5CK controller.

3. Results

When interpreting our experimental data, we refer to energy level schemes for Sm³⁺, Tb³⁺, and Dy³⁺ ions gathered in Figure 1. The energy levels within respective 4f⁵, 4f⁸, and 4f⁹ configurations are labeled by symbols ^2S+1L_J of corresponding multiplets.

It can be seen that there is a correspondence between term structures for Sm³⁺ and Dy³⁺ ions in agreement with the principle of electrons and holes in the 4fⁿ configurations. Actually, each multiplet of rare earth ions incorporated in the crystal host is split by the crystal field into crystal field components. In depth analysis of the crystal field splitting of multiplets for Sm³⁺ and Dy³⁺ ions in LiNbO₃ has been performed based on low temperature absorption and emission spectra and reported in [45,46], respectively. These results were used to construct corresponding energy level schemes shown in Figure 1. To our knowledge the information concerning the crystal field splitting of Tb³⁺ in LiNbO₃ is not available. Therefore, we include in Figure 1 the data for Tb³⁺ in GAGG garnet reported in [29]. It should be noticed here that the agreement between theoretical and experimental crystal field splitting inferred from low temperature spectra of rare earth ions in LiNbO₃ is poor owing to multi-site location of incorporated ions combined with a strong inhomogeneous line broadening of their optical spectra. Other consequences result from a strongly defective congruent composition of the hosts crystal. The LiNbO₃ forms crystals that belong to trigonal/rhombohedral system, R3c space group, hence they are optically uniaxial. Incorporated rare earth ions enter sites with the C₃ local symmetry in the stoichiometric composition but in congruent LiNbO₃ they are located in several nonequivalent sites with strongly dissimilar local symmetries. As a consequence, the anisotropy of their transition intensities is no longer consistent with theoretical predictions preventing thereby the reliable interpretation of polarized optical spectra, as observed in [45,46]. To avoid the difficulties mentioned above and remembering that effect of temperature on integrated luminescence intensity is independent of polarization of optical spectra we infer our results from unpolarized luminescence spectra.

In the limit of low doping level, the population of an excited multiplet decays by competing radiative transitions and nonradiative multiphonon relaxation consisting of simultaneous emission of several phonons. The contribution of the latter process is commonly assessed from the phenomenological energy gap law which relates the rate of multiphonon relaxation to the order of the process defined as the ratio of energy separation between the excited level in question and a next lower level to the cut-off phonon energy available in the host. It follows from the Raman spectra analysis of LiNbO₃ and LiTaO₃ crystals that the highest energy stretching vibrations Nb-O are around 610 cm⁻¹ (597 cm⁻¹ for Ta-O) [47]. Thus, Sm³⁺, Tb³⁺, and Dy³⁺ ions in LiNbO₃ have single metastable levels able to relax radiatively without multiphonon contribution providing thereby intense luminescence. Downward arrows in Figure 1 indicate radiative transitions observed in the visible region.

3.1. Effect of Temperature on Luminescence of Sm³⁺ and Dy³⁺ in LiNbO₃

On the right side of Figure 2 we compare survey room temperature luminescence spectra of Sm³⁺ and Dy³⁺ ions in LiNbO₃ recorded within the 450–800 nm spectral region. Sm³⁺ luminescence was excited at 402 nm. Its luminescence spectrum consists of bands located at about 567, 600, 646, and 708 nm related to transitions between the metastable 4G_5/2 level and ⁶H_J (J = 5/2, 7/2, 9/2, 11/2) terminal levels, respectively. Very weak contribution of the ⁴G_5/2 → ⁶H_13/2 transition can be discerned around 800 nm. It should be noticed here that there are other transitions from the ⁴G_5/2 level, namely to the multiplets of the ⁶F term and the ⁴G_5/2 → ⁶H_15/2 transition, all of them located in the near IR. The contribution of these transitions to luminescence spectra of samarium ions in crystals and glasses is marginal and therefore is commonly neglected. Dy³⁺ luminescence shown in Figure 2 was excited at 356 nm. Its luminescence spectrum consists of bands located at about 486, 580, 665, and 754 nm related to transition between the metastable 4F_9/2 level and ⁶H_J (J = 15/2, 13/2, 11/2, and 9/2) terminal levels, respectively. Remaining transitions from the metastable ⁴F_9/2 level are located in the IR and terminate on multiplets of the ⁶F term and on the ⁶H_7/2 and ⁶H_5/2 multiplets. Their contribution to dysprosium luminescence is very weak, as for samarium luminescence.

On the left side of Figure 2 we compare excitation spectra of Sm³⁺ luminescence and Dy³⁺ luminescence in LiNbO₃, monitored at 611 and at 573 nm, respectively. Excitation spectra for the two ions show very complex structure of band components corresponding to closely spaced transitions from the ground states to high energy multiplets derived from the ⁴F, ⁴G, ⁴H, ⁴I, ⁴K, ⁴L, ⁴M quartet and the ⁶P sextet terms. It should be noticed here that despite a rich spectrum, the intensity of absorption in this spectral region is very low because spin forbidden sextet-quartet transitions are involved. It can be seen that the highest energy bands contributing to excitation spectra in Figure 2 are located around 411 and 356 nm although the energy level schemes of two ions contain levels at higher energy. Rather curiously, the wavelength 335 nm correspond to 3.7 eV, a value of the most frequently cited energy gap for LiNbO₃. In view of theoretical calculations mentioned above in Section 1 the short wavelength limit of the excitation spectra may not be due to the fundamental UV absorption edge of LiNbO₃. It may result also from an absorption of color centers, likely to exist in the matrix and/or transitions between the ground states of incorporated rare earth ions and trapped exciton states, as proposed for LiTaO₃:Pr system [47,48].

Figure 3 compares luminescence spectra of Sm³⁺ in LiNbO₃ recorded at different temperatures between 295 K and 775 nm. Excitation was at 402 nm. It can be seen that with increasing temperature the contribution of narrow band components to the spectrum vanishes gradually, the bands become smoother and their intensities diminish. To assess the effect of temperature on the overall Sm³⁺ luminescence the spectra recorded at different temperatures were integrated numerically within the 450–750 nm region. Results of this assessment presented in the inset on the right side of Figure 3 reveal the adverse thermally induced quenching of Sm³⁺emission. The inset on the left side of Figure 3 shows spectra for the high energy part of the spectrum between 460 and 550 nm, where the luminescence grows monotonously.

Examination of luminescence spectra of Dy³⁺ in LiNbO₃ excited at 356 nm and recorded at different temperatures between 295 and 723 K reveal similar thermal effect.

Namely, the contribution of narrow band components to the spectrum vanishes gradually, the bands become more smooth and their intensities diminish, except for the high energy part of the spectrum between 440 and 500 nm, where the luminescence grows steadily. For the sake of brevity, the spectra shown in Figure 4 are restricted to the short wavelength parts encompassing bands within the 440–550 nm region. The Dy³⁺ luminescence intensities determined by a numerical integration of spectra within the 440–750 nm region are plotted versus temperature in the inset on the right. The inset on the left side shows the onset of steep rise of the sample absorption in the UV region observed at several different temperatures. Examination of plots in insets on the right side of Figure 3 and Figure 4 reveals a thermally enhanced quenching of both the Sm³⁺ and Dy³⁺ luminescence in LiNbO₃. This phenomenon may be due to a thermally induced decrease of absorption efficiency in the optical pump region and/or to thermally enhanced increase of nonradiative relaxation rates of the metastable 4G_5/2 and ⁴F_9/2 levels of Sm³⁺ and Dy³⁺, respectively. The contribution of these quenching factors can be assessed based on examination of Figure 5.

Points in this figure show the temperature dependence of the ⁴G_5/2 and ⁴F_9/2 experimental lifetimes determined experimentally from respective exponential luminescence decay curves. It can be seen that initially, i.e., up to about 700 K for Sm³⁺ and up to about 600 K for Dy³⁺, the luminescence lifetimes do not depend on temperature implying that the contribution of the latter factor can be neglected. Therefore, we attribute the decrease of integrated luminescence intensity observed in this initial temperature region to thermally induced increase of a broad-band absorption that affects adversely the efficiency of optical pumping. In fact, the onset of this absorption shifts towards longer wavelengths when the temperature increases, as shown in the inset on the left side of Figure 4. Further increase of the temperature brings about a very steep decrease of the lifetime values. At 775 K, the highest temperature provided by our experimental setup, the Dy³⁺ lifetime is close to zero whereas the Sm³⁺ lifetime attains about 70% of its value at room temperature.

To interpret the effect of temperature on the ⁴G_5/2 and ⁴F_9/2 lifetimes, shown in Figure 5 one should remember that the measured luminescence lifetime τ_exp of an excited level of rare earth ion.

$${\tau }_{exp}=1/[W_r+ W_{nr}]$$

(1)

where W_r is the temperature independent radiative decay rate and W_nr denotes the rate of nonradiative decay affected by the temperature. Following the interpretation of data acquired for LiTaO₃:Pr³⁺ [47] and recent generalization on spectroscopy of Pr³⁺ in niobate-titanates [48] we interpret the steep decrease of Sm³⁺ and Dy³⁺ lifetimes in terms of temperature-dependent charge transfer (CT) transitions within the modified model (TDCT) proposed by Nikolic et al. [40]. According to the TDCT model the rate W_nr(T) of nonradiative charge transfer transitions

$$W_{nr}\left(T\right)= W_{nr}\left(0\right)\times \left(\frac{1}{T^{*\frac{1}{2}}}\right)\times \exp [(-E_a+\alpha T)/k_{B }{T}^{*}]$$

(2)

where T denotes the temperature, W_nr(0) denotes the rate of the process at 0 K, E_a is the energy barrier to be crossed by phonons, k_B is the Boltzmann constant. The factor αT represents the thermally induced change of the energy E_a assumed to be linear with the slope α.

The factor

$$T^{* }=\left(\frac{\mathrm{ћ}\omega }{2k_{B }}\right)\times \cot \mathrm{h}(\mathrm{ћ}\omega /2k_B\mathrm{T})$$

(3)

included to the above relation accounts for the non-Arrhenius nature of the excitation energy flow through CT states [49]. The ħω denotes the energy of the host phonons involved. Solid lines in Figure 5 represent theoretical temperature dependence of Sm³⁺ and Dy³⁺ lifetimes determined from Equation (2) with parameters gathered in

Table 1. Spectral parameters governing the temperature dependence of ⁴G_5/2 (Sm³⁺) and ⁴F_9/2 (Dy³⁺) lifetimes in LiNbO₃ within the framework of the TDCTmodel.

	LiNbO3:Sm	LiNbO3:Dy
Wnr (1/s)	855	508
Ea (cm−1)	16,159	14,795
α (cm−1/K)	23.3	26.1
ħω (cm−1)	672	680
Wr (1/s)	1540	5164

. It can be seen that their agreement with experimental data points is reasonable.

In the following we focus our attention to temperature dependent changes of luminescence bands related to transitions that terminate on the ⁶H_5/2 ground state of Sm³⁺ and the ⁶H_15/2 ground state of Dy³⁺. It follows from Figure 3 and Figure 4 that there are contributions of luminescence that grow monotonously with increasing temperature at the expense of the ⁴G_5/2 → ⁶H_5/2 and ⁴F_9/2 → ⁶H_15/2 bands located at slightly longer wavelengths. Examination of low temperature absorption spectra reported in the past [45,46] reveals that these growing contributions in the Sm³⁺ spectrum is due to transitions from thermally populated ⁴F_3/2 level and that in the Dy³⁺ spectrum from thermally populated ⁴I_15/2 level. These levels are located, respectively, above the ⁴G_5/2 and ⁴F_9/2 metastable levels by about 1000 cm⁻¹ only and therefore their populations are governed by the Boltzmann statistics. Accordingly, a thermally induced change of fluorescence intensity ratio (FIR) between the ⁴F_3/2 → ⁶H_5/2 and ⁴G_5/2 → ⁶H_5/2 emission bands for Sm³⁺ and between the ⁴I_15/2→ ⁶H_15/2 and ⁴F_9/2 → ⁶H_15/2 emission bands for Dy³⁺can be applied as the temperature sensing parameter. It is known that the luminescence intensities are proportional to the population of involved energy levels and FIR of two thermally coupled levels can be defined by following equation [39]:

$$FIR=\frac{I_{{}^4{I}_{15/2}}}{I_{{}^4{F}_{9/2}}}=B\exp \left(-\frac{\mathrm{\Delta }E}{kT}\right)$$

(4)

where B is temperature independent constant, ΔE is the energy gap between the two thermally coupled levels, and k is the Boltzmann constant. Plots of FIR versus temperature for LiNbO₃:Sm³⁺ and LiNbO₃:Dy³⁺ are compared in Figure 6.

Points indicate experimental data determined from spectra in Figure 3 and Figure 4 and the solid lines represent fits of Equation (4) with ΔE = 1139 cm⁻¹ (for Sm³⁺) and ΔE = 1146 cm⁻¹ (for Dy³⁺).

Optical thermometer may be quantitatively characterized with absolute and relative thermal sensitivity. The former parameter reveals the absolute FIR change with temperature variation and is expressed as:

$$S_A=\frac{dFIR}{dT}=FIR\frac{\mathrm{\Delta }E}{k{T}^2}$$

(5)

To reliably compare the thermometers quality, relative sensitivity is usually used because this parameter determines normalized change of FIR with temperature variation and is defined as [39]:

$$S_R=\frac{1}{FIR}\frac{dFIR}{dT}\cdot 100\%=\frac{\mathrm{\Delta }E}{k{T}^2}\cdot 100\%$$

(6)

Figure 6 compares also plots of S_A and S_R parameters versus temperature for LiNbO₃:Sm³⁺ (left) and LiNbO₃:Dy³⁺ (right). Examination of the S_R plots indicates that the LiNbO₃:Sm³⁺ is mostly suitable for the optical sensor within the 500–750 K temperature region whereas LiNbO₃:Dy³⁺ offers the highest sensitivity at lower temperatures between 300 and 400 K. However, the most significant shortcoming of these optical sensors resides in that their luminescence is quenched at temperatures markedly lower as compared to other hosts doped with Sm³⁺ and Dy³⁺.

3.2. Effect of Temperature on Luminescence of Tb³⁺ in LiNbO₃ and LiTaO₃

Points in Figure 7 show the temperature dependence of the ⁵D₄ lifetime determined experimentally from luminescence decay curves. Unlike LiNbO₃:Sm³⁺ and LiNbO₃:Dy³⁺ systems the LiNbO₃:Tb³⁺ shows luminescence below ambient temperature only but still the dependence of its lifetime on the temperature consists of the initial part weakly affected by the temperature up to about 130 K and of subsequent steep decrease. For a comparison the experimental ⁵D₄ lifetime for LiTaO₃:Tb³⁺ plotted versus temperature also in Figure 7 is nearly constant at temperatures up to about 480 K and next decreases steeply. Solid lines in Figure 7 represent theoretical temperature dependence of the ⁵D₄ lifetime for LiNbO₃:Tb³⁺ and LiTaO₃:Tb³⁺ determined from Equation (2) with parameters gathered in

Table 2. Spectral parameters governing the temperature dependence of ⁵D₄ (Tb³⁺) lifetimes in LiNbO₃ and LiTaO₃ within the framework of the TDCT model.

	LiNbO3:Tb	LiTaO3:Tb
Wnr (1/s)	1064	807
Ea (cm−1)	1984	8128
α (cm−1/K)	17.1	17.6
ħω (cm−1)	693	778
Wr (1/s)	1874	1464

. It is worth noticing here that LiNbO₃ and LiTaO₃ compounds form isostructural crystals characterized by very similar physicochemical properties. Incorporated luminescent rare earth ions are located in sites with the same symmetry in these hosts. They interact with lattice phonons having similar energy distribution with nearly the same cut-off energy corresponding to Nb-O or Ta-O stretching vibrations [47]. Thus, rather small disparity between spectral features of Tb³⁺ in the two hosts can be supposed. Indeed, the survey luminescence spectra of LiNbO₃:Tb³⁺ and of LiTaO₃:Tb³⁺ compared in Figure 8 corroborate this supposition.

3.3. Discussion

Results presented above imply that the location of the metastable 5D₄ level of Tb³⁺ with respect to the bottom of conduction band of the host is a factor that governs the luminescence quenching. Obviously, for a given band-gap the energy difference between the bottom of the conduction band and the metastable 5D₄ level results from the energy difference between the Tb³⁺ ground state and the top of the valence band. The latter difference can be assessed based on generalizations proposed by Dorenbos et al. and by Dorenbos in a series of published papers, e.g., [50,51]. In particular, the ground state of Tb³⁺ located previously 0.7–1.0 eV higher above the valence band than that of Pr³⁺ has been adjusted based on new experimental data showing that these locations are at about the same energy [51]. The band-gap of LiTaO₃ has been determined in the past to be 4.59 eV, a value corresponding to the UV absorption edge located at 271 nm [52]. The available information on band-gap of LiNbO₃ is not reliable in view of the controversy mentioned above in Section 1 but the disparity of band-gap values for the two hosts is likely be small. In any case, we suppose that peculiarities of luminescence quenching in LiNbO₃:Tb³⁺ and LiTaO₃:Tb³⁺ are not governed by the band-gaps but stem from the fact that in LiTaO₃ the ground state of Tb³⁺ is located lower, i.e., closer to the valence band that in LiNbO₃. It follows from the zigzag curves connecting the ground state levels of rare earth ions [50,51] that among rare earth ions considered here the ground state of Tb³⁺ is located at the highest energy, that of Dy³⁺ is located markedly lower and that of Sm³⁺ at the lowest energy. In principle, the energy difference between the top of the valence band and the ground state level of the rare earth ion in question can be determined provided the energy of the charge transfer (denoted also IVCT = intervalence charge transfer) transition and the band-gap of the host are known. Unfortunately, we were not able to obtain these data from our measurement. Figure 2 shows that the IVCT transitions do not contribute to excitation spectra of Sm³⁺ and Dy³⁺ luminescence. Additionally, the IVCT transition does not contribute to the excitation spectrum of Tb³⁺ luminescence in LiNbO₃. In [53] the IVCT transition energy of about 3.4 eV for Pr³⁺ in LiNbO₃ has been mentioned. Assuming that the band-gap of LiNbO₃ equals to 3.8 eV we locate the ground state of Pr³⁺ at about 0.4 eV above the top of the valence band. With this assumption and supposing that the ground states of Pr³⁺ and Tb³⁺ are located at the same energy we locate the ⁵D₄ metastable level of Tb³⁺ at around 0.86 eV below the conduction band. This calculation is rather speculative and the reliability of the ⁵D₄ location is not certain. However, it is worth noticing that it locates the ground states of Sm³⁺ and Dy³⁺ below the top of the valence band accounting for the absence of IVCT band in excitation spectra of their luminescence. The onset of the steep decrease of luminescence lifetime, related with the IVCT, occurs at about 700, 600, and 150 K for Sm³⁺, Dy³⁺, and Tb³⁺ ions in LiNbO₃, respectively. The ⁴G_5/2 metastable level of Sm³⁺is located at about 17,600 cm⁻¹, i.e., about 2.18 eV above the ground state level. The ⁴F_9/2 metastable level of Dy³⁺ is located at about 21,100 cm⁻¹ i.e., about 2.61 eV above the ground state level. These values, combined with those inferred from the zigzag curve predict that the energy barrier to be crossed by phonons would be higher, hence the onset temperature would be also higher for Sm³⁺, in agreement with experimental data. The ⁵D₄ metastable level of Tb³⁺ is located at about 2.55 eV above the ground state level, a value slightly smaller than that for Dy³⁺. However, this location combined with a relatively large energy inferred from the zigzag curve results in markedly smaller energy barrier hence the lower onset temperature.

4. Conclusions

The effect of temperature on experimental luminescence lifetimes consists of the initial temperature-independent stage followed by a steep decrease with the onset at about 700, 600, and 150 K for Sm³⁺, Dy³⁺, and Tb³⁺ ions, respectively. Experimental results were interpreted in terms of temperature-dependent charge transfer (CT) transitions within the modified phenomenological model TDCT. Disparity of the onset temperatures and their sequence were explained based on the location of familiar zigzag curves connecting the ground state levels of rare earth ions with respect to the band-gap of the host. It was concluded also that LiNbO₃:Sm³⁺ is suitable as an optical sensor within the 500–750 K temperature region whereas LiNbO₃:Dy³⁺ offers the highest sensitivity at lower temperatures between 300 and 400 K.