Luminescence of self-trapped excitons in alkali halide crystals at low temperature uniaxial deformation

Nuclear Inst. and Methods in Physics Research B 464 (2020) 95–99

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Luminescence of self-trapped excitons in alkali halide crystals at low temperature uniaxial deformation

T

L. Myasnikova , K. Shunkeyev, N. Zhanturina, Zh. Ubaev, A. Barmina, Sh. Sagimbaeva, Z. Aimaganbetova ⁎

K. Zhubanov Aktobe Regional State University, Moldagulova Av. 34, 030000 Aktobe, Kazakhstan

ARTICLE INFO

ABSTRACT

Keywords: Alkali-halide crystals Self-trapped exciton Luminescence Exciton-phonon interaction Elastic uniaxial deformation

The processes of temperature quenching of the luminescence of the self-trapped excitons (STEs) from 93 to 200 K in the conditions of applied elastic low-temperature deformation for several alkali-halide crystals (AHCs) are presented in the paper. The theoretical explanation of the influence of low-temperature uniaxial deformation on the formation of the luminescence of the STEs was done. The increase in activation energy of the temperature quenching of the luminescence of the STEs, narrowing of the emission bands, increase in the frequency of the active oscillations of the STEs and the decrease in the Huang-Rhys parameter are shown as the results. These changes indicate a weakening of the exciton-phonon interaction in elastically deformed AHCs, which, in turn, leads to an increase in the probability of radiative annihilation of excitons.

1. Introduction In nuclear technology, materials in operation are exposed simultaneously to ionizing radiation, deformation and temperature. However, the details of the processes occurring in the materials are not fully understood. Simple in structure alkali-halide crystals (AHCs), in which the processes/mechanisms of evolution of electronic excitations (EEs) are studied in detail, are “model objects” for understanding similar processes in other materials with a predominantly ionic bond [1–8]. It should be noted that alkali-halide scintillation detectors as a reliable detector are used in such relevant experiments as recording the energy of dark matter particles [9–10]. On the basis of AHCs, the fundamental laws of the stages of the evolution of EEs in the anionic sublattice are established: how they can be created, migrate, trapped/self-trapped, and, finally, decay channels of EEs. Moreover, in addition to radiative decay of self-trapped excitons (STEs) with the appearance of the so-called σ- and π-luminescence, nonradiative annihilation of STEs also takes place, their decay into primary Frenkel defects in the anionic sublattice of the crystal – socalled F,H and α,I-pairs of neutral or charged Frenkel defects [2–7,11–17]. Reducing the lattice symmetry of AHCs is possible under the action of local disturbances in the crystal lattice due to the presence of impurity ions of various sizes and valencies, as well as Frenkel defects (interstitial-vacancy pairs of point defects) and dislocations, especially by applied plastic and elastic deformation [18–24].

⁎

The pre-decay structure of STEs, the hole component of which is the molecular halide ion (X2−), is very sensitive to changes in the symmetry of the surrounding particles, i.e. to the reduction in lattice symmetry. According to the generally accepted Kanno classification [25], depending on the displacement of the centers of mass of electron and hole components of an exciton, there are three configurations of STEs: a symmetric configuration I – type of on-STEs, and also with a weakly or strongly displaced X2− molecule relative to the two involved anion sites – STEs of type II (weak off-center) and III (strong off-center). Plastic deformation of AHCs (ε > 1%), as is known [26–29], causes a number of reversible and irreversible processes, as a result of which various structural defects, such as divacancies, are also created. Note that defects caused by plastic deformation play the role of traps/stabilizers of anionic radiation defects; for example, near divacancies, the efficiency of the association of interstitial halogen atoms with the formation of X2 centers increases. On the other hand, low-temperature elastic uniaxial deformation (ε ≤ 1%) does not create structural defects, but only changes the lattice parameters and, therefore, significantly affects on the process of radiative annihilation of STEs of three different configurations. Under conditions of uniaxial elastic deformation of AHCs, the mean free path of excitons and, accordingly, the excitation/luminescence efficiency of impurity centers should be significantly reduced, but at the same time, the probability of self-trapping of excitons at regular lattice sites and the intensity of STEs luminescence during their subsequent radiative

Corresponding author. E-mail address: [email protected] (L. Myasnikova).

https://doi.org/10.1016/j.nimb.2019.12.014 Received 19 November 2019; Received in revised form 10 December 2019; Accepted 16 December 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.

Nuclear Inst. and Methods in Physics Research B 464 (2020) 95–99

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annihilation will increase sharply. The effect of enhancing the intrinsic luminescence of AHCs and weakening the transfer of EEs energy to impurities during low-temperature uniaxial deformation was recorded in our previous works [30,31]. It should be noted that hydrostatic compression of AHC leads to the opposite effect – weakening of the STEs luminescence and intensification of the excitation of impurity centers by free excitons [32,33]. An increase in the luminescence intensity of AHCs at 93 K under the action of elastic deformation [26,31,34] is probably due to a decrease in the efficiency of the nonradiative channel for exciton decay into primary radiation defects [1,2]. In the temperature range under consideration (about 93 K), almost all excitons exist only in the self-trapped state [1,2]. In the temperature range in which quenching of the STEs luminescence occurs, the efficiency of the creation of radiation defects increases [1,2]. This effect is due to the fact that nonradiative decay of excitons into radiation defects is now predominant. It is possible to obtain information on the annihilation of excitons simultaneously through two channels by lowering the symmetry of the lattice of AHCs by low temperature uniaxial elastic deformation. We think that this problem can be experimentally realized by measuring the temperature quenching of the luminescence of the STEs of AHCs. With this approach, the activation energy values are identified with the height of the potential barrier between the radiative and nonradiative decay channels of the STEs. In the KI crystal at 100 K π- and σ-luminescence of STEs are not quenched. From [1,2,35] it is known that in the absence of deformation, the KI crystal activation energy (Ea) of the π- and σ-luminescence of the STEs is 132–136 meV and 46.2 meV, respectively. For the NaCl crystal consisting of two σ (5.35 eV) and π (3.35 eV) STEs luminescence bands, the intensity of σ-luminescence is quenched by more than one order up to 100 K in comparison with the intensity of π-luminescence of the STEs. So, π-luminescence is dominant in the Xray spectra at 100 K. In [1,35] for π-luminescence of NaCl in the absence of deforming factors, the activation energy of 99 meV was determined upon excitation by photons with energies of 8.0 eV and 8.3 eV, creating excitons and electron-hole pairs in the crystal, respectively. The luminescence intensity (especially σ) of the STEs of the KBr crystal is quite detectable at 100 K. The data from [1,2,35] show that Ea = 23 meV for σ-luminescence of the STEs of the undeformed KBr crystal. In the CsBr crystal, the STEs luminescence spectrum consists of three bands: σ-emission with maxima of 4.74 eV and 5.8 eV and πemission with a maximum of 3.55 eV. A quantitative analysis of the temperature quenching of the π-luminescence of the STEs shows that this process is well described by the Mott formula and the activation energy is 0.1 eV, which remains constant upon photoexcitation in all long-wavelength exciton absorption bands with energies of 6.8 eV, 7.1 eV and 7.2 eV [36,37]. From the three STEs luminescence bands in CsBr, we selected π-luminescence by the same criterion as for NaCl. It follows from [35] that the temperature quenching of π-luminescence (4.62 eV) of the STEs in NaBr crystals begins in the region of 100 K. According to [1,2], the activation energy of quenching of πluminescence of the STEs in undeformed NaBr crystals is 160 meV. Studies of X-ray and photoluminescence showed that the luminescence spectrum of the STEs in the RbI crystal consists of at least three bands, the maxima of which are located at 3.89 eV (σ); 3.1 eV (EX) and 2.3 eV (π). In the region of 100 K, the decrease in intensity from the maximum value for π-luminescence is more than one order, and for σluminescence up to one order, and its quenching is extended up to high temperatures – 120 K. Activation energy of quenching of σ-luminescence intensity in zone purified RbI crystal in the absence of deformation, is 43 meV [38]. According to the experimental data obtained in [39], by the method of pulsed electron irradiation, the activation energy between two

exciton configurations in the CsI crystal is 50 meV. The aim of the work was to study the processes of temperature quenching of STEs luminescence under applied elastic low temperature deformation as well as to analyze theoretically the obtained experimental data for a number of AHCs. 2. Experimental Single AHCs were grown at the Institute of Physics of the University of Tartu, according to the Kyropoulos method in a helium atmosphere [29]. It should be emphasized that all experiments on measuring the temperature dependence of X-ray luminescence were performed on zone-purified crystals. This circumstance is associated with the fact that to prevent the transfer of excitation energy to impurities, and register the change in the intensity of only the intrinsic emission of the STEs. For the experiment, the samples with typical size of 5 × 5 × 2 mm3, freshly cleaved along the (1 0 0) plane were used. Elastic uniaxial deformation of samples was carried out by the crystallographic direction 〈1 0 0〉 in a special evacuated cryostat (10−4 Torr). The crystals were deformed after their cooling to a temperature of 93 K. The design of the cryostat allows to set the required degree of deformation. The degree of deformation of the objects did not exceed 1% so that the deformation remained in the elastic region obeying Hooke's law. The X-ray luminescence spectra were recorded in spectral region from 2 to 6 eV on the base of the MSD-2 monochromator and photomultiplier tube such as a Hamamatsu (Japan) H 8259 device. X-ray luminescence spectra with specified parameters were scanned automatically with SpectraSCAN software. The MSD-2 monochromator and copper-constant thermocouple controllers are based on high-performance 32-bit STM32 microprocessors with an ARM Cortex-M core manufactured by STMicroelectronics (Switzerland). The sample heating to a certain temperature (the limit at 200 K) was carried out at a rate of 0.2 0/s, and every 5–10 degrees the X-ray spectrum was measured with a scanning speed of 50 nm/s. RUP-120 Xray apparatus worked in a mode (W, 3 mA, 100 kV), providing hard gamma radiation penetrating through the entire thickness of the crystal. 3. Main results The exposed uniaxial deformation of AHCs leads to a strong increase in the luminescence of STEs [34,40]. Fig. 1 shows, by way of example, the X-ray spectra of KI crystals measured at 93 K for both an

Fig. 1. The spectra of X-ray luminescence of KI crystal at 93 K. Curve 1 – before applied deformation; 2 – at uniaxial stress ε = 0.8% at 93 K; 3 – at uniaxial stress ε = 1% at 93 K; 3′ – decomposition into Gaussian components of curve 3. Inset demonstrates dependence of the intensity of π-luminescence of the STEs on the degree of deformation ε. 96

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undeformed sample (curve 1) and samples subjected to uniaxial elastic deformation of different degrees ε = 0.8% and ε = 1% (curves 2 and 3, respectively). Two luminescence bands with peaks at 3.31 eV and 4.16 eV, corresponding to the π- and σ-luminescence of STEs, are clearly manifested in the spectrum. In the KI crystal that is elastically deformed at 93 K (ε = 1%), the π-luminescence intensity increases by about an order of magnitude, and the decomposition of the spectrum (Fig. 1, curve 3′) into elementary Gaussians makes it possible to isolate the so-called EX-band of luminescence with a maximum at about 3.0 eV. The EX-band is considered as with the largest Stokes-shift energy (2.8 eV) and the longest decay time (1.5 × 10−6 s) and it’s nature is triplet [32]. Inset in Fig. 1 it demonstrates the influence of the relative degree of uniaxial deformation ε on the intensity of the maximum of the π-luminescence band of STEs, I = f (ε). A linear section can be distinguished with an increase in deformation up to ε = 1%, which characterizes the elastic part of the deformation and corresponds to Hooke's law. The second section for large values of ε describes the saturation of the function I = f (ε) and belongs to the region of plastic deformation of the crystal. Similar results were obtained for other AHCs [34,40–42]. The enhancement of the luminescence of STEs in an elastically deformed crystal is most likely due to a decrease in the efficiency of the nonradiative exciton decay channel with the formation of Frenkel defects. An analysis of the temperature quenching of luminescence of STEs in AHCs, whether or not subjected to elastic deformation, allows us to estimate the activation energy of the nonradiative transition in STEs (see Fig. 2). Consider the luminescence of STEs, by analogy with the Mott model for the impurity center of luminescence [43–46], i.e. under conditions of local interaction of the exciton with lattice vibrations. The quantum yield of the luminescence of STEs in AHCs is described by the probability of radiative (f0) and nonradiative (d) transitions: η = I (T)/ T = f0/(f0 + d). According to Mott [47–49], the probability of radiative transitions does not depend on temperature and is determined by the STEs lifetime f0 = 1/τ0. The probability of a nonradiative transition in the STEs depends on temperature according to the law d = d0 exp(–Ea/ kBT) [2,4]. Therefore, the expression for η can be represented as follows:

= I (T )/ I0 = 1/[1 +

0 d 0 exp(

Ea/ kB T )],

temperature range 93–200 K with a step of 5–10 K. Fig. 2 demonstrates the temperature dependences of the π-luminescence intensity of STEs (3.31 eV) for undeformed (curve 1) and deformed (curve 2) KI crystals. Note that the luminescence intensity of STEs increases by an order of magnitude in a crystal subjected to uniaxial low-temperature deformation. Inset in Fig. 2 demonstrates dependence of inverse temperature on lg[(1/η) − 1] for the π-luminescence of the STEs of the KI crystal before (curve 1′) and at deformation (curve 2′). From the slope of the straight line lg[(1/η) – 1] ~ (1/T) it is possible to estimate the activation energy of the nonradiative transition Ea during elastic deformation of the lattice. According to our calculations, this energy before deformation of the KI crystal was Ea = 132 meV, and at deformation it increased to Ea = 169 meV. Apparently, the height of the barrier separating the channels of the radiative and nonradiative (with the formation of Frenkel defects) decay of the STEs in an elastically deformed KI crystal increases. A similar methodology was used to evaluate the effect of elastic deformation on the value of Ea for a number of AHCs, the data obtained are presented in Table 1 [50]. It is known [35] that the spectral characteristics of STEs in AHCs are well described by the one-oscillator harmonic approximation. Therefore, it is reasonable to analyze, within the framework of this approximation, changes in the spectral characteristics caused by elastic deformation. As a result, it is possible to obtain additional information on the parameters of potential STEs curves in AHCs. The width of the π-band of the STEs radiation at half maximum (δ) was not determined from the X-ray spectra measured at a certain temperature for both an unstressed and a deformed KI crystal. The shape of the elementary X-ray bands of the KI crystal is Gaussian (Fig. 1) both before and at low-temperature deformation. However, low-temperature uniaxial elastic deformation (up to ε = 1%) leads to a narrowing of the STEs emission band at 100 K (see Table 1). For example, according to our experimental data, it turned out that the πluminescence band of the STEs of the KI crystal at 100 K had δ = 0.63 eV before deformation, and δ = 0.60 eV after applying elastic deformation. Note that in [32], a similar value before hydrostatic compression was δ = 0.62 eV. With increasing temperature, the width of the STEs emission bands increases. In the harmonic approximation, the temperature dependence of the optical bandwidth is described by the formula:

(1)

(T ) = (0){cth (h

where I(T) is the intensity of the luminescence of the STEs at a certain temperature T; I0 is the intensity of the STEs emission before the temperature quenching; τ0 is the STEs lifetime; d0 is the probability of nonradiative transitions; Ea is the activation energy of the nonradiative transition; kB is the Boltzmann constant. For a number of AHCs, X-ray spectra were measured in the

0/

4 kB T )}1/2,

(2)

where δ (T) is the value of the emission bandwidth at its half height at temperature T; δ (0) is the radiation bandwidth at extremely low temperatures; h is the Planck constant; ω0 is the frequency of active vibrations of the STEs [28,32]. Note that within high temperatures (hω0 ≪ kBT), the width grows proportionally to T , and at low temperatures (hω0 ≫ kBT) the δ is constant. Each electronic state of the STEs is associated with a harmonic oscillator with its frequency and its equilibrium coordinate. The adiabatic potentials in this approximation are parabolas having different minimum positions for various electronic states. In [32], the values of the frequencies of active exciton vibrations ω0 were calculated for πand σ-luminescence (ω0(π) = 7.20 × 1012 s−1 and ω0(σ) = 1.80 × 1013 s−1, respectively) of the KI crystal before hydrostatic compression. Moreover, as was noted by the authors, the value of ω0 for π-luminescence is much smaller than for σ-luminescence. For an undeformed KI crystal, we obtained the values of the active vibration frequencies ω0(π) = 7.19 × 1012 s−1 and ω0(σ) = 1.82 × 1013 s−1 for π- and σ-luminescence of the STEs, respectively. In addition, for the first time, the dependence of ω0 on the degree of low-temperature uniaxial elastic deformation (up to ε = 1%) was studied and the values ω0(π) = 7.63 × 1012 s−1 and ω0(σ) = 2.23 × 1013 s−1 were obtained. Changes in frequencies indicate a change in the form of the adiabatic potential curve of the

Fig. 2. Temperature dependences of the intensity I = f (ε) of the π-luminescence of STE in the KI crystal (1) before deformation and (2) at low-temperature uniaxial deformation (ε = 1%, 93 K). Inset demonstrates temperature dependences of the Arrhenius coordinates T−1 – lg[(1/η) – 1]. 97

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Table 1 The spectral characteristics of AHCs. The positions of the maximum of the STEs emission band [1,35]; activation energies (Ea) of luminescence quenching for selftrapped excitons; the half-width of the emission band (δ) at 100 K; the frequency of active vibrations of the STEs (ω0); the value of the Stock losses (Sst) [1,35]; the Huang-Rhys parameter (S). Column 1 – before deformation; 2 at elastic uniaxial deformation (ε = 1%) at 93 K. Crystal

KI RbI CsI KBr CsBr NaBr NaCl

Maximum of the emission bandof the STEs (eV)

3.31 4.16 3.89 3.67 4.42 3.55 4.62 3.35

Activation energy, Ea (meV)

Half-width, δ (eV) (at 100 K)

The frequency of active vibrations of the STEs, ω0 (s−1)

1

1

1

132 47.3 43.7 50 26 95 154.7 103

2 169 81 113.2 66 33 140 192.7 138

0.63 0.44 0.47 0.59 0.57 0.63 0.55 0.94

2 0.60 0.42 0.44 0.55 0.55 0.60 0.52 0.87

2 12

7.19 × 10 1.82 × 1013 1.19 × 1013 9.01 × 1012 1.89 × 1013 7.87 × 1012 2.20 × 1013 1.103 × 1013

7.63 2.23 1.44 1.05 1.93 8.83 2.76 1.15

× × × × × × × ×

12

10 1013 1013 1013 1013 1012 1013 1013

The value of the Stock losses, Sst (eV)

Huang-Rhys parameter, S 1

2

2.56 1.69 1.83 2.14 2.31 3.33 2.06 4.62

540 141 233 360 185 413 142 294

509 115 193 308 181 368 113 282

temperature elastic deformation (ε = 1%, 93 K) in crystals RbI, CsI, KBr, CsBr, NaBr, and NaCl (see Table 1). The knowledge of ω0 of the STEs relevant in the exciton-phonon interaction and the Stokes losses (Sst) allows us to estimate the HuangRhys parameter: S = 2πSst/hω0 [51]. This parameter characterizes both the degree of localization of the wave function and the exciton-phonon interaction, showing the number of phonons generated in a single radiative electronic-vibrational transition. For example, for π-luminescence of the STEs S = 540 and S = 509 for an undeformed and deformed KI crystal, respectively. The difference between the Huang-Rhys parameters before deformation and at uniaxial deformation of the KI crystal ΔS(π) = 31 and ΔS(σ) = 26, therefore, at applied deformation at 93 K, the number of phonons emitted by the crystal in one electronvibrational act in the form of π- and σ-luminescence of the STEs decreases by 31 and 26, respectively. This also explains the sharp increase in the π- and σ-luminescence intensities of the STEs in the KI crystal at uniaxial elastic deformation. In AHCs subjected to uniaxial low-temperature elastic deformation, the parameter S decreases in comparison with undeformed samples (see Table 1).

Fig. 3. Temperature dependences of the half-width of the emission bands of KI crystals before (1) and at low-temperature deformation ε = 1% (2).

4. Conclusions In fact, at low-temperature elastic deformation (ε = 1%, 93 K) by 〈1 0 0〉 direction from the AHC end, the actual compression of the lattice occurs in the crystallographic direction 〈1 1 0〉, which corresponds to the direction of the STEs in face-centered AHCs. As a result of which, there is an increase in the luminescence of STEs in AHCs. However, at hydrostatic compression, the opposite effect occurs. In [32,33], a decrease in intensity is observed with increasing hydrostatic pressure. The results obtained in this work confirm the significant effect of low-temperature uniaxial deformation on the formation of the luminescence of the STEs. If the deformation did not affect the structure of the STEs, the differences in the dynamic characteristics for AHCs would be less significant, and there would be no significant qualitative changes in the optical spectra themselves. Thus, an increase in the activation energy of the temperature quenching of the luminescence of the STEs, narrowing of the STEs emission bands, an increase in the frequency of active STEs oscillations, and a decrease in the Huang-Rhys parameter indicate a weakening of the exciton-phonon interaction in elastically deformed AHCs, which, in turn, leads to an increase in the probability of radiative annihilation of excitons.

Fig. 4. Temperature dependences of the half-width of the emission bands of NaBr and RbI crystals before (1) and at low-temperature deformation ε = 1% (2).

excited state. Figs. 3 and 4 demonstrate the temperature dependences of the width of the luminescence bands of the STEs of KI, NaBr, and RbI crystals. Curve 1 corresponds to the state of an undeformed crystal, curve 2 – to an elastically deformed crystal up to ε = 1% at 93 K of the crystal. The solid line in the figures represents the function δ (0){cth (hω0/4πkBT)}1/2, which corresponds quite well to the experimental points (○, ●, Δ, ▲). Using a similar technique, we calculated the values of the frequencies of active vibrations of the STEs before and after low-

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 98

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