Optical properties of γ-sensing β-GaLaS3:Er crystal

An erbium-doped β-GaLaS3 crystal has been grown by the solution-melt method, and the effect of γ-irradiation on their vibrational and radiative properties has been investigated. Experiments have demonstrated that the crystal is transparent in the near and mid-infrared (IR) regions (transparency ∼62% in the range of 350–7100 cm−1), which allows it to be used as an effective matrix for creating lasers in this spectral range. For the first time, the vibrational spectrum of the crystal and the density of phonon states have been calculated using the density-functional theory (DFT) method. Both the original and γ-irradiated β-GaLaS3:Er crystals were investigated by Raman and IR spectroscopy. It has been established that irradiation with a dose of up to 5000 Gray does not lead to structural changes in the crystals. The effect of the formed defects is more clearly manifested in the IR reflection spectra, compared to the Raman spectra. The mechanism of the occurrence of excited states and the emission of Er ions embedded in the lattice has been established, and the effect of γ-irradiation on the radiative properties of β-GaLaS3:Er due to the occurrence of radiation-induced defects has been analyzed. A model has been constructed that explains the Stokes and anti-Stokes radiation of the erbium ions in the crystal. It has been demonstrated that the grown crystal has good prospects for sensor and laser technology of the near and mid-IR ranges due to relatively high values of optical transparency and the intense radiative capacity of the erbium ions.

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Introduction
In recent years crystal [1] and glass-like [2] chalcogenide materials have been the subject of extensive study for their use in optical electronics in visible (Vis-) and infrared (IR) spectral bands. Such interest in this class of compounds is caused by the combination of unique structural and optical properties, in particular a low absorption coefficient in a wide range of the spectrum, high values of the refraction indexes and quantum emission, resistance to aggressive environments, etc. Doping of chalcogenides by rare earth (RE) metals makes them effective materials for radiation detection. Nowadays active and passive elements in laser technology based on glass and crystal chalcogenides doped with RE metals are being explored [1,2] as elements of various sensor devices [3][4][5], high-resolution laser distance sensors, and telecommunications [6][7][8]. It is especially worth noting the possibility of using such materials to create lasers on their basis, which emit in so-called 'atmospheric windows' of transparency in the ranges 3-5 and 8-14 µm.
The radiation resistance of these semiconductor materials and their use as γ-ray sensors [9] are particularly attractive. A number of publications reported the influence of γrays irradiation on optical absorption spectra of such materials [10][11][12]. However, only few works are devoted to the analysis of changes in the photoluminescence (PL) intensity under the action of γ-radiation, establishing the mechanism of the PL and the nature of radiation-induced defects at different radiation doses [13][14][15]. This especially applies to the analysis of crystal structures of chalcogenides doped with RE metals.
In our previous work [16], devoted to the study of the GaLaS 3 :Er 3+ single crystal, the band gap was determined, and the Stokes PL mechanism excited by 532 nm laser radiation was studied. At the same time, for the profound practical application of such crystals, it is also important to establish the effect of γ-irradiation on the PL properties and local crystal structure, which is the main goal of this work.

Methods
Single crystal GaLaS 3 :Er 3+ was grown according to the technique described earlier in [16] with the solution-melt method based on the La 2 S 3 -Ga 2 S 3 phase diagram published in [17] from the (Ga 69.5 La 29.5 Er) 2 S 300 batch as a source melt. According to the x-ray phase analysis of the sample ground into powder, no other phases were found [16].
The obtained sample was indexed in the orthorhombic syngony, space group Pna2 1 , with lattice parameters a = 1.0427, b = 2.1986, c = 0.6062 nm. According to [16], small concentrations of Er did not affect the crystal structure, which was confirmed by comparing the diffractogram obtained for the grown single crystal and the calculated diffractogram for the β-GaLaS 3 sample according to [18] data. For the following discussion, it is important to mention that doping with erbium enters the crystal as a substitution for La atoms, as both elements have close ionic radii. It is very unlikely that Er atoms substitute Ga in the lattice, which has coordination number 4, while the coordination number of La atoms is known to be 8 [18]. It should be noted that for the sake of simplicity in this article we use the formula β-GaLaS 3 :Er 3+ when mentioning the composition of the crystal under investigation.
The image of the surface of the crystal [16] obtained by the scanning electron microscopy method is shown in figure 1(a). The energy dispersive x-ray spectroscopy analysis confirmed the composition (Ga 42.10 La 46.13 Er 0.93 ) 2 S 302.75 of the grown crystal, which is in good agreement with the calculated composition (Ga 49.5 La 49.5 Er 1 ) 2 S 300 for the crystal in [16]. The quantitative distribution of the elements is shown in figure 1 PL spectra were studied at room temperature using a MDR-204 monochromator equipped with Si and PbS photodetectors. PL spectra were excited by laser radiation with the wavelength of 805 nm. γ-irradiation of the single crystal was carried out by a 60 Co source in the air at ambient temperature with the average energy of the γ-rays about 1.25 MeV. The method of γ-irradiation and the assessment of the dose were described in detail in [19]. IR transmission spectra were studied using a Fourier spectrometer 'IRAffinity-1S Shimadzu'.
Raman spectra were excited with the line 457 nm of solidstate laser and acquired using a single-stage spectrometer MDR-23, equipped with a cooled CCD detector (Andor iDus 420, UK). The laser power density on the samples was limited to 10 3 W cm −2 , to preclude any thermal modification of the samples. A spectral resolution of ∼3 cm −1 was determined from the Si phonon peak width of a single crystal Si substrate. The Si phonon peak position of 520.5 cm −1 was used as a reference for determining the frequencies of the Raman peaks.
IR reflectance spectra in the spectral region of 50-500 cm −1 were recorded at room temperature using Bruker Vertex 70 V FTIR spectrometer equipped with a Globar source and a deuterated triglycine sulfate (DLaTGS) detector with a polyethylene window, the angle of incidence was 11 • .
First principle calculations of the electronic ground state of monocrystals were performed within the generalized gradient approximation using the Perdew-Burke-Ernzerhof functional [20] as implemented in the CASTEP code [21]. Before performing the calculations, the positions of atoms within the unit cell were relaxed so that forces on atoms in their equilibrium position did not exceed 3 meV Å −1 and residual stress was below 0.01 GPa. The total energy convergence tolerance was set to 2 × 10 −7 eV atom −1 . For calculations of the electronic structure, integration within the Brillouin zone was performed over a 1 × 1 × 2 Monkhorst-Pack grid [22] in the reciprocal space. Norm-conserving pseudopotentials were used. Lattice dynamics properties were further calculated within the density functional perturbation approach. For β-GaLaS 3 crystal, we also calculated the phonon density of states.

IR and Raman spectroscopy
In our previous work [16], the optical absorption spectrum of the grown crystal β-GaLaS 3 :Er 3+ was analyzed in the spectral range of 1.2-2.1 eV, and the band gap energy was found  to be equal to 1.99 eV. A similar band gap value of 2.2 eV was calculated for β-GaLaS 3 [18] by the DFT method [16], which is typically only an estimate unless special measures are considered. Figure 2 shows the transmission spectrum of the β-GaLaS 3 :Er 3+ crystal in the range of 350-7100 cm −1 . The transmission coefficient varies in the region of 60%-64% over a very wide spectral interval down to 750 cm −1 . A sharp decrease in the crystal transparency from the low-frequency side is associated with the absorption by lattice phonons and multiphonon processes [23]. Absorption bands (6250-6750 cm −1 or 1.48-1.6 µm) were detected in the spectrum, which are due to the transitions 4 I 15/2 → 2 I 13/2 in Er 3+ ions. In addition, the impurity absorption of the H 2 O, CO 2 , S-H, and O-H groups was recorded [24][25][26][27][28]. The detected impurities had a technological origin and are related to the purity of the raw materials.
In order to reveal the influence of γ-irradiation on the local structure of the crystal and, accordingly, on its radiative properties, the crystal was investigated by means of Raman and IR spectroscopies.
The structure and electronic properties of the β-LaGaS 3 compound were reported in [18]. The authors did not study the vibrational properties of the β-LaGaS 3 , but at the same time, they indicated that the crystal structure was formed by wavy chains of GaS 4 tetrahedra extended along the [100] direction and separated by La 3+ cations. The Ga-centered polyhedra were interconnected through shared sulfur atoms.
From our previous experience of the investigation of Raman and IR spectra of Ga-containing multicomponent chalcogenides AgGaGeS 4 [29] and PbGa 2 GeS 6 [30], their phonon spectra were largely determined by the vibrations of the structural groups formed by GaS 4 tetrahedra.
The β-GaLaS 3 :Er 3+ unit cell consists of 12 formula units and contains 60 atoms, i.e. possesses 180 vibrational degrees of freedom, of which 3 are acoustic and 177 are optical. According to the group-theoretic analysis for the point group C 2v , to which this crystal belongs, for the Brillouin zone center, the vibrational representations are as follows: Despite rather large unit cell of the crystal under consideration, the DFT calculation allowed us to reliably obtained frequencies of all vibrational modes of the β-GaLaS 3 :Er 3+ crystal and assigned them to four types of vibrations: A 1 , A 2 , B 1 , and B 2 . As far as an experimental spectrum, it shows a number of rather broad bands (figure 3), due to the superposition of a large number of Raman-active modes. Their assignment to the specific lattice eigenmodes seems to be difficult to perform.
γ-ray irradiation of the crystal with a 5000 Gy dose only slightly changes the Raman spectrum apart from a decrease in the intensity of the band at 187 cm −1 and some other less intense bands ( figure 3). γ-irradiation might generate some lattice defects and influence the Raman line intensities and their broadening, and, consequently, 'smearing' of certain spectral features. In order to characterize the most intense bands in the Raman spectrum, polarization studies were conducted. Figure 4 shows the experimental Raman spectra recorded in different polarizations. The spectra are denoted with Porto indices, which are traditionally used for polarization measurements. Given the reference to the Cartesian coordinate system (xyz), the first and last letters in the designations in figure 4 are related to the directions of the exciting laser and scattered beams, while the letters in brackets indicate the directions of the electric vector of the beams. According to these notations, the fully symmetric A 1 modes are active in the (xx), (yy), and (zz) configurations, as follows by the selection rules, the A 2 modes appear in the spectra at the (xy) and (yx) polarizations, B 1 modes-in (xz) and (zx), and, finally, B 2 modes-in (yz) and (zy) polarizations. The most intense bands in experimental Raman spectra appear at 187, 321, 342, and 358 cm −1 . Their significant intensity is due partly to a superposition of several bands, which is further confirmed by their asymmetric line shapes ( figure 4). Moreover, they include components of fully symmetric oscillatory modes A 1 , which, as a rule, are the most intense in the spectra. Comparing these frequencies with our previously published data for quaternary chalcogenides [29,30], all of them can be related to the vibrations of the GaS 4 tetrahedra.
As follows from the DFT calculation, 2 vibrational modes B 2 (187 cm −1 ) and A 1 (189 cm −1 ) make the largest contribution to the intensity of the band in the Raman spectrum with the frequency of 187 cm −1 . Depending on the polarization, the contribution of each of them changes, and the total intensity of the total band and its frequency position also changes, depending on the intensity of the components with different frequencies. The band with the frequency of 321 cm −1 is formed by the contribution of the vibrational modes with frequencies B 1 (318 cm −1 ) and B 2 (322 cm −1 ), and the intensity of the experimentally registered band only weekly depends on polarization.
According to the selection rules, the number of vibrational modes appearing in the IR absorption spectra is smaller but is still very large-132. Figure 5 shows the experimental IR reflection spectra of the crystal before and after γ-irradiation with a dose of 5000 Gy. It can be seen, γ-irradiation gives a more significant effect on the change of the IR reflection spectrum in comparison with the Raman spectrum. γ-irradiation of the crystal does not lead to the knocking out of atoms from the crystal structure and, accordingly, to amorphization and mixing of the component composition, but only breaks the bonds between the chains of the Ga-S tetrahedra, increasing the number of different vacancies in the crystal structure and changing the charge of the atoms. Therefore, this effect is more noticeable in the IR spectra, since a change in the dipole moment is important for the appearance of the vibrational modes, while in the Raman spectra-a change in polarizability. Interestingly, the experimental IR spectrum shows a larger number of vibrational modes in comparison with Raman spectra.
Using the DFT calculation, the IR absorption spectrum has been obtained (figure 6(a), curve 1). As can be seen, the calculated bands correlate well with the bands of the imaginary part of the dielectric function (curve 2), which has been obtained from the experimental IR reflection spectrum, shown in figure 5. The structure of these spectra in the range of frequencies >250 cm −1 is entirely related to GaS 4 tetrahedra. The same applies to bands in the region of 120 and 200 cm −1 , as follows from the analysis of spectra of related compounds [29,30]. Figure 6(b) shows the spectrum of the density of phonon states (DOSs) of the crystal calculated by the DFT method. The phonon DOS is one of the important characteristics of the energy spectrum of solids and determines not only the thermodynamic characteristics associated with the lattice subsystem of the crystal but also the kinetic effects. It describes the number of phonon modes of a selected frequency ω(k) in a certain frequency interval. In our case, most of the characteristic features of calculated phonon DOS do not contradict the spectral features of the Raman scattering plots presented in figures 3, 4, and IR absorption (figure 6(a)).

FL spectra
Secondary radiation of the β-GaLaS 3 :Er crystal in the Vis-and near-IR ranges under 805 nm laser excitation are displayed in figure 7. One anti-Stokes PL band (650 nm) and two intense Stokes PL bands (980 and 1540 nm) were registered. They are known to correspond to the transitions in the f-shell of the Er 3+ ions: 650 nm-4 F 9/2 → 4 I 15/2 ; 980 nm-4 I 11/2 → 4 I 15/2 ; 1540 nm-4 I 13/2 → 4 I 15/2 . As can be seen from the spectra, irradiation of the crystal with γ-rays leads to a decrease in the intensity of all PL bands, while their frequency position does not change.
Based on the diagram of energy transitions, we have considered the mechanism of the occurrence of excited states and radiation in the Er 3+ ions (figure 8). When the crystal is irradiated with radiation of 805 nm, the erbium ions pass from the main 4 I 15/2 to the excited state 4 I 9/2 and from 4 I 13/2 to the 2 H 11/2 state. The radiation with a maximum of 1540 nm arises as a result of CR1 cross-relaxation, while the maxima at 980 and 650 nm are due to CR2 cross-relaxation transitions. The Er 3+ ions cannot pass from the 4 S 3/2 state to the 4 F 9/2 state nonradiatively, as in the case of the closely located 2 H 11/2 and 4 S 3/2  states. This is because the energy separation between them is ∼3000 cm −1 [18], and the maximum energy of phonons, according to the vibrational spectra (figures [3][4][5][6], is only about 420 cm −1 . Therefore, the occurrence of excited states 4 F 9/2 , 4 I 11/2 , and 4 I 13/2 can be associated with cross-relaxation processes, as one of the important elements of the emission mechanism in Er-doped sulfide semiconductors. Note that the role of energy exchange processes through cross-relaxation increases as a result of an increase in the Er 3+ concentration from 0.5 to 1.0%, as shown in [31,32]. This causes a decrease in the distance between neighboring Er 3+ ions and, accordingly, an increase in the probability of cross-relaxation. This leads to the dominance of the PL bands, which are associated with cross-relaxation processes. Irradiation with γ-rays leads to the appearance of radiationinduced defects that interfere with the processes of energy exchange between erbium ions and, as a consequence, with an increase in the radiation dose, the PL intensity decreases. According to our previous investigation of similar systems, Ga-V S can be the γ-induced defect centers [15].

Conclusions
It has been established that β-GaLaS 3 :Er crystal exhibits transparency 60%-64% in the IR range of 350-7100 cm −1 , which is important for the use of such crystals as laser emitters as well as in the creation of effective and radiation-resistant sensors in the near and mid-IR spectral regions.
The research of the vibrational spectra of the nonirradiated and γ-irradiated crystal shows that γ-irradiation leads to minor (local) structural changes in the crystal, which are more clearly present in the IR reflection spectra compared to the Raman spectra.
The frequencies of the vibrational modes of the crystal, the spectrum of the DOSs, and the IR absorption spectrum have been calculated using the DFT method. Based on the performed polarization Raman studies and the comparison of the obtained results with DFT calculations, the most intense Raman and IR bands have been assigned to the corresponding vibrational modes.
Based on the diagram of energy levels of the Er 3+ ions, the mechanism of the occurrence of excited states and radiation has been established, in which cross-relaxation processes play an important role. It has been demonstrated that γ-irradiation leads to a decrease in the PL intensity due to the presence of radiation-induced defects.
Taking into account relatively high values of optical transparency, intense PL in the IR range, and sensitivity to γ-rays, we can conclude that β-GaLaS 3 :Er crystal has good prospects for use in sensor technology in the near and mid-IR ranges.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).