Raman and Infrared Vibrational Spectra of PbGa2 S4 Crystal


N.N. Syrbu*

Citation: Raman and Infrared Vibrational Spectra of PbGa2 S4 Crystal, American Research Journal of Materials Science, vol 1, no. 1, pp. 1-9.

Copyright This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


Abstract:

Raman scattering spectrain different geometries at temperatures 10 - 300 K and infrared vibrational spectra in polarizations Ec and Ec for range 50 - 4000 cm-1 at 300 K were investigated for PbGa2 Se crystals. Contours of reflection spectra in polarizations Ec and Ec were calculated and parameters of phonons and dielectric constants were determined. Temperature dependences of Raman spectra were investigated and soft modes with different temperature dependences in intervals 80 - 150 and 150 - 370 K were discovered. A group of lines attributed to Davydovmultiplets was found out and their polarization dependences temperature changes were investigated. Effective ion charges were calculated and a polarizability of ionic charges of Pb, Ga and S in PbGa2 S4 lattice was determined.

Keywords: Raman scattering; infrared reflection spectra; vibrational modes; Davydovmultiplets; effective ion charges;

Description:

INTRODUCTION

A lead thiogallate (PbGa2 S4) compound belongs to a wide class of ternary chalcogenide materials of AIIB2 IIIC4VI group, which uses in different devices of linear and nonlinear optics [1 - 6]. The PbGa2 S4 compound crystallizes in rhombic lattice and is a layered semiconductor crystal with well pronounced anisotropy of optical properties. Crystal is easy cleaved along (100) plane. Excitonic states at room temperature with binding energies of 292 meV (Frenkelexcitons) were discovered [7, 8].
A new technology of grow for crystals of lead thiogallate was proposed by authors of Ref. [1 - 4]. The lead thiogallate crystals in consequence of a nonisovalent substitution of bivalent lead ions by trivalent rare-earth ions and a crystalline structure peculiarity can contain active ions in positions with different local surroundings. A change of such positions amount and its population by active impurity allows to operate on spectroscopic and generation properties of lasers in dependence of synthesis technology. It is possible to make a management of spectroscopic properties of optical centers and correspondingly parameters of laser emission in crystals activated by dysprosium ions at the expense of nanosize modifications of nearest surroundings of rare-earth ions.

Lead thiogallate is a semiconductor with three kind of atoms and has a strong anisotropy of optical properties and in visible and in infrared spectral regions [7 - 10].An investigation of dynamic properties of such crystals presents an interest for ascertainment an ionicity of each atom and anisotropy of chemical bond in crystal lattice. Long-wavelength vibrational spectra is used for calculations of force constants and effective ion charges of cations and anions in compound crystals. Raman scattering and IR vibrational spectra of PbGa2 S4 crystals were investigated in Ref. [11, 12].

New information about dynamic of lead thiogallate lattice is presented in this work. Raman scattering in actual geometries at 10, 77 and 300 K was investigated. Reflection spectra in E||c and E⊥c (E||a) polarizations were measured for 50 - 4000 cm-1 range. Contours of reflection spectra were calculated by help of dispersion equations and main phonon parameters were determined. A temperature influence on vibrational modes of Raman scattering were discussed for temperature interval from 10 to 300 K. An effective Szigeti and effective Pb, Ga and S ion charges for both polarization were calculated.

EXPERIMENTAL METHODS

Investigated monocrystals were grown by Bridgman method at temperature gradient 20 - 30 K/cm and pulling speed 0.25mm/hour [2, 3]. Crystals have 2×2×5 cm3 size and easy cleave along direction [100] withsmooth facets formation with excellent mirror-like surfaces. Raman scattering spectra were measured at temperatures 10 - 300 K on a double spectrometer DFS - 32 with resolution 5 Å/mm in a cryostat LTS-22 C330 Workhorse. A bandwidth of spectrometer slit at scattering spectra measurements does not exceed a value 0.1 Å. Spectra were excited by lines of Spectra-Physics Ar+ laser. IR reflection spectra were measured on Specord M 80 spectrometer (7000 - 200 cm-1) and vacuum spectrometer KSDI-82 (300 - 50 cm-1).All spectrometers were computerized.

EXPERIMENTAL RESULTS

A lead thiogallate is crystallized in the lattice of rhombic system with space group D2h 24, lattice parameters a = 20.706, b = 20.380 and c = 12.156 and z = 32 [14]. An amount of phonons of different symmetries is equal to 192 for symmetry D2h 24, at z = 32.In center of Brillouin zone vibrational modes can be presented as Г=24Аg + 24Аu + 24В1g + 24 В2g + 24В3g+ 23 В1u + 23 В + 24B3u + (В + В2u3u). According selection rules the modes with Аg , В1g, В2g and В3gsymmetry are Raman-active and В1и, B2u and B3u are active in IR absorption and reflection [11, 12]. Phonons of Au symmetry are not active neither in IF nor in Raman scattering. Phonons of B1u symmetry and phonons of B2u+B3u are active in E||c and E⊥c polarizations, respectively. A first order tensor of light Raman scattering for crystals with D2h symmetry has a next view:


Figure 1 shows Raman scattering spectra in y(zz)x and y(xz)x geometries measured at 77 K and 300 K. Phonons with symmetries Ag and B2g are discovered at temperature 300 K in y(zz)x and y(xz)x geometries, respectively. Frequencies of observed vibrational modes are represented on the figure. The high-frequency mode Ag (411 cm-1) at temperature reducing from 300 K to 77 R splits on two components 399 and 408.2 cm-1. The high frequency mode B1g split also at temperature decreasing in y(xz)x geometry but its intensity is considerably small.

Temperature decreasing down to 77 K leads to insignificant change of Raman spectra and vibrational modes in frequency interval 411 - 270 cm-1 shift on a value of the order of few inverse centimeters for both investigated geometries. Intensive lines 164 and 181 cm-1 due to Ag mode and lines 154 and 188 cm-1due to B2g mode are marked out at temperature 300 K. On four modes for each symmetry (Ag and B2g 149, 159, 177 and 187 cm-1) are recognized at 77 K.Raman lines 149 and 187 cm-1 in y(xz)x geometry caused by B2g modes are more intensive than lines 159 and 177 cm-1. Spectra changes insignificantly in low-frequency mode region (18 - 100 cm-1) with temperature decreasing (see Fig. 1).

Figure 2 shows Raman scattering spectra in y(xy)x geometry measured at temperatures 300K, 77 K and 10 K. Twelve vibrational modes of B1g symmetry are observed at room temperature. A temperature reduction from 77 to 10 K leads to Raman scattering lines narrowing and new lines arising. Four lines are observed at 300 K and six lines at 10 K at high-frequencies interval 250 - 425 cm-1. The vibrational modes 278 cm-1 and 369 cm-1 shift most strongly no 9 cm-1 and 12 cm-1, respectively with temperature decreasing from 300 to 10 K. Two lines 162 and 179 cm-1 are discovered at room temperature in frequencies interval 150 - 240 cm-1. Four lines 151, 161, 180 and 188 cm-1 are observed at 77 K, these lines are presented and at temperature 10 K (see Fig. 2, B and C).


A temperature change from 77 to 10K has the greatest influence on 188 cm-1 vibrational mode it shifts on 12 cm-1. The over three lines practically do not shift. Six vibrational modes are observed in frequencies interval 10 - 100 cm-1 (Fig.2 and Fig. 3, A). Low frequency vibration mode B1g (23 cm-1 at 300 K and 16 cm-1 at 10 K) shifts the most distant at temperature decreasing. Vibration mode B1g (4) is discovered at 87 cm-1 at 300 K. It shifts to 84 and 63.3 cm-1 frequencies at temperature decreasing to 77 and 10 K, respectively. The line 124 cm-1 is observed at 77 K and it isn’t discovered at temperatures 10 and 300 K. It should be noted that frequencies of vibrational modes at high-frequency increase with temperature decreasing but in low-frequency the situation is opposite. Figure 3 shows a temperature dependence of frequency square for two vibrational modes B1g (1) and B1g (4).


Thus all examined Raman scattering lines at temperature reduction shift to higher energies on a value of few cm-1 or practically do not shift. Simultaneously with this general regularity an opposite dependence takes place for some lines. Four lines (49, 45, 34 and 23 cm-1) appear at low-frequencies. Lines 49, 45 and 34 cm-1 do not practically shift with temperature decreasing. The most low-frequency band 23 cm-1 (mode B1g) has the biggest temperature shift coefficient and moves to long-wavelength side in contrast to high-frequency lines. A temperature decreasing from 370 to 10 K leads to this band frequency reduction on 7 cm-1. This change is monotonic and has two parts with different shift coefficients (Fig. 3, B). The similar dependence is revealed for vibrational mode B1g (87 cm-1 at 300 K).

Such change of vibrational mode in PbGa2 S4 crystals indicates about structural instability of this crystal. We suppose that PbGa2 S4 crystal suffers a phase transition at low temperatures. One can speculate that lines 23 and 87 cm-1are soft modes. Figure 3 shows temperature dependences of frequency square (υ2 ) and line 23 cm1 half-width (γ). Two linear segments with different shift coefficients in temperature intervals 10 - 150 K and 150 - 300 K is revealed for frequency square. The temperature dependence of frequency square for both lines is described by relationship , where γ = 1.1±0.1 for interval 10 - 150 К and γ = 0.7±0.1 in interval 150 - 300 К. At the same time a line 278 cm-1 shifts strongly to high-frequency part with temperature decreasing (Fig. 2). The dependence of soft mod half-width on temperature for this crystals takes place linearly and is explained by usual broadening of scattering line at temperature rising. The damping of soft mode (17 and 87 cm-1) isn’t found out in investigated temperature interval.