## Raman and Infrared Vibrational Spectra of PbGa2 S4 Crystal

**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 E⊥c and E⊥c for range 50 - 4000 cm^{-1}at 300 K were investigated for PbGa

_{2 }Se crystals. Contours of reflection spectra in polarizations E⊥c and E⊥c 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 PbGa

_{2}S

_{4}lattice was determined.

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

## Description:

**INTRODUCTION **

A lead thiogallate (PbGa_{2} S_{4}) compound belongs to a wide
class of ternary chalcogenide materials of A^{II}B_{2} ^{III}C_{4}^{VI}
group, which uses in different devices of linear and nonlinear optics [1 - 6].
The PbGa_{2} S_{4} 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 PbGa_{2} S_{4} 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 cm^{3}
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 D_{2h}^{ 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 D_{2h}^{ 24}, at z = 32.In center of
Brillouin zone vibrational modes can be presented as Г=24А_{g} + 24А_{u}
+ 24В_{1g} + 24 В_{2g} + 24В_{3g}+ 23 В_{1u }+
23 В_{2и} + 24B_{3u} + (В_{1и} + В_{2u}+В_{3u}).
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 D_{2h} symmetry has a next view:

_{g} and B_{2g} 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 A_{g} (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^{-1}due to B_{2g} 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 B_{1g} (23 cm^{-1} at 300 K
and 16 cm^{-1} at 10 K) shifts the most distant at temperature
decreasing. Vibration mode B_{1g} (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 B_{1g} (1) and B_{1g} (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 B_{1g}) 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 B_{1g}
(87 cm^{-1} at 300 K).

Such
change of vibrational mode in PbGa_{2} S_{4} crystals indicates
about structural instability of this crystal. We suppose that PbGa_{2}
S_{4} 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 (υ^{2} ) and line 23 cm^{1}
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.