Raul Paucar

Phase transitions and Raman scattering spectra of TlGaSe2

In this work we examined the phase transitions in the ternary thallium chalcogenide TlGaSe2 by studying the temperature dependence of the Raman spectra with the aid of confocal microscopy. The unpolarized Raman scattering spectra of TlGaSe2 single crystals were measured over the temperature range 78- 300 K (which includes the range of the successive phase transitions) in the frequency region of 50 – 300 cm-1. The Raman spectra exhibited 12 lines at 300 K, but the number of lines rose to 17 at 78 K. In the temperature interval between 107 K and 120 K, where the phase transitions take place, the temperature dependence of the phonon frequencies showed discontinuities for several of the Raman lines.

Optical phonons in CdGa2S4xSe4(1 − x) alloys

The Raman spectra of CdGa2S4xSe4(1 − x) alloys (x = 0.1, 0.2, … 0.9) are studied. Both the singlemode and double-mode behavior of optical phonons are observed in CdGa2S4xSe4(1 − x) alloys. The observed optical mode at 138 cm−1 is independent of the composition. It seems likely that this mode is the “breathing mode” and is caused by atomic motion in the anion sublattice relative to vacancies. It is shown that the high-frequency modes of symmetry B1(LO, TO) and B2(LO, TO) are caused by the in-phase motion of atoms in the anion sublattice along the tetragonal axis c relative to trivalent Ga atoms. The doubly degenerate symmetry modes E1(LO, TO) and E2(LO, TO) are caused by the in-phase motion of atoms in the anion sublattice relative to trivalent Ga atoms of the cation sublattice in the xy plane (dipole 2Ga–4C), where C is S, Se. The optical symmetry modes B3(LO, TO) and B4(LO, TO) are associated with the motion of atoms in the anion sublattice relative to Cd atoms along the tetragonal axis c. The doubly degenerate modes E3(LO, TO) and E4(LO, TO) are associated with the motion of atoms in the anion sublattice relative to Cd atoms (dipole Cd–4C). The low-frequency modes B5(LO, TO) and E5(LO, TO) are the analogs of acoustic phonons at the edge of the Brillouin zone of sphalerite.

Evaluation of crystal structure in TlInS2 by optical second- harmonic generation

Second-harmonic generation (SHG) in layered TlInS2 crystals was studied over the temperature range of 77-300 K using a confocal laser microscope system. As expected, the SHG signal was observed in the low temperature ferroelectric phase of the layered compound. In addition, the polarization properties of the SHG signals of TlInS2 were investigated in the 80-180 K range. The results are in good agreement with those of the symmetric space group C32 in the ferroelectric phase.

Optical phonons in CdGa2S4x Se4(1 − x) alloys

The Raman spectra of CdGa2S4xSe4(1 − x) alloys (x = 0.1, 0.2, … 0.9) are studied. Both the singlemode and double-mode behavior of optical phonons are observed in CdGa2S4xSe4(1 − x) alloys. The observed optical mode at 138 cm−1 is independent of the composition. It seems likely that this mode is the “breathing mode” and is caused by atomic motion in the anion sublattice relative to vacancies. It is shown that the high-frequency modes of symmetry B1(LO, TO) and B2(LO, TO) are caused by the in-phase motion of atoms in the anion sublattice along the tetragonal axis c relative to trivalent Ga atoms. The doubly degenerate symmetry modes E1(LO, TO) and E2(LO, TO) are caused by the in-phase motion of atoms in the anion sublattice relative to trivalent Ga atoms of the cation sublattice in the xy plane (dipole 2Ga–4C), where C is S, Se. The optical symmetry modes B3(LO, TO) and B4(LO, TO) are associated with the motion of atoms in the anion sublattice relative to Cd atoms along the tetragonal axis c. The doubly degenerate modes E3(LO, TO) and E4(LO, TO) are associated with the motion of atoms in the anion sublattice relative to Cd atoms (dipole Cd–4C). The low-frequency modes B5(LO, TO) and E5(LO, TO) are the analogs of acoustic phonons at the edge of the Brillouin zone of sphalerite.

Optical phonons in CdGa 2 S 4 x Se 4(1 − x ) alloys

The Raman spectra of CdGa2S4xSe4(1 − x) alloys (x = 0.1, 0.2, … 0.9) are studied. Both the singlemode and double-mode behavior of optical phonons are observed in CdGa2S4xSe4(1 − x) alloys. The observed optical mode at 138 cm−1 is independent of the composition. It seems likely that this mode is the “breathing mode” and is caused by atomic motion in the anion sublattice relative to vacancies. It is shown that the high-frequency modes of symmetry B1(LO, TO) and B2(LO, TO) are caused by the in-phase motion of atoms in the anion sublattice along the tetragonal axis c relative to trivalent Ga atoms. The doubly degenerate symmetry modes E1(LO, TO) and E2(LO, TO) are caused by the in-phase motion of atoms in the anion sublattice relative to trivalent Ga atoms of the cation sublattice in the xy plane (dipole 2Ga–4C), where C is S, Se. The optical symmetry modes B3(LO, TO) and B4(LO, TO) are associated with the motion of atoms in the anion sublattice relative to Cd atoms along the tetragonal axis c. The doubly degenerate modes E3(LO, TO) and E4(LO, TO) are associated with the motion of atoms in the anion sublattice relative to Cd atoms (dipole Cd–4C). The low-frequency modes B5(LO, TO) and E5(LO, TO) are the analogs of acoustic phonons at the edge of the Brillouin zone of sphalerite.

Optical phonons in CdGa{sub 2}S{sub 4x}Se{sub 4(1-x)} alloys

The Raman spectra of CdGa2S4xSe4(1 − x) alloys (x = 0.1, 0.2, … 0.9) are studied. Both the singlemode and double-mode behavior of optical phonons are observed in CdGa2S4xSe4(1 − x) alloys. The observed optical mode at 138 cm−1 is independent of the composition. It seems likely that this mode is the “breathing mode” and is caused by atomic motion in the anion sublattice relative to vacancies. It is shown that the high-frequency modes of symmetry B1(LO, TO) and B2(LO, TO) are caused by the in-phase motion of atoms in the anion sublattice along the tetragonal axis c relative to trivalent Ga atoms. The doubly degenerate symmetry modes E1(LO, TO) and E2(LO, TO) are caused by the in-phase motion of atoms in the anion sublattice relative to trivalent Ga atoms of the cation sublattice in the xy plane (dipole 2Ga–4C), where C is S, Se. The optical symmetry modes B3(LO, TO) and B4(LO, TO) are associated with the motion of atoms in the anion sublattice relative to Cd atoms along the tetragonal axis c. The doubly degenerate modes E3(LO, TO) and E4(LO, TO) are associated with the motion of atoms in the anion sublattice relative to Cd atoms (dipole Cd–4C). The low-frequency modes B5(LO, TO) and E5(LO, TO) are the analogs of acoustic phonons at the edge of the Brillouin zone of sphalerite.