Directional Dispersion of the Phonon Modes in Optically Uniaxial Solids, Far‐Infrared Reflection Spectra, Dielectric and Optic Constants, Dynamic Effective Ionic Charges of the Defect Chalcopyrites CdGa₂S₄, CdGa₂Se₄, HgGa₂S₄, and HgGa₂Se₄

Abstract

The polarized far-infrared reflection spectra of single crystals of the defect chalcopyrites CdGa,S, CdGa,Se,, HgGa,S,, and HgGa,Se, grown by vapour phase transport technique are presented in the range from 40 to 600 cm-'. The spectra reveal ten reststrahlen bands as predicted by group theoretical treatment. The oscillator parameters cam, mxf, eaf, and y a f , the transverse and longitudinal optical phonon frequencies WTO and WLO, and the dynamic effective ionic charges are calculated using classical oscillator fit method. The high-frequency dielectric constants cam show an anisotropic behaviour, the effective charges (Szigeti charges), however, are approximately scalar quantities. From the obtained oscillator parameters the directional dispersion of the extraordinary polar phonons is determined. The reflection spectra calculated for oblique phonons are discussed and compared to those of pellets hot-pressed from polycrystalline samples.Die polarisierten FIR-Reflexionsspektren von Einkristallen der Thiogallate CdQa,S,, CdGa,Se4, HgGa,S, und HgGa,Se, werden im Bereich von 40 bis 600 cm-l untersucht. Die Darstellung der Kristalle erfolgt durch Gasphasentransport. Nach der Methode des Oszillatorfits werden die Oszillatorprameter cam, oaf, emf und yaf, die transversalen und longitudinalen optischen Gitterschwingungsfrequenzen WTO und @LO sowie die dynamischen effektiven Ionenladungen (Szigeti-Ladung) ermittelt. Wlihrend sich die effektiven Ionenladungen annahernd wie Skalare verhalten, zeigen die hochfrequentenDielektrizitatskonstanten cam ein anisotropes Verhalten. Mit Hilfe der Oszillatorparameter wird die Richtungsdispersion der aulierordentlichen polaren Phononen bestimmt. Die Spektren polykristalliner Proben werden mit Spektren, die fur nicht-orthogonale Phononen erhalten wurden, verglichen und diskutiert.

Vibrational study of HgGa2S4 under high pressure

“…Curiously, these two Raman-active modes were not reported by Haeuseler 30 and were reported by Razzetti and Lottici at 318 and 365 cm À1 , respectively. 32 The mode at 318 cm À1 is close to the modes, we have measured at 315 and 316 cm À1 (that Haeuseler et al report at 323 and 324 cm À1 ) and that can be attributed to E 4 and B 4 modes on the basis of their frequencies and pressure coefficients that agree with polarization measurements of Haeuseler et al 30,31 On the other hand, the mode at 365 cm À1 is close to our modes of 360 and 364 cm À1 (that Haeuseler et al report at 363 cm À1 ) and that can be attributed to the E 5 mode.…”

“…Our experimental Raman mode frequencies at zero pressure are compared to those reported in previous works. [30][31][32] In general, a good agreement between the different experimental values at zero pressure is observed as well as the theoretical and experimental frequencies and pressure coefficients. This agreement supports the symmetry assignment of the experimental modes on the basis of our theoretical calculations.…”

“…The only first-order Raman-active mode not followed is the weak B 3 mode. [30][31][32] It can be observed that above 18 GPa there are three extra Raman peaks marked with asterisks in Fig. 2(a) that suggest the occurrence of a phase transition to the DS structure which will be commented later.…”

“…In this context, we have assigned the low-frequency modes below 220 cm À1 in the same way as previous works. [30][31][32] However, there are different assignments of symmetry in the Raman modes above this frequency. These discrepancies are common in the literature of AB 2 X 4 compounds in the high-frequency region because of the overlapping of relatively broad and weak modes.…”

Infrared reflectivity of ZnIn2Te4

“…Infrared reflectivity studies of compounds with Bm' = Ga and C" = S have shown that the high-energy infrared active modes have practically the same transverse mode frequencies in both types of compounds (HAEUSELER et al;NEUMANN et al 1984). Recently the same trend has been established for the compound sequence ZnIn,Se,-CuInSe,-AgInSe, .…”