High-pressure Raman study of the ternary chalcogenides TlGaS2, TlGaSe2, TlInS2, and TlInSe2

Abstract

The Raman spectra of tetragonal TlIn${\mathrm{Se}}_{2}$ and monoclinic TlGa${\mathrm{S}}_{2}$, TlGa${\mathrm{Se}}_{2}$, and TlIn${\mathrm{S}}_{2}$ have been investigated as a function of pressure at constant temperatures (300 and 110 K, TlIn${\mathrm{Se}}_{2}$ only at 110 K). Contrary to earlier assignment the structure of the monoclinic compounds can be described in the space group $\frac{C2}{c}$. The appearance and disappearance of Raman modes with increasing pressure are due to a crossing of bands with ${A}_{g}$ and ${B}_{g}$ symmetry and do not indicate the occurrence of a structural phase transition. In TlIn${\mathrm{S}}_{2}$ we observe modes with a negative pressure dependence indicating an impending phase transition at pressures above the maximum pressure of our measurements. The dependence of $(\frac{1}{\ensuremath{\nu}})(\frac{d\ensuremath{\nu}}{\mathrm{dp}})$ on $\ensuremath{\nu}$ of the observed Raman bands in these compounds reveals that a hierarchy of bonding forces is present.