High-pressure Raman study of the ternary chalcogenides TlGa<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">S</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>, TlGa<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Se</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>, TlIn<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">S</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>, and TlIn<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Se</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

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.

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