The high-pressure and low-temperature behaviour of the GeSe x Te 1Àx system (x = 0, 0.2, 0.5, 0.75, 1) was studied using a combination of powder diffraction measurements and first-principles calculations. Compounds in the stability field of the GeTe structure type (x = 0, 0.2, 0.5) follow the high-pressure transition pathway: GeTe-I (R3m) ! GeTe-II (f.c.c.) ! GeTe-III (Pnma). The newly determined GeTe-III structure is isostructural to -GeSe, a high-pressure and high-temperature polymorph of GeSe. Pressure-dependent formation enthalpies and stability regimes of the GeSe x Te 1Àx polymorphs were studied by DFT calculations. Hexagonal Ge 4 Se 3 Te is stable up to at least 25 GPa. Significant differences in the high-pressure and low-temperature behaviour of the GeTetype structures and the hexagonal phase are highlighted. The role of GeÁ Á ÁGe interactions is elucidated using the crystal orbital Hamilton population method. Finally, a sketch of the high-pressure phase diagram of the system is provided.

We have performed a detailed structural analysis of several crystals from the Sb2Te3−xSex solid solution (with x = 0–1.55) using a combination of single-crystal and powder X-ray diffraction and high-resolution scanning transmission electron microscope imaging combined with energy-dispersive X-ray spectroscopic mapping. The experimental study was supported by quantum-chemical calculations. All compounds crystallize in the rhombohedral tetradymite structure type in which the atoms occupy three symmetrically independent crystallographic sites; the two chalcogenide sites A1 and A2, and the antimony site Sb. They form quintuple A2–Sb–A1–Sb–A2 layers perpendicular to the [001] direction, held together via A2–A2 chalcogenide-chalcogenide interactions. The results of our experimental study show unambiguously, and in agreement with quantum-chemical calculations, a preferred incorporation of selenium into the A2 position inside the layers. An unexpected increase of the van-der-Waals inter-layer distances is observed for low Se contents (x < 1) and discussed with the help of DFT calculations.

The capability to perform high‐pressure low‐temperature nuclear inelastic scattering on 125Te and 121Sb with a sapphire backscattering monochromator is presented. This technique was applied to measure nuclear inelastic scattering in TeO2 at pressures up to 10 GPa and temperatures down to 25 K. The evaluated partial Te densities of phonon states were compared with theoretical calculations and with Raman scattering measured under the same conditions. The high‐pressure cell developed in this work can also be used for other techniques at pressures up to at least 100 GPa.

The lattice dynamics of SbTe Se (x  =  0, 0.6, 1.2, 1.8, 3) mixed crystals have been studied by a combination of low-temperature heat-capacity measurements between 2-300 K and first-principles calculations. The results from the experimental and theoretical investigations are in excellent agreement. While SbSe can be considered as a harmonic lattice oscillator in this temperature range, for the isostructural compounds SbTe, SbSeTe, SbSeTe and SbSeTe (tetradymite structure type; R [Formula: see text] m) a small anharmonic contribution to the total heat capacity has to be taken into account at temperatures above 250 K. For the compounds which crystallize in the tetradymite structure type the experimental and theoretical data show unambiguously that the exchange of Te by Se leads to an increase of the bonding polarity and consequently to a hardening of the bonding which is reflected in an increase of the Debye temperatures with increasing Se contents. In addition, our studies clearly demonstrate that the mixed crystals in the stability field of the tetradymite structure type are characterized by a strong non-ideal mixing behavior.

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