Crystal structures and vibrational properties of chalcogenides: the role of temperature and pressure

Phase-change materials (e. g. GeTe, GeSe, Sb$_{2}$Te$_{3}$ and Sb$_{2}$Se$_{3}$) are characterized by an ultra-fast switching between an amorphous and a crystalline phase and thus, these materials are considered to be hopeful candidates for applications in future computer-based memories which allow a more reliable, more efficient and more capable data storage. In the crystalline phase a strong properties contrast between the tellurides and selenides is observed and and it was concluded that this is due todifferent bonding schemes. While the tellurides are believed to exhibit a metavalent bonding scheme, the selenides are covalently bonded. However, up to now, the metavalent bonding scheme is still under discussion and it is not completely understood. This study is focused on the GeSe$_{x}$Te$_{1-x}$ and Sb$_{2}$Te$_{3-x}$Se$_{x}$ solid solutions which are both relevant systems of phase-change materials. According to the conception of the metavalent bonding scheme it is expected that composition-induced changes and the application of low-temperature and/or high-pressure lead to either a weakening or a collapse of this bonding, however, an experimental conformation is missing. The influence of composition-induced changes, low-temperature and high-pressure on the structural parameter of compounds from the GeSe$_{x}$Te$_{1-x}$ (x=0, 0.2, 0.5, 0.75, 1) solid solution was investigated by a combination of powder and single crystal diffraction. At ambient conditions, three phases, a rhombohedral (0$\le$x(Se)$\le$0.52), a hexagonal (0.58$\le$x(Se)$\le$0.86) and an orthorhombic (0.91$\le$x(Se)$\le$1) one, exist in this system. All phases are stable down to at least 25K and no evidences for structural phase transition have been observed. The thermal behavior of the rhombohedral compounds differs significantly from the one of hexagonal GeSe$_{0.75}$Te$_{0.25}$ which is explained by the additional Ge$\cdot$ $\cdot$Ge and Se/Te$\cdot$ $\cdot$Se/Te interactions which are only present in the crystal structure of the hexagonal phase. Compounds from the stability field of the rhombohedral phase (x=0, 0.2, 0.5) follow the pressure transition pathway: GeTe-I (R3$\textit{m}$)$\rightarrow$GeTe-II (FCC)$\rightarrow$GeTe-III (Pnma). A first-order nature for all observed phase transitions is suggested. It was found that all phase transitions are reversible. For the GeTe-III polymorph, a new crystal structure was determined and it was demonstrated that earlier structural models are erroneously. GeTe-III is isostructural to $\beta$-GeSe, a high-pressure and high-temperature polymorph of GeSe, and crystallizes in the orthorhombic space group $\textit{Pnma}$ with lattice parameter of $\textit{a}$=7.3690(18) Å, $\textit{b}$=3.9249(10)Å and $\textit{c}$=5.698(9) Å. In the structure, Ge$\cdot$ $\cdot$Ge and long-ranged Te$\cdot$ $\cdot$Te interactions are present. Hexagonal GeSe$_{0.75}$Te$_{0.25}$ is [...