Anisotropic to Isotropic Transition in Monolayer Group-IV Tellurides

Chemical Reactions and Phase Stabilities in the Si-Te System at High Pressures and High Temperatures

Chemical reactions and phase stabilities in the Si-Te system at high pressures were explored using in situ angle-dispersive synchrotron powder diffraction in a large-volume multianvil press together with density functional theory-based calculations. Cubic and rhombohedrally distorted clathrates, with the general formula Te₈@(Si₃₈Te₈) and wide compositional range, preceded by a hexagonal phase with the composition Si_0.14Te, are formed for different mixtures of Si and Te as starting materials. Si_0.14Te, with the structural formula Te₂(Te_0.74Si_0.26)₃(Te_0.94Si_0.06)₃, is the very first chalcogenide with the Mn₅Si₃-type structure. Silicon sesquitelluride α-Si₂Te₃ decomposes into a mixture of phases that includes the clathrate and hexagonal phases at high pressures and high temperatures. The higher the pressure, the lower the temperature for the two phases to occur. Regardless of the starting compositions, only the clathrate is quenched to atmospheric conditions, while the hexagonal phase amorphizes on decompression. The rhombohedral clathrates Te₈@(Si₃₈Te₈) form on quenching of the cubic phases to ambient conditions. There is a high degree of interchangeability of Si and Te not only in the clathrates but also in the Mn₅Si₃-type structure. The theoretical calculations of enthalpies indicate that the reported decomposition of α-Si₂Te₃ is energetically favorable over its transformation to another polymorph of the A₂X₃ type at extreme conditions.

Electronic, Optical, and Thermoelectric Properties of Bulk and Monolayer Germanium Tellurides

Electronic, optical, and thermoelectric properties of germanium tellurides (GeTe) were investigated through a series of first-principles calculations of band structures, absorption coefficients, and thermoelectric transport coefficients. We consider bulk GeTe to consist of cubic and rhombohedral phases, while the two-dimensional (2D) GeTe monolayers can form as a 2D puckered or buckled honeycomb crystals. All of the GeTe variants in the bulk and monolayer shapes are excellent light absorbers in a wide frequency range: (1) bulk cubic GeTe in the near-infrared regime, (2) bulk rhombohedral GeTe and puckered monolayer GeTe in the visible-light regime, and (3) buckled monolayer GeTe in the ultraviolet regime. We also found specifically that the buckled monolayer GeTe exhibits remarkable thermoelectric performance compared to the other GeTe phases due to a combination of electronic band convergence, a moderately wide band gap, and unique 2D density of states from the quantum confinement effect.