Despite its simple chemical constitution and unparalleled technological importance, the phase-change material germanium telluride (GeTe) still poses fundamental questions. In particular, the bonding mechanisms in amorphous GeTe have remained elusive to date, owing to the lack of suitable bond-analysis tools. Herein, we introduce a bonding indicator for amorphous structures, dubbed "bond-weighted distribution function" (BWDF), and we apply this method to amorphous GeTe. The results underline a peculiar role of homopolar Ge-Ge bonds, which locally stabilize tetrahedral fragments but not the global network. This atom-resolved (i.e., chemical) perspective has implications for the stability of amorphous "zero bits" and thus for the technologically relevant resistance-drift phenomenon.

Density-functional theory (DFT) computations are reported for the (111) crystal surfaces of the phase-change material germanium telluride in its stable rhombohedral modification (dubbed α-GeTe). Atomic structures and surface energies are evaluated using a custom-tailored slab model and periodic plane-wave basis sets in the PBE-GGA approximation. Independent of the chemical surrounding, a pristine Te-covered (111) surface is energetically favorable among competing models, whereas a purely Ge-terminated surface is about 60 meV Å–2 higher in energy and predicted to undergo a structural reconstruction. Several atomic motifs for such reconstructions lie closely together energetically and are expected to coexist at finite temperatures. Formation of Ge vacancies in the subsurface layers is investigated in detail. Scanning tunneling microscopy (STM) images are simulated from the DFT wave functions for comparison with experiments to be performed in the foreseeable future.

Germanium telluride (GeTe) is an iconic functional material, both in itself and as the parent compound for a range of ternary phase-change data-storage alloys. Long taken to be a “simple” AB compound, crystalline GeTe is today known to contain a large number of germanium vacancies which directly affect the material’s macroscopic properties. Here, we use atomistic simulations to elucidate local mechanisms behind the motion of Ge atoms (and thus, vacancy diffusion) in crystalline GeTe. Transition pathways are computed using the nudged elastic band (NEB) approach at the gradient-corrected level of density-functional theory (GGA-DFT), both for the idealized rhombohedral (R3m) crystal and a number of defective configurations. Besides obvious structural arguments (i.e., beyond a simple rigid-sphere model), the diffusion barriers show a delicate dependence on the material’s electronic structure. The latter is controlled by vacancy formation, Sb adatoms, and charge injection, all of which is discussed in a unified framework.

The experimentally known perovskite-like materials BaYMn2O5+δ (δ = 0, 0.5, 1) are characterized by a remarkably reversible oxygen-storage capacity at a moderate 500 °C. We try to elucidate the local structures of the vacancy arrangements in these compounds taking place after an oxygen release. This is done for the three compounds with the help of both ab initio total-energy calculations of density-functional quality and using classical structure rationale. Our results are compared with experimental structure findings. We further calculate oxygen-vacancy formation energies and predict the pathways of the oxygen atoms through the crystal by using NEB (nudged elastic band) calculations. Structure diagrams of the most likely energy pathways for oxygen migration are presented. Finally, thermodynamic considerations of the oxygen intake are carried out based on quasiharmonic phonon calculations and compared with experimental data. The theoretical molar reaction enthalpy for oxidizing BaYMn2O5 to BaYMn2O6 matches the experimental value.

Germanium dioxide (GeO2 ) takes two forms at ambient pressure: a thermodynamically stable rutile-type structure and a high-temperature quartz-type polymorph. Here, we investigate the phase stability at finite temperatures by ab initio phonon and thermochemical computations. We use gradient-corrected density-functional theory (PBE-GGA) and pay particular attention to the modeling of the "semicore" germanium 3d orbitals (ascribing them either to the core or to the valence region). The phase transition is predicted correctly in both cases, and computed heat capacities and entropies are in excellent agreement with thermochemical database values. Nonetheless, the computed formation energies of α-quartz-type GeO2 (and, consequently, the predicted transition temperatures) differ significantly depending on theoretical method. Remarkably, the simpler and cheaper computational approach produces seemingly better results, not worse. In our opinion, GeO2 is a nice test case that illustrates both possibilities and limitations of modern ab initio thermochemistry.