41 research outputs found
Ab initio phonon structure of h-YMnO3 in low-symmetry ferroelectric phase
We present a systematic first-principles study of the phonon spectrum of
hexagonal YMnO3 in ferroelectric (multiferroic) phase. We investigated in
detail the low-energy phonon modes, their dispersion, symmetry, as well as the
infrared optical properties of the crystal and determined the phonon density of
states
First-principles prediction of oxygen octahedral rotations in perovskite-structure EuTiO3
We present a systematic first-principles study of the structural and
vibrational properties of perovskite-structure EuTiO3. Our calculated phonon
spectrum of the high-symmetry cubic structural prototype shows strong M- and
R-point instabilities, indicating a tendency to symmetry-lowering structural
deformations composed of rotations and tilts of the oxygen octahedra.
Subsequent explicit study of 14 different octahedral tilt-patterns showed that
the I4/mcm, Imma, and R\bar{3}c structures, all with antiferrodistortive
rotations of the octahedra, have significantly lower total energy than the
prototype Pm\bar{3}m structure. We discuss the dynamical stability of these
structures, and the influence of the antiferrodistortive structural distortions
on the vibrational, optical, and magnetic properties of EuTiO3, in the context
of recent unexplained experimental observations
Routes for increasing endurance and retention in HfO 2 -based resistive switching memories
We investigate metastable and thermodynamically stable phases that can be expected to occur in electroformed filaments in resistively switching hafnia, and discuss their relevance for the switching process. To this end, we conduct a study, based on density functional theory combined with an evolutionary algorithm determining the composition-dependent (meta)stable phases in HfOx, focusing on the region 0<x<2. We find that oxygen vacancies in hafnia tend to form regular patterns, which leads to periodic metastable structures featuring one-dimensional open channels, thus favoring ionic conductivity in the host material, i.e., oxygen migration. The band gap of such structures is systematically lowered with increasing oxygen deficiency, resulting in metallic behavior when oxygen migrates out of the channels. Moreover, we find that the solubility of oxygen in metallic Hf is very high, up to one oxygen per six metallic atoms, the concentration corresponding to a thermodynamically stable and ordered metallic compound, Hf6O. Therefore, thick enough metallic capping of Hf could play the role of an active electrode for hosting oxygen which migrates out of HfO2. In combination with reversible oxygen migration in predicted suboxide phases, this should lead to robust resistive memory cells with high endurance and long retention