Exploring the High-Temperature Stabilization of Cubic Zirconia from Anharmonic Lattice Dynamics

Finite-temperature stability of crystals is of continuous relevance in solid-state chemistry with many important properties only emerging in high-temperature polymorphs. Currently, the discovery of new phases is largely serendipitous due to a lack of computational methods to predict crystal stability with temperature. Conventional methods use harmonic phonon theory, but this breaks down when imaginary phonon modes are present. Anharmonic phonon methods are required to describe dynamically stabilized phases. We investigate the high-temperature tetragonal-to-cubic phase transition of ZrO2 based on first-principles anharmonic lattice dynamics and molecular dynamics simulations as an archetypical example of a phase transition involving a soft phonon mode. Anharmonic lattice dynamics calculations and free energy analysis suggest that the stability of cubic zirconia cannot be attributed solely to anharmonic stabilization and is thus absent for the pristine crystal. Instead, an additional entropic stabilization is suggested to arise from spontaneous defect formation, which is also responsible for superionic conductivity at elevated temperatures

Low-Cost Vibrational Free Energies in Solid Solutions with Machine Learning Force Fields

The rational design of alloys and solid solutions relies on accurate computational predictions of phase diagrams. The cluster expansion method has proven to be a valuable tool for studying disordered crystals. However, the effects of vibrational entropy are commonly neglected due to the computational cost. Here, we devise a method for including the vibrational free energy in cluster expansions with a low computational cost by fitting a machine learning force field (MLFF) to the relaxation trajectories available from cluster expansion construction. We demonstrate our method for two (pseudo)­binary systems, Na1–xKxCl and Ag1–xPdx, for which accurate phonon dispersions and vibrational free energies are derived from the MLFF. For both systems, the inclusion of vibrational effects results in significantly better agreement with miscibility gaps in experimental phase diagrams. This methodology can allow routine inclusion of vibrational effects in calculated phase diagrams and thus more accurate predictions of properties and stability for mixtures of materials

Low-Cost Vibrational Free Energies in Solid Solutions with Machine Learning Force Fields

The rational design of alloys and solid solutions relies on accurate computational predictions of phase diagrams. The cluster expansion method has proven to be a valuable tool for studying disordered crystals. However, the effects of vibrational entropy are commonly neglected due to the computational cost. Here, we devise a method for including the vibrational free energy in cluster expansions with a low computational cost by fitting a machine learning force field (MLFF) to the relaxation trajectories available from cluster expansion construction. We demonstrate our method for two (pseudo)­binary systems, Na1–xKxCl and Ag1–xPdx, for which accurate phonon dispersions and vibrational free energies are derived from the MLFF. For both systems, the inclusion of vibrational effects results in significantly better agreement with miscibility gaps in experimental phase diagrams. This methodology can allow routine inclusion of vibrational effects in calculated phase diagrams and thus more accurate predictions of properties and stability for mixtures of materials

WMD-group/MacroDensity: MacroBack

The code has been completely re-written and modularised. The package is now pip installable. This release is for Band Alignment Bootcamp.</p

Variation in Surface Ionization Potentials of Pristine and Hydrated BiVO₄

Bismuth vanadate (BiVO₄) is a promising material for photoelectrochemical water splitting and photocatalytic degradation of organic moieties. We evaluate the ionization potentials of the (010) surface termination of BiVO₄ using first-principles simulations. The electron removal energy of the pristine termination (7.2 eV) validates recent experimental reports. The effect of water absorption on the ionization potentials is considered using static models as well as structures obtained from molecular dynamics simulations. Owing to the large molecular dipole of H₂O, adsorption stabilizes the valence band edge (downward band bending), thereby increasing the ionization potentials. These results provide new understanding to the role of polar layers on complex oxide semiconductors, with importance for the design of efficient photoelectrodes for water splitting

Models of Oxygen Occupancy in Lead Phosphate Apatite Pb₁₀(PO₄)₆O

Lead phosphate apatite, the parent compound of the proposed room-temperature superconductor LK-99, features a [Pb10(PO4)6]II scaffold with a charge-compensating oxide ion. This O–II occupies a 4e site in the P63/m unit cell, with 25% probability on average. We model the occupancy of this site from substoichiometric (x = 0) to superstoichiometric (x = 4) regimes in Pb10(PO4)6Ox. Doping is predicted by adjusting the oxygen composition within the ⟨0001⟩ channel, with evidence for strong O–O correlation. This behavior introduces a sensitivity to the crystal growth and annealing conditions, with an opportunity for novel functionality to emerge

Supporting data; sulfur vapour modelling

<p>Supporting data for work on thermodynamic equilibrium of sulfur vapours: raw files from density functional theory (DFT) calculations with FHI-aims; structure set from evolutionary search; summary of frequency data by sulfur allotrope and calculation method.</p

WMD-group/Eris: Dysnomia

The version of ERIS used to create the data for the first publication in 2018</p

Data from global structure search of sulfur clusters

<p>Raw data (structures and energies) from global structure search of sulfur clusters. The search was carried out using an evolutionary algorithm as implemented in the USPEX code, with VASP as the calculator and energies within density-functional theory with the PBE exchange-correlation functional as the fitness criteria. This is supporting data for a study of the free energy of mixed sulfur vapours.</p

Summary of calculated frequency data

<p>Summary of frequency data from all-electron DFT calculations with FHI-aims. The species are a set of sulfur allotropes; the data is used to form a thermodynamic model of the mixed sulfur vapour phase. Frequencies are given in reciprocal cm. The lowest six frequencies (or five frequencies in the case of S2) are the spurious translational and rotational terms; ideally these are equal to zero. They have been retained in order to give a sense of the convergence level of the calculations.</p> <p>Generated using open-source code (see link).</p