Ronald

Cohen

We clarify the nature of hafnia as a proper ferroelectric and show that there is a shallow double well involving a single soft polar mode as in well-known classic ferroelectrics. Using symmetry anal-ysis, density-functional theory (DFT) structural optimizations with and without epitaxial strain, and density functional perturbation theory (DFPT), we examine several important possible haf-nia structures derived ultimately from the cubic fluorite structure, including baddeleyite (P21/c), tetragonal antiferroelectric P42nmc, Pbca (OI and brookite), and ferroelectric rhombohedral (R3m), Pmn21 and Pca21 structures. The latter is considered to be the most likely ferroelectric phase seen experimentally, and has an antiferroelectric parent with space group Pbcn, with a single unstable polar mode and a shallow double well with a well depth of 24 meV/atom. Strain is not required for switching or other ferroelectric properties, nor is coupling of the soft-mode with any other modes within the ferroelectric Pca21 phase.

(Mg, Fe)SiO3 post-perovskite is the highest-pressure silicate mineral phase in the Earth's interior. The extreme pressure and temperature conditions inside large extrasolar planets will likely lead to phase transitions beyond post-perovskite. In this work, we have explored the high-pressure phase relations in Mg2SiO4 using computations based on density functional theory. We find that a partially disordered I4 over bar 2d-type structure would be stable under the conditions expected for the interiors of super-Earth planets. We have explored the mechanism of the phase transition from the ordered ground state and the effect of disordering on the electronic properties of the silicate phase. The discovery of a structure where two very dissimilar cations, Mg2+ and Si4+, occupy the same crystallographic site opens up a domain of interesting crystal chemistry and provides a foundation for other silicates and oxides with mixed occupancy.

(Mg,Fe)SiO3 post-perovskite is the highest-pressure silicate mineral phase in the Earth’s interior. The extreme pressure and temperature conditions inside large extrasolar planets will likely lead to phase transitions beyond post-perovskite. In this work we have explored the high- pressure phase relations in Mg2SiO4 using computations based on density functional theory. We find that a partially disordered I4̅2d-type structure would be stable under the conditions expected for the interiors of super-Earth planets. We have explored the mechanism of the phase transition from the ordered ground state and the effect of the disordering on electronic properties of the silicate phase. The discovery of a structure where two very dissimilar cations, Mg2+ and Si4+ occupy the same crystallographic site opens up a domain of interesting crystal chemistry and provides a foundation for other silicates and oxides with mixed occupancy.

The thermal equation of states for fcc iridium (Ir) is obtained from first-principles molecular dynamics up to 3000 K and 540 GPa. The equation of state (EoS) is globally fitted to a simplified free-energy model and various parameters are derived. The theoretical principal Hugoniot is compared with shock wave experiments, where discrepancy suggests formation of new Ir phases. A few representative EoS parameters, such as bulk modulus KT, thermal expansivity alpha, Gruneisen parameter gamma, constant pressure capacity CP, and Debye temperature OD, are computed to compare with experimental data.

We use globular cluster data from the Resolved Stellar Populations Early Release Science (ERS) program to validate the flux calibration for the Near Infrared Camera (NIRCam) on the James Webb Space Telescope (JWST). We find a significant flux offset between the eight short wavelength detec-tors, ranging from 1–23% (∼0.01-0.2 mag) that affects all NIRCam imaging observations. We deliver improved zeropoints for the ERS filters and show that alternate zeropoints derived by the community also improve the calibration significantly. We also find that the detector offsets appear to be time variable by up to at least 0.1 mag.