Polymorphism and phase transitions in NaUO from density functional perturbation theory.

Polymorphism and phase transitions in sodium diuranate, NaUO, are investigated with density functional perturbation theory (DFPT). Thermal properties of crystalline α-, β- and γ-NaUO polymorphs are predicted from DFPT phonon calculations, , the first time for the high-temperature γ-NaUO phase (3̄ symmetry). The standard molar isochoric heat capacities predicted within the quasi-harmonic approximation are for 2/ α-NaUO and 2/ β-NaUO, respectively.

Crystal and Electronic Structures of ANaIO Periodate Double Perovskites (A = Sr, Ca, Ba): Candidate Wasteforms for I-129 Immobilization.

The synthesis, structure, and thermal stability of the periodate double perovskites ANaIO (A= Ba, Sr, Ca) were investigated in the context of potential application for the immobilization of radioiodine. A combination of X-ray diffraction and neutron diffraction, Raman spectroscopy, and DFT simulations were applied to determine accurate crystal structures of these compounds and understand their relative stability. The compounds were found to exhibit rock-salt ordering of Na and I on the perovskite B-site; BaNaIO was found to adopt the 3 aristotype structure, whereas SrNaIO and CaNaIO adopt the 2/ hettotype structure, characterized by cooperative octahedral tilting.

Molecular dynamics simulation of zirconium tungstate amorphization and the amorphous-crystalline interface.

Classical molecular dynamics (MD) simulations were performed to provide a conceptual understanding of the amorphous-crystalline interface for a candidate negative thermal expansion (NTE) material, ZrWO. Simulations of pressure-induced amorphization at 300 K indicate that an amorphous phase forms at pressures of 10 GPa and greater, and this phase persists when the pressure is subsequently decreased to 1 bar. However, the crystalline phase is recovered when the slightly distorted 5 GPa phase is relaxed to 1 bar.