Atomistic-geometry inspired structure-composition-property relations of hydrogen sII hydrates

Gas hydrates are crystalline inclusion compounds formed by trapping gas molecules inside water cages at high pressures and low temperatures. Hydrates are promising materials for hydrogen storage, but their potential depends on understanding their mechanical properties. This work integrates density functional theory (DFT) simulations with a geometry-inspired composite material model to explore the bulk moduli of structure II hydrogen hydrates subjected to pressure loads of - 0.2 to 3 GPa, representative of the hydrogen hydrate formation conditions. Our findings reveal that structure II hydrate comprises a bi-continuous composite of small and large cages with nearly equal volume fractions. The bulk modulus increases with rising pressure but decreases with increasing composition. Notably, these results align closely with the ideal laws of mixtures, especially at low pressures and compositions, where cage interactions are minimal. This integrated DFT-laws of mixtures methodology provides a key database for fast estimation of hydrate mechanical properties without costly computations.

The crystal structure of ZrCr2D≈4 at 50 K ≤ T ≤ 200 K

Many Laves phases take up considerable amounts of hydrogen to form metallic Laves phase hydrides. They frequently undergo phase transitions driven by ordering phenomena for the hydrogen atom distribution. The cubic Laves phase ZrCr2 takes up hydrogen to form a hydride with almost four hydrogen atoms per formula unit, which undergoes a phase transition to a monoclinic modification at a critical temperature T c = 250.2 K. Its crystal structure was refined based on neutron powder diffraction data on the deuteride (ZrCr2D3.8 type [T = 1.6 K, C2/c]) at four temperatures in the range 50 K ≤ T ≤ 200 K. The monoclinic low-temperature modification features a strongly distorted square anti-prism ZrD8 and three CrD4 polyhedra with almost fully occupied deuterium sites in saddle-like, distorted tetrahedral and planar configurations. Zr–D distances are in the range 201.4(7) pm ≤ d(Zr–D) ≤ 208.5(8) pm and Cr–D distances in the range 172.9(7) pm ≤ d(Cr–D) ≤ 182.4(8) pm.