X-ray diffraction study of the cubic-to-tetragonal structural transformation in substoichiometric lanthanum trihydride and trideuteride

A low-temperature powder x-ray diffraction study of 13 compounds in the series $\mathrm{La}{\mathrm{H}}_x$ and $\mathrm{La}{\mathrm{D}}_x$ ($2.50\le x\le 3.00$) indicates the presence of a cubic-to-tetragonal distortion of the lanthanum sublattice for a limited concentration range centered about $x=2.75\mathrm{}$. Transformation temperatures occur from 197 to 256 K, with a maximum distortion from cubic symmetry of about 1%. Possible influence on the metal sublattice from the hydrogen (deuterium) sublattices is considered. Precision room-temperature lattice parameters are also reported for 22 samples in the extended series $\mathrm{La}{\mathrm{H}}_x$ and $\mathrm{La}{\mathrm{D}}_x$ ($2.00\le x\le 3.00$).