Origin of unexplained reflections in the XRD pattern of hexagonal (La,Ce)H12 at ~152 GPa

Determine the origin and correct crystallographic indexing of the reflections in the powder X-ray diffraction pattern of hexagonal (La,Ce)H12 synthesized at approximately 152 GPa that are not accounted for by the P63/mmc structural model used in the Le Bail refinement, and ascertain whether adoption of a different hexagonal space group for the heavy-atom sublattice is required.

Background

To enable structural characterization, a La–Ce hydride sample synthesized from La86Ce14 and NH3BH3 at high pressure was examined by powder X-ray diffraction in a wide-opening DAC. The diffraction pattern was analyzed via Le Bail refinement using a P63/mmc structural model assigned to a phase denoted (La,Ce)H12 at ~152 GPa.

While the P63/mmc model provides a reasonable fit, the authors note an unusual c/a ratio and the presence of additional diffraction features that are not captured by the model, indicating unresolved structural details. Clarifying the origin of these extra reflections is necessary to establish the correct symmetry and phase composition of the high-pressure La–Ce hydride.