High-pressure synthesis and structural characterization of Na1-xKxMgH3 perovskite hydrides

Perovskite hydrides containing light elements are attractive for hydrogen storage. We describe a new series of composition NaKMgH (0 ≤ x ≤ 1) prepared by an original synthesis method based on the direct reaction of simple hydrides under high-pressure and high-temperature conditions. Mixtures of MgH, NaH and KH were enclosed in gold capsules and treated in a piston-cylinder hydrostatic press at moderate pressures of 2 GPa and temperatures around 775 °C. Well-crystallized hydride samples were analyzed by x-ray and neutron powder diffraction, collecting excellent patterns even in non-deuterated samples that provided an accurate description of the crystal structure features. For x ≤ 0.75 the perovskites are isostructural with GdFeO-type oxides, characterized by the same tilting scheme of the MgH octahedra. For the end-member KMgH the structure is cubic, adopting the canonical aristotype of the perovskite structure. The analysis of the tolerance factor across the series accounts for the stability of those samples in the studied compositional range.
This research was supported by the MICINN and MINECO under projects MALTA-Consolider Team (CSD2007-00045, MAT2015-71070-REDC), MAT2013-41099-R and CTQ2015-67755-C2-1-R (MINECO/FEDER). A. A-C. acknowledges support from a MINECO-FPI grant (BES-2013-066112).