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

Abstract

Perovskite hydrides containing light elements are attractive for hydrogen storage. We describe a new series of composition Na1-xKxMgH3 (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 MgH2, 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 GdFeO3-type oxides, characterized by the same tilting scheme of the MgH6 octahedra. For the end-member KMgH3 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.