High-Pressure and High-Temperature Synthesis of Anion-Disordered Vanadium Perovskite Oxyhydrides.

Crystallographic order-disorder phenomena in solid state compounds are of fundamental interest due to intimate relationship between the structure and properties. Here, by using high-pressure and high-temperature synthesis, we obtained vanadium perovskite oxyhydrides SrNaVOH ( = 0, 0.05, 0.1, 0.2) with an anion-disordered structure, which is different from anion-ordered SrVOH synthesized by topochemical reduction. High-pressure and high-temperature synthesis from nominal composition SrVOH yielded the anion-disordered perovskite SrVOH ( ∼ 0.4) with a significant amount of byproducts, while Na substitution resulted in the almost pure anion-disordered perovskite SrNaVOH with an increased amount of hydride anion ( ∼ 0.7 for = 0.2). The obtained disordered phases for = 0.1 and 0.2 are paramagnetic with almost temperature-independent electronic conductivity, whereas anion-ordered SrVOH is an antiferromagnetic insulator. Although we obtained the anion-disordered perovskite under high pressure, a first-principles calculation revealed that the application of pressure stabilizes the ordered phase due to a reduced volume in the ordered structure, suggesting that a further increase of the pressure or reduction of the reaction temperature leads to the anion ordering. This study shows that anion ordering in oxyhydrides can be controlled by changing synthetic pressure and temperature.