Synthesis, Structure, and Properties of 2O-BaPtO₃, a Phase Derived from Hexagonal Perovskite

We have made the compound 2O-BaPtO₃ by high-pressure, high-temperature synthesis, determined its structure, and tested its catalytic activity. Compounds of the same stoichiometry have been reported and tentatively identified as hexagonal perovskites, and although no structural model was ever established, 2O-BaPtO₃ is clearly different and, to the best of our knowledge, unique. It features continuous chains of face-sharing PtO₆ octahedra, like the well-known 2H hexagonal perovskite type, but with a staggered offset between the chains that breaks hexagonal symmetry and disrupts the close-packed array of A = Ba and X = O that is a defining characteristic of ABX₃ perovskites. We investigated this structure and its stability vs the conventional 2H form using X-ray and neutron diffraction, X-ray absorption spectroscopy, and ab initio calculations. Catalytic testing of 2O-BaPtO₃ showed that it is active for hydrogen evolution.