Na₃H(ZnH₄) Antiperovskite: A Large Octahedral Distortion with an Off-Centering Hydride Anion Coupled to Molecular Hydride

Inclusion of molecular ions with additional characteristics, including anisotropic shape, magnetic moments, and bonding, widens the possibilities for controling and tuning the structure and properties of extended solids. In contrast to the zealous efforts at the perovskite ferroelectrics, the ferroelectric-like feature of antiperovskite containing asymmetrical inorganic polyanion receives less research interest. Here, we report the high-pressure synthesis of a Na3H(ZnH4) antiperovskite containing the (ZnH4)2– polyanion which crystallizes into the P4/ncc space group, in contrast to other antiperovskites containing larger alkali ions with the I4/mcm symmetry. One of the distinct structural differences lies in the heavily distorted HNa6 octahedra with the off-centered hydride (H–) anion on the B site. The ferroelectric-like off-center displacement arises from the repulsive interaction exerted by the (ZnH4)2– polyanion at the A site. A pressure-induced phase transition from P4/ncc to I4/mcm is theoretically demonstrated, with concerted motion between the B-site single anion and the A-site tetrahedral polyanion.