High-Pressure Polymorphs of LaHO with Anion Coordination Reversal

Two novel high-pressure polymorphs of lanthanum oxyhydride have been successfully predicted and stabilized under pressure. When reacted at 3 GPa, the fluorite structure of LaHO with anion-centered tetrahedral (HLa 4 /OLa 4 ) geometry is transformed to the PbCl 2 -type structure involving coordination number increase of H - o five (HLa 5 square pyramids). Upon further application of pressure to 5 GPa, LaHO changed into the anti Fe 2 P-type structure. Interestingly, the 5 GPa phase contains tetrahedral HLa 4 and square-pyramidal OLa 5 geometry, meaning coordination switching, as confirmed by ab initio calculations. The structural analysis shows that this unprecedented phenomenon is enabled by higher compressibility of hydride anion and emphasizes its potential in the search for new high-pressure forms of hydride-based mixed anion materials.