Stabilization Factor of Anion-Excess Fluorite Phase for Fast Anion Conduction

Anion-excess fluorite is a unique structure type of inorganic crystals and is well known as an appropriate crystal structure for fast anion conduction. In particular, the introduction of excess anion and charged defect by chemical doping significantly enhances the conductivity. However, the clustering of dopants and defects is the main obstacle for further enhancement of conductivity. We investigated the pressure–chemical composition phase diagram of the LaHO–LaH3 system, in which the highly H– conducting LaH1+2xO1–x phase with the anion-excess fluorite structure appears. The sample at x = 0 crystallizes in a distorted fluorite structure with a monoclinic symmetry. For 0 < x ≤ 1, the fluorite lattice is maintained while H– and O2– ions are disordered at the regular anion position of fluorite, and an excess H– is distributed at the interstitial site. Comparing the phase diagram and crystal structure with those of the La–F–O, Y–H–O, and Y–F–O systems, we found that the large radius ratio of cations and anions in LaH1+2xO1–x alleviates the intrinsic Coulomb repulsion between the anions at regular positions in fluorite and the interstitial atoms. This is crucial in stabilizing the anion-excess fluorite structure without forming defect clustering and enabling fast anion conduction. These results provided guidelines for avoiding cluster formation and achieving higher conductivity in the fluorite structure.