High-pressure phase behavior of LnHO (Ln = lanthanides): Entropy stabilization of the fluorite structure

Abstract

In this study, we investigate the phase behavior in LnHO (Ln = Nd, Gd, and Er) at external pressures up to 5 GPa. In contrast to LaHO, we observed a direct phase transformation from the fluorite-type structure to the anti-Fe2P-type structure in all the compounds examined. In the high-pressure anti-Fe2P-type structure above 3 GPa, the H− and O2− anions are coordinated four-fold and five-fold, respectively, as observed in LaHO. The absence of the PbCl2-type structure by chemical pressure (i.e., smaller Ln3+ substitution) is supported by DFT calculations. We further found a tendency for the fluorite-type structure to stabilize toward the higher-pressure side as the Ln ion becomes smaller; in ErHO and GdHO, the fluorite-type structure survives at the secondary phase even at 5 GPa. This suggests that chemical pressure plays a different role in the structural stabilization than physical pressure. The unexpected observation is interpreted in terms of entropy stabilization since, unlike LaHO and NdHO, the fluorite-type ErHO and GdHO compounds are anion disordered.