A high-pressure study of the heavy alkaline earth hydrides

Abstract

The heavy alkaline earth hydrides consist of the Group II (alkaline earth metal) hydrides, namely CaH2, SrH2, and BaH2, which adopt the cotunnite-type structure at ambient pressure and temperature conditions. There has been much recent research interest in these compounds; in the past decade numerous experimental and theoretical studies exploring the hydrides in both applied and fundamental respects have been added to the literature. The present work constitutes a study of the heavy alkaline earth hydrides subjected to extreme pressure conditions (at ambient temperature). Specifically, pressure-dependent angle-dispersive powder x-ray diffraction and Raman spectroscopy experiments, as well as first-principles calculations, have been carried out to explore the structural stability, compressibility, and optical phonon spectra of the heavy alkaline earth hydrides. To complement the work on the hydrides, a companion study on BaF2---a structural analogue---is presented, as well as a description of some selected experimental techniques used to carry out the research. The powder x-ray diffraction results reveal that all of the heavy alkaline earth hydrides undergo a pressure-induced, first-order structural phase transition. The proposed Ni2ln structure for the high-pressure phase is based on both the results of first-principles calculations and an isostructural comparison with BaF2. The transition pressures and the proposed high-pressure structure are corroborated by the results of the Raman spectroscopy measurements. Where applicable, equation of state parameters from both the experimental and theoretical studios are reported, and a comparison of these respective parameters at ultra-high pressures (greater than 50 GPa) suggests that the experimentally observed bulk moduli are overestimated due to the presence of non-hydrostatic pressure conditions.