High-density strontium hydride: An experimental and theoretical study

Powder x-ray diffraction experiments and first-principles calculations have been carried out to investigate the possibility of a structural phase transition, characterized by a change from ionic to covalent bonding, in strontium hydride at pressures greater than 50 GPa. The powder x-ray diffraction results confirm a previously reported transition from the cotunnite structure to the Ni2In structure at approximately 8 GPa. The Ni2In phase remained stable up to the maximum experimental pressure of 113 GPa. The first-principles calculations, however, predict that under hydrostatic conditions a transition from the Ni2In structure to the AlB2 structure will occur at 115 GPa. A comparison of the pressure-dependent volume yielded by the respective experimental and theoretical studies suggests that in many cases the bulk modulus obtained from experiments carried out under non-hydrostatic conditions may be overestimated. Raman spectroscopy experiments corroborated the previously proposed Ni2In structure, as the spectra obtained at pressures greater than 8 GPa exhibited two Raman-active modes, consistent with those expected from the Ni2In structure.