Formation of PbCl2-type AHF (A = Ca, Sr, Ba) with partial anion order at high pressure. Issue 24 (25th May 2021) - Tsuchiya, Yumi
Abstract : PbCl2 -type AHF (A = Ca, Sr, Ba) with partial anion order is obtained by pressurization of fluorite-type AHF whose anions are fully disordered. Abstract : The high-pressure structures of alkaline earth metal hydride-fluorides (AHFs) (A = Ca, Sr, Ba) were investigated up to 8 GPa. While AHF adopts the fluorite-type structure ( Fm 3̄ m ) at ambient pressure without anion ordering, the PbCl2 -type (cotunnite-type) structure ( Pnma ) is formed by pressurization, with a declining trend of critical pressure as the ionic radius of the A 2+ cation increases. In contrast to PbCl2 -type LaHO and LaOF whose anions are fully ordered, the H − /F − anions in the high-pressure polymorph of SrHF and BaHF are partially ordered, with a preferential occupation of H − at the square-pyramidal site ( vs. tetrahedral site). First-principles calculations partially support the preferential anion occupation and suggest occupation switching at higher pressure. These results provide a strategy for controlling the anion ordering and local structure in mixed-anion compounds.