Formation of PbCl2-type AHF (A = Ca, Sr, Ba) with partial anion order at high pressure

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 (Fmm) 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 A²⁺ 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.