Pressure-induced structural phase transition in BaHCl

We have investigated the structural evolution of BaHCl under high pressure using in-situ synchrotron X-ray diffraction up to 30 GPa. The in-situ observation found that the ambient pressure BaHCl phase crystallizing in the PbFCl-type structure (P4/nmm) is transformed into an orthorhombic phase at 29 GPa and 300 K. Rietveld refinements of synchrotron X-ray and neutron diffraction data for the quenched sample revealed that BaHCl adopts PbCl2-type structure (Pnma), which consists of a distorted hcp framework of Ba2+ ions, with H- and Cl- ions at tetrahedral and octahedral interstitials, respectively. The complete ordering of anions in the PbCl2-type BaHCl contrasts with the partial ordering in the PbCl2-type AHF (A =1/4 alkali earth metal). This result may promote further exploration of hydride-based mixed anion materials and understanding and control of anion order/disorder.