Structural phase transitions of SrF2 at high pressure

K3Na(TaF7)(SiF6): a mixed-anion pentanary fluoride with zero-dimensional anions exhibiting a large band gap

Metal halides have potential applications in many optics-related fields, such as birefringence materials. Here, a pentanary fluoride K₃Na(TaF₇)(SiF₆) has been obtained through a facile hydrothermal route. It crystallizes in the orthorhombic space group Immm, and its zero-dimensional framework features isolated SiF₆ octahedra and TaF₇ monocapped trigonal prisms, a new type of combination representing a new structure-type. It exhibits two wide optical band gaps of 4.46 eV and 5.67 eV which is consistent with DFT calculation. K₃Na(TaF₇)(SiF₆) exhibits a larger birefringence (0.045 at 253.7 nm) compared with the commercial MgF₂ crystal.

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

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 (Fm3[combining macron]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 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.

High H⁻ ionic conductivity in barium hydride

With hydrogen being seen as a key renewable energy vector, the search for materials exhibiting fast hydrogen transport becomes ever more important. Not only do hydrogen storage materials require high mobility of hydrogen in the solid state, but the efficiency of electrochemical devices is also largely determined by fast ionic transport. Although the heavy alkaline-earth hydrides are of limited interest for their hydrogen storage potential, owing to low gravimetric densities, their ionic nature may prove useful in new electrochemical applications, especially as an ionically conducting electrolyte material. Here we show that barium hydride shows fast pure ionic transport of hydride ions (H(-)) in the high-temperature, high-symmetry phase. Although some conductivity studies have been reported on related materials previously, the nature of the charge carriers has not been determined. BaH2 gives rise to hydride ion conductivity of 0.2 S cm(-1) at 630 °C. This is an order of magnitude larger than that of state-of-the-art proton-conducting perovskites or oxide ion conductors at this temperature. These results suggest that the alkaline-earth hydrides form an important new family of materials, with potential use in a number of applications, such as separation membranes, electrochemical reactors and so on.