Pressure-induced crystalline-to-noncrystalline transformations of barium fluorozirconates: A probe of the medium range order of noncrystalline solids

The reversible pressure-induced transformation of crystalline barium fluorozirconates to noncrystalline solids is reported. The transformation is observed by in situ high pressure Raman spectroscopy. Since the reported crystal-to-noncrystalline transformations occur reversibly, the medium range order (MRO) of the noncrystalline solid formed can be determined; there are very few experimental methods for determining the MRO of amorphous materials. Specifically, it is reported that crystalline β-BaZr₂ F ₁₀ becomes noncrystalline at 35-45 kb, α-BaZrF₆ at 65-75 kb, and β-BaZrF₆ at 100-120 kb. The medium range order for the noncrystalline phases formed at high pressures is as follows: those formed from β-BaZrF₆ crystals consist of kinked chains of zirconium fluoride; noncrystalline materials formed from crystals of α-BaZrF ₆ contain chains which are connected in many directions forming a "net" of zirconium fluoride polyhedra; and crystals of β-BaZr₂ F₁₀ form a solid composed of distorted layers of zirconium fluoride polyhedra. More succinctly, the high pressure noncrystalline phase of β-BaZrF₆ is one dimensional, of α-BaZrF₆ is somewhat three dimensional, and the solid formed from β-BaZr₂ F₁₀ is fully two dimensional. This is the first report of a crystalline-to-noncrystalline transformation for a material which can be formed at ambient pressure as both a crystalline solid and as a melt-quenched glass. The relationship between the pressure-induced noncrystalline phases and their ambient pressure melt-quenched analogues is discussed.