Quenchable amorphous glass-like material from VF3

Abstract

The quite simple but relatively stable VF₃-type compounds are known to be of major interest due to their building blocks – octahedra that are extremely important in perovskites as well. Here, we show that the VF₆ octahedron in VF₃ varies over a fairly wide pressure range (0–50 GPa), maintaining undisturbed rhombohedral crystal symmetry. Half of this pressure, VF₆ rotates easily while the other undergoes strong uniaxial deformation in a “super-dense” condition. The congested sphere packing ultimately does not endure and drives the material to amorphize. We observed that the amorphous state could be quenched and acquire a transparent glass-like appearance when unloaded to ambient conditions. Dramatic, pressure-induced changes are clarified by phonon dispersion curves with the imaginary phonon mode, the so-called phonon soft mode, which indicates the structural instability. The distortion of the VF₆ octahedra is attributed to the distinctive amorphization that could be further searched for throughout the whole almost identical VF₃-type series providing metal trifluorides of various amorphous species.