High-pressure transformations of NbO2F

The ReO3-type structure NbO2F, niobium dioxyfluoride, has been studied at high pressures using diamond anvil cells and synchrotron X-ray radiation. High-pressure powder diffraction measurements have been performed up to 40.1 GPa. A phase transition from the cubic (Pm (3) over bar m) ambient pressure structure to a rhombohedral (R (3) over bar c) structure at 0.47 Cpa has been observed. Rietveld refinements at 1.38, 1.96, 3.20, 6.23, 9.00 and 10.5 GPa showed that the transition involves an a(-)a(-)a(-) tilting of the cation-anion coordination octahedra and a change of the anion-anion arrangement to approach hexagonal close packing. Compression and distortion of the Nb(O/F)(6) octahedra is also revealed by the Rietveld refinements. At 17-18 Gpa, the diffraction pattern disappears and the structure becomes X-ray amorphous.