High-pressure transformations of NbO2F

The ReO3-type structure NbO2F, niobium dioxyfluoride, has been studied at h igh pressures using diamond anvil cells and synchrotron X-ray radiation. Hi gh-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 b een 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 cat ion-anion coordination octahedra and a change of the anion-anion arrangemen t 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 G pa, the diffraction pattern disappears and the structure becomes X-ray amor phous.