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Structural Aspects of NaV 2O 5 under High Pressure

Anomalous lattice properties and structural phase transition of NaV 2 O 5 have been studied under hydrostatic high pressure at room temperature by using powder X-ray diffraction techniques. Upon compression the c -axis along the stacking direction of V 2 O 5 -layers contracts rapidly and the b -axis along the one-dimensional magnetic ladder direction expands, while the a -axis contracts up to about 4 GPa and then begins to expand. Structural analysis of NaV 2 O 5 and the previous data of V 2 O 5 indicate that Na atoms between the V 2 O 5 -layers govern these lattice properties. A structural phase transition is found to take place around 13 GPa. A detailed structure analyzed by the Rietveld method reveals that the approach of an apical oxygen of the VO 5 -pyramid to the V atom in the upper VO 5 -pyramid plays an important role in the phase transition. The present crystal field calculation shows the stability of the d x y -like orbital under the high pressure.