Metallic to half-metallic transition driven by pressure and anion composition in niobium oxyfluoride

Half-metallic materials play a key role in the development of spintronics, but few suitable materials are known. Continuously varying the ratio of two anions in a single material offers one route to enhance half-metallic behaviour. Here we report the effect of varying the oxide-fluoride ratio in ReO3-type niobium oxyfluoride NbO2-xF1+x using ab initio calculations. Increasing the fluorine content from NbO2F to NbF3 leads to a transition from semiconducting, through metallic, to half-metallic behaviour as x increases. The effect of pressure on this behaviour is also investigated, finding a transition from cubic to rhombohedral symmetry for all compositions below a maximum transition pressure of 5 GPa for x = 0.35. Additionally, we reveal that compositions close to NbF3 are expected to retain rhombohedral symmetry under ambient pressure. The structural phase transition and electronic behaviour are independent for most compositions except those close to NbO1.65F1.35. Here, we discover that the application of moderate pressure can drive the material from metallic to half metallic, offering a new possibility for controlling spin currents.