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

Abstract

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 NbO_2−xF_1+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 NbO_1.65F_1.35. 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.