Effects of nitrogen and oxygen co-doping on α to β phase transition in tungsten

beta-W is a potential candidate for spintronic devices, but its formation is a challenge as it is metastable and is stabilized by O doping. Here, using ab initio calculations, we study N doping in W and find it to favor octahedral interstitial sites in alpha-W but tetrahedral interstitial sites in beta-W. The solution energy of N in both alpha- and beta-W is endothermic that makes a transition from alpha to beta-W difficult. However, calculations on interaction of N and N2 on small clusters of W for reactive deposition of W films show that N2 dissociates on W clusters leading to the incorporation of N in the W film. Further ab initio molecular dynamics simulations on N and O co-doping in alpha- and beta-W show that the beta phase becomes energetically favorable over alpha-W making it possible to form beta-W with low O doping and its applications in devices.