Substoichiometry and tantalum dependent thermal stability of α-structured W-Ta-B thin films

Physical vapor deposited (PVD) WB₂ thin films crystallize in the α-AlB₂-prototype structure rather than in their thermodynamically stable ω-W₂B_5-z-prototype structure. Contrary to the majority of α-AlB₂-type transition metal diborides (TMB₂), α-WB₂ exhibits a more ductile character. Combining density functional theory and sophisticated experiments, we show that the stability of α-WB₂ thin films is basically influenced by point defects such as vacancies present in PVD materials. With the help of α-TaB₂ (one of the most ductile TMB₂ with high preference for α-AlB₂-type), the thermally driven decomposition and phase transformation of α-W_1-xTa_xB_2-z to the ω-W₂B_5-z-type can be shifted to temperatures above 1200 °C.