Local atomic and electronic arrangements in ${\mathrm{W}}_{\mathrm{x}}$${\mathrm{V}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{O}}_2$

Low-temperature magnetic susceptibilities yield an effective magnetic moment per W atom of (3.24±0.10)${\mu }_B$ for both ${\mathrm{W}}_{0.05}$ ${\mathrm{V}}_{0.95}$ ${\mathrm{O}}_2$ and ${\mathrm{W}}_{0.08}$ ${\mathrm{V}}_{0.92}$ ${\mathrm{O}}_2$. This value suggests a transfer of two d-shell electrons from the W to neighboring V ions to form ${\mathrm{V}}^{3+}$-${\mathrm{W}}^{6+}$ and ${\mathrm{V}}^{3+}$-${\mathrm{V}}^{4+}$ pairs along the a axis of the monoclinic low-temperature phase. Studies of the extended x-ray-absorption fine-structure pattern for various compositions of different W-cation fraction x, and of the area of the W $L_{\mathrm{III}}$-edge resonance as well, confirm that the valence of W is 6+. Also, examination of the position of the V K edge versus composition for these alloys and other compounds indicates that the concentration of ${\mathrm{V}}^{3+}$ ions increases with W-cation fraction x. Thus, upon alloying ${\mathrm{VO}}_2$ with W, the reduction in the temperature of the metal-semiconductor transition by W appears to be due to charge transfer. From x-ray small-angle scattering it was found that there are no W-atom clusters present. The high-field magnetization at low temperature suggests that alloying causes this oxide to become a spin glass.