High-frequency EPR experiments in niobium-doped vanadium dioxide

EPR spectra of localized ${\mathrm{V}}^{3+}$ ions in the insulating monoclinic phase of ${\mathrm{V}}_{1-x}{\mathrm{Nb}}_x{\mathrm{O}}_2$ single crystals have been observed at frequencies from 73 to 310 GHz, in magnetic fields up to 58 kG, and at helium temperatures. The parameters of the effective spin Hamiltonian have been determined. Values of the spin-orbit coupling constants and the position in energy of the first-excited crystal-field levels for ${\mathrm{V}}^{3+}$ ions in V${\mathrm{O}}_2$ are given. The results are interpreted in terms of a charge-transfer mechanism ${\mathrm{V}}^{4+}$ + ${\mathrm{Nb}}^{4+}$ → ${\mathrm{V}}^{3+}$ + ${\mathrm{Nb}}^{5+}$ with a nearest-neighbor ${\mathrm{Nb}}^{5+}$ which ensures a local charge compensation. Some covalence between the ${\mathrm{V}}^{3+}$ ions and the neighboring oxygens is shown to exist.