g hyperfine and superhyperfine tensors of pentavalent tungsten in polycrystalline tin dioxide

Some of the tungsten ions impregnated in polycrystalline tin dioxide are found to be stabilized in the pentavalent state at substitutional positions after a heat treatment in air at 1273 K. The following ESR parameters were obtained for W⁵⁺ ions: gₓₓ=1.671, gyy=1.500, and gzz=1.732; Aₓₓ=48×10^-4 cm^-1, Axy=56×10^-4 cm^-1, and Azz=96×10^-4 cm^-1. Two superhyperfine structures were also detected. One was due to the two tin ions (a) located along the crystallographic c axis: aₓₓ=335×10^-4 cm^-1, ayy=363×10^-4 cm^-1, azz=363×10^-4 cm^-1, and the other from the four tin ions (b) lying in the diagonal plane of the unit cell containing the four equatorial oxygens of the octahedron around the W⁵⁺ ion: bₓₓ=58×10^-4 cm^-1, byy=56×10^-4 cm^-1, and bzz=52×10^-4 cm^-1. The g tensor is computed by a variational approach in the frame of the parametrized-crystal-field theory, and is in agreement with experiment. A self-consistent (on charge and configuration) extended H\"uckel calculation has been performed for insight on the unpaired-spin-electron molecular orbital. A contribution of the next-nearest neighbors is evidenced, but only qualitative evaluation of the superhyperfine structure is given. Delocalization of the unpaired-spin electron and local distortion of the host crystal are discussed.