H. Mizoguchi, H. Kawazoe, T. Ueda, S. Hayashi,H. Hosono,
and N. Ueda
[Bull. Chem. Soc. Jpn., Vol. 69, pp. 111-115, 1996]

The new thallium mixed valence compound Tl₂Nb₂O_6+x (0.1 <x < 1.1) with a defect pyrochlore structure was synthesized by inserting excess oxygen under an oxygen atmosphere up to 2 MPa. A distinct red shift with ca. 0.25 eV of absorption edge was observed on going from x = 0.1 to x = 1.1, and a new and weak absorption band was observed for the samples with large x, at 0.6 eV for x = 1.1 and at 0.8 eV for x = 0.9. The induced band was tentatively ascribed to be due to intervalence charge transfer between Tl⁺ and Tl₃⁺. As x increased, full width at half maximum of Tl4f_7/2 increased from 2.4 to 3.0 eV in X-ray photoemission spectra. ²⁰⁵Tl NMR spectra showed the unusual behavior that the absorption due to Tl⁺ vanished as x increased to ca. 1. Electrical conducting properties of these compounds were semiconductive, independent of x. The optical and electrical properties demonstrated the presence of the mixed valence states of Tl in these oxides.