Formation and optical absorption spectra of mixed valence state of Tl in Tl2Nb2O6+x with pyrochlore structure

The new thallium mixed valence compound Tl2Nb2O6+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. (205)TINMR spectra showed the unusual behavior that the absorption due to Tl+ vanished as x increased ts 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 T1 in these oxides.