ISSP - The institute for Solid State Physics

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MDCL Special Seminar: First Observation of electronic conductivity in mixed valence tellurium oxides
標題: MDCL Special Seminar: First Observation of electronic conductivity in mixed valence tellurium oxides
日時: 2013年5月15日(水)  午後1:30~午後2:30
場所: 物性研究所本館6階 第5セミナー室(A615)  
講師: Theeranun Siritanon氏
所属: Suranaree University of Technology
要旨: Superconductivity has been observed in mixed-valence oxides with heavy post-transition metals. Sleight et al. reported the superconductivity in BaBi1-xPbxO3 perovskites with transition temperatures of 13K. BaBiO3 itself is insulating with distorted perovskite structure because charge disproportionation of Bi4+ splits the Bi 6s conduction band into a completely filled band (Bi3+) and an empty band (Bi5+). Substitution of Pb disrupts charge disproportionation and the resulting cubic perovskite oxide is superconducting. The superconductivity of this compound is related to diffused s orbital in Bi. Although Te has similar diffused s orbital, conducting Te oxides are not known. Most of oxides containing Te are insulating. Moreover, oxides that seem to have Te5+ in the chemical formula always contain mixture of Te4+ and Te6+. In other word, Te5+ always disproportionate to Te4+ with completely filled band and Te6+ with an empty band.
In my talk, I will present the studies we carried out in attempt to prepare electronically conducting Te oxides. In this work, we have prepared series of pyrochlore-related compounds with general formula Cs(M,Te)2O6 (M = Al, Ga, Cr, Al, Fe, Co, In, Ho, Lu, Yb, Er, Ge, Rh, Ti, Zn, Ni, and Mg). These compounds have very dark color which is different from most of Te oxides. They can be viewed as type II mixed valence compound according to the classification of Robin and Day. Electronic conductivity was observed in some phases and could be as high as 2S/cm (M=Ge). Seebeck coefficients of conducting samples show negative values which suggests that electrons are the major charge carriers. Based on results from several measurements, the observed conductivity in these compounds is explained based on Te4+/Te6+ mixed valency.
世話人: 廣井 善二 (内線:63445)
e-mail: hiroi :at: issp.u-tokyo.ac.jp