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Structure-property relationships in oxides containing tellurium

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  • Oxides of post-transition metals often show unique structures and properties due to the presence of lone pair electrons and the diffused s orbitals. The present work focuses on synthesis and characterizations of oxides containing Te, a heavy post transition metal. New series of pyrochlore oxides of the formula Cs(M,Te)₂O₆ (M = Al, Ga, Cr, Fe, Co, In, Ho, Lu, Yb, Er, Ge, Rh, Ti, Zn, Ni, and Mg) have been prepared. The samples were highly colored (ranging from black to dark green) indicating a possible mixed valency for Te with appreciable charge transfer between them in the octahedral sites. 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 suggest that electrons are the major charge carriers. Temperature dependence of conductivity indicates that the samples are semiconductors with, in some cases, degenerate semiconducting behavior. Detailed studies on the conduction mechanism indicate the mixed valency of tellurium which leads to semiconducting behavior and the color of the compounds.Systematic studies of cesium tellurate with CsTe₂O[subscript 6-x] where x = 0, 0.15, 0.25, 1.5 have been investigated. On heating at slightly above 600ºC, CsTe₂O₆ loses oxygen resulting in cubic structure with disordered Te⁴⁺/Te⁶⁺ and oxygen vacancies. Two novel phases of CsTe₂O[subscript 6-x] were prepared with orthorhombic structure. The first phase with x value of about 0.2-0.3 crystallizes in Pnma symmetry. At higher values of x, a new compound was discovered with a structure related to Rb₄Te₈O₂₃. Optical properties of the compounds are consistent with their colors. CsTe₂O₆ belongs to class II mixed valency according to Robin and Day classification. However, structures and properties of CsTe₂O[subscript 6-x] phases indicate that they are class I mixed valence compounds. Series of compounds with formula CsTe[subscript 2-x]W[subscript x]O₆ with x=0.2-0.5 have been made which can be considered as solid solution of CsTe₂O₆ and CsTe₀.₅W₁.₅O₆. Although the two end members adopt rhombohedral and trigonal structure, these solid solution phases crystallize in cubic defect pyrochlore structure with W⁶⁺, Te⁶⁺, and Te⁴⁺ randomly occupying 16c octahedral site. The compounds show no electronic conductivity at room temperature. Novel cubic pyrochlore with the formula (CdBi)(MTe)O₇, M= Al, Cr, Ga, In, Fe, Mn, and Sc were synthesized by solid state reaction using oxides of the constituent elements. Magnetic properties analyses show paramagnetism in M=Cr and Mn but antiferromagnetism with short-range correlation in M= Fe phase. All compositions are insulating. Dielectric measurements show relatively low dielectric constants which are independent of temperature and frequency. Metallic Tl₂TeO₆ and insulating In₂TeO₆ are both known to crystallize in the Na₂SiF₆-type structure. We have now prepared a complete Tl[subscript 2-x]In[subscript x]TeO₆ series in a search for a compositionally controlled metal-insulator transition that might be expected if a complete solid solution can be obtained. Unit cell edges and volume vary monotonically with no indication of miscibility gap. The metal-insulator transition occurs at an x value of about 1.4, which can be rationalized on a percolation model. No superconductivity could be detected down to 5K. Rh₂MO₆, M = Mo, W, and Te were synthesized by solid state reaction. Electronic properties as well as thermoelectric properties were investigated and discussed. The compounds crystallize in rutile-related structure and all show relatively high electronic conductivities with Rh₂TeO₆ showing the highest electronic conductivity (~500 S/cm at room temperature) despite localized electrons in Rh³⁺ and Te⁶⁺. Measurable magnetic moments also indicate valence degeneracy between Rh and the M cation. The measured Seebeck coefficients are relatively low and positive indicating hole-type conduction.
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