Structure-property relationships in oxides containing tellurium

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

Refractive Index and Optical Dispersion of In2O3, InBO3 and Gahnite

Refractive indices of In₂O₃, In₂₋ₓSnₓO₃, InBO₃ and 2 different gahnite crystals (ZnAl₂O₄ and Zn.₉₅Fe.₀₅Al₂O₄) were measured at wavelengths of 435.8 to 643.8 nm and were used to calculate n at λ = 589.3 nm ( nD ) and at λ = ∞ (n∞) using the one-term Sellmeier equation 1/(n²-1) = -A/λ² + B. Total polarizabilities, αₜₒₜₐₗ, were calculated from n∞ and the Lorenz-Lorentz equation. Refractive indices, nD and dispersion values, A, were, respectively, 2.093 x 10-16 m² and 133 x 10⁻¹⁶ m² for In₂O₃; 2.0755 and 138 x 10⁻¹⁶ m² for In₂₋ₓSnₓO₃; 1.7940 and 57 x 10⁻¹⁶ m² for ZnAl₂O₄ and 1.7995 and 56 x 10⁻¹⁶ m² for Zn.₉₅Fe.₀₅Al₂O₄ and n₀ = 1.8782 and ne = 1.7756 and x 10⁻¹⁶ m² for InBO₃.Please note: Authors differ slightly in the published version

Refractive index and optical dispersion of In_2O_3, InBO_3 and gahnite

Refractive indices of In_2O_3, In_(2−x)Sn_xO_3, InBO_3 and 2 different gahnite crystals (Zn_(0.95)Fe_(0.05)Al_2O_4 and Zn_(0.91)Mg_(0.04)Mn_(0.03)Fe_(0.03)Al_(1.99)Fe_(0.01)O_4) were measured at wavelengths of 435.8–643.8 nm and were used to calculate n (n_D) at λ = 589.3 nm and (n_∞) at λ = ∞ with the one-term Sellmeier equation 1/(n^2 − 1) = −A/λ^2 + B. Total polarizabilities, α_(total), were calculated from n_∞ and the Lorenz–Lorentz equation. Refractive indices, n_D and dispersion values, A, are, respectively, 2.093 and 133 × 10^(−16) m^2 for In_2O_3; 2.0755 and 138 × 10^(−16) m^2 for In_(2−x)Sn_xO_3; 1.7995 and 56 × 10^(−16) m^2 for Zn_(0.95)Fe_(0.05)Al_2O_4; 1.7940 and 57 × 10^(−16) m^2 for Zn_(0.91)Mg_(0.04)Mn_(0.03)Fe_(0.03)Al_(1.99)Fe_(0.01)O_4 and n_o = 1.8782 and n_e = 1.7756 and 〈63〉 × 10^(−16) m^2 for InBO_3. The lack of consistency of the polarizabilities of Zn^(2+) in ZnO and In^(3+) in In_2O_3 with the Zn_(2+) and In3+ polarizabilities in other Zn- and In-containing compounds is correlated with structural strain and very high dispersion of ZnO and In_2O_3

Modulation of the Internal Electric Field in Te-Doped Bi₂MoO₆ Nanosheets: Implication for the Photocatalytic Degradation of Rhodamine B and Photooxidation of Benzylamine

Despite its considerable potential to address energy and environmental challenges, the practical application of photocatalysis is restricted by the limited efficiency of the photocatalysts. This research aims to improve the photocatalytic efficiency of Bi2MoO6 by adjusting the internal electric field within the crystal lattice via Te doping. Te-Bi2MoO6 nanosheets were prepared by a one-step hydrothermal method. Although the band gap energy and the long-range crystal structure are not affected upon doping, Te substitution provides several benefits to the performance of Bi2MoO6. First, the introduction of Te significantly reduces the thickness of the nanosheets, resulting in a larger surface area. Te states are also introduced into the conduction band, thereby increasing the carrier mobility. More importantly, the reduced cell parameters upon doping and the presence of Te in the lattice not only increase the potential difference between Bi–O and Mo(Te)–O layers in the lattice but also create an asymmetric potential difference. As a result, an internal electric field is enlarged, leading to increased carrier separation. Consequently, 5.0 mol % Te-doped Bi2MoO6 could degrade 99.7% of Rhodamine B in 1 h under visible light, with 6.4 times higher rate constant than pristine Bi2MoO6. Additionally, the doped sample also exhibits about two times enhancement in benzylamine photooxidation (85% conversion vs 39% conversion) while maintaining excellent stability. Thus, this research highlights the efficacy of modulating the internal electric field through chemical modification as a viable strategy for enhancing the performance of photocatalysts