Coexistent Phases and their Electrical and Structural Behaviour in the Ternary Re/Mo/O System

In order to complete and improve phase diagrams of transition metal oxides the existence and properties of compounds in the Re/Mo/O system have been investigated. Single crystalline solid solutions Re_1-xMo_xO₂ (0 < x < 0.37) and Mo_1-yRe_yO₂ (0 < y < 0.42) were prepared by Solid State Reactions and Chemical Transport Reactions (CTR) and characterized by means of X-ray-diffraction, ICP-AES, and EPMA. In addition, it was shown that the trioxides Re_1-xMo_xO₃ or Mo_1-yRe_yO₃ respectively, do not prove a distinct solubility. First results on Re-doped Magneli phases of Mo are also discussed. There were found two series of solid solutions Mo_4-zRe_zO₁₁ (0 < z < 0.40) which derive from γ-Mo₄O₁₁ and η-Mo₄O₁₁. The other Magneli phases did not show a significant Re content. Using X-ray diffraction, AFM and optical microscopy the structure and morphology of single crystals has been studied. It was steated that the dioxide solid solutions were very homogeneous and structural phase transitions depending on x or y did not occur. That is in good agreement with measurements of electrical conductivity σ in the temperature range from 20 K to 300 K in which the samples showed the behaviour of metals with lattice defects. The Magneli phases Mo₈O₂₃ and Mo₉O₂₆ turned out to be “narrow gap semiconductors” in the low temperature range with activation energies of less than 0.1 eV. The γ-Mo₄ O_11- and γ-Mo₄O₁₁ crystals, the pure and Re-doped ones, were found as metallic conductors at higher temperatures and below a transition temperature they show a semiconductive behaviour.     

Die Berechnung von Kriterien für die Abscheidung reiner Phasen binärer Systeme beim chemischen Transport und der Transport im System Cu-O

A thermodynamical analysing method is described of the deposition of binary solids AB_x, which specially takes into account the influence of the coexistence equilibria in the A—B-system. This analysis is applicated to the Cu—O system. It permits to calculate the conditions, under which Cu₂O is deposited without deposition of any other phases. The deposition of cuprous oxide by CTR-techniques is investigated using hydrogen halides as transport media. The experiments result in the growth of Cu₂O-monocrystals (2 × 2 × 2 mm) only if HCl is used as transport medium. Using HI the simultaneous deposition of Cu₂O and Cu is observed.     

Halbleiter-Metall-Übergang in V3O5

V₃O₅ belonging to the Magnéli phases V_nO_2n−1 shows a conductivity transition at 154 °C. The character of the transition, the changes of the electric, thermoelectric, magnetic, structural and other properties and the influence of hydrostatic pressure and ternary components (dopings) on the transition are described. The properties of V₃O₅ settle down well in the behaviour of the other Magnéli phases. The transition is characterized as a charge order-disorder transition with important effect of electron correlation.     

Electrical properties of ZnO single crystals annealed in a zinc atmosphere

The electrical conductivity and the Hall coefficient of ZnO single crystals annealed in a zinc atmosphere after growth were measured in a temperature range from 77 K to 1000 K. The results are compared with those of ZnO crystals from the zinc rich phase boundary. A shallow level with an activation energy at about 15 meV dominates in both groups of samples. Its concentration was found to correlate directly with the degree of non-stoichiometry.