Solid-state equilibrium relations in the Ternary Systems TeO2?MoO3?MoO2 and TeO2?MoO3?Te

In a study of the solid-state reactions in the ternary systems TeO₂? MoO₃? MoO₂ and TeO₂? MoO₃? Te, approximately 70 selected compositions were sintered at 550°C to attain equilibrium conditions, and solid-state equilibrium relations were characterized by x-ray diffraction. In a large composition range, the interaction of TeO₂ and MoO₃ with the reducing agents MoO₂ or Te leads to the reduced ternary oxide TeMo₄O₁₃ (m. p. 748°C), in addition to Te₂MoO₇, Te and (intermediate) molybdenum oxides. The compatibility relations for the binary systems TeO₂? MoO₂ and MoO₃? Te are presented for the first time. In the TeO₂? MoO₂ system, three-phase regions are found: (Te₂MoO₇? TeO₂? Te) on the TeO₂? and (TeMo₄O₁₃? MoO₂? Te) on the MoO₂-rich sides with (TeMo₄O₁₃? Te₂MoO₇? Te) in the intermediate region. In the MoO₃? Te system, three-phase regions (TeMo₄O₁₃? MoO₂? Te), (TeMo₄O₁₃? Mo₄O₁₁? MoO₂) and (TeMo₄O₁₃? MoO₃? Mo₄O₁₁) were detected. TeMo₄O₁₃ presents two allotropic forms (α′ for T < 450°C, α for T > 450°C). Both structures have been characterized by I.R. and optical reflectance spectroscopy. Unit cell dimensions are also given.     

The Binary Oxide System TeO2?MoO3

Phase analysis of the mixed oxide system TeO₂? MoO₃ by means of x-ray diffraction and optical microscopy indicates the formation of a new phase, α-Te₂MoO₇, stable at room temperature. Below 500°C mixtures of crystalline products are obtained or complete devitrification can easily be induced in the system TeO₂? MoO₃. Above this temperature, tendency to glass formation is observed under the conditions employed, due to the liquidus temperature effect. Quenching of a melt of Te₂MoO₇ yields a dark yellow glass, α-Te₂MoO₇. X-ray and density measurements were used to explore the range of stoichiometry and exclude formation of solid solutions in the system; no apparent relationship exists between the crystal structures of the component oxides and the binary compound.     

Solid-State Equilibria in the MoO3 – TeO2 – Al2O3 System

The binary and ternary equilibrium reactions of Al₂O₃ with TeO₂ and MoO₃ were studied by X-ray diffraction methods and the following compatibility ranges were determined in the TeO₂ – MoO₃ – Al₂O₃ system at 750°C in air: TeO₂, Te₂MoO₇, Al₂TeO₆; Te₂MoO₇, MoO₃, Al₂TeO₆; MoO₃, Al₂(MoO₄)₃, Al₂TeO₆; Al₂(MoO₄)₃, Al₂O₃, Al₂TeO₆. Ternary compound formation was not observed in the temperature range investigated (450—750°C). Phasengleichgewichte im System MoO₃—TeO₂—Al₂O₃ .     

Das Schmelzdiagramm des Systems TeCl4 ? TeO2

The Melting Point Diagram of the System TeCl₄? TeO₂ The barogram of the system TeCl₄? TeO₂ was obtained by measurements of the total pressure in this system. From the characteristic points of the barogram and DTA measurements the melting point diagram TeCl₄? TeO₂ was derived. A congruent melting composition Te₆O₁₁Cl₂ exist in the system. The subsystems TeCl₄? B and B? TeO₂ are eutectic, with 48 Mol-% TeO₂ and 185°C and 92,5 Mol-% TeO₂ and 560°C, respectively. The melting point of Te₆O₁₁Cl₂ is 585°C.     

Structure of Glasses of the TeO2 – MoO3 System

Structural models for glasses of the TeO₂–MoO₃ system are suggested. On the basis of X-ray and infrared spectral investigations, by comparing with known crystalline structures of TeO₂, MoO₃ and Te₂MoO₇(T₂M), it is shown that the glasses from TeO₂ to Te₂MoO₇ possess [TeO₄] and [MoO₅] groups as basic structural units. The latter are connected to form [Mo₂O₈] complexes. The glasses in the MoO₃-rich compositional range are built up of [TeO₃] and [MoO₆] polyhedra. The glass-formation tendency is discussed in relation to the role of the free electron pair and the disruption of secondary and weak primary bonds in the crystals.     

Glass formation in the TeO2?MoO3?CeO2 System

Glass formation in the TeO₂? MoO₃? CeO₂ system was investigated and low melting stable glasses with up to 30 mole-% CeO₂ were synthesized. Infrared spectral investigations were used to develop structural models for the vitreous ternary system. CeO₂ mainly acts as a modifier without affecting appreciable changes to the glass network and coordination of the glass formers. Glasses in the molybdenum-rich compositional range are mainly composed of [MoO₆] and [TeO₃] polyhedra, whereas low MoO₃-containing glasses consist of [TeO₄] groups and isolated [MoO₄] units. On the whole, the basic structural polyhedra participating in the formation of the three-dimensional glass forming network are therefore [TeO₄], [TeO₃], [MoO₆], [MoO₄], and [Mo₂O₈] (or [MoO₅]) units. The structural affinity of some ternary glasses to crystalline Ce₄Mo₁₁Te₁₀O₅₉ is pointed out. The high electrical conductivity of the ternary glasses is interpreted on the basis of electron hopping between transition ions in different valence states and contributions due to the Te(IV) network.     

Phase diagram of the system PbTe-As2Te3

TheT — x phase diagram of the pseudobinary system PbTe-As₂Te₃ was constructed from DTA data and results of X-ray diffraction analysis and electron-probe microanalysis. No new compound was found in the system PbTe-As₂Te₃. The phase diagram of this system is of an eutectic type with an eutectic temperature of 350±5°, the eutectic composition corresponding to 10 mole% PbTe. Two solid phases with compositions near to As₂Te₃ and PbTe, respectively, coexist in the system below the eutectic temperature. The solubility of PbTe in As₂Te₃ is smaller than 2 mole% PbTe, and that of As₂Te₃ in PbTe is smaller than 0.5 mole% As₂Te₃ at 290°.     

Tellurium-Cerium Oxides

Phase equilibria in the (Ce, Te) O system, originating from interaction between Ce(NO₃)₃ · 6 H₂O and H₆TeO₆ followed by calcination up to 700°C, have been studied by methods of physico-chemical analysis (DTA, TGA, x-ray diffraction, XPS). Evidence is given for the formation of non-stoichiometric fluorite-type mixed-crystals (Ce, Te)O₂ in the 450–500°C range, up to a tellurium solubility limit corresponding to a lattice parameter of 5.666(3) Å. At higher temperatures (500–600°C) Ce₂(TeO₄)₃ is stable and Ce(TeO₃)₂ is formed above about 550°C. Ce(TeO₃)₂ is also obtained by solid-state interaction between CeO₂ and TeO₂ and is identical with the previously reported CeTe₃O₈ phase. Preparative conditions of the various compounds are described and x-ray diffraction data are reported. The TeO₂? CeO₂ phase diagram was established in the temperature interval from about 650 to 1100°C. Eutectic temperatures are 689°C and 794°C at the TeO₂ and CeO₂-rich sides, respectively.     

Additive telluromolybdates. Structure and catalysis in oxidation

Additive telluromolybdates, MoO₃·2TeO₂ and M^IIO·TeO₂·MoO₃ (M^IITeMoO₆; M^II = Co, Mn, Zn), converted ethyl lactate selectively to pyruvate in a vaporphase fixedbed flow system at 250–300 °C. A synergy in activity was observed for binary TeO₂–MoO₃, and crystalline Te₂MoO₇ was suggested as the active species. The Rietveld analysis of powder XRD patterns of ternary CoTeMoO₆ revealed the layer structure quite different from that of the reference Te₂MoO₇, but tellurium was again located adjacent to molybdenum linked through lattice oxygen.     

Paramagnetic resonance absorption in reduced tellurium?vanadium oxides

Hydrogen reduction of Te₂V₂O₉ at temperatures ranging from 300 to 450°C was studied by means of x-ray diffraction as well as ESR and IR spectroscopies. The reduction sets in through the formation of oxygen vacancies in Te₂V₂O₉ giving several types of transient VO²⁺ species. Subsequently, TeVO_4·17 and β-TeVO₄ (the latter only in minor amounts) are detected, in accordance with the phase diagram of the V₂O₅? TeO₂? VO₂ ternary system. The reduction stages leading to TeVO_4·17 and β-TeVO₄ are slow until 350°C, whereas rapid decomposition of the binary oxides (to give V₂O₃) is observed at higher temperatures. The magnetic interactions between paramagnetic V⁴⁺ ions in the reaction products are related to the structure of the reduced oxides.     

CoTeMoO6, Co4TeMo3O16: Two new cobalt molybdotellurates

It has been found that a solid-state reaction of CoMoO₄ with TeO₂ at 500°C yields a new compound of the formula CoTeMoO₆. This compound is also formed in the course of annealing of CoMoO₄H₆TeO₆ mixtures. Another new compound, the cobalt molybdotellurate containing Te⁶⁺, was prepared by a solid-state reaction of Co₅TeO₈ with MoO₃. It has the formula Co₄TeMo₃O₁₆. Both CoTeMoO₆ and Co₄TeMo₃O₁₆ have been characterized by X-ray method. The latter has the structure of a wolframite type with the unit cell dimensions a = 4.66, b = 5.67, c = 4.96 Å, β ? 90°.     

Isothermal Reduction Behaviour of Some Metal Molybdates. Selective light alkane oxydehydrogenation and/or olefins partial oxidation

The reduction profile of several unpromoted and promoted metal molybdate catalysts was investigated correlating their reducibility with the reactivity in catalysis. Using the stoichiometric α- and β-nickel molybdate compounds it was observed that the reduction rate was significantly affected by the nature of the phase. The results show thatβ-NiMoO₄ phase led to a significant increase in the reduction rate with respect to α phase. The increased resistance to reduction by hydrogen due to the structure of the catalytic system is reported. It was found that there is a relationship between the reducibility of the catalysts and selectivity to dehydrogenation products, indicating that the lattice oxygen plays an important role in the reaction. The effect of MoO₃, TeO₂ and Te₂MoO₇ added to NiMoO₄ systems onthe reducibility of the catalyst and on the propylene oxidation were also studied. It wasobserved that the reduction rate was significantly affected by the nature of the doping element. The results show that NiMoO₄–MoO₃ combination led toa significant increase of the reduction resistance of the nickel molybdate while TeO₂ or Te₂MoO₇ addition increases its oxygen depletion rate.Ni–Mo–O systems (Mo/Ni>1) were found to favour low CO_x selectivity, high selectivity to C₃H₄O and C₃H₄O₂ and good propylene conversion. In presence of TeO₂ and Te₂MoO₇ doped Ni–Mo–O system both acrolein and propylene conversion were increased with respect to the undoped system. Ni–Mo–Te–O catalysts have been found to have a reducibility trend which fits well with the acrolein and acrylic acid formation from propylene oxidation in presence of molecular oxygen. This revised version was published online in July 2006 with corrections to the Cover Date.     

Investigation on the TeO2–MoO3–V2O5 System. II. Properties of the obtained glasses

Some of the properties of glasses obtained in the systems TeO₂–MoO₃ and TeO₂–MoO₃–V₂O₅ had been studied. A good correlation between the properties and the phase diagram of the TeO₂–MoO₃ system was established. The glass resistance-composition function varied between 6.85 · 10⁹ ohm · cm and 2.93 · 10¹⁰ ohm · cm. The isolines of the properties (softening temperature, density, resistance at room and higher temperatures and activation energy) of the glasses obtained from the TeO₂–MoO₃–V₂O₅ system were ploted. The electrical resistance is influenced by the concentration of V₂O₅ and MoO₃ and by temperature. The glass absorption characteristics of thin layers were determined in the visible range.     

Untersuchungen zum System Te/O/Br

Investigations on the System Te/O/Br The melting point diagram of the system TeBr₄? TeO₂ was obtained by total pressure measurements and DTA measurements. A Congruent melting composition Te₆O₁₁Br₂ exists, the melting point is 570°C. The enthalpy of formation and the standard entropy of the species TeOBr₂,g was derived from measurements of the total pressure over Te₆O₁₁Br₂/TeBr₄ and from the transport behaviour of the TeO₂ with Br₂. From the decomposition-pressure measurements over Te₆O₁₁Br₂/TeO₂ follow the partial pressure. The enthalpy of formation ΔH°(Te₆O₁₁Br₂,f,298) = ?453.5 kcal/Mol was obtained from the enthalpy of solution. The transport-behaviour of TeO₂ with HBr, with Tebr₄ and Br₂ and that of Te₆O₁₁Br₂ is clear with the thermodynamic data of TeObr₂.     

Glass formation study within the TeO2–TlF and Tl2Te3O7–TlF systems

Large glassy domains have been observed at 800 °C within the TeO₂–TlF (from 7 to 65 mol% of TlF) and Tl₂Te₃O₇–TlF (from 0 to 75 mol% of TlF content) systems. The density, glass transition and crystallisation temperatures of glassy samples have been determined. The main features of a structural evolution within the TlF–TeO₂ glasses have been interpreted by analysing the relevant Raman spectra, and comparing them with those previously observed for the Tl₂O-TeO₂ system.     

Compound Formation in the Systems TeO2?SeO2 and TeO2?SeO2?H2O

The solid state reaction between TeO₂ and SeO₂ in inert atmosphere was studied by x-ray diffraction techniques and compound formation was observed. The pale-white reaction product α-TeSeO₄, obtained at 300°C, is not isostructural with the component oxides. The substance is stable at room temperature under exclusion of moisture but decomposes above about 320°C in dry atmosphere. Evidence is given for the formation of 3 TeO₂ · SeO₂ · nH₂O and other hydrated mixed oxides in the system TeO₂? SeO₂? H₂O; d-spacings are reported.     

The compounds and the phase diagram of MoO3-Rich Bi2O3MoO3 system

The equilibrium phase diagram between 0 and slightly above 50 mole% Bi₂O₃ in the Bi₂O₃MoO₃ system has been studied by differential thermogravimetric analysis (DTA) and X-ray diffraction measurements on fused mixtures and single crystals. The results confirm the existence of the four compounds α (Bi₂O₃·3MoO₃), β (Bi₂O₃·2MoO₃), γ (Bi₂O₃·MoO)₃ and ? (～1.3Bi₂O₃·MoO₃) in the system. However, the phase diagram as well as the nature of melting of the α and γ were found disagreed with previous results. The γ compound melts incongruently at 947°C, whereas the α compound melts congruently at 662°C. The crystal class and lattice parameters of the compounds were determined based on the single crystal as well as powder pattern techniques. The results show that all four compounds have the monoclinic structure. The unit cell parameter of the β, γ, and ? compounds were found to be quite different from previously reported data. The lattice parameters obtained from X-ray analysis were also verified by density measurements of the single crystals. The polymorphism of the compounds was also investigated with single crystal samples. No polymorphic transformations for the α, β, and γ phases were detected in the work.     

Sc2Te5O13 und Sc2TeO6: Die ersten Oxotellurate des Scandiums

Sc₂Te₅O₁₃ and Sc₂TeO₆: The First Oxotellurates of Scandium Sc₂Te₅O₁₃ and Sc₂TeO₆ are the first oxotellurates of scandium that could be structurally elucidated by X‐ray diffraction using single crystals. The scandium(III) oxotellurate(IV) Sc₂Te₅O₁₃ was synthesized by reacting Sc₂O₃ with TeO₂ at 850 °C and crystallizes in the triclinic system with space group (no. 2) and the lattice parameters a = 660.67(5), b = 855.28(7), c = 1041.10(9) pm, α = 86.732(8), β = 86.264(8), and γ = 74.021(8)° (Z = 2). The crystal structure contains chains respectively strands of alternatingly edge‐ and vertex‐sharing [ScO₆]^9? and [ScO₇]^11? polyhedra. These strands are connected by [TeO₃₊₁]^(2+2)? oxotellurate(IV) anions. The coordination spheres of Sc³⁺ appear markedly smaller than those of M³⁺ cations in the other known compounds of the formula type M₂Te₅O₁₃ (M = Y, Dy – Lu), therefore Sc₂Te₅O₁₃ is not really isotypic, but only isopuntal with these compounds. Single crystals of the scandium(III) oxotellurate(VI) Sc₂TeO₆ were obtained through the fusion of a mixture of Sc₂O₃ and TeO₃ at 850 °C. It crystallizes trigonally (a = 874.06(7), c = 479.85(4) pm and c/a = 0.549) with the Na₂SiF₆‐type structure in space group P321 (no. 150) and three formula units per unit cell. Its crystal structure is built up by a hexagonal closest packing (hcp) of oxide anions with the Sc³⁺ cations residing in ¹/₃ and the Te⁶⁺ cations in ¹/₆ of the octahedral interstices in a well‐ordered occupation pattern. Thus one can address the structural situation in Sc₂[TeO₆] as a stuffed β‐WCl₆‐type arrangement.     

Solid state reactions to CdTeMoO6 and its structural characterization

CdTeMoO₆ has been obtained by solid state reactions of CdMoO₄ with orth. TeO₂ at 425°C, with tetr. TeO₂ at 470°C, and with H₆TeO₆ at 490°C. Its crystal structure belongs to the tetragonal system (space group $\text{P4}\text{n}$ or $\text{P4}\text{nmm}$ with unit cell dimensions a = 5.279(2) Å, c = 9.056(2) Å. The specificity of this compound in the allylic oxidation reactions should be strictly related to its structural features, among which the presence of cis MoO₂ groups could be very important.     

Diagramme de phases du système Ag2Te-In2Te3

The phase diagram of the Ag₂Te-In₂Te₃ quasibinary system was established by differential thermal analysis and X-ray diffraction, in particular by the Guinier — Lenné method. This study confirms the existence of three intermediate ternary phases: AgInTe₂, which crystallizes in a chalcopyrite-type structure and possesses a homogeneity range, undergoes a peritectic decomposition at 650 °C; AgIn₅Te₈, which possesses a large homogeneity range, shows a phase transition at 699 °C and congruently melts at 725 °C; Ag₃In₉₇Te₁₄₇, which crystallizes in a cubic structure, incongruently melts at 672 °C.