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.     

Investigations Concerning the Quasi-binary System V2O5 – Nb2O5

In examining the pseudobinary system V₂O₅ – Nb₂O₅ it was possible to isolate, by systematic transport reactions, just one stable compound of the mean composition V_1.057Nb_8.943O₂₅ in addition to the boundary phases, which is in contrast to the data given in the literature. The phase width of this compound is very small and within the confidence limits of the analytical method of determination of 0.2 p.c. A maximum of 50 ppm Nb₂O₅ only dissolve in V₂O₅. Determining the solubility of V₂O₅ in Nb₂O₅ failed, because the two-phase products resulting during the chemical transport of the respective samples could not be separated exactly.     

Crystal structure of β′-Zn2V2O7

The crystal structure of the high-temperature modification of zinc pyrovanadate, namely, β′-Zn₂V₂O₇, is refined by the full-profile Rietveld method (GSAS) using the high-temperature X-ray diffraction data. The crystals are monoclinic (space group C2/m, Z = 2); the unit cell parameters are as follows: a = 6.9324(2) Å, b = 8.4394(2) Å, c = 5.0326(1) Å, and β = 108.272(2)°. Comparative analysis of the crystal structures of β′-Zn₂V₂O₇, β-Mn₂V₂O₇, and Cd₂V₂O₇ is performed.     

Growth studies of YVO4 crystals (II). Changes in Y?V?O-stoichiometry

Incongruent vanadium oxide vaporization of yttrium orthovanadate (YVO₄) melt generates changes in both oxygen and yttrium-vanadium (Y V) stoichiometry. Slightly modified YVO₄ or YVO₄ + Y₈V₂O₁₇ phases are formed from continuously changing melt systems depending on their starting compositions. The occurrence of Y₈V₂O₁₇ in yttrium orthovanadate crystal fibers grown by laser heated pedestal growth (LHPC) technique can be simply eliminated by utilization of suitable V₂O₅-excess starting compositions. In contrast, the oxygen-deficient YVO_4–x black phase is an inherently oxygen deficient phase with limited solid solution in the YVO₄ YVO₃ subsystem of Y₂O₃ V₂O₅ V₂O₃ complex ternary phase diagram close to the congruent YVO₄ composition (49.3 mol% V₂O₅–50.7 mol% Y₂O₃). The greater oxygen deficiency YVO_4–x specimens have smaller lattice parameters as determined from detailed XRD data. Ceramic feed rod and fiber crystal grown from the feed rod with slight yttrium oxide excess starting composition were characterized by electron microprobe analysis. Changes in both Y V and oxygen stoichiometry were observed along the fiber, due to double limited solid solution system of three dimensional ternary phase diagram. Consequently, both the presence of Y₈V₂O₁₇ phase and change of Y V stoichiometry over oxygen deficiency cause difficulties for YVO₄ crystal growth from yttrium oxide excess melt.     

Über die Zerfallsstruktur des Ni-Katalysators und die Ni(C)-Phase des Diamanten bei der Diamantsynthese

The nature of intermediate phases in the Ni C system is discussed and the concentration dependence of the specific atomic volume is determined. Decomposition structures of Ni C samples exposed to high pressures and temperatures are studied metallographically and by X-ray diffraction. The specific atomic volume of Ni(C) decomposition phase and of Ni(C) satellite phase of Diamond is measured by the asymmetric powder method and from this the concentration of carbon is ascertained. It is stated that the Ni(C_y) satellite phase of diamond — the terminal solid solution of carbon in nickel approximately contains 3 at.% of carbon. It is concluded from this that this phase is the result of the last stage of decomposition of carbon supersaturated catalyst. Thus, the intermediate phase is playing no important role in diamond nucleation.     

A Novel Coordination Structure of Dichromates Bonding to Copper (II): [Cu(bipy)2(Cr2O7)] · 2 H2O

Dark-green multimetal compound crystal [Cu(bipy)₂(Cr₂O₇)]· 2 H₂O was obtained from aqueous solution of Cu(NO₃)₂, K₂Cr₂O₇ and bipyridine. The crystal structure was determined by X-ray crystallography: triclinic P1 , a = 7.716(3) Å, b = 9.656(3) Å, c = 15.517(5)Å, α = 77.41(3)°, β = 81.04(3)°, γ = 82.33(3)°, Z = 2. In this compound, two chromium atoms and a copper atom are linked by two oxo bridges (Cu(II) O Cr(VI) O Cr(VI)). The copper coordination polyhedron corresponds to a five-coordinated distorted trigonal bipyramid.     

Thermodynamische Berechung des Systems Ge/H2O/H2 und experimentelle Untersuchung mit der Nahabstandsanordnung

In the temperature range from 950 to 1200 K K_p-values result from 5 · 10⁻⁵ to 6,9 · 10⁻³ for the heterogeneous reversible reaction Ge_(s) + H₂O_(g) ⇋ GeO_(g) + H₂O_(g), the average reaction enthalpy being 46,5 ± 05 kcal/mol. Etching rates calculated with these equilibrium constants for closed systems are 20-25% larger than the experimental etching rates for (110)-Ge in the sandwich device. The ratio of the etching rates is for (111)-, (100)- and (110)-Ge 1:1,4:1,8; the average reaction enthalpy calculated experimentally is 46 ± 1 kcal/mol for the temperature range 1000-1200 K. The dependence of transport rate on distance between source and substrate characterizes the sandwich device as a quasi-closed system with the diffusion as the rate controlling step of the material transport.     

Abscheidung von Cu2O aus der Gasphase im strömenden Medium

CVD-experiments in the system Cu O H Cl N result in the growth of Cu₂O single crystals. The characteristic faces of the various forms are the {100} ones. A thermodynamic analysis permits to predict such conditions under which Cu₂O is deposited without deposition of any other phases.     

Experimenteller Nachweis der Temperaturfelder beim Schaltvorgang in Cu2O und Zeitlupenstudie des thermischen Durchschlages

The temperatures before and during the breakdown (switching) in planar Ag Cu₂O Ag structures were determined experimentally. They were measured by means of liquid crystals and thermo-chromatic cryons as temperature indicators as well as measuring the intensity distribution of the emitted IR radiation. Structural changes during electrical switching were observed using a slow-motion camera. The formation of the switching channel and the movement of the liquid phase in it were observed. The results show that the electrical switching is due to a thermal mechanism.     

On the Bindings in VOm Phases

The phenomenological plural correlations model permits to attribute to the intermediate phases of the mixture VO_M bonding types (bindings) which indicate a low energy of the empirically found structures. In the phases various electron subsystems exist which contain lattice-like spatial correlations, and when the corresponding cells are in energetically low commensurabilities (in harmonies) to the crystal and to one another then the phase becomes stable. A fundamental assumption of a binding analysis is the electron count being here V^1,4,8O^0,6,2. As a consequence the phases V₈O, V₁₆O₃, and V₇O₄ consist of a tetra-gonally deformed V(B1) partial structure with interstices partly filled by O atoms. The composition of a phase determines the electron concentrations in the subsystems, and these influence the harmony of the correlations in the binding (BF_U2 for V₁₆O₃, e.g.). In VO, V₁₃O₁₆, V₂O₃, and V₃O₅ an essentially complete close packed partial structure of O accepts V into its octahedral interstices. Once more harmonies of electron correlations determine favourable bindings (FF'2 for VO e.g.). The Magnéli phases V₄O₇ up to V₈O₁₅ being shear homeotypes of VO₂ · h may be considered as homeodesmic to TiO₂ · r, which is stabilized by a CF_U2 binding. From the binding it may be concluded how much Hund insertion is present in a phase. Hund insertion is for instance responsible for the transformation VO₂ · h → r. The phases V₆O₁₃, V₄O₉ and V₃O₇ are homeotypic to V₂O₅. This last phase permits presumably a UH_T3 binding. The results of the binding analysis of VO_M shed some new light on the interpretation of properties of the VO_M phases.     

The Crystal Structure of the Cluster Complex [{Co(phen)2}2V4O12] · H2O

The crystal structure of [{Co(phen)₂}₂V₄O₁₂] · H₂O consists of hexanuclear bimetallic clusters [{Co(phen)₂}₂V₄O₁₂]. The cyclic [V₄O₁₂]^4‐ anion acts as a bidentate bridging ligand between the two [Co(phen)₂]²⁺ cations. The π‐π stacking interactions between the parallel 1,10‐phenantroline (phen) groups play a significant role in stabilizing this structure. The title compound crystallizes in the P2₁/c space group.     

Effect of mixed transition-metal ions in glasses. Part III: The P2O5–V2O5–MnO system

Electrical and optical properties of phosphate glasses containing vanadium and manganese ions in the xP₂O₅–[(100 − x)(V₂O₅ + MnO)] (PVM) system have been investigated. This is the last article of a III-part series devoted to the electronic properties of phosphate glasses containing a mixture of transition ions. The first article was devoted to the electrical conductivity of glasses having the general composition: xP₂O₅–[(100 − x)(V₂O₅ + Fe₂O₃)] (PVF). Competitive transport of small polarons on V and Fe ion sites was found to contribute to a mixed transition-ion effect (MTE) in PVF glasses. Several features of MTE were found to be similar to the well known mixed alkali effect, observed in glasses containing two alkali ions. In the second article, optical absorption and electronic conduction of xP₂O₅–[(100 − x)(Fe₂O₃ + MnO)] (PFM) glasses were reported. In the absence of competitive transport between the two transition ions (since Mn ions were determined not to contribute to dc conduction), MTE was not observed. The most important feature of PFM glasses was a sharp increase in resistivity at a critical concentration of iron ions, similar to ‘metal–insulator transition’ (MIT). In the present article, we report a resistivity transition in PVM glasses which is similar to that exhibited by the glasses of the PFM series. While Fe ions contributed the carriers in the PFM glasses, V ions serve the same purpose in the PVM compositions. As the concentration of vanadium ions, n_V, is decreased in the composition range 0.82 > n_V > 0.40, resistivity (ρ) increases marginally. For glasses with 0.2 < n_V < 0.40, resistivity and the activation energy for dc conduction (W) increase sharply with decreasing n_V, marking the incidence of an MIT-type transition. As in the PFM glasses, the observation of MIT coincides with the transformation of small polarons to small bipolarons, which is confirmed by the shifting of the small polaron optical absorption band to higher energies with decreasing V concentration.     

The cation distributions in the spinel phases of the mixed vanadium oxides LiZnx V2-xO4 and LiMgx V2-xO4

Cation distributions and values of the oxygen parameter in the spinel phases of mixed vanadium oxides with the general formula Li_xMeV_2–xO₄, where Me is Zn or Mg, respectively, were studied by X-ray diffraction measurements. The distribution of cations on the sublattices in the zinc series can be expressed by the formula Li_1–x–δ Zn_x V_δ [Li_x+δ V_2–x-δ] and for the magnesium series by the formula Li_1–δ V_δ [Li_δ Mg_x V_2–x-δ]. For the zinc series a general uniform increase of the oxygen parameter was determined. The substitution of magnesium ions leads to an anomal course of this parameter, which is possible to explain by substitution of magnesium ions on intersticial sites. The results are discussed using the values of the Madelung energy. The recently observed tetragonal deformation of the spinel lattice for the bulk composition Li Mg_0.375 V_1.625 O₄ is explained by a cooperative Jahn Teller effect of V⁴⁺ ions.     

Über Strukturbeziehungen bei Erdalkalivanadin (IV,V)-oxiden

It is reported on compounds and their structural relations in the systems MeV^v-kO_3,5–k/2 with Me = Ca, Sr, Ba. The index k shows values from zero to one. According to the kind of Me-ions different phases are found. CaVO₃ und SrVO₃ crystallize in a perovskit-type. BaVO₃ crystallizes in a structure deriving from the Ba₃V₂O₈-type. X-ray analysis is confirmed by density- and NMR-measurements. Compounds of mixed oxidation numbers for Vanadium are only observed in the Ba and Sr-systems. Likewise their structures are derived from the Ba₃V₂O₈-type.     

Crystal structure and electrochemical behavior of a novel polyoxometalate [Ni(bpy)3]2[W4V2O19] with Lindqvist‐type structure

A novel polyoxometalate [Ni(bpy)₃]₂[W₄V₂O₁₉] (Ni₂V₂W₄) has been synthesized by the hydrothermal method and the structure determined by X‐ray crystallography. Ni₂V₂W₄ crystallizes in a trigonal system with space group R ‐3c (a = 15.8984 (5) Å, b = 15.8984 (5) Å, c = 43.855 (3) Å). In the structure of the compound, terminal and bridging oxygen atoms are bond to the metal centers by σ or π bonds. The W⁶⁺ and V⁵⁺ ions in isopolyanion [W₄V₂O₁₉]^4‐ statistically distribute over the six metal centers. Using cathodic adsorptive voltammetric method with a carbon paste electrode, the redox property and the electron transferring process were studied. The results show that electrochemical behavior about W(VI) and V(V) atoms give one‐electron, three‐electron and two‐electron reduction waves. Three successive oxidation waves are observed too. The compound was also characterized by thermal gravimetric analysis and IR spectra. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

An effect of oxygen pressure on the process of monocrystallization of the MnaZnbFe2O4 manganese-zinc ferrites

The present paper relates theoretical assumptions concerning the pressure of oxygen required for the process of monocrystallization of the Mn_aZn_bFe₂O₄ manganese-zinc ferrites. A general analysis of the equilibrium diagram Zn Mn Fe O was carried out, taking into account the pseudobinary ZnFe₂O₄ MnFe₂O₄ system for the case of the formation of the above mentioned ferrite. At the same time, it was assumed that the base material (a polycrystal) for the unidirectional solidification in a Bridgman system would be formed of two compounds, i.e.f. ZnFe₂O₄ and MnFe₂O₄. It was stated that the monocrystallization of manganese-zinc ferrites should be carried out in the working chamber of a furnace at a pressure of oxygen amounting to from several tenth to several megapascals, the values of these pressures depending on the composition of the crystalline phase (the values of X_{2, MnFe2O4} and X_{1, Fe2O4} used in Equation (12)).     

Single crystal preparation of MnxCr3–xO4 by flux method

Mn_xCr_{3 x}O₄ was prepared by the flux method. Melts of PbO PbF₂, Bi₂O₃ B₂O₃, B₂O₃, Na₂B₄O₇, and Na₂W₂O₇ Na₂WO₄ were used. The best results could be yielded with the PbO PbF₂ flux, from which crystals with 2–4 mm in thickness were grown. The Bi₂O₃ B₂O₃ flux produced crystals with 1–2 mm in thickness. The spinell structure of the chromite was proved by X-ray investigation.     

Synthesis and characterization of Bis(μ-sulfido)bis[{O,O-dimethyldithiophosphato}oxomolybdenum(V)]. Crystal structure of Mo2O2(Φ-S2)[S2P(OMe)2]2.C6H6

Bis(μ-sulfido)bis[{O,O-dimethyldithiophosphato}oxomolybdenum(V) was prepared by dropwise addition of an ethanolic solution of the ammonium salt of O,O-dimethyldithio-phosphoric acid to a hot aqueous solution of MoCl₅. The X-ray structure of Mo₂O₂(Φ-S₂)[S₂P(OMe)₂]₂.C₆H₆ was determined. Crystal data: Monoclinic, P2₁/n, a = 6.9788(6), b = 22.616(3), c = 14.633(1) Å, β = 101.193(9)°, V = 2265.5(4) Å⁻³, Z = 4. The immediate environment around the two molybdenum atoms is essentially square pyramidal if the Mo—Mo interaction is ignored. The terminal oxygen atoms are in the syn conformation.     

Specific features of phase transitions and the conduction of La<Subscript>2</Subscript>Mo<Subscript>2</Subscript>O<Subscript>9</Subscript> oxide-ion conducting compound doped with vanadium

The X-ray powder analysis, calorimetric studies, and conductivity measurements of a series of ceramic La₂Mo_2−x V _x O _y specimens with different vanadium content are performed with the aim of following the dynamics of phase formation of the low-temperature α, high-temperature β, and metastable β _ms phases. At x ≥ 0.06, the cubic phase becomes stable and the monoclinic phase vanishes; therefore, the main α → β transition is suppressed. According to the data of differential thermal analyses, a weak thermal anomaly is observed in the range 450–470°C at x ≥ 0.06. This anomaly is indicative of the β _ms → β transition due to the conversion of the cubic phase with statically disordered oxygen atoms into the cubic phase with dynamic disorder. The conductivity of the high-temperature β phase obeys the Vogel-Tammann-Fulcher law.     

Darstellung von VO2-einkristallen der oberen und unteren phasengrenze durch chemischen transport

The chemical transport of VO₂ with TeCl₄ was investigated in the temperature range of about 1000°C and 600°C. In the first case there is TeO₂ existing in the gaseous phase, in the second case the TeO₂ has no noticeable vapour pressure. The transport takes place from the hot to the cool zone. In both cases we obtained high transport rates of 10 to 20 mg h^-1. The composition of the gaseous phase is calculated and the transport mechanisms are discussed. It is possible to obtain VO₂ with the upper and lower phase boundary by variation of the initial compositions. Starting with compositions lower than VO_1.99 we obtained near 1000°C in the deposition zone V₈O₁₅ first and after a time VO₂ with the lower phase boundary. Compositions higher than VO_2.00 give vanadium dioxide with the upper phase boundary. In the temperature range of about 600°C however we obtained the phase with the higher oxygen content first. The results are compared with those, which have been obtained by the transport of VO₂ with HCl and Cl₂ — which takes place from the cool to the hot zone.