A series of lead tungsten bronzes

The phases in samples of gross composition Pb_xWO₃ (0.01 ? x ? 0.28) heated at temperatures between 973 and 1373°K have been investigated. At all temperatures a nonstoichiometric tetragonal tungsten bronze phase forms for compositions x > 0.18. At temperatures up to 1273°K a series of orthorhombic intergrowth bronzes also forms, but these appear to be unstable at higher temperatures and were not found in the preparations made at 1373°K. Aspects of the crystal chemistry of these latter materials are discussed, including structure, crystal habit, valence of the Pb atoms in these phases, and the relation of the phases found here to other related intergrowth bronze phases.     

The stability of the perovskite-type zirconium tungsten bronze ZrxWO3

It has been found that a perovskite-related zirconium tungsten bronze Zr_xWO₃ (with 0 < x ? 0.08) forms readily at temperatures between 973 and 1573° K. Prolonged heating causes the bronze to decompose to other oxide products at all the temperatures investigated. These results are summarized in phase diagrams. Possible reasons for the decomposition of the bronze are discussed.     

Phase equilibria,thermal analysis,and reactivity of tin tungsten bronzes and related phases

Crystal chemistry and phase relations of the bronze forming region of the SnWO system have been investigated. Above 780°C the tin bronzes Sn_xWO₃ are shown to be thermally unstable and an equilibrium diagram is established at 700°C which shows that the composition limits of the tetragonal phase are 0.21 ? x ? 0.29. Below x = 0.21 a series of single and two phase regions containing orthorhombic bronzes exists for which the composition limits have been established. In the range 0.29 ? x ? 0.76 the system comprises the tetragonal bronze, Sn₂W₃O₈ and SnWO₄, while above 0.76 there is no bronze, only Sn₂W₃O₈, SnWO₄ and free Sn. The phase Sn₂W₃O₈ has been isolated and shown to have a hexagonal unit cell, a = 7.696 Å, c = 18.654 Å. The evidence of differential thermal analysis and X-ray studies suggests that this hexagonal phase arises from the decomposition of the tungsten bronze phase and is itself decomposed to cubic SnWO₄ above 700°C. Small thermal effects observed in the DTA scans of tin-containing tetragonal bronzes are interpreted in terms of an order-disorder phenomenon arising from asymmetric tunnel occupancy by Sn²⁺ ions caused by the presence of the lone pair of electrons. Hydrogen reduction of Sn_xWO₃ has been shown to result in complete removal of oxygen, producing Sn + α-W in the range 600–850°C. Some activation energy data are given for the reduction process.     

Phase relations in the dioxide-trioxide region of some 3d transition Metal-WO ternary systems

The phases occurring in the MnWO, FeWO, CoWO, and NiWO systems at 1373°K have been determined using X-ray diffraction and electron and optical microscopy. Experimentally most attention was given to the MnWO system, where it was found that Mn entered as the Mn²⁺ ion into the WO₃ host matrix and formed a perovskite-related bronze Mn_xWO₃. The highest observed x-value in the bronze is about 0.027. In addition a metastable θ_w(Mn) oxide with the Mo₅O₁₄ structure and a disordered oxide of overall composition approximately (Mn, W)O_2.82 were found. The FeWO system was similar to the MnWO system but significant differences occurred in the CoWO and NiWO systems where M_xWO₃ bronze phases were not observed to form at 1373°K. The stability of the M_xWO₃ and the θ_w(M) oxides formed are discussed in terms of the ionic size of the M ions involved. It is suggested that M_xWO₃ bronzes are metastable if these M ions are small.     

The preparation,phase relationships,and Eu-151 Mössbauer spectroscopy of europium tungsten bronzes and related phases

Crystal chemistry and phase relations for the bronze-forming region of the EuWO system have been investigated. A bronze Eu_xWO₃ is stable up to 1000°C when x ? 0.125 and in the region 0.085 ? x ? 0.125 the symmetry is cubic. A tetragonal bronze exists at x = 0.05, and an orthorhombic bronze with a structure closely related to the orthorhombic form of WO₃ exists below x = 0.01. Mössbauer spectra at room temperature and at 80 K indicate that in all these phases the europium is highly ionized as Eu(III) with no electron localization to give (EuII) even at low values for x. The decomposition products of the bronzes have been established, and the Mössbauer parameters for the highly nonstoichiometric tungstates Eu_xWO₄ were determined. Both Eu(II) and Eu(III) resonances were obtained, and a cation vacancy model for Eu_xWO₄ was found to fit the data best. In conformity with the foregoing data, a sample of composition “Eu₂W₂O₇” was found not be be a pyrochlore but to comprise a mixture of Eu₆WO₁₂, Eu_xWO₄, and W. The phase relationships for the europium bronze system Eu_xWO₃ are compared with those of other ionic bronzes Na_xWO₃, Li_xWO₃, and Al_xWO₃.     

Tungsten bronze formation by the Group IIIA metals

Attempts have been made to prepare tungsten bronze phases from the Group IIIA metals, Al, Ga, and In. Of these, only In seems to from bronzes with any facility and three distinct compounds were characterized. Two of these were perovskite-type phases, one of tetragonal symmetry, with lattice parameters a = 0.3714 nm, c = 0.3870 nm, which forms below 1173 K and one of orthorhombic (pseudotetragonal) symmetry, with lattice parameters a = 0.3696 nm, b = 0.3722 nm, and c = 0.3859 nm, which forms above 1173 K. Both of these have a composition of approximately In_0.02WO₃. The third phase which formed in this system was a hexagonal tungsten bronze which has been characterized already. In neither the AlWO or the GaWO systems were stable bronzes formed, but some evidence suggested that metastable perovskite bronzes may form in the GaWO system in some circumstances. The formation of these phases is discussed and related to the formation of tungsten bronzes in general.     

Preparation and studies of a new ammonium vanadium bronze, (NH4)xV2O5

A new bronze-type phase of composition (NH₄)_0.40±0.02V₂O₅ is obtained around 230°C during the thermal decomposition of NH₄VO₃ in hydrogen atmosphere. The bronze intermediate is characterized by X-ray diffraction, electrical conductivity, magnetic susceptibility, and ESR studies. It is found to be isostructural with other known β-type vanadium bronzes of general formula M_xV₂O₅, where M is usually a monovalent metal. Electrical conductivity and magnetic studies indicate the localized character of conduction electrons at V⁺⁴ sites. At high temperatures (>400°C), the bronze undergoes decomposition and subsequent reduction to V₂O₃ in hydrogen atmosphere.     

The reaction of cubic sodium tungsten bronzes,NaxWO3, with metallic iron

The reaction of the cubic sodium bronzes, Na_xWO₃, with powdered iron metal has been studied by heating samples in vacuo and also at high pressure. Evidence for reaction is found at unexpectedly low temperatures. The reaction is an overall reduction which proceeds via an increase in the sodium content of the bronze phase up to some temperature-dependent limiting composition for which x < 1. The existence of this limit, its temperature dependence, and the identity of the other products of reduction have been explained in terms of the partial oxygen pressure of the system. The course of the reduction has been followed through the evolution of the bronze lattice parameter and a reaction mechanism is postulated. No evidence of significant incorporation of iron into a stable cubic sodium bronze phase has been found.     

Specific heat and phase diagram of nonstoichiometric ceria (CeO2−x)

The phase diagram for nonstoichiometric ceria, CeO_2?x, was determined from specific heat measurements in the temperature range 320–1200 K and composition range CeO₂CeO_1.72. Coexistence temperatures of three phases are found at 722, 736, 766, 913, and 1084 K. There is some indication for the existence of two other coexistence temperatures at 850 and at 880 K. The maximum of the miscibility gap occurs at T = 910 K and 2 ? x = 1.93. The phase diagram exhibits some phases in the homologous series Ce_nO_2n?2 with n = 7, 10, 11, and two phases at 2 ? x = 1.79 and 2 ? x = 1.808 not belonging to this series.     

Preparation and Some Properties of Lithium Vanadium Bronzes

Lithium vanadium bronzes with composition formula Li_xV₂O₅ (0.04 ≤ × ≤ 0.92) have been prepared by solid‐state reaction at 650 °C in argon atmosphere. The obtained products were characterized by X‐ray powder diffraction and IR spectroscopy. The results reveal that four phases are present in the range from x = 0.04 to 0.92, namely α, β, β′, and γ phase. The magnetic susceptibility for the investigated bronzes was measured using the conventional Gouy's method. The values of the effective magnetic moments, as calculated from experimental data, indicate the presence of V⁴⁺ ions in all bronze samples. The electrical conductivity as a function of temperature and lithium content was measured in the temperature range from room temperature to 483 K. The electrical conductivity of the bronzes is found to be affected by lithium content. The values of the electrical conductivity increase with temperature for the prepared samples and both electronic and ionic conduction are discussed.     

Mössbauer study of the thermal decomposition products of BaFeO4

A pure sample of a hexavalent iron compound, BaFeO₄, was decomposed at temperatures below 1200°C at oxygen pressures from 0.2 to 1500 atm. In addition to the already known BaFeO_x (2.5 ≦ x < 3.0) phases with hexagonal and triclinic symmetry, two new phases were obtained as decomposition products at low temperatures. One of the new phases, with composition BaFeO_{2.61 – 2.71}, has tetragonal symmetry; lattice constants are a₀ = 8.54 Å, c₀ = 7.29 Å. The phase is antiferromagnetic with Néel temperature estimated to be 225 ± 10 K. Two internal fields observed on its Mössbauer spectra correspond to Fe³⁺ and Fe⁴⁺. In the other new phase, with composition BaFeO_2.5, all Fe³⁺ ions had the same hyperfine field; it too is antiferromagnetic with a Néel temperature of 893 ± 10 K. Mössbauer data on the hexagonal phase coincided with earlier results of Gallagher, MacChesney, and Buchanan [J. Chem. Phys.43, 516 (1965)]. In the triclinic-I BaFeO_2.50 phase, internal magnetic fields were observed at room temperature, and it was supposed that there were four kinds of Fe³⁺ sites. The phase diagram of BaFeO_x system was determined as functions of temperature and oxygen pressure.     

Phase relations in the SnWO ternary system near to WO3

Phase relations in the SnWO system for compositions near to WO₃ and temperatures up to 1173 K have been determined by electron microscopy and X-ray diffraction. The phase limits for the bronzes previously reported in this system have been determined. For the orthorhombic I bronzes the phase limits are from Sn_0.04WO₃ to Sn_0.06WO₃. Two orthorhombic II bronze phases form, one at a composition of Sn_0.13WO₃ to Sn_0.15WO₃, and another at Sn_0.16WO₃. These bronzes have structures which consist of lamellae of WO₃ united by fault planes. The other bronze phase to form, with the tetragonal tungsten bronze structure, has a lower composition limit of Sn_0.21WO₃.     

Synthesis and Characterization of Niobium Substituted Hexagonal Tungsten Bronzes

Alkali metal tungsten bronzes, M_xWO₃, and its niobium substituted forms, M_xNb_yW_1‐yO₃, have been prepared with M = K and Rb and nominal compositions of x = 0.20, 0.25, 0.30 and 0.0 ≤ y ≤ 0.20 at temperatures between 600 and 900?C. The X‐ray powder patterns reveal that single phases of niobium substituted hexagonal tungsten bronze (HTB) can be prepared for x = 0.2, y ≤ 0.05 ; x = 0.25, y ≤ 0.125 and x = 0.3, y ≤ 0.15. Investigations of the optical reflectivity and the infrared absorption of Rb_0.3Nb_yW_1‐yO₃ indicate a decreasing concentration of free carrier with increasing niobium content.     

Hydrothermal synthesis of potassium molybdenum oxide bronzes: structure-inheriting solid-state route to blue bronze and dissolution/deposition route to red bronze

The hydrothermal syntheses of the alkali metal molybdenum bronzes from starting solids (H_xMoO₃) with structural affinities to the desired products were investigated. Single-phase potassium blue and red bronzes were prepared by the hydrothermal treatments at around 430 K, and characterized by powder X-ray diffraction, IR spectroscopy, and SEM. The formation processes of these two bronzes during the hydrothermal treatments were found to differ. The blue bronze was formed by a structure-inheriting solid-state route from H_xMoO₃ with x<0.3, whereas the red bronze was formed for x>0.3 through a solution dissolution/deposition route via the formation of MoO₃+MoO₂.     

Stoichiometry and physical properties of ternary molybdenum chalcogenides MxMo6X8 (X = S,Se; M = Li,Sn, Pb)

Chemical and electrochemical insertion of Li at room temperature, as well as insertion of lead and tin at moderate temperatures (500°C), into the binary phase Mo₆X₈ forms ternary molybdenum chalcogenides M_xMo₆X₈ (X = S, Se). Crystallographic parameters, superconducting properties, and magnetic susceptibility are reported. The stoichiometry x for lead and tin is shown not to exceed x = 1, while for Li, x can reach approximately 4.0. For the lead and tin sulfide series, the hexagonal lattice parameters and superconducting critical temperatures (T_c) are invariant to changes in the nominal composition of 0.8 < x < 1.2, while both an increase in T_c and a small decrease in c_h is observed for the selenides; a narrow homogeneity range exists near x = 1 below 500°C for both these sulfides and selenides, the single-phase region being somewhat larger in the selenides. In contrast, several single-phase regions and large unit cell changes are observed in Li_xMo₆X₈ (0 < x < 3.2). Magnetic susceptibility measurements of the lithiated compounds at x ～ 3.2 reveals a structural phase transition at 140 and 185 K for the sulfide and selenide, respectively; but neither superconducts down to 1.5 K. At lower lithium concentration near x ～ 1.0, the T_c of the sulfide is raised from that of Mo₆S₈ (1.8 K) to 5.2 K but the T_c of Mo₆Se₈ (6.5 K) is depressed to 3.9 K.     

The Ba1−ySryMnO3−x system

Subsolidus phase relations at ambient atmospheric pressure and elevated temperatures in the Ba_1?ySr_yMnO_3?x system were investigated by quenching, gravimetric, and X-ray diffraction methods. The system is not binary above ～1035°C because of reactions with atmospheric oxygen. The air isolar, P_O2 = 0.2 atm, was characterized at 1225, 1375, 1490, and 1610°C. Seven oxygen-deficient phases including a perovskite phase characterize the system. Their stability depends on the values of y and x in Ba_1?ySr_yMnO_3?x. The cell dimensions of these phases expand as x increases at fixed y. These seven modifications can be retained in stoichiometric form by oxidation at lower temperatures.     

Heat capacity of pure and doped V2O3 single crystals

Heat capacities have been measured for single crystals of V₂O₃, either pure or doped with 1 and 1.4 mole% Cr₂O₃ and Al₂O₃ over the temperature range 100–700°K. V₂O₃ undergoes a fairly sharp transition at low temperatures (～170°K) but fails to exhibit any thermal anomaly above 300°K. The thermal behavior of (M_xV_1?x)₂O₃, M = Cr, Al, is manifested by two transitions: one at low temperatures, 170–180°K for x = 0.01 and 180–190°K for x = 0.014, and the other at high temperatures. For x = 0.01, the high-temperature (HT) anomaly extended over the range 325–345°K (Cr-doped V₂O₃) and 345–365°K (Al-doped V₂O₃), respectively. The corresponding ranges for x = 0.014 were found to be 260–280°K and 270–290°K, respectively. Further, the HT anomaly was characterized by a large hysteresis (～50°K). The values of lattice heat capacity of pure and doped V₂O₃ were, however, found to be almost the same and could be empirically represented by the Debye (D)?Einstein (E) function $\text{D}\text{(}\text{580}\text{T}\text{) + 4}\text{E}\text{(}\text{θ}\text{T}\text{)}$ with θ values 430°K (T = 100–230°K) and 465°K (T > 230°K), respectively. Further, the enthalpy change ΔH associated with the HT anomaly in doped V₂O₃ (80 ≤ ΔH ≤ 510 J/mole) was 5–10 times smaller than the ΔH corresponding to the lower-temperature transition. The results cited here appear incompatible with the Mott transition model that has been invoked to explain the HT anomaly.     

Magnetic susceptibility and the spin-glass transition of Cd1−xMnxS and Zn1−xMnxS at low temperatures

Low-field magnetic susceptibility of the diluted magnetic semiconductors Cd_1?xMn_xS and Zn_1?xMn_xS was measured between 4.2 and 30 K for the Mn concentration range 0.25 < x < 0.40. When x > 0.25, both of these ternary systems show a spin-glass transition in the above temperature range, as evidenced by a somewhat rounded cusp in the susceptibility and by the presence of irreversible effects. Because these materials are insulators at low temperatures, and the interactions between the Mn ions are only antiferromagnetic, the observed spin-glass behavior is attributed to frustration inherent in the hcp lattices of these compounds. The phase diagrams for the boundary of the paramagnetic and the spin-glass phases are presented for the two alloy systems, and the difference between the two phase diagrams is discussed.     

Electrical conductivity in single-crystals of vanadium oxide bronzes

Single-crystal electrical conductivity of vanadium oxide bronzes of the type M_xV₂O₅ (β-phase), where M = Li, Na, K has been studied from 300 to 900°K. These compounds are found to exhibit a reversible semiconductor-metal transition along [010] in the region of 340°K. The phase transition observed seems to be caused by vanishing of the activation energy of carrier mobility due to the shift of some vanadium atoms in the planes perpendicular to the B axis.     

Influence of solid-phase ferritization method on phase composition of lithium-zinc ferrites with various concentration of zinc

Methods of X-ray phase analysis (XRPA) and differential thermogravimetry in a magnetic field (DTG(M)) are used to investigate the phase composition of Li_0.5(1?x)Fe_2.5?0.5x Zn _x O₄ (x _Zn?=?0.2, 0.4, and 0.6) ferrite spinels synthesized at a temperature of 700?°C during 120?min by thermal annealing of a reagent mixture in a furnace and heating of the mixture using high-power beam of accelerated electrons with energy of 2.4?MeV. Thermal ferritization of all compositions leads to the formation of phases whose composition is close to simple monoferrites. Lithium?Czinc ferrite phases are formed during annealing under electron irradiation. It is concluded that the rate of controllable diffusion interaction of monoferrite phases significantly increases under conditions of high-power electron irradiation.