Thermodynamic analysis of the metal-insulator transitions in V2O3 alloy systems

An overview is presented concerning the entropies (ΔS) and enthalpies (ΔH) associated with metal-insulator transitions in the (V_1?x M _x )₂O₃ M=Al, Cr, Ti systems, 0?x?0·12 and in non-stoichiometric V_{2(1 ?y} )O₃, 0?y?0·01. The recent literature and new data are analyzed with respect to gradiations in ΔH and ΔS withx ory, and their implications concerning the mechanism of the transitions. A revised phase diagram is presented.     

Thermal studies on Cr-doped V2O3 crystals

Studies on heat capacity (C_p) of (Cr_xV_1?x)₂O₃ system, 0 ≤ x ≤ 0.12, revealed a sharp C_p anomaly in each composition in the range 150–190 K. With increasing temperature, the transition temperature at first increased from 173 K (x = 0) to 187 K (x = 0.033) and then decreased steadily to 151 K for x = 0.12. Similarly, the enthalpy of transition increased from 2025 J/mole for V_2.3O to 2355 J/mole-CrV₂O₃ for x = 0.02 and then decreased steadily to 1130 J/mole-CrV₂O₃ x = 0.12. 1 at. % Cr-doped samples exhibited an additional anomaly in the range 325–360 K. The C_p transitions may be correlated with corresponding electrical anomalies in the same temperature ranges. Our study on V₂O₃ failed to confirm the anomalies reported by Jaffray and Lyand.     

Composition,constitution and stability of the synthetic hollandites AxM4−2xN2xO8, M=Ti,Ge, Ru,Zr, Sn and N=Al,Sc, Cr,Ga, Ru,In and the system (A,Ba)xTiyAlzO8 with A=Rb,Cs, Sr

Synthetic hollandite compounds of composition Ba_xM(IV)_4−2xN(III)_2xO₈ (M=Ti, Ge, Ru, Zr, Sn; Ni=Al, Sc, Cr, Ga, Ru, In) and Sr_xM(IV)_4−2xN(III)_2xO₈ (M=Ti, Ge, Ru; N=Al, Cr, Ga) and (A, Ba)_xTi_yAl_zO₈ with A=Rb, Cs, Ca, Sr have been studied by electron and X-ray diffraction and high resolution electron microscopy. They have been found to be stable only within certain ranges of x, depending on the M and N ions. The lower value of x is never less than 0.56, the upper x level is about 0.73. Higher x are correlated with larger radii of M and N. The variation in x gives rise to an incommensurate occupation by the A ions in the tunnels. Correlation among tunnel sequences varies widely, also depending on the nature of the M and N ions. The lower and upper x, the cell dimensions, the correlation between the tunnels and the stability in water at 300°C of the existing hollandites are given. It was found that Rb, Cs and Sr can only be incorporated in the hollandite phase if Ba is not present in excess. Ca cannot be incorporated in the hollandite structure.     

New rutile solid solutions,Ti1−4xLixM3xO2: M = Nb,Ta, Sb

Three extensive new rutile solid solution series have been prepared in which Ti⁴⁺ is replaced by a combination of Li⁺ and a pentavalent cation: Nb⁵⁺, Ta⁵⁺, Sb⁵⁺. The formulae are Ti_1?4xLi_xM_3xO₂: 0 < x ? 0.15, M = Ta; 0 < x ? 0.17, M = Nb; 0 < x ? 0.12, M = Sb. The solid solutions were characterised by X-ray powder diffraction and density measurements. In addition to the rutile solid solutions, LiNb₃O₈ forms a limited range of solid solutions, Li_1?yNb_3?3yTi_4yO₈: 0 < y ? 0.06.     

Study of the oxidation kinetics of finely-divided magnetites. II - Influence of chromium substitution

The oxidation kinetics of magnetites substituted by chromium (Fe²⁺Fe³⁺_2?xCr³⁺_x)O^2?₄ (0 < x < 2) into γ(Fe³⁺_1?yCr³⁺_y)₂O^2?₃, (x = 3y), which is a metastable phase, was found out to be ruled by the law of diffusion, under variable working conditions, of vacancies generated at the surface. The chemical diffusion coefficient is a function of the substitution ratio, crystallite size and the number of vacancies in the spinel lattice. Contrary to magnetites substituted by aluminum, the activation energy varies irregularly with the substitution ratio.     

Etude au microscope electronique des phenomenes de transition de phases dans des monocristaux d'oxyde V2O3 pur et dope au chrome

An investigation by electron microscopy confirms the low temperature transition in single crystal of V₂O₃ and V_2?xCr_xO₃ (x = 0,048). The twin planes were identified in the corundun structure as (1 1 2 0)_M. An explanation is proposed.     

Conductivite ionique dans les systems Na1+xZr2−xLx(PO4)3 (L = Cr,Yb)

Ionic conductivity measurements in the solid solution Na_1+xZr_2?xL_x(PO₄)₃ (L = Cr, Yb) have been carried out. The materials have a Nasicon-type structure in a 0 ? x ? x^max._L range (x^max._Cr = 2.0 and x^max._Yb = 1.9). A small monoclinic distortion appears at low temperature for Na₃Cr₂(PO₄)₃. As in the Na_1+xZr₂P_3?xSi_xO₁₂ system a strong increase of the conductivity with rising x has been observed. The results are discussed in connection with temperature and structural parameters.     

Intercalation properties and electrochemical reduction of chromium oxide chloride

Intercalation complexes M_x(DMSO)_y[Cr_1?x⁺³ Cr_x⁺²OCl] have been obtained by cathodic reduction of chromium oxide chloride in a solution of M⁺ (Li⁺, Na⁺, K⁺) in dimethyl-sulfoxide (DMSO). From galvanostatic experiments upper limits for x were determined to be x = 0.12 for Li⁺ and x = 0.05 for Na⁺ and K⁺. Correlations between the degree of reduction (x), ionic radii of M⁺, amount of solvent intercalation (y), lattice expansion and M⁺-coordination are discussed.Chromium oxide chloride samples with either chloride deficiency or Cl^? by O^2? replacement showed a degree of reduction of only x = 0.05 upon Li⁺-intercalation.     

Preparation and electronic properties of ceramic (V1−xCrx)2O3

Compacts of ceramic (V_1?xCr_x)₂O₃ for 0.006 ? x ? 0.013 were made by sintering material prepared by a co-precipitation method. Low temperature resistivity measurements were in general agreement with the single crystal results of McWhan and Remeika (1), but the paramagnetic insulator to metal transition was not sharp. High pressure Hall measurements through the ‘Mott’ transition have been measured for the first time. The Hall constant remained the same in sign and magnitude on both sides of the transition.     

Heat capacity of Al-doped V2O3 alloys

Heat capacities of single crystal (Al_xV_1?x)₂O₃ alloys in the composition range 0 < x ≤ 0.10 have been measured in the temperature range 100–740 K. Each of the compositions exhibited a sharp heat capacity maximum lying in the range 170–190 K. With increasing temperature, the transition temperature increases from 171 K (x = 0.0033) to 187 K for x = 0.033 and then steadily decreases to 148 K for x = 0.10. The value of the enthalpy change (δH) increases from 570 cal./mole Al-V₂O₃ for x = 0.0033 to 650 cal./mole Al-V₂O₃ for x = 0.01 and then decreases steadily to 250 cal./mole Al-V₂O₃ for x = 0.10. These transitions have been correlated with the electrical (metal → insulator) transformations occurring in these alloys. The 1 at. % Al-doped V₂O₃ alloys revealed an additional heat capacity anomaly extending over the range 325–370 K. This is identified with the anomalous electrical behavior in this temperature region. The associated δH value was found to be only 30 cal./mole Al-V₂O₃. Analysis of the calorimetric data reveals that the temperature-composition phase diagram for the Al-doped V₂O₃ system is similar to that constructed by McWhan and Remeika (Phys. Rev. $\text{B2}$, 3734, 1970) for the Cr-doped V₂O₃ system.     

Phase Composition, Electrical Conductivity, and Stability of ZrO2–Sc2O3–Cr2O3 Solid Electrolytes

The phase composition, electrical conductivity, and structural and electrical stability of ZrO₂–Sc₂O₃–Cr₂O₃ solid electrolytes prepared by solid-state reactions involving three-step firing at 1350, 1850 (vacuum), and 1300°C were studied for compositions along two lines: x(0.91ZrO₂ + 0.09Sc₂O₃)–yCr₂O₃ (I) andx(0.89ZrO₂ + 0.11Sc₂O₃)–yCr₂O₃ (II), x + y = 1, y = 0–0.04. The results indicate that the ternary solid solutions withy= 0.01–0.02 retain a cubic structure in a broad temperature range, down to room temperature. This increases the low-temperature (<600°C) conductivity of the solid electrolytes, especially in system II. In both systems, Cr₂O₃ solubility is about 3 mol %. Stability tests at 900°C for 200 h reduce the conductivity of the solid electrolytes, particularly at the lower Sc₂O₃ content and in the presence of Cr₂O₃. The reduction in conductivity is due to the decomposition of the high-temperature tetragonal phase and the formation of a tetragonal phase with a low stabilizer content.     

Phase diagram of FexV1−xO2 in the 0≤x≤0.25 region

The phase diagram of the Fe_xV_1?xO₂ system (0≤x≤0.25) has been studied in detail by means of X-ray diffraction and differential scanning calorimetry (DSC). As the result, there appear succesively six phases, M₁→T→M₂→M₄→O→X (the same notation as the Cr_xV_1?xO₂ system. O; orthorhombic, X; unknown) with increasing x. Change of the phase transformation temperature Tt and the heat of transformation ΔH with the impurity concentration x have been determined.     

Le systeme V1−xFexO2: Proprietes structurales et magnetiques

The system V_1?xFe_xO₂ (x ? 0.020) was investigated by X-ray diffraction, DTA, magnetic susceptibility measurements and Mössbauer spectroscopy. The phase diagram and the magnetic properties are similar to those of the V_1?xCr_xO₂ and V_1?xAl_xO₂ systems previously studied. In oxygen deficient materials appears a Magnéli type shear phase, the Fe³⁺ ions occupying the shear planes.     

Some thermodynamic aspects of the high-temperature transition in doped V2O3

Measurements of the entropy change are reported for the high-temperature metal-insulator (MI) transitions in the (V_1–xCr_x)₂O₃ and (V_1–xAl_x)₂O₃ systems. It is emphasized that the entropy of the I phase exceeds that of the M phase. Evidence is presented to show that the M and I phases coexist over a narrow temperature range. The transformation is attended by enormous hysteresis effects; these indicate that the lattice plays an important role in the transition. The probable role of Cr³⁺ and Al³⁺ as a dopant in the V₂O₃ lattice is briefly discussed. A phase diagram for the dilute V₂O₃-Al₂O₃ alloy system is presented.     

Über die systeme BaRu1−xMxO3−y (M = Rh,Ir, Mn,Fe, Co,Ni)

In the systems BaRu_1?xM_xO_3?y (M = Rh, Ir, Mn, Fe, Co, Ni) several perovskite stacking polytypes are present. The influence of the crystal structure and Ru/M-ratio on the electrical conductivity and catalytic activity is investigated.     

The new pyrochlores Pb2(M0 · 5Sb1 · 5)O6 · 5 (M = Al,Sc, Cr,Fe, Ga,Rh)

From mixtures of PbO, M₂O₃ (M = Al, Sc, Cr, Fe, Ga, Rh), and Sb₂O₃, the new oxides Pb₂(M_{0 · 5}Sb_{1 · 5})O_{6 · 5} have been obtained as coloured powders giving X-ray diffraction patterns typical of cubic pyrochlores, S.G. Fd3?m (No. 227), and $\text{a}\text{A}\text{?}$ values from 10.3964(1) (M = Al) to 10.5558(1) (M = Sc). The best R factors were obtained for Pb in 16(c) positions, M and Sb (1:3) randomly distributed in 16(d), oxygen in 48(f) and one half of the 8(a) sites, and $\text{x}$ values for the oxygen positional parameter (origin at center, 3?m) from 0.430 (M = Sc) to 0.436 (M = Al). Apparent interatomic distances compare with those calculated. The angles of the coordination polyhedra around the metals are reported for the compounds of Al and Sc. Seven-coordination of Pb(II) is accounted for by the non-bonded electron pair effect.     

Sauerstoff - spinelle mit ruthenium und iridium

In the systems Co/1bRu/1bO and Co/1bIr/1bO cubic spinel type phases of composition Co_3?xRu_xO₄ (～0, 4≤ x ≤1) and Co_2,5Ir_0,5O₄ have been synthesized. The cobalt can be partially substituted by iron and manganese (Co_2,3?yM_yRu_0,7O₄; 0≤y～0,7; M = Mn, Fe) or copper. By vibrational spectroscopic investigations a transition from low to high concentration of free electrons can be detected.     

Structure and electrical conductivity of (La0.9Sr0.1)[(Ga1 − x Crx)0.8Mg0.2]O3 − δ (x = 0–0.35) solid solutions

(La_0.9Sr_0.1)[(Ga_{1 ? x} Cr_x)_0.8Mg_0.2]O_{3 ? δ} (x = 0–0.35) perovskite-like solid solutions have been prepared by ceramic processing route, and their structural parameters, microstructure, and electrical conductivity have been determined. The results indicate that the cation substitutions reduce the unit-cell volume of the solid solutions and raise their total conductivity via an increase in electronic conductivity, while their ionic conductivity seems to decrease with increasing x.     

Etude de la cinetique d'oxydation de magnetites finement divisees. I - Influence de la substitution par l'aluminium

The kinetics of finely divided, (Fe²⁺Fe³⁺_2?xAl³⁺_x)O₄ type (0 ? x ? 2), substitution magnetites oxidation into metastable phase γ(Fe³⁺_1?yAl³⁺_y)₂O₃ (x = 3y) has been accounted for by means of the law of diffusion, under variable working conditions, of the cationic vacancies generated at solid-gas interface with a diffusion coefficient that decreases when the stoichiometric difference is high. The crystallites size and the trivalent substituant percentage increases, in both cases, slow down the reaction rate.     

New oxide pyrochlores: A2[B2−xAx]O7−y (A = Pb,Bi; B = Ru,Ir)

A new series of mixed metal oxides with the pyrochlore structure has been discovered and can be described by the general formula A₂[B_2?xA_x]O_7?y where A = Pb or Bi, B = Ru or Ir, 0 < x ? 1 and 0 ≤ y ≤ 0.5. Values of x appreciably greater than zero may only be obtained at relatively low temperatures. It has been observed that when A = Pb and B = Ru, y = 0.5 for all values of x. Linear correlations between lattice parameter and x are presented for both Pb₂[Ru_2?xPb_x]O_6.5 and Bi₂[Ru_2?xBi_x]O_7?y.