Tom Mathews

Phase relations in the system BiO1.5–SrO–CuO at 1123 K

Phase relations in the system Bi–Sr–Cu–O at 1123 K have been investigated using optical microscopy, electron-probe microanalysis (EPMA) and powder X-ray diffraction (XRD) of equilibrated samples. Differential thermal analysis (DTA) was used to confirm liquid formation for compositions rich in BiO1.5. Compositions along the three pseudo-binary sections and inside the pseudo-ternary triangle have been examined. The attainment of equilibrium was facilitated by the use of freshly prepared SrO as the starting material. The loss of Bi2O3 from the sample was minimized by double encapsulation. A complete phase diagram at 1123 K is presented. It differs significantly from versions of the phase diagram published recently.

Phase relations in the system Cu-Gd-O

The phase relations in the system Cu-Gd-O have been determined at 1273 K by X-ray diffrac- tion, optical microscopy, and electron microprobe analysis of samples equilibrated in quartz ampules and in pure oxygen. Only one ternary compound, CuGd2O4, was found to be stable. The Gibbs free energy of formation of this compound has been measured using the solid-state cell Pt, Cu2O + CuGd2O4 + Gd2O3 // (Y2O3) ZrO2 // CuO + Cu2O, Pt in the temperature range of 900 to 1350 K. For the formation of CuGd2O4 from its binary component oxides, CuO (s) + Gd2O3 (s) → CuGd2O4 (s) ΔG° = 8230 - 11.2T (±50) J mol-1 Since the formation is endothermic, CuGd2O4 becomes thermodynamically unstable with respect to CuO and Gd2O3 below 735 K. When the oxygen partial pressure over CuGd2O4 is lowered, it decomposes according to the reaction 4CuGd2O4 (s) → 4Gd2O3 (s) + 2Cu2O (s) + O2 (g) for which the equilibrium oxygen potential is given by Δμo2 = −227,970 + 143.2T (±500) J mol−1 An oxygen potential diagram for the system Cu-Gd-O at 1273 K is presented.

Seebeck Coefficient of Magnetite: A Reinterpretation Invoking Jahn-Teller Entropy

The Seebeck coefficient of stoichiometric

Phase relations in the systems Cu–O–R2O3(R = Tm, Lu) and Gibbs energies of formation of Cu2R2O5 compounds

Fe_3O_4

Equilibrium oxygen potential for the decomposition of YBa2Cu4O8

has been measured at high temperature in a controlled

High temperature seebeck measurements on YBa2Cu3O7-δ

CO + CO_2

Decomposition temperatures of Cu 2 Ln 2 O 5 (Ln = Tb, Dy, Ho, Er, Tm, Yb, and Lu) compounds

gas atmosphere. Assuming small polaron transport via octahedral sites, the cation distribution between the tetrahedral and octahedral sites of the spinel is obtained as a function of temperature from the Seebeck coefficient. The temperature dependence of cation distribution is explained by a new model in which the entropy arising from randomization of the Jahn-Teller distortion is incorporated. It is unnecessary to postulate that the enthalpy change for the cation interchange reaction is a linear function of the cation disorder parameter.

Variation of the partial thermodynamic

The phase relations in the systems Cu–O–R2O3(R = Tm, Lu) have been determined at 1273 K by X-ray diffraction, optical microscopy and electron probe microanalysis of samples equilibrated in evacuated quartz ampules and in pure oxygen. Only ternary compounds of the type Cu2R2O5 were found to be stable. The standard Gibbs energies of formation of the compounds have been measured using solid-state galvanic cells of the type, Pt|Cu2O + Cu2R2O5+ R2O3‖(Y2O3)ZrO2‖CuO + Cu2O‖Pt in the temperature range 950–1325 K. The standard Gibbs energy changes associated with the formation of Cu2R2O5 compounds from their binary component oxides are: 2CuO(s)+ Tm2O3(s)→Cu2Tm2O5(s), ΔG°=(10400 – 14.0 T/K)± 100 J mol–1, 2CuO(s)+ Lu2O3(s)→Cu2Lu2O5(s), ΔG°=(10210 – 14.4 T/K)± 100 J mol–1 Since the formation is endothermic, the compounds become thermodynamically unstable with respect to component oxides at low temperatures, Cu2Tm2O5 below 743 K and Cu2Lu2O5 below 709 K. When the chemical potential of oxygen over the Cu2R2O5 compounds is lowered, they decompose according to the reaction, 2Cu2R2O5(s)→2R2O3(s)+ 2Cu2O(s)+ O2(g) The equilibrium oxygen potential corresponding to this reaction is obtained from the emf. Oxygen potential diagrams for the Cu–O–R2O3 systems at 1273 K are presented.

Electrical transport in magnesium aluminate

On lowering the chemical potential of diatomic oxygen, the compound YBa2Cu4O8 was found to decompose into a mixture of YBa2Cu3O6+x and CuO. The equilibrium oxygen potential corresponding to this decomposition has been measured from 870 to 1150 K using a solid‐state cell incorporating yttria‐stabilized zirconia as the solid electrolyte and pure oxygen at a pressure of 1.01×105 Pa as the reference electrode: Pt,u2009O’2‖YBa2Cu4O8+YBa2Cu3O6+x+CuO∥(Y2O3) ZrO2∥O2(1.01×105u2009Pa),u2009Pt. The low oxygen potential boundary for the stability of YBa2Cu4O8 can be represented by ΔμO2=−238u2009900+207.7 T(±800) Ju2009mol−1. At the standard pressure of oxygen (1.01×105 Pa) the decomposition occurs at 1150 K.

Potentiometric determination of the stability of BaCu2O2

The Seebeck coefficient (S) of YBa2Cu3O7-δ was measured in the temperature range 450 – 1200 K in air and in pure oxygen in order to derive information on charge carrier concentration. The orthorhombic to tetragonal phase transition manifests as maxima in the variation of (dS/dT) with temperature. Seebeck coefficient in air decreases beyond ∼ 1130K corresponding to a value of δ = 0.73.