Thermal expansion studies of substituted CTP derivatives

Several new Na, Y and Zr substituted derivatives of Ca0·5Ti2(PO4)3 (CTP) have been synthesized. These derivatives retain the hexagonal structure of the parent (CTP) compound with minor changes in lattice parameters. Linear thermal expansion coefficients(α) have been obtained using a high sensitivity dilatometer

Measurement of Gibbs energy of formation of Ca<SUB>2</SUB>PbO<SUB>4</SUB> using a solid-state cell with three electrodes

Phase relations in the system Ca–Pb–O at 1100 K have been determined by equilibrating 18 compositions in the ternary and identifying the phases present in quenched samples by X-ray diffraction and energy dispersive X-ray analysis (EDX). Only one ternary compound Ca2PbO4 was found to be present. The compound coexists with CaO and PbO. The intermetallic compounds Ca2Pb, Ca5Pb3 and CaPb and liquid alloys are in equilibrium with CaO. The standard Gibbs energies of formation of Ca2PbO4 (880–1100 K) and Pb3O4 (770–910 K) were determined using solid-state cells based on yttria-stabilized zirconia as the solid electrolyte. Pure oxygen gas at 0.1 MPa was used as the reference electrode. For measurements on Ca2PbO4, a novel cell design with three electrodes in series, separated by solid electrolyte membranes, was used to avoid polarization of the electrode containing three solid phases. Two three-phase electrodes were used. The first absorbs the electrochemical flux of oxygen from the reference electrode to the measuring electrode. The other three-phase electrode, which is unaffected by the oxygen flux through the solid electrolyte, is used for electromotive force (EMF) measurement. The results from EMF studies were cross-checked using thermogravimetry (TG) under controlled oxygen partial pressures. The stability of Pb3O4 was investigated using a conventional solid-state cell with RuO2 electrodes. The results can be summarized by the following equations: 2CaO+PbO+&#189;O2&#8594;Ca2PbO4 &#916;rG&#176;/J mol-1=(-128 340+93.21T /K)&#177;200 3PbO+&#189;O2&#8594;Pb3O4 &#916;G&#176;/J mol-1=(-70 060+77.5T /K)&#177;150

System Sr-Pb-O: phase equilibria and thermodynamics using solid-state cells with buffer electrodes

The isothermal section of the phase diagram for the system Sr-Pb-O at 1100 K is established by equilibrating 18 compositions in the ternary and analyzing quenched samples using optical and scanning electron microscopy, EDX and XRD. Two ternary oxides, Sr2PbO4 and SrPbO3, containing lead in the tetravalent state, are found to be stable. They coexist with PbO. Alloys and intermetallics are in equilibrium with SrO. The high-temperature thermodynamic properties of the ternary oxides are measured for the first time with a novel apparatus. Yttria-stabilized zirconia is used as the solid electrolyte and pure diatomic oxygen gas as the reference electrode. The new feature is the buffer electrode, which is introduced between the reference and measuring electrodes to absorb the electrochemical flux of oxygen through the solid electrolyte. This prevents the polarization of the measuring electrode and ensures accurate data. Measurements are made in the temperature range 860-1090 K. The standard Gibbs energies of formation of Sr2PbO4 and SrPbO3 from SrO, PbO, and O2 are obtained directly from the reversible cell emfs. The results can be represented by the equations: Sr2PbO4, &#916;Gof(ox)/ J mol-1 = -168 650 + 97.87 T/K (&#177;330), and SrPbO3, &#916;Gof(ox)/J mol-1 = -141 925 + 94.30 T/K (&#177;290)

Phase relations in the system Cu-La-O and thermodynamic properties of CuLaO₂ and CuLa₂O₄

Phase relations in the system Cu-La-O at 1200 K have been determined by equilibrating samples of different average composition at 1200 K, and phase analysis of quenched samples using optical microscopy, XRD, SEM and EDX. The equilibration experiments were conducted in evacuated ampoules, and under flowing inert gas and pure oxygen. There is only one stable binary oxide La₂O₃ along the binary La-O, and two oxides Cu₂O and CuO along the binary Cu-O. The Cu-La alloys were found to be in equilibrium with La₂O₃. Two ternary oxides CuLaO₂ and CuLa₂O_4+δ were found to be stable. The value of δ varies from close to zero at the dissociation partial pressure of oxygen to 0.12 at 0.1 MPa. The ternary oxide CuLaO₂, with copper in monovalent state, coexisted with Cu, Cu₂O, La₂O₃, and/or CuLa₂O_4+δ in different phase fields. The compound CuLa₂O_4+δ, with copper in divalent state, equilibrated with Cu₂O, CuO, CuLaO₂, La₂O₃, and/or O₂ gas under different conditions at 1200 K. Thermodynamic properties of the ternary oxides were determined using three solid-state cells based on yttria-stabilized zirconia as the electrolyte in the temperature range from 875 K to 1250 K. The cells essentially measure the oxygen chemical potential in the three-phase fields, Cu + La₂O₃ + CuLaO₂, Cu₂O + CuLaO₂ + CuLa₂O₄ and La₂O₃ + CuLaO₂ + CuLa₂O₄. Although measurements on two cells were sufficient for deriving thermodynamic properties of the two ternary oxides, the third cell was used for independent verification of the derived data. The Gibbs energy of formation of the ternary oxides from their component binary oxides can be represented as a function of temperature by the equations: 1/2Cu₂O+1/2La₂O₃(A rare-earth) CulaO₂ &#916;_f.oxG°/Jmol^-1=-4335+1.32 T/K(&#177;45) CuO+La₂O₃(A rare-earth) CuLa₂O₄ &#916;_f.oxG°/Jmol^-1=-19600-4.01 T/K(&#177;240

Trends in the stability of ternary oxides: systems M-Pb-O (M = Ca, Sr, Ba)

Recent experimental investigations of phase equilibria and thermodynamic properties of the systems M-Pb-O, where M = Ca, Sr or Ba, indicate a regular increase in thermodynamic stability of ternary oxides, MPbO₃ and M₂PbO₄, with increasing basicity of the oxide of the alkaline-earth metal. Number of stable interoxide compounds at 1100 K in the systems M-Pb-O (M = Mg, Ca, Sr, Ba) increases in unit increments from Mg to Ba. In this paper, experimentally determined standard Gibbs energies of formation of M₂PbO₄ (M = Ca, Sr, Ba) and MPbO₃ (M = Sr, Ba) from their component binary monoxides and oxygen gas are combined with an estimated value for CaPbO₃ to delineate systematic trends in thermodynamic stability of the ternary oxides. The trends are interpreted using concepts of tolerance factor and acid-base interactions. All the ternary oxides in these systems contain lead in the tetravalent state. The small Pb⁴⁺ ions polarize the surrounding oxygen ions and cause the formation of oxyanions which are acidic in character. Hence, the higher oxidation state of lead is stabilized in the presence of basic oxides of alkaline-earth group. A schematic subsolidus temperature-composition phase diagram is presented for the system BaO-PbO-O-₂ to illustrate the change in oxidation states in binary and ternary oxides with temperature

Combined Use of oxide and fluoride solid electrolytes for the measurement of gibbs energy of formation of ternary oxides: system Bi-Ca-O

Phase equilibrium studies of the ternary system Bi-Ca-O at 1000 K and ambient pressure indicate that all the ternary oxides lie along the pseudo-binary line Bi2O3-CaO. Four ternary oxides, Bi2Ca2O5, Bi6Ca4O13, Bi2CaO4 and Bi14Ca5O26 and two phases of variable composition (δ and β) are identified. The chemical potential of CaO in two-phase fields of the pseudo-binary Bi2O3-CaO is determined as a function of temperature using solid state cells based on single crystal CaF2 as the electrolyte. The chemical potential of Bi2O3 in the two-phase fields is measured using cells incorporating (Y2O3)ZrO2 as the solid electrolyte. The standard Gibbs free energy of formation of each ternary oxide from the binary oxides is calculated independently from the chemical potentials of CaO and Bi2O3 in two-phase fields on either side of the compound composition. The independent assessments agree closely; the maximum difference in the value of ΔGf° (Bi2mCanO3m+n)/(m+n) is 170 J/mol of the component binary oxides. The results are discussed in the light of the phase diagram and compared with calorimetric and free energy measurements reported in the literature. The combined use of emf data from cells incorporating fluoride and oxide electrolytes enhances the reliability of derived data. Free energies of formation of ternary oxides from component binary oxides are given by the following equations: Bi2Ca2O5:ΔG°f,ox(J/mol)=-43800+5.79T(±900) Bi6Ca4O13:ΔG°f,ox(J/mol)=-109800+7.04T(±2300) Bi2CaO4:ΔG°f,ox(J/mol)=-31910+2.31T(±650) Bi14Ca5O26:ΔG°f,ox(J/mol)=-184560+12.76T(±4320) Bi1.4Ca0.3O2.4 (β):ΔG°f,ox(J/mol)=-12290(±300) at 900 K Bi1.56Ca0.22O2.56 (β):ΔG°f,ox(J/mol)=-9890(±180) at 900 K

Stability of Cu₂Ln₂O₅ compounds: Comparison, assessment and systematics

Phase diagram studies show that at ambient pressure only one ternary oxide, Cu2Ln2O5, is stable in the ternary systems Cu-Ln-O (Ln = Tb, Dy, Ho, Er, Tm, Yb, Lu) at high temperatures. The crystal structure of Cu2Ln2O5 can be described as a zig-zag arrangement of one-dimensional Cu2O5 chains parallel to the a-axis with Ln atoms occupying distorted octahedral sites between these chains. Four sets of emf measurements on Gibbs energy of formation of Cu2Ln2O5 (Ln = Tb, Dy, Ho, Er, Tm, Yb, Lu; Y) from component binary oxides and one set of high-temperature solution calorimetric data on enthalpy of formation have been reported in the literature. Except for Cu2Y2O5, the measured values for the Gibbs energies of formation of all other Cu2Ln2O5 compounds fall in a narrow band (±1 kJ mol-1) and indicate a regular increase in stability with decreasing ionic radius of the lanthanide ion. The values for the second law enthalpy of formation, derived from the temperature dependence of emf obtained in different studies, show larger differences, as high as 25 kJ mol-1 for Cu2Tm2O5. Though associated with an uncertainty of ±4 kJ mol-1, the calorimetric measurements help to identify the best set of emf data. The trends in thermodynamic data correlate well with the global instability index (GII) based on the overall deviation from the valence sum rule. Low values for the index calculated from crystallographic information indicate higher stability. Higher values are indicative of the larger stress in the structure

Phase relations in the system Cu-Eu-O and thermodynamic properties of CuEu₂O₄ and CuEuO₂

Phase relations in the system Cu-Eu-O have been determined by equilibrating samples of different average composition at 1200 K and by phase analysis after quenching using optical microscopy (OM), x-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive x-ray (EDX). The equilibration experiments were conducted in evacuated ampoules and under flowing inert gas and pure oxygen. The Cu-Eu alloys were found to be in equilibrium with EuO. The higher oxides of europium, Eu₃O₄ and Eu₂O₃, coexist with metallic copper. Two ternary oxides CuEu₂O₄ and CuEuO₂ were found to be stable. The ternary oxide CuEuO₂, with copper in the monovalent state, can coexist with Cu, Cu₂O, Eu₂O₃ and CuEu₂O₄ in different phase fields. The compound CuEu₂O₄ can be in equilibrium with Cu₂O, CuO, CuEuO₂, Eu₂O₃, and O₂ gas under different conditions at 1200 K. Thermodynamic properties of the ternary oxides were determined using three solid-state cells based on yttria-stabilized zirconia as the electrolyte in the temperature range from 875 to 1250 K. The cells essentially measure the oxygen chemical potential in the three-phase fields: Cu+Eu₂O₃+CuEuO₂, Cu₂O+CuEuO₂+CuEu₂O₄, and Eu₂O₃+CuEuO₂+CuEu₂O₄. The thermodynamic properties of the ternary oxides can be represented by the equations: 1/2Cu₂O+1/2Eu₂O₃(C−rareearth)&#8594;CuEuO₂ Δ_f,oxG∘/Jmol⁻¹=−570−0.463T/K(±20) CuO+Eu₂O₃(C−rareearth)→CuEu₂O₄ Δ_f,oxG∘/Jmol⁻¹=−3530−5.96T/K(±110) Thermogravimetric analysis (TGA) studies in Ar+O₂ mixtures confirmed the results from emf measurements. An oxygen potential diagram for the system Cu-Eu-O at 1200 K was evaluated from the results of this study and information available in the literature on the binary phases