The thermodynamic properties of Ln2BaO4 (Ln = Sm, Dy, Ho)

The thermodynamic properties of the Ln₂BaO₄ phases (Ln = Dy, Ho, Sm) were studied by the electromotive force method with a fluoride electrolyte (890–1180 K), solution calorimetry in 1.07 N hydrochloric acid at 298.15 K, and differential scanning calorimetry (298–860 K). The experimental data were jointly processed, and the thermodynamic functions of the compounds over the temperature range 298–1200 K were calculated.     

Crystal structure of the pyrovanadate K4V2O7

The crystal structure of anhydrous K₄V₂O₇ (I) is determined by powder X-ray diffraction. The compound crystallizes in the monoclinic system (a = 10.222(1) Å, b = 6.2309(8) Å, c = 7.282(1) Å, β = 101.31(1)°, space group C2/m, Z = 2). The structure contains layers of isolated V₂O₇ pyrovanadate groups separated by layers of potassium cations. The hydration and dehydration of I are studied by thermal analysis and high-temperature X-ray diffraction. The dehydration is accompanied by decomposition of the starting crystal hydrate to give intermediate compounds. Anhydrous compound I undergoes a reversible phase transition at 740°C. The high-temperature phase is assumed to have a hexagonal unit cell (a = 6.169(4) Å, c = 15.72(1) Å, Z = 2).     

Inducing superconductivity and structural transformations by fluorination of reduced YBCO

Bulk superconductivity with T_c up to 94 K has been induced by fluorination of non-superconducting YBa₂Cu₃O_6.11 using XeF₂ as a fluorination agent. Strong changes on X-ray patterns were found after fluorination of reduced YBCO. High resolution electron microscopy of superconducting samples showed the presence of a new phase with c ≈ 13 Å which exists as noticeable areas included within a matrix of the 123 structure or which occurs as isolated defects with a limited extension. All fluorinated compounds exhibited a strong disorder along the c-direction resulting in a ‘loss’ of c-parameter on X-ray patterns. The present results demonstrate that fluorine indeed enters the YBCO structure with a significant structural rearrangement for the high level of fluorination. The structure of the fully fluorinated YBa₂Cu₃O₆F₂ phase, possibly responsible for superconductivity, has been deduced from high resolution electron microscopy.     

Phase diagram and thermodynamic properties of compounds in the SrO-SrF2-Mn2O3-O2 system

Temperature dependences of the Gibbs energy of the formation of strontium manganite (SrMn₃O_{6 ? δ}, SrMnO₃, Sr₂MnO₄) and fluorine manganite (Sr_2.5Mn₆O_{12.5 ? δ}F₂) from simple oxides, oxygen, and SrF₂, respectively, are determined by EMF with fluorine- and oxygen-ion electrolytes in the 1000–1300 K range of temperatures. Phase relations in the SrO-SrF₂-Mn₂O₃-O₂ system at p(O₂) = 1 atm are calculated on the basis of the obtained data.     

Thermodynamic properties of nonsuperconducting Ln2CuO4 (Ln = Nd, Sm, Eu), Ho2Cu2O5 and Ln2BaCuO5 (Ln = Nd, Sm, Eu, Ho, Yb) cuprates

Thermodynamic properties of rare-earth Ln₂CuO₄ (Ln = Nd, Sm, Eu), Ho₂Cu₂O₅ and Ln₂BaCuO₅ (Ln = Nd, Sm, Eu, Ho, Yb) cuprates were determined using the electromotive force method with a fluorine-ion electrolyte in the temperature interval of 1000–1230 K. The thermodynamic data obtained by different experimental methods were compared.     

The thermodynamic properties of yttrium and dysprosium manganites

The temperature dependences of the Gibbs energies of formation of RMnO₃ (R = Y, Dy) from the coexisting phases R₂O₃ + 2RMn₂O₅ = 4RMnO₃ + 0.5O₂ were determined by the electromotive force (EMF) method with a fluorine ion conducting electrolyte. The temperature dependences of log P (O₂) for the dissociation reactions RMn₂O₅ ? RMnO₃ + (1/3)Mn₃O₄ + (1/3)O₂ and Mn₂O₃ ? (2/3)Mn₃O₄ + (1/6)O₂ were obtained by jointly applying the methods of heterogeneous equilibria and EMF with an oxygen ion conducting electrolyte. The results were used to calculate the standard thermodynamic functions of formation of yttrium and dysprosium manganites from simple oxides.