Yu. Ya. Skolis

Thermal analysis of mercury superconductor HgBa2CuO4 + x and its precursor Ba2CuO3 + y☆

The superconducting compound HgBa2CuO4 + x and its precursor Ba2CuO3 + y have been studied in an oxygen atmosphere by TG-DTA in the temperature interval 20–1100°C. It was supposed that the succession of chemical processes taking place during the heating (20° min−1, 1 atm O2) of HgBa2Cu1.05O4.12 is as follows: It was supposed that the Ba2CuO3.23 compound during heating (1° min−1, 0.21 atm O2) and cooling (20° min−1, 0.21 atm O2) undergoes following stages:

Thermodynamic properties and oxygen stoichiometry of Ba2Cu3O5 + δ

The thermodynamic properties and oxygen stoichiometry of Ba2Cu3O5 + δ are studied by means of the electromotive force (EMF) with a fluoride electrolyte, dissolution calorimetry, and thermogravimetry. It is shown that the temperature dependence of the Gibbs energy of the formation of barium cuprate from simple oxides and oxygen in the temperature range of 860–1120 K can be described by the polynomial Δf, oxG∘(Ba2Cu3O5 + δ) ± 0.1 (kJ/mol) = −291.78 + 1.127T − 0.13207 TlnT (kJ/mol).

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.

The synthesis and thermodynamic properties of strontium fluoromanganite Sr2.5Mn6O12.5 − δF2

The existence of the [SrF0.8O0.1]2.5[Mn6O12] = Sr2.5Mn6O12.5 − δF2 compound was established in the SrO-Mn2O3-SrF2 system at 900°C and p(O2) = 1 atm. The crystal structure of strontium fluoromanganite was determined from the X-ray powder diffraction data, electron diffraction, and high-resolution electron microscopy. It can be described in the monoclynic system with four Miller hklm indices: hklm: H = ha* + kb* + lc1* + mq1, q1, q1 = c2* = γc1*, γ ≈ 0.632, a ≈ a ≈ 9.72 Å, b ≈ 9.55 Å, c1 ≈ 2.84 Å, c2 ≈ 4.49 Å, monoclinic angle γ ≈ 95.6°. The electromotive force method with a solid fluorine ion electrolyte was used to refine the composition of fluoromanganite and determine the thermodynamic functions of its formation from phases neighboring in the phase diagram (SrMn3O6, Mn2O3, SrF2, and oxygen), ΔG°, kJ/mol = −(111.7 ± 1.9) + (89.5 ± 1.5) × 10−3T.

Solid solution in the system BaCuO2CuOO2

The existence of an homogenity range of BaCu1 + yO2 + x in respect of copper was established by EMF measurement and DTA, and the nonstoichiometry index (xt) of oxygen versus temperature (t) was measured by TG in static dry oxygen. The values obtained for x and y were: 0≤y(±0.02)≤0.15 0.05 (1000°C) ≤ xt(pO2 = 1 atm, y = 0.15) ± 0.03 < 0.25 (450°C) significant attention was paid to phase and composition analysis of samples synthesized.

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 (SrMn3O6 − δ, SrMnO3, Sr2MnO4) and fluorine manganite (Sr2.5Mn6O12.5 − δF2) 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.