N.I. Matskevich

Heat capacities and thermodynamic functions of BaCuO2 in the temperature range 8–305 K

Abstract The heat capacity of two samples of BaCuO 2 was measured in a vacuum adiabatic calorimeter in the temperature range 8–305 K. The thermodynamic functions of barium cuprate (entropy, enthalpy, Gibbs free energy) were calculated on the basis of C p - T dependences. Both samples exhibited an anomalous rise in C p at low temperatures (below 12 K). Our data are compared with previously measured data.

Thermochemistry of Gd2BaCuO5 and LuBa2Cu3Ox.

Solution calorimetry, using 6.0 M HCl as a solvent, is used to study the thermochemistry of Gd2BaCuO5 and the high-temperature superconductor LuBa2Cu3O6.92. For the first time, the standard formation enthalpies of these phases have been determined as follows: DeltafH(o)(Gd2BaCuO 5, s, 298.15 K) = -2618.6 +/- 7.4 kJ/mol; DeltafH(o)(LuBa2Cu3O6.92, s, 298.15 K) = -2693.1 +/- 11.9 kJ/mol. The thermodynamic stability at room temperature has been assessed. The results show that Gd211 and Lu123 are thermodynamically stable with respect to binary oxides and unstable with respect to interaction with CO 2 at ambient temperatures. Lu123 is thermodynamically stable with respect to assemblages containing combinations of Lu2O3, CuO, and BaCuO2 and thermodynamically unstable with respect to interactions with water.

The thermodynamics of some reactions in the GdBaCuO system

Abstract In this study, enthalpies of solution for Gd 2 O 3 , CuO, BaCO 3 and GdBa 2 Cu 3 O 6.9 phases have been measured by solution calorimetry in 6N HCl at 323 K. Experimental and literature data have been used to calculate the enthalpy of the reaction 0.5Gd 2 O 3 + CuO + 2BaCuO 2 + 0.2O 2 = GdBa 2 Cu 3 O 6.9 . Δ r H was found to be −37.3 ± 8.2 kJ mol −1 . Based on these data, we came to the conclusion that the GdBa 2 Cu 3 O 6.9 phase appeared to be thermodynamically more favourable than the mixture of gadolinium oxide, copper oxide, and barium cuprate at room and lower temperatures. The experimental and literature data have allowed us to conclude that the GdBa 2 Cu 3 O 6.9 phase is thermodynamically stable, whereas the YBa 2 Cu 3 O 6.9 phase appears to be thermodynamically unstable at room and lower temperatures.

Phase Transitions in YBa2Cu3Ox AT 300–900 K: Thermochemical and Structural Aspects

The thermochemical and structural aspects of the occurrence of two anomalies in the temperature dependence of the specific heat of YBa2Cu3O6.91 are considered. The anomalies were found at T = 300–500 K and T > 600 K. The specific heat was measured by mixing calorimetry in the temperature range 300–900 K. Analytical expressions for the temperature dependences of enthalpy and specific heat were obtained. The observed anomalies were confirmed by structural data and measurements of the temperature dependence of the linear‐expansion coefficient. The appearance of the anomalies is explained by the presence of apical oxygen.

Formation enthalpy and thermodynamic stability of 247 compound in the YBaCuO system

Abstract Solution calorimetry (6 N HCl solvent, 323 K) was used to obtain the enthalpies of certain reactions including the 247 substance (Y 2 Ba 4 Cu 7 O 15 ) on the basis of solution enthalpies of Y 2 O 3 , BaCO 3 , CuO, Y 2 BaCuO 5 and Y 2 Ba 4 Cu 7 O 15 . It was established that Y 2 Ba 4 Cu 7 O 15 was thermodynamically more favourable than Y 2 O 3 + 4BaCuO 2 + 3CuO, Y 2 BaCuO 5 + 3BaCuO 2 + 3CuO mixtures at room and lower temperatures, whereas, in accordance with our earlier papers, the 123 phase in the YBaCuO system was metastable. This work continues our research on thermodynamic stability of superconducting phases in the Y(Gd, Ho)BaCuO systems.

Thermodynamic study of compounds of the Nd-Ba-Cu-O system

Standard enthalpies of formation for solid solutions of composition Nd1 + xBa2 − xCu3Oy (x = 0–0.8, y = 6.65–7.24) from oxides were determined by solution calorimetry. The heat capacity of NdBa2Cu3O6.87 phase was measured in the range 5–320 K by low-temperature adiabatic calorimetry. The absolute entropy So(T), the difference of enthalpies Ho(T)-Ho(0 K), and the reduced Gibbs energy Φo(T) = So(T)–[Ho(T)–Ho(0)]/T were calculated on the basis of smoothed dependence Cp(T) in the 0–320 K range. An assessment was made for the heat capacities and the absolute entropies of solid solutions Nd1+xBa2−xCu3Oy. The obtained set of thermodynamic parameters can be used for the calculation of phase equilibria in the Nd-Ba-Cu-O system.

Size effect in rare earth stabilization of Bi2BaLnCuO6+δ family

Abstract Using the solution calorimetry technique, the formation enthalpies for the Bi 2 MLaCuO 6.5 (M  Ba, Ba 0.5 Sr 0.5 and Sr) phases at 323 K were obtained. The values are equal to 2201, 2217 and 2227 kJ mol −1 for M  Ba, Ba 0.5 Sr 0.5 and Sr respectively. On the basis of these data and an estimation of the lattice energies for perovskite-like BaBi 0.5 Ln 0.5 O 3 phases, a simulation of the reaction Bi 2 BaLnCuO 6.5 =BaBi 0.5 Ln 0.5 O 2.5 +CuO+Bi 1.5 Ln 0.5 O 3 for Ln  La, Pr, Nd, Sm, Eu, Gd, Dy and Y was performed. The steeper dependence of the lattice energies of perovskite-like phases on the rare earth metal radii relative to the BaBi 0.5 Ln 0.5 O 3 phases determines the possibility of the formation of 2201-type structures for different rare earth elements in the BiBaLnCuO system near the 2:1:1:1 composition.