Yu. G. Stenin

On the thermodynamic properties of germanium-iodide compounds

Abstract The syntheses of high purity GeI 2 and GeI 4 from the elements are described. The vapour pressure over solid germanium (II) iodide has been measured by the static method with a membrane-gauge manometer from T =550 K to T =650 K and the molar enthalpy and entropy of sublimation calculated by second-law and third-law methods. Enthalpy differences for solid and liquid GeI 2 were measured in the temperature interval (330 to 770) K by drop calorimetry. The temperature, enthalpy and entropy of melting of germanium (II) iodide were determined. The values of the standard molar thermodynamic properties for solid and liquid GeI 2 are presented from T =298.15 K to T =781 K. We have used the maximum likelihood method in the treatment of the data obtained together with previous reported values. On the basis of this work we recommend a set of standard enthalpies of formation and absolute entropies for the Ge–I compounds.

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.

Extrapure WO3 for the preparation of CdWO4 single crystals

A process has been developed for the preparation of extrapure WO suitable for crystal growth. The method involves thermal decomposition of ammonium paratungstate (APT) powder at 230–250° C, resulting in the formation of soluble ammonium metatungstate (AMT). The AMT solution is then purified and hydrolyzed. The resultant purified APT is converted to WO3. The reaction yield is at a level of 95%. The contents of metallic impurities, especially those of polyvalent impurities (Fe, Mn, Cu, Pb, V, Cr, and others), capable of influencing the optical properties of crystals, was comparable to their detection limit: 10−6 to 10−5 wt %. The purification process was tested using a pilot-scale system. The purity was confirmed by the synthesis of high-quality CdWO4 crystals.

A calorimetric study of germanium dibromide

The temperature dependence of the heat capacity of germanium dibromide was studied over the temperature range 8.3–314 K by adiabatic calorimetry. The enthalpy of formation of solid germanium dibromide was determined by solution calorimetry. Scanning calorimetry was used to obtain the thermodynamic characteristics of fusion of GeBr2. The thermodynamic characteristics of the Ge-Br system studied by us earlier were used to consistently calculate the consistent standard enthalpies of formation and absolute entropies of germanium bromides in the solid, liquid, and gaseous states.

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.

The thermodynamic properties of talyl-and phenylsilanes

The enthalpies of fusion of C6H5SiH3, C6F5Si(CH3)3, and (C6H5)4Si were obtained in scanning calorimetry measurements. Pressure over the condensed talylsilane 4-CF3C6F4SiCH3, phenylsilane C6H5SiH3, and pentafluorophenylsilane C6F5SiCH3, (C6F5)2Si(CH3)2, and (C6F5)4Si phases was measured by the static method with the use of membrane null manometers. Equations approximating the dependences of saturated vapor pressures on temperature and the enthalpy and entropy of vaporization were obtained.

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.