Arkadij Popovič

Vapour pressure and mixing thermodynamic properties of the KNbO3–NaNbO3 system

Equilibrium vapour pressures of sodium and potassium over a KxNa1−xNbO3 solid solution within its whole compositional range at temperatures between 1173 K and 1303 K were determined by Knudsen Effusion Mass Spectrometry. It should be noted that the thermodynamic equilibrium between the condensed and the vapour phase could be established only after prolonged annealing (more than 10 h at 1263 K). The equilibrium vapour pressure of potassium over K0.5Na0.5NbO3 (KNN) is a few times larger than that of sodium, i.e., 8 × 10−3 Pa as compared to 3 × 10−3 Pa at 1263 K. From the obtained results, the excess thermodynamic functions for the pseudo-binary KNbO3–NaNbO3 system were evaluated. The excess Gibbs energy was found to be positive, the excess enthalpy is close to zero, while the negative excess entropy indicates a partial ordering of alkaline ions in the solid solution. The comparison of the obtained results to the well-established lead-based piezoelectric systems revealed, that the vapour pressure of alkalis over the respective niobates at 1200 K is almost three orders of magnitude lower as compared to the values reported for lead oxide over Pb(Zr,Ti)O3.

MASS SPECTROMETRIC INVESTIGATION OF THE EVAPORATION PROPERTIES OF LEAD OXIDE

Partial pressures of molecular species over solid PbO were measured in the temperature interval between 890 K and 1100 K using Knudsen-cell mass spectrometry. The assignment of ions is discussed in detail. Structural and vibrational parameters of the gas phase for (PbnOn) (n = 1,2,3,4) were calculated by the ab initio molecular orbital method at different approximation levels. Third law enthalpies of dissociation of Pb2O2 and Pb4O4 to monomeric PbO were obtained as (250.6 ± 6) kJ/mol and (835.0 ± 15) kJ/mol, respectively. The possibility of direct measurement of the dissociation pressure of PbO by a mass spectrometer is demonstrated.

Knudsen cell mass spectrometric investigation of the PbO-ZrO2-TiO2 system

Knudsen effusion mass spectrometry (KEMS) was used for direct determination of lead oxide activity as a function of temperature in various regions of the PbO-ZrO(2)-TiO(2) system. From the results, the enthalpy, Gibbs free energy and entropy of formation of PbTiO(3) (PT), PbZrO(3) (PZ) and Pb(Zr,Ti)O(3) (PZT) were evaluated. In addition, the single phase widths of Pb(Zr(0.5)Ti(0.5))O(3) and PbTiO(3) perovskite structures were determined at 1100 K. The reaction rate of PZT synthesis in vacuo was followed by direct measurement of the change of PbO activity with time. Lead oxide activity in stoichiometric Pb(Zr(0.5)Ti(0.5))O(3), PbTiO(3) and Pb(0.968)(Zr(0.5)Ti(0.5))O(2.968) (3% lead deficient) at 850 degrees C was found to be 0.40, 0.45 and 0.1, respectively. PZT, PT and PZ powder samples prepared by a solid state procedure were also measured, all revealing lead deficiency. Copyright 1999 John Wiley & Sons, Ltd.

THE EVAPORATION THERMODYNAMICS OF LITHIUM IODIDE. MASS SPECTROMETRIC AND AB INITIO STUDIES

The vaporization of LiI was investigated in the range between 583 and 726 K by Knudsen effusion mass spectrometry (KEMS). The ionization or the appearance energies (IE or AE) of all kinds of ions formed from the equilibrium vapour over solid lithium iodide were determined using Vogt and Pascuals deconvolution method. For the determination of vapour pressure two methods, viz. mass-loss Knudsen effusion and Knudsen effusion mass spectrometry were applied. The mean natural logarithms of the equilibrium vapour pressures (in Pa) of monomer and all kinds of oligomers, that can be detected using KEMS, as a function of temperature, can be expressed as follows: The molecular structure and the harmonic vibrational frequencies of (LiI)n species (n = 1,2,3,4) were predicted using ab initio molecular orbital methods. Both the bond dissociation energies and the enthalpy changes of dissociation of (LiI)n oligomers were evaluated and compared with the measured enthalpy change data. Using the calculated geometry and the vibration frequencies, the thermodynamic functions of (LiI)n could be calculated, making it possible to compare the second and third law thermodynamic data.

Mass spectrometric studies of tungsten bromides and oxybromides

Abstract The reaction products obtained by the bromination of tungsten oxides, metallic tungsten and tungsten hexacarbonyl were studied by means of the Knudsen effusion mass spectrometric method. The fragmentations of WO 2 Br 2 , WBr 4 , WOBr 4 , WBr 4 and W 2 Br 6 were determined, the appearance potentials of ions present in higher intensities were measured and the heats of formation of these ions calculated. The sublimation enthalpies of the compounds were determined. During vaporization of WBr 4 , an intense W 2 Br 6 + ion was detected. For the formation of metal-metal bonded cluster compounds, the following reaction was proposed: 2WBr 4( g ) ⇌W 2 Br 6( g ) . The enthalpy change of this reaction and the heat of formation of W 2 Br 6( g ) were determined. According to vaporization experiments in vacuo , no WBr 6 exists in the gaseous phase. On the basis of the experiments, the following decomposition process seems to be probable: WBr 6( s ) → WBr 5( s ) + 1 2 Br 2( g ) . By measuring the change of Br 2 pressure as a function of temperature, the thermodynamic data for this reaction were calculated. From the ionisation potential measurements the average WBr bond strength was determined.

Mass spectrometric investigation of reaction between tungsten and bromine at high temperature

The reaction between tungsten and bromine has been studied in the temperature interval between 1000 and 1500 K by means of a Knudsen double cell connected to a mass spectrometer. At pressures of bromine between 6 and 50 torr in the reaction cell only WBr4(g) is formed while at higher pressures formation of W2Br6(g) could be observed as the result of a secondary gas-phase reaction. From the measured intensities of different ions the pressure-temperature curve obtained for WBr4(g) exhibits abnormalities which could be explained in terms of the reaction kinetics leading to equilibrium conditions above 1270 K. On the basis of these considerations ΔH4o (−36.7 ± 1.7 kcal/mole), ΔGTo(−8.6 ± 0.6 kcal/mole) and ΔSTo(−21.3 ± 1 e.u.) have been obtained and compared with some thermodynamic data calculated using the third-law method.

Mass spectrometric determination of appearance energies for ions formed from CoF4 and CoF3 molecules

Knudsen cell mass spectrometry was applied to the evaluation of the ionization efficiency curves for the ions originating from CoF4 molecules. Cobalt tetrafluoride was obtained in the gas phase over the CoF 3(s)‐ TbF4(s) system in the temperature range from 640 to 690 K. From the ionization efficiency curves the appearance energies of the ions formed from the CoF4 molecules were determined by means of Vogt’s deconvolution method. Clausius-Clapeyron plots for the ions from CoF4 molecules were measured. Evaporation of pure CoF3(s) was carried out, and the appearance energies of the ions formed from CoF3 molecules were determined. The ionization energies for CoF4 and CoF3 molecules were found to be (14.3 0.2) and (13.3 0.1) eV, respectively. Copyright # 2000 John Wiley & Sons, Ltd.

Mass spectrometric measurements of the reaction rate in a gas-flow system: The kinetics of the tungsten-bromine reaction

Abstract A mass spectrometric method with a gas-flow system for investigation of the tungsten-bromine (oxygen) reaction kinetics is described. Results for activation energies and reaction rates of the various reactions are given for the temperature interval between 400 and 800 K.