MASS-SPECTROMETRIC INVESTIGATION OF THE EVAPORATION PROPERTIES OF BA2CAWO6 AND BA2SRWO6

The vaporization of Ba2CaWO6 and Ba2SrWO6 was investigated by Knudsen- cell mass spectrometry. According to the results obtained with a Knuds en cell made of tungsten, the reduction of barium-calcium tungstates i s a non-equilibrium process 2Ba2CaWO6(s) + W(s) --> 3BaWO4(s) + Ba(g) + 2Ca(g) whereas the sublimation of BaWO4 and its decomposition in the gas phase to BaO(g) and WO3(g) are equilibrium reactions. From the te mperature dependence of the partial pressure of BaWO4(g) In(p(BaWO4(g) /Pa) = (-56.4 +/- 4.0) x (10(3)/T) + (28.1 +/- 2.5) the enthalpy of su blimation is obtained as DELTAH(T)-degrees-II = 469 +/- 4 kJ/mol, whic h is in excellent agreement with the value obtained by third-law calcu lations. The origin of the most abundant ions in the gas phase, i.e., BaWO4+, BaWO3+, BaO+, Ba+ etc, has been explained from measurements of the corresponding ionization energy (IE) and appearance energy (AE) v alues. The difference between the AE values for Ba2CaWO6 and Ba2SrWO6 is negligible. The presence of BaWO4 in the condensed phase was confir med by X-ray diffraction measurements. Neither X-ray analysis nor the mass spectra show the presence of CaWO4 or SrWO4 in the solid phase or in the gas phase.