STUDY OF THE VAPOR OF THE NAL-SCL3 SYSTEM - VAPOR COMPOSITION, THERMOCHEMISTRY OF THE GASEOUS HETEROCOMPLEXES NASCL4 AND NA2SCL5, AND THEIR POTENTIAL FOR CHEMICAL VAPOR TRANSPORT

The vapor of the NaI-ScI3 system was investigated under equilibrium conditions in the temperature range from 613 to 848 K by using Knudsen effusion mass spectrometry with a one- and a two-compartment Knudsen cell. NaI/ScI3 samples of the compositions xNaI = 0.25 and 0.75 were studied with the one-compartment cell. The gaseous species NaI, (NaI)2, ScI3, (ScI3)2, NaScI4, and Na2ScI5 were present in the vapor. Their partial pressures were employed in a second-law evaluation yielding for the first time reliable thermodynamic data on the gas phase chemistry of the NaI-ScI3 system. The enthalpy and entropy changes of the reactions NaScI4(g) ⇄ NaI(g) + ScI3(g) [1] Na2ScI5(g) ⇄ 2NaI(g) + ScI3(g) [2] (NaI)2(g) + (ScI3)2(g) ⇄ 2NaScI4(g) [3] resulted as ΔH°298 = 228 ± 5 kJ mol−1, ΔS°298 = 165 ± 8 J mol−1 K−1 for reaction Eq. [1]; ΔH°298 = 388 ± 9 kJ mol−1, ΔS°298 = 310 ± 13 J mol−1 K−1 for reaction Eq. [2]; and AH°298 = −87 ± 6 kJ mol−1, AS°298 = −31 ± 8 J mol−1 K−1 for reaction Eq. [3]. The following equilibrium constants were obtained for the dissociation reactions Eq. [1] and [2] log Kp = −11,652 1/(T/K) + 8.0533 for Eq. [1] (613–848 K) and log Kp = −19,747 1/(T/K) + 15.088 for Eq. [2] (623–848 K) The data are discussed generally as well as with respect to their significance for chemical vapor transport in metal halide lamps and for the prediction of the dissociation enthalpy of gaseous heterocomplexes by a semiempirical rule. © 1990, The Electrochemical Society, Inc. All rights reserved.