ABSTRACT
The thermal emission of alkali metal ions from Al30-pillared montmorillonite in comparison with its natural form was studied by mass spectrometry in the temperature range 770-930 K. The measurements were carried out on a magnetic mass spectrometer MI-1201. For natural montmorillonite, the densities of the emission currents (j) decrease in the mass spectrum in the following sequence (T = 805 K, A/cm2): K+ (4.55 · 10-14), Cs+ (9.72 · 10-15), Rb+ (1.13 · 10-15), Na+ (1.75 · 10-16), Li+ (3.37 · 10-17). For Al30-pillared montmorillonite, thermionic emission undergoes temperature-time changes. In the low-temperature section of the investigated range (770-805 K), the value of j increases substantially for all ions in comparison with natural montmorillonite (T = 805 K, A/cm2): Cs+ (6.47 · 10-13), K+ (9.44 · 10-14), Na+ (3.34 · 10-15), Rb+ (1.77 · 10-15), and Li+ (4.59 · 10-16). A reversible anomaly is observed in the temperature range 805-832 K: with increasing temperature, the value of j of alkaline ions falls abruptly. This effect increases with increasing ionic radius of M+. After a long heating-up period, this anomaly disappears and the lnj - 1/T dependence acquires a classical linear form. The results are interpreted from the point of view of the dependence of the efficiency of thermionic emission on the phase transformations of pillars.
ABSTRACT
RATIONALE: The vaporization of Sm, Eu, and Yb tri- and dibromides is accompanied by decomposition and disproportionation reactions. These result in complex vapor compositions whose analysis is an intricate problem for experimentalists. Approaches have been developed to interpret mass spectra and accurately determine the vapor composition of thermally unstable compounds. METHODS: A sector type magnet instrument was used. A combined ion source allowed the study of both the molecular and ionic vapor compositions in the electron ionization (EI) and the thermionic emission (TE) modes. The methodological approaches were based on a joint analysis of the ionization efficiency functions, the temperature and time dependences of the ion currents, and special mathematical data evaluation. RESULTS: The vaporization of SmBr3 , YbBr3 , SmBr2 , EuBr2 , and YbBr2 was studied in the temperature range of 850-1300 K. An initial stage of incongruent vaporization was observed in the case of the tribromides, SmBr2 , and YbBr2 . This eventually changed to a congruent vaporization stage. Various neutral (Ln, Br, Br2 , LnBr, LnBr2 , LnBr3 , Ln2 Br4 , Ln2 Br5 , and Ln2 Br6 ) and charged (Br(-), LnBr3 (-), LnBr4 (-)) species were detected at different vaporization stages. CONCLUSIONS: The quantitative vapor composition of Sm, Eu, and Yb tri- and dibromides was determined. It was found that only EuBr2 was stable in the studied temperature range. The developed approaches can be useful in the case of other thermally unstable compounds.
