D. N. Sergeev

Knudsen effusion mass spectrometric determination of the complex vapor composition of samarium, europium, and ytterbium bromides

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.

Formation enthalpies of molecules and negative ions of ytterbium bromides

A procedure for determining the formation enthalpies of LnXn (n = 1–3) molecules of thermally unstable lanthanide di- and trihalides that is based on measuring the equilibrium constants of reactions in Ln-X systems of various content and solving a system of thermochemical equations is suggested. The procedure is used to determine the enthalpies of formation ΔfH298o of molecules and negative ions found in the vapors of ytterbium bromides: YbBr (20 ± 3), YbBr2 (−135 ± 10), YbBr3 (−233 ± 12), YbBr3− (−615 ± 31), and YbBr4− (−766 ± 23) kJ/mol.

Thermodynamic parameters of vaporization of EuBr2

The vaporization process of europium dibromide was studied using high-temperature mass spectrometry. It was ascertained that saturated vapor in temperature range of 1049–1261 K was represented mainly by EuBr2 molecules; the fraction of dimer molecules Eu2Br4 was less than 1%. Heat capacities of solid and liquid europium dibromide, as well as the melting enthalpy were measured by means of differential scanning calorimetry in temperature range 300–1100 K; using these data thermodynamic functions for EuBr2 in condensed state were calculated. For all experimental data, including the literature data, thermodynamic characteristics of the vaporization of europium dibromide were determined using a unified set of thermodynamic functions according to the methods of the second and third laws of thermodynamics. The value of ΔsHo(298.15 K) = 354 ± 5 kJ/mol was recommended for the reaction of sublimation of EuBr2(cr.) = EuBr2.

Formation energies of molecules and anions of europium bromides

The content of saturated vapors above europium dibromide and Eu-EuBr2, Eu-Ba-BaBr2, EuBr2-LaBr3 systems is investigated by means of high-temperature mass-spectrometry in the electron ionization and thermoionic emission regimes. On the basis of the measured equilibrium constants for reactions with participation of molecules and negative ions, the enthalpies of formation ΔfH298° (kJ/mol) are determined using the method of the third law of thermodynamics: −59 ± 13 (EuBr), −349 ± 19 (EuBr2), and −861 ± 24 (EuBr3−).

Composition of saturated vapor over Ytterbium bromides

The vaporization process of ytterbium di- and tribromide was studied using high-temperature mass spectrometry over the temperature range of 850 to 1300 K. It was ascertained that, at the early vaporization stages, the vapor contained molecules YbBr3, YbBr2, YbBr, Br2, Yb2Br2, Yb2Br3, Yb2Br4, Yb2Br5, Yb2Br6, and atoms Yb and Br. The partial pressures of all components of saturated vapor were calculated. It was found that vapor composition reflected the course of the reactions of decomposition of tribromide and disproportionation of dibromide in the condensed phase. It was concluded that vaporization of di- and tribromide was incongruent at the initial stages; vaporization of both agents acquired a congruent character with the Yb: Br = 1.0: 1.9±0.2 ratio with time.