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The molecular and ionic composition of vapor over lanthanum tribromide was studied by high-temperature mass spectrometry under the conditions of sublimation from a Knudsen effusion cell and from an open single crystal surface. The partial pressures of LaBr 3 and La 2 Br 6 in saturated vapor and the ratio between their sublimation coefficients under free vaporization conditions were determined. The second and third laws of thermodynamics were used to calculate the enthalpies of sublimation of LaBr 3 in the form of monomers and dimers. In ionic sublimation studies, the emission of the Br -, , and negative ions was recorded in both regimes. Studies of the ionic sublimation of LnX 3 single crystals were shown to give reliable data on the thermochemical properties of negative ions.

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The thermodynamic stability of the anion was for the first time studied by high-temperature mass spectrometry and nonempirical quantum-chemical methods. The experimental and theoretical enthalpies of the reaction = Br -+ LaBr 3 were ∆ r H °(298.15 K) = 302 ± 14 and 303 kJ/mol, respectively. The value ∆ f H ° ( , g, 298.15 K) = -1105 ± 14 kJ/mol was recommended as the enthalpy of formation of the anion.