Molecular and ionic sublimation of neodymium tribromide polycrystals and single crystals

Abstract

The molecular and ionic sublimation of polycrystals and single crystals under Knudsen effusion and Langmuir evaporation conditions is reported. In both sublimation regimes, the sublimation product at 780–1050 K contains neodymium tribromide monomer and dimer molecules, as well as the negative ions NdBr ⁻₄ , Nd₂Br ⁻₇ , and Br⁻. The dimer-to-monomer flux ratio j(Nd₂Br₆)/j(NdBr₃)is larger in the molecular beam coming out of the effusion hole, while the ratio of the sublimation fluxes of the negative ions, j(Nd₂Br ⁻₇ )/j(NdBr ⁻₄ ), is independent of the sublimation conditions. The partial pressures of the neutral components of the vapor have been determined, and the enthalpies and activation energies of sublimation of neodymium tribromide as monomer and dimer molecules and NdBr ⁻₄ and Nd₂Br ⁻₇ ions have been calculated. The equilibrium constants of ion-molecule reactions have been measured, and the enthalpies of these reactions have been determined. Based on these data, values of the thermodynamic properties Δ _s H ⁰(298.15) and Δ _f H ⁰(298.15) are recommended for the monomer and dimer molecules and the NdBr ⁻₄ and Nd₂Br ⁻₇ ions.