Chemischer Transport von Cr2O3 mit Brom und mit CrBr3/Br2 — Experimente und Modellrechnungen zur Beteiligung von CrOBr2,g und CrO2Br2,g

On the Chemical Transport of Cr₂O₃ with Br₂ and CrBr₃/Br₂ — Experiments and Model Calculations for Participation of CrOBr_2,g and CrO₂Br_2,g Gaseous chromium oxybromides that were unknown up to now cause the migration of the starting material Cr₂O₃ in the temperature gradient from T₂ = 1000°C to T₁ = 900°C when Br₂ or Br₂/CrBr₃ respectively is added. Model calculations show that under the influence of H₂O (from the wall of the silica ampoule) or O₂ (from a homogenous equilibrium between H₂O/Br₂) the transport takes place via the oxybromide CrO₂Br₂ of the hexavalent chromium (eq. (1) and (2)). For thermodynamical reasons eq. (2) seems to be more favourable. At higher temperature the less oxygen containing gas species CrOBr_2,g has also to be taken into account if H₂O is excluded. An addition of CrBr₃ lowers the partial pressure of oxygen (and of H₂O as well) in the system Cr₂O₃/Br₂. Under this conditions CrOBr_2,g becomes an important species for the transport of the solid phase (eq. (4)) and CrBr_4,g has to be considered as transport agent. Estimated values of the enthalpies of formation were fixed more precisely by thermodynamic model calculation. For CrOBr_2,g (system Cr₂O₃/CrBr₃/Br₂) Δ_fH°₂₉₈ = ?70 kcal/mol and for CrO₂Br₂ (Cr₂O₃/Br₂) Δ_fH°₂₉₈ = ?107,4 kcal/mol was found. The estimated limits of error for the enthalpies of formation given for both oxybromides are smaller than ±5 kcal/mol.