ON THE CHEMICAL-TRANSPORT OF CR2O3 WITH B R2 AND CRBR3 BR2 - EXPERIMENTS AND MODEL-CALCULATIONS FOR PARTICIPATION OF CROBR2,G AND CRO2BR2,G/

Gaseous chromium oxybromides that were unknown up to now cause the mig ration of the starting material Cr2O3 in the temperature gradient from T2 = 1000-degrees-C to T1 = 900-degrees-C when Br2 or Br2/CrBr3 respe ctively is added. Model calculations show that under the influence of H2O (from the wall of the silica ampoule) or 02 (from a homogenous equ ilibrium between H2O/Br2) the transport takes place via the oxybromide CrO2Br2 of the hexavalent chromium (eq. (1) and (2)). For thermodynam ical reasons eq. (2) seems to be more favourable. (1) Cr2O3,s + H2Og 3 Br2,g = 2CrO2Br2,g + 2HBrg, (2) Cr2O3,s + 1/202,g + 2Br2,g = 2CrO2B r2,g At higher temperature the less oxygen containing gas species CrOB r2,g has also to be taken into account if H2O is excluded. (3) Cr2O3,s + 2Br2,g CrO2Br2,g + CrOBr2,g An addition of CrBr3 lowers the partial pressure of oxygen (and of H2O as well) in the system Cr2O3/Br2. Unde r this conditions CrOBr2,g becomes an important species for the transp ort of the solid phase (eq. (4)) and CrBr4,g has to be considered as t ransport agent. (4) Cr2O3,s + 3CrBr4,g = 3CrOBr2,g + 2CrBr3,g Estimate d values of the enthalpies of formation were fixed more precisely by t hermodynamic model calculation. For CrOBr2,g (system Cr2O3/CrBr3/Br2) DELTA(f)H298-degrees = -70 kcal/mol and for CrO2Br2 (Cr2O3/Br2) DELTA( f)H298-degrees = -107,4 kcal/mol was found. The estimated limits of er ror for the enthalpies of formation given for both oxybromides are sma ller than +/-5 kcal/mol.