ON THE CHEMICAL-TRANSPORT OF CR2O3 WITH C RI3 I-2 - EXPERIMENTS AND MODEL-CALCULATIONS FOR PARTICIPATION OF CROI2,G/

Gaseous chromium oxyiodides that were unknown up to now cause the migr ation of Cr2O3 in the temperature gradient 1000 degrees C --> 900 degr ees C when iodine (e. g. 0.1 mmol/ml) and CrI3 is added (eg. (1)). Tra nsport agent for Cr2O3 is gaseous CrI4. (1) Cr2O3,s + 2CrI(4,g) = 3CrO I(2,g) + CrI2,g With a smaller concentration of iodine (D(I-2) less th an or equal to 0.016 mmol/ml) and lower temperatures (e.g. (T) over ba r = 850 degrees C) the influence of H2O (from the wall of the silica a mpoule) becomes more important. Under these conditions the transport o f Cr2O3 is a result from the endothermic reactions (2), (3) and (4). ( 2) 1/2Cr(2)O(3,s) + 3HI(g) = CrI2,g + 3/2H(2)O(g) + I-g (3) 1/2Cr(2)O( 3,s) + 3HI(g) + 1/2CrI(4,g) = 3/2CRI(2,g) + 3/2H(2)O(g) + 2I(g) (4) 1/ 2Cr(2)O(3,s) + 1/2H(2,g) + 2HI(g) = CrI2,g + 3/2H(2)O(g) H-2,H-g has o n the basis of the decomposition of HIg a positive difference of the s olubility and H-2,H-g should not to be considered as a transport agent . Because of the range of equilibrium-values the reaction 4 has to be taken into consideration. Estimated value of the enthalpie for CrOI2,g is fixed more precisely by thermodynamic model calculation to Delta(f )H(298)degrees(CrOI2,g) = -51.4kcal/mol. The estimated limit of error for the enthalpie of formation is smaller than +/-5 kcal/mol. Without an addition of CrI3 is in the system Cr2O3/I-2 a migration of Cr2O3 no t observable.