CHEMICAL VAPOR TRANSPORT OF SCNBO4 WITH CL2 OR NBCL5/CL2 - EXPERIMENTS AND CALCULATIONS

Crystals of ScNbO4 can be grown by CVT using Cl2 (added as PtCl2) or a mixture of NbCl5/Cl2 as transport agent (1100-degrees-C --> 1000-degrees-C). As a result of thermodynamic calculations the evaporation and deposition of ScNbO4 in the presence of Cl2 can be expressed by the endothermic equilibrium: ScNbO4,f + 3 Cl2,g = ScCl3,g + NbOCl3,g + 3/2 O2,g. With addition of NbCl5 the following endothermic reactions are relevant: ScNbO4,f + 3 NbCl5 g = 1/n (ScCl3)n,g + 4 NbOCl3,g (n = 1-3) Although the equilibria (n = 1, 2) are more favourable using NbCl5/Cl2 as transport agent than using Cl2 alone, the amount of deposited crystals is strongly reduced by the exothermic reaction ScCl3,g + Sc2Cl6,g = Sc3Cl9,g, so that the mass obtained is only a little larger than with Cl2 as sole transport agent. Assuming DELTA-BH-degrees-298(ScNbO4,s) = -472.1 kcal/mol, a satisfying agreement between thermodynamical calculation and experimental results can be reached.