On the effect of oxygen partial pressure on the chromium distribution coefficient in melt-grown ruby crystals

Small ruby crystals were grown by the Czochralski technique in different atmospheres and their actual chromium content was analysed by a wet chemical method. The chromium distribution coefficient k was found to be strongly dependent on oxygen partial pressure p(O2). It ranges from k=0.3 in a reducing atmosphere to k=1.2 in a slightly oxidizing atmosphere and to a good approximation k depends linearly on log(p(O2)). The experimental data are discussed on the basis of thermodynamic equilibrium calculations.