Crystals of the cationic clathrate Si46-2xP2xTex can be grown via chemical vapour transport by adding tellurium tetrachloride. The transport proceeds in the temperature gradient T-1 = 923 K -> T-2 = 1003 K and with increasing temperature under reversion of the transport direction T-2 = 1173 K -> T-1 = 1073 K. By means of the thermodynamic modelling of solid state-gas phase equilibria, the transport mechanism for both directions was clarified; in the direction T-1 -> T-2 (T-1 < T-2) HCI(g) acts as the transport agent and in the other direction T-2 -> T-1 (T-2 > T-1) SiCl4(g) becomes active. Using the modelling the composition of the crystals Si46-2xP2xTex can be predicted depending on the experimental conditions, i.e. the temperature range, the direction of temperature gradient and the composition of the starting material. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.