towards 0 006 u00d7 10 5 pa 0 006 atm which is approximately 600 pa 6 mbar the significance of this pressure is that it corresponds to the vapour pressure of pure stoichiometric vapour p cd 2 p te2 the gradual pressure decrease is considered to result from a gradual refining of the stoichiometry of the charge material within the region of solubility of te in cdte the heat treatment is stopped at around 20 hours when the stoichiometric pressure of 500 pa 5 mbar is attained this is ideal if the heat treated charge is to be used as source material for growth of crystalline cdte by physical vapour transport pvt if the heat treatment were continued after reaching the pressure of stoichiometric vapour 500 pa i e 5 mbar in the case of te 2 vapour from cdte no further refinement would occur only wasteful loss of material as in the prior art where it is not possible to make any in situ measurement of the vapour pressure during heat treatment the in situ monitoring of vapour pressure during the anneal also provides the possibility of stopping the heat treatment at any target vapour pressure above the stoichiometric pressure in general it is expected that the vapour pressure of a relevant gas species associated with many other compounds will show the same basic functional form as illustrated in fig4 namely after a compound has become free of precipitates there will be a gradual pressure reduction from an elevated pressure.