Study of p-T-x-phase diagram of BiI3-Bi2Te3 system

The p-T-x phase diagram of the system BiI3-Bi2Te3 is studied in order to get some general physico-chemical rules for the growing of single crystals of this compound. The T-x projection of the system was investigated with the methods of differential-thermal, x-ray-phase and microstructural analysis. The temperature of congruent melting of BiTeI is 555+-50C. The composition of BiTeI does not correspong to a 1:1 relation of its components but is shifted to the side of BiI3. Micropolishing of all samples with 50% BiTe3 showed two phases. In x-ray photographs besides the lines of BiTeI lines of Bi2Te3 are also found. The two phases of the samples with 45% of Bi2Te3 conform that BiTeI decomposes between 45 and 50% Bi2Te3. The parameters of the elementary cell are a=4.376+-0.007 and C equals 6.847+-0.007A for BiTeI. The p-T diagram of state of the system BiI3-Bi2Te3 is obtained from data on the vapor pressure and corresponds completely to the T-x projection. The qualitative spectrometric investigation of the vapor composition on BiTeI at 2500C showed that there are no molecules in the vapour with three different elements. Therefore one can assume that the dissociation goes in the way 3BiTeI (solid) = BiTe3 (solid) + BiI3 (gas) which is analogous to the dissociation of other compounds of the type Asub(V)Bsub(VI)Csub(VII). The calculated enthalpy and entropy of dissociation for 1 mol BiTeI are: ΔH0sub(T)=10.07+-0.28 kcal/mol and ΔS0sub(T)=11.57+-0.35 cal/mol grad. On the basis of a general theory of the representation of the p-T-x-diagram all three projections of the diagram of state with approximate lines of vapour composition are constructed