757(x), enlargement of the wavefunction of aggregated localized states and corresponding enhancement of the polarizability or type of the electronic transition, first or second order, further more precise measurements of el (0, x) and E~( W , X ) are necessary. A precise control of the x-dependence by coulometric titration is possible and a continuous variation of the w-dependence down t o low energies seems feasible. These experiments are underway. That measurements under the necessary high temperature conditions can be successfully performed is demonstrated in this paper. Phase Diagrams J Semiconductors / Solid Solutions 1 ThermodynamicsThe phase diagram of the quasiternary system ZnkHal -k)SelTe(l -1) was investigated by X-ray diffraction and DTA measurements. The phase diagrams of the quasibinary subsystems were recalculated using the interaction parameters determined from the isothermal quasiternary sections. The subsolidus region of the quasiternary system shows an extended miscibility gap inspite of complete solid solutions on the quasibinary edges. The tie line field is fan shaped converging on the ZnSe comer of the phase square. The enthalpies of pair formation in the sublattices are calculated using the model of 4-particle-clusters. It is also shown, that the assumption of random distribution for the clusters is a rather good approximation for the calculation of the quasiternary phase diagram.

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