You need a ternary phase diagram for Bi-Te-Zn. Springer Materials database has an isothermal section at 500C. There appears to be little solubility of Zn in Bi2Te3 and this compound forms an equilibrium 2-phase structure with ZNTe. However if your composition is slightly off the tie line between Bi2Te3 and ZnTe you will get 3 phases, either Bi2Te3+ZnTe+Te(liquid at 500C) or on the Te deficient side Bi2Te3+ZnTe+BiTe