CuZnSnSe formation and reaction enthalpies in molten NaI starting from binary chalcogenides

The present study deals with chemical reactions and enthalpies during the synthesis of CuZnSnSe (CZTSe) from CuSe, SnSe, and ZnSe in molten NaI as flux material in closed degassed ampoules. Differential thermal analysis (DTA) at heating rates 5 °C min and cooling rates 10 °C min were used for the determination of temperatures of phase transitions and/or chemical reactions. XRD and Raman analyses confirmed that the formation of CZTSe starts already at 380 °C after the melting of Se that deliberates from the transformation of CuSe to CuSe, and the CZTSe formation process impedes to a great extent due to the presence of solid NaI. After the melting of NaI, the formation of CZTSe is completed. For the determination of enthalpy values, the calibration with pure NaI was performed. The thermal effects and enthalpies were compared with the available known thermodynamical values. The specific enthalpy of exothermic CuZnSnSe formation at 661 °C in NaI -36 ± 3 kJ mol was determined experimentally for the first time. Ternary compound NaSnSe was formed during the synthesis process. NaI·2HO, if present in NaI, was found to be a critical issue in the synthesis process of CZTSe monograin powders in molten NaI - it gave rise to the formation of oxygen-containing by-products NaSeO and NaCu(OH). The complete dehydration of NaI·2HO at T ≤ 70 °C in vacuum is necessary to avoid the formation of oxygen-containing compounds.