Synthesis, Characteristics and Standard Molar Enthalpies of Formation of MZn 2 HPO 4 PO 4 (M=Na + , K + )

MZn 2 HPO 4 PO 4 (M=Na + , K + ) double salts were synthesized by a solid‐state reaction at low temperature and a hydrothermal method, respectively, and characterized by XRD, FT‐IR, TG and SEM. Thermochemical study was performed by an isoperibol solution calorimeter. According to Hess's law, a new thermochemical cycle was designed. The results show that the obtained products are layered compounds with an isostructure of triclinic system. The decomposition temperatures are 415 and 430 °C by TG, respectively. With the dissolution enthalpies and other standard chemical data, the standard molar formation enthalpies of the MZn 2 HPO 4 PO 4 (M=Na + , K + ) were worked out as follows: = f H ⊖ m [NaZn 2 HPO 4 PO 4, s] =(−3042.38±0.31) kJ·mol −1 and = f H ⊖ m [KZn 2 HPO 4 PO 4, s] =(−3093.46±0.27) kJ·mol −1 .