Energetics of phosphate frameworks containing zinc and cobalt: NaZnPO4, NaH(ZnPO4)2, NaZnPO4·H2O, NaZnPO4·43H2O, and NaCoxZn1−xPO4·43H2O

Differential scanning calorimetry and high temperature oxide melt solution calorimetry were used to study the enthalpy of the α–β phase transformation of NaZnPO4 and enthalpies of formation of α-NaZnPO4, NaH(ZnPO4)2, NaZnPO4·H2O, and NaCoxZn1−xPO4·43H2O (x=0, 0.1, 0.2, 0.3). The enthalpies of formation from the oxides of cobalt substituted in NaZnPO4·43H2O do not depend on cobalt content, confirming similar acid–base interactions for Zn-PO4 and Co-PO4. While water molecules stabilize zinc phosphate frameworks through solvating a cation or forming extra hydrogen bonds, the partial substitution of water for sodium oxide to form NaH(ZnPO4)2 represents the formation of an acidic compound with weaker acid–base interactions and less exothermic enthalpy of formation from oxides than Na2(ZnPO4)2.

Graphical abstractRelative stability of NaZnPO4 dense phases, open frameworks, and hydrated frameworks. Enthalpy of interaction between water and NaZnPO4 frameworks is presented by reaction: NaZnPO4 (cr, open framework) + nH2O (l) → NaZnPO4·nH2O (cr, hydrated framework).Figure optionsDownload full-size imageDownload as PowerPoint slide