Energetics of phosphate frameworks containing zinc and cobalt: NaZnPO4, NaH(ZnPO4)2, NaZnPO4.H2O, NaZnPO4.4/3 H2O, and NaCoxZn1-xPO4.4/3 H2O

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-xPO4.4/3 H2O (x=0, 0.1, 0.2, 0.3). The enthalpies of formation from the oxides of cobalt substituted in NaZnPO4.4/3 H2O 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 abstract: Relative 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)