Transition metal phosphates show extensive polymorphism, including many structures having anionic frameworks similar to aluminosilicate zeolites. These phosphates are considered to be an important class of materials in science and technology because of their potential applications in gas adsorption and separation, catalysis, conductivity, and magnetism. Thermodynamic data for these materials, not studied much till now, can provide insight in to their fundamental structure, bonding, stability, as well as guide their syntheses. This work is devoted to thermal behavior, relative stabilities, and enthalpies of formation of some cobalt, zinc, and copper phosphates by using differential scanning calorimetry, high temperature oxide melt solution calorimetry, and acid solution calorimetry.
Generally, the extensive polymorphism seen in cobalt and zinc phosphates is allowed by small energy differences among polymorphs. There is a strong relationship between enthalpy of formation from oxides and acid - base interaction for cobalt, zinc, and copper phosphates. Cobalt and zinc phosphates exhibit similar trends in enthalpies of formation from oxides as the corresponding aluminosilicates, but their enthalpies of formation from oxides are more exothermic than those of aluminosilicates because of their stronger acid - base interactions. The similar basicity of CoO and ZnO results in the similar enthalpies of formation from oxides of cobalt and zinc phosphates, as well as in the lack of dependence on cobalt content of the enthalpies of formation from oxides of cobalt substituted zinc phosphate, NaCo x Zn 1-x PO 4. 4/3H 2 O.
Water plays an important role in acid - base interactions and the formation of zinc and copper phosphates. Hydrate water in NaZnPO 4. H 2 O and NaZnPO 4. 4/3H 2 O stabilizes the zinc phosphate frameworks through solvating a cation or forming extra hydrogen bonds. But the partial substitution of water for sodium oxide in NaZnPO 4 , or that for copper oxide in Cu 2 P 2 O 7 results in the formation of acidic compounds, NaH(PO 4 ) 2 or Cu 3 (P 2 O 6 OH) 2 , with weaker acid - base interactions and less exothermic enthalpies of formation from oxides. It must be noted that, although acid - base chemistry may dominate the enthalpy of formation, other factors, such as ionic radius and electron configuration, help determine the actual structure formed.