Energetics of cobalt phosphate frameworks: alpha, beta, and red NaCOPO4

摘要

Thermal behavior. relative stability, and enthalpy of formation of alpha (pink phase), beta (blue phase), and red NaCoPO4 are studied by differential scanning calorimetry, X-ray diffraction, and high -temperature oxide melt drop solution calorimetry. Red NaCoPO4 with cobalt in trigonal bipyramidal coordination is metastable, irreversibly changing to a NaCoPO4 at 827 K with an enthalpy of phase transition of -17.4 +/- 6.9 kJ mol(-1). alpha NaCoPO4 with cobalt in octahedral coordination is the most stable phase at room temperature. It undergoes a reversible phase transition to the beta phase (cobalt in tetrahedra) at 1006 K with an enthalpy of phase transition of 17.6 +/- 1.3 kJ mol(-1). Enthalpy of formation from oxides of alpha, beta, and red NaCoPO4 are -349.7 +/- 2.3, -332.1 +/- 2.5, and -332.3 +/- 7.2 kJ mol(-1); standard enthalpy of formation of alpha, beta, and red NaCoPO4 are -1547.5 +/- 2.7, -1529.9 +/- 2.8, and -1530.0 +/- 7.3 kJ mol(-1), respectively. The more exothermic enthalpy of formation from oxides of beta NaCoPO4 compared to a structurally related aluminosilicate, NaAlSiO4 nepheline, results from the stronger acid-base interaction of oxides in beta NaCoPO4 (Na2O, CoO: P2O5) than in NaAlSiO4 nepheline (Na2O, Al2O3, SiO2). (c) 2006 Elsevier Inc. All rights reserved.