Thermochemistry of the alkali rare-earth double phosphates, A3RE(PO4)2

Abstract

The formation enthalpies for alkali rare-earth compounds of the type K3RE(PO4)2where RE = Sc, Y, Lu, Er, Ho, Dy, Gd, Nd, or Ce and for A3Lu(PO4)2compounds with A = K, Rb, or Cs were determined using high-temperature oxide-melt solution calorimetry. Structural phase transitions were observed and characterized using differential scanning calorimetry and high-temperature x-ray diffraction. The formation enthalpy of the K3RE(PO4)2phases from oxides becomes more exothermic with increasing rare-earth radius for the K3RE(PO4)2series and with increasing alkali radius for the A3Lu(PO4)2compounds. The K3RE(PO4)2phases are stable with respect to anhydrous K3PO4and REPO4. The monoclinic K3RE(PO4)2compounds undergo a reversible phase transition to a hexagonal (glaserite-type) structure with a phase transition temperature that increases from −99 to 1197 °C with increasing RE ionic radius going from Lu to Ce.