PHASE EQUILIBRIUM IN THE SYSTEMS Ln₂O₃ - P₂O₅ - H₂O: REGULARITIES OF FORMATION AND SOME PROPERTIES OF RARE EARTH PHOSPHATES

Abstract

Phase equilibrium in the systems Ln₂O₃ - P₂O₅ - H₂O, (Ln = La-Lu and Y, molar ratio P₂O₅/Ln₂O₃ = (3÷10)/1) has been studied within the temperature range of 25-350 °C by thin layer method. The following phosphates have been synthesized: LnPO₄·nH₂O (Ln = La-Eu), LnH₃(PO₄)₂· 2H₂O (Ln = Sm-Dy, Lu, Y), LnH₃(PO₄)₂·0.5H₂O (Ln = Ho-Lu, Y), LnH₃(PO₄)₂ (Ln = Tb-Lu, Y), Ln(H₂PO₄)₃ (Ln = Nd-Dy), LnHP₂O₇ (Ln = Eu-Lu, Y), Ln₂P₄O₁₃-II (Ln = Ho-Lu, Y), Ln₄(P₄O₁₂)₃ (Ln = Tm-Lu), Ln(PO₃)₃-I (Ln = Ln-Gd), Ln(PO₃)₃-II (Ln= Tb-Lu, Y), Ln(PO₃)_3-C (Ln = Sm-Lu, Y), LnH(PO₃)_4-I (Ln = Sm, Eu), LnH(PO₃)_4-II (Ln = Eu-Er, Y), LnP₅O_14-I (Ln = La-Gd), LnP₅O_14-II (Ln = Tb- Lu, Y), Ln₂P₄O₁₃-I (Ln = Gd-Lu, Y); Ln₄(P₂O₇)₃ (Ln = Gd-Lu, Y). Thermal transformations of these compounds have been studied within the temperature range of 25-1000 °C. Lattice parameters of some synthesized phosphates have been calculated. Comparative diagram of crystallization fields for stable phases of rare earth phosphates has been constructed.