Anhydrous Europium Phosphates: A Comprehensive Report on Syntheses, Crystal Structures, and Phase Relations

In the ternary system Eu/P/O the anhydrous phosphates Eu-3(II)(PO4)(2), (Eu3EuIII)-Eu-II(PO4)(3), (Eu3O3)-O-III(PO4), (EuPO4)-P-III, Eu-III(PO3)(3), and (EuP5O14)-P-III exist at equilibrium conditions (theta approximate to 1000 degrees C). A superstructure model for (Eu3EuIII)-Eu-II(PO4)(3) [Eulytite structure family, Fdd2, Z = 24, a = 14.298(1) angstrom, b = 42.123(3) angstrom, c = 10.3887(9) angstrom] is proposed. The results of single-crystal X-ray diffraction (SXRD) studies on (Eu10-xEuxIII)-Eu-II(PO4)(6)O1+x/2 (x approximate to 1.5), (Eu3O3)-O-III(PO4) and ht-Eu-III(PO3)(3) are reported. The crystal structures of the thermodynamically metastable phases (Eu2P4O13)-P-III (isotypic to (Y2P4O13)-P-III) and lt-Eu-III(PO3)(3) (isotypic to Gd-III(PO3)(3)) were refined from X-ray powder diffraction data (XRPD). According to our investigation for the oxide phosphide (Eu4OP2)-O-II an europium deficiency has to be assumed according to (Eu4-xEu2x/3OP2)-Eu-II-O-III (x approximate to 1.5).