Preparation, structure determination, and redox characteristics of new calcium copper phosphates

New calcium copper phosphates o-Ca19Cu2(PO4)(14), r'-Ca19Cu2H1.42(PO4)(14), and r"-Ca19Cu2-yH2.24(PO4)(14) (0.64 less than or equal to y less than or equal to 0.7) were synthesized and characterized. The structures of these phosphates were refined using Rietveld analysis. The compound o-Ca19Cu2(PO4)(14) has beta-Ca-3(PO4)(2)-like structure (Space Group R3c, z = 3). Compounds r'-Ca19Cu2H1.42(PO4)(14) and r"-Ca19Cu2-yH2.24(PO4)(14) (0.64 less than or equal to y less than or equal to 0.7) have the whitlockite-like structure (Space Group R3c, z = 3). The corresponding crystal data are (i) o-Ca19Cu2(PO4)(14): a = 10.3633(1) Angstrom, c = 37.242(2) Angstrom, V = 3463.8(9) Angstrom(3), R-p = 4.24%, R-wp = 5.75%; (ii) r'-Ca19Cu2H1.42(PO4)(14): a = 10.3987(1) Angstrom, c 37.300(2) Angstrom, V = 3493.0(9) Angstrom(3), R-p = 4.89%, R-wp = 6.50%; (iii) r"-Ca-19 Cu2-yH2.24(PO4)(14) (0.64 less than or equal to y less than or equal to 0.7): a = 10.3975(1) Angstrom, c = 37.274(2) Angstrom, V = 3489.7(9) Angstrom(3), R-p = 4.57%, R-wp = 6.04%. Redox reactions in calcium/copper double phosphates were investigated by XRD, DTA, DTG, and IR spectroscopy under hydrogen-containing and oxygen-containing atmospheres. These reactions proceed reversibly in the temperature ranges 753-825 K for the reduction process and 973-1173 K for the oxidation process. The redox cycles can be repeated continuously without destroying the crystal lattice. (C) 1999 Academic Press.