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≤y≤0.7) were synthesized and characterized. The structures of these phosphates were refined using Rietveld analysis. The compound o-Ca19Cu2(PO4)14 has β-Ca3(PO4)2-like structure (Space Group R3c, z=3). Compounds r′-Ca19Cu2H1.42(PO4)14 and r″-Ca19Cu2−yH2.24(PO4)14 (0.64≤y≤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) Å, c=37.242(2) Å, V=3463.8(9) Å3, Rp=4.24%, Rwp=5.75%; (ii) r′-Ca19Cu2H1.42(PO4)14: a=10.3987(1) Å, c=37.300(2) Å, V= 3493.0(9) Å3, Rp=4.89%, Rwp=6.50%; (iii) r″-Ca19 Cu2−yH2.24(PO4)14 (0.64≤y≤0.7): a=10.3975(1) Å, c= 37.274(2) Å, V=3489.7(9) Å3, Rp=4.57%, Rwp=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.