Synthesis and crystal structure of Ca9Cu1.5(PO4)7 and reinvestigation of Ca9.5Cu(PO4)7

Ca9Cu1.5(PO4)7 was synthesized by the solid-state method at 1273 K followed by quenching in air. Crystal structure of Ca9Cu1.5(PO4)7 was refined by the Rietveld method: space group R3c, a = 10.3379(1) Å, c = 37.1898(3) Å, Z = 6 with Rwp = 4.87%, Rp = 3.67%, S = 1.58, RI = 2.42%, RF = 1.07%. The structure of Ca9Cu1.5(PO4)7 is isotypic with β-Ca3(PO4)2 and Ca9.5Cu(PO4)7. The M(5) site is fully occupied, and the M(4) site is half occupied by copper cations. The M(1)-M(3) sites are occupied by calcium cations. Copper cations in the M(4) site are displaced from the threefold axis due to long distances M(4)—O. Reinvestigation of crystal structure of Ca9.5Cu(PO4)7 showed that the M(5) site is occupied by 0.79(1)Cu2+ + 0.21Ca2+, and other copper cations locate in the M(4) site.