Crystal structures of new triple Ca9CoM(PO4)7 (M = Li, Na, K) phosphates

New triple phosphates Ca9CoM(PO4)7 (M = Li, Na, K) were synthesized by solid state method. Their crystal structures were determined by Rietveld analysis. They are related to tricalcium phosphate and crystallize into trigonal system (space group R3c) with unit-cell parameters a = 10.3276(1) إ, c = 37.100(1) إ, M = Li; a = 10.3515(1) إ, c = 37.073(1) إ, M = Na; a = 10.4017(1) إ, c = 37.009(1) إ, M = K. All five independent cation sites are filled in Ca9CoM(PO4)7 (M = Li, Na, K). Cobalt occupies the octahedral site M(5). Alkali metal cations occupy the M(4) site.