Crystallization of MIGe2(PO4)3 (MI – Na, K, Ag) from molten phosphate media

A number of NASICON-related MIGe2(PO4)3 (MI – Na, K, Ag) have been prepared using a high-temperature solution method in the molten system MI-Ge-P-O (MI – Na, K) based on the solubility data for GeO2 at 1123 K for the Na-P-O system and 1173 K for the K-P-O one. The single crystals of AgGe2(PO4)3 were obtained after crystallization of a melt with Ag/P = 1.4. It crystallizes in the trigonal system, space group R-3, cell parameters a = 8.06340(1) A, c = 21.8385(4) A, Z = 6. The framework is built up from two GeO6 units that are isolated from each other by PO4 tetrahedra to form the main building block of the structure [Ge2(PO4)3]−. Two types of silver cations M1 and M2 are found to have different coordination requirements shown by the bond valence sum calculations.