Asabina, E. A.; Petkov, V. I.; Kovalsky, A. M.; Kotelnikov, A. R. Synthesis and Hydrothermal Study of (Na,K)M2(PO4)3 (M = Ti, Zr, Hf) Solid Solutions. Eurasian Chem.-Technol. J. 2010, 12, 195-199.