This article studies the regularities of formation of isostructural to langbeinite phosphates from solutions in melts under crystallization conditions for K2O-P2O5-MIII2O3-SnO2 (MIII - Fe, Cr) and K2O-P2O5-LnF3-SnO2 (Ln - Y, REE) systems. Based on X-ray powder diffraction method the phase composition of synthesized samples and lattice parameters have been determined for a number of new compounds with common composition K2MIIISn(PO4)3. Analysis of obtained results shows that the crystallochemical criterion for formation of langbeinite-type framework on the basis of different multivalent metals is the difference in their ionic radii up to 35 %.
