Rubidium tetramanganese tris(phosphate), RbMn4(PO4)3, has been synthesized as single crystals under hydrothermal conditions. The crystal structure was refined in the space group Pnnm (D2h12). It is argued that the size factor RM/RA, i.e. the ratio of the A+ ionic radius to the M2+ ionic radius, within the morphotropic series AM4(TO4)3 corresponds to a specific type of crystal structure. At low temperatures, the antiferromagnet superimposed on a buckled kagomé network in RbMn4(PO4)3 experiences a transition into a long-range ordered state with finite spontaneous magnetization. First principles calculations provide the dominant magnetic exchange interactions both within and between the kagomé layers. The analysis of these interactions allows us to suggest a model of alternating ferromagnetic and antiferromagnetic arrangements within chains of Mn3 atoms.
Keywords: antiferromagnetism; canted antiferromagnet; crystal structure; frustration; kagomé network; morphotropic series.