Jahn-Teller Distorted Frameworks and Magnetic Order in the Rb-Mn-P-O System

Two previously uncharacterized members of the Rb-Mn-P-O system, RbMnP2O7 and beta-RbMnHP3O10, have been synthesized using a phosphoric acid flux synthetic route and their crystal and magnetic structures determined using neutron powder diffraction. The crystal structure of RbMnP2O7 (space group P2(1)/c, a = 7.3673(2) angstrom, b = 9.6783(2) angstrom, c = 8.6467(2) angstrom, and beta = 105.487(1)degrees) was found to be isostructural with RbFeP2O7. The polymorph beta-RbMnHP3O10 was also isolated as a single phase and found to crystallize in the space group C2 (a = 12.2066(5) angstrom, b = 8.5243(3) angstrom, c = 8.8530(4) angstrom, beta = 107.233(2)degrees). Both structures consist of frameworks of corner-sharing MnO6 octahedra linked together by condensed phosphate anions, with Rb+ cations located in the intersecting channels. In both cases the Mn3+ octahedra exhibit unusual Jahn-Teller distortions indicative of a plasticity effect driven by the steric requirements of the condensed phosphate anions, and this causes a strong violet coloration similar to that observed in the manganese violet pigment; the structure of this has yet to be determined. Magnetic susceptibility measurements show that both RbMnP2O7 (T-N = 20 K) and beta-RbMnHP3O10 (T-N = 10 K) undergo a phase transition at low temperatures to an antiferromagnetically ordered state. Low-temperature neutron powder diffraction studies show that the magnetic ground states of each of these materials involve both ferromagnetic and antiferromagnetic super-superexchange interactions between orbitally ordered Mn3+, which are mediated by PO4 tetrahedra. These interactions are compared and discussed.