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, RbMnP(2)O(7) and beta-RbMnHP(3)O(10), 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 RbMnP(2)O(7) (space group P2(1)/c, a = 7.3673(2) A, b = 9.6783(2) A, c = 8.6467(2) A, and beta = 105.487(1) degrees) was found to be isostructural with RbFeP(2)O(7). The polymorph beta-RbMnHP(3)O(10) was also isolated as a single phase and found to crystallize in the space group C2 (a = 12.2066(5) A, b = 8.5243(3) A, c = 8.8530(4) A, beta = 107.233(2) degrees). Both structures consist of frameworks of corner-sharing MnO(6) octahedra linked together by condensed phosphate anions, with Rb(+) cations located in the intersecting channels. In both cases the Mn(3+) 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 RbMnP(2)O(7) (T(N) = 20 K) and beta-RbMnHP(3)O(10) (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 Mn(3+), which are mediated by PO(4) tetrahedra. These interactions are compared and discussed.