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

• Published in: Inorganic chemistry 2010-02, Vol.49 (3), p.934-942
• Main Authors: Coomer, Fiona C, Checker, Neal J, Wright, Adrian J
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic901668u

Two previously uncharacterized members of the Rb−Mn−P−O system, RbMnP2O7 and β-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 P21/c, a = 7.3673(2) Å, b = 9.6783(2) Å, c = 8.6467(2) Å, and β = 105.487(1)°) was found to be isostructural with RbFeP2O7. The polymorph β-RbMnHP3O10 was also isolated as a single phase and found to crystallize in the space group C2 (a = 12.2066(5) Å, b = 8.5243(3) Å, c = 8.8530(4) Å, β = 107.233(2)°). 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 β-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.