Magnetic properties of the RbMnPO4 zeolite-ABW type material: a frustrated zigzag spin chain

The crystal structure and magnetic properties of the RbMnPO4 zeolite-ABW type material have been studied by temperature-dependent neutron powder diffraction, low temperature magnetometry and heat capacity measurements. RbMnPO4 represents a rare example of a weak ferromagnetic polar material, containing Mn2+ ions with TN = 4.7 K. The neutron powder diffraction pattern recorded at T = 10 K shows that the compound crystallizes in the chiral and polar monoclinic space group P21 (No. 4) with the unit-cell parameters: a = 8.94635(9) {\AA}, b = 5.43415(5) {\AA}, c = 9.10250(8) {\AA} and beta = 90.4209(6){\deg}. A close inspection of the crystal structure of RbMnPO4 shows that this material presents two different types of zigzag chains running along the b axis. This is a unique feature among the zeolite-ABW type materials exhibiting the P21 symmetry. At low temperature, RbMnPO4 exhibits a canted antiferromagnetic structure characterized by the propagation vector k1 = 0 resulting in the magnetic symmetry P21. The magnetic moments lie mostly along the b axis with the ferromagnetic component being in the ac plane. Due to the geometrical frustration present in this system, an intermediate phase appears within the temperature range 4.7 - 5.1 K characterized by the propagation vector k2 = (kx, 0, kz) with kx/kz = 2. This ratio is reminiscent of the multiferroic phase of the orthorhombic RMnO3 phases (R = rare earth). This suggests that RbMnPO4 could present some multiferroic properties at low temperature. Our density functional calculations confirm the presence of magnetic frustration, which explains this intermediate incommensurate phase. Taking into account the strongest magnetic interactions, we are able to reproduce the magnetic structure observed experimentally at low temperature.