The Magnetic Structure of the Novel Hexagonal Perovskite Ba3NdMo2O9

Oxides with the general formula Ba3AM2O9 crystallising in the 6H perovskite structure, with magnetic cations, can show geometrically frustrated states because the arrangement of the metal cations creates triangular motifs parallel to the ab planes. We have recently synthesised the novel phase Ba3NdMo2O9. Preliminary neutron diffraction data indicate that there is no change in crystal symmetry upon cooling, but very weak magnetic diffraction peaks are observed below 10 K. This set of reflections can be indexed by the k = (1/2, 1/4, 0) propagation vector, which is unusual for hexagonal perovskites. Using representational analysis, the best fit to the data suggests that the Nd moments order ferromagnetically along the c-axis, while the two Mo sublattices order antiferromagnetically with respect to each other with moments aligned parallel to the a-axis. The preliminary results also suggest that only half of the Mo4+/5+ spins order, which is highly unusual. Unfortunately, due to the low statistical data, weak nature of the magnetic reflections, combined with structural/magnetic contributions coming from the Ba2NdMoO6 impurity phase we are unable to determine if this is the only configuration possible to describe the data. We have now made a phase pure sample of Ba3NdMo2O9. In order to conclusively determine the magnetic structure of Ba3NdMo2O9, we propose to perform a variable temperature neutron diffraction experiment on the high resolution, long-wavelength WISH diffractometer.