Single crystal neutron and magnetic measurements of Rb2Mn3(VO4)2CO3 and K2Co3(VO4)2CO3 with mixed honeycomb and triangular magnetic lattices

Two new alkali vanadate carbonates with divalent transition metals have been synthesized as large single crystals via a high-temperature (600 °C) hydrothermal technique. Compound I, Rb2Mn3(VO4)2CO3, crystallizes in the trigonal crystal system in the space group P3[combining macron]1c, and compound II, K2Co3(VO4)2CO3, crystallizes in the hexagonal space group P63/m. Both structures contain honeycomb layers and triangular lattices made from edge-sharing MO6 octahedra and MO5 trigonal bipyramids, respectively. The honeycomb and triangular layers are connected along the c-axis through tetrahedral [VO4] groups. The MO5 units are connected with each other by carbonate groups in the ab-plane by forming a triangular magnetic lattice. The difference in space groups between I and II was also investigated with Density Functional Theory (DFT) calculations. Single crystal magnetic characterization of I indicates three magnetic transitions at 77 K, 2.3 K, and 1.5 K. The corresponding magnetic structures for each magnetic transition of I were determined using single crystal neutron diffraction. At 77 K the compound orders in the MnO6-honeycomb layer in a Néel-type antiferromagnetic orientation while the MnO5 triangular lattice ordered below 2.3 K in a colinear 'up-up-down' fashion, followed by a planar 'Y' type magnetic structure. K2Co3(VO4)2CO3 (II) exhibits a canted antiferromagnetic ordering below TN = 8 K. The Curie-Weiss fit (200-350 K) gives a Curie-Weiss temperature of -42 K suggesting a dominant antiferromagnetic coupling in the Co2+ magnetic sublattices.