Single crystal neutron and magnetic measurements of Rb₂Mn₃(VO₄)₂CO₃ and K₂Co₃(VO₄)₂CO₃ with mixed honeycomb and triangular magnetic lattices _auth 1,2*Smith Pellizzeri*T.M.*Tiffany M.

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 P31c, 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 T_N = 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.