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

Abstract

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 P1c, 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.