Honeycomb-like S = 5/2 Spin–Lattices in Manganese(II) Vanadates

New complex manganese vanadate materials were synthesized as high-quality single crystals in multi-millimeter lengths using a high-temperature, high-pressure hydrothermal method. One compound, Mn₅(VO₄)₂(OH)₄, was grown from Mn₂O₃ and V₂O₅ in 3 M CsOH at 580 °C and 1.5 kbar. Changing the mineralizer to 1 M CsOH/3MCsCl leads to the formation of another product, Mn₆O­(VO₄)₂(OH). Both compounds were structurally characterized by single-crystal X-ray diffraction (Mn₅(VO₄)₂(OH)₄: <i>C</i>2/<i>m</i>, <i>Z</i> = 2, <i>a</i> = 9.6568(9) Å, <i>b</i> = 9.5627(9) Å, <i>c</i> = 5.4139(6) Å, β = 98.529(8)°; Mn₆O­(VO₄)₂(OH): <i>P</i>2₁/<i>m</i>, <i>Z</i> = 2, <i>a</i> = 8.9363(12) Å, <i>b</i> = 6.4678(8) Å, <i>c</i> = 10.4478(13) Å, β = 99.798(3)°), revealing interesting low-dimensional transition-metal features. Mn₅(VO₄)₂(OH)₄ possesses complex honeycomb-type Mn–O layers, built from edge-sharing [MnO₆] octahedra in the <i>bc</i> plane, with bridging vanadate groups connecting these layers along the <i>a</i>-axis. Mn₆O­(VO₄)₂(OH) presents a more complicated structure with both octahedral [MnO₆] and trigonal bipyramidal [MnO₅] units. A different pattern of planar honeycomb sheets are formed by edge-shared [MnO₆] octahedra, and these sublattices are connected through edge-shared dimers of [MnO₅] trigonal bipyramids to form corrugated sheets. Vanadate groups again condense the sheets into a three-dimensional framework. Infrared and Raman spectroscopies indicated the presence of OH groups and displayed characteristic Raman scattering due to vanadate groups. Temperature-dependent magnetic studies indicated Curie–Weiss behavior above 100 K with significant anti-ferromagnetic coupling for both compounds, with further complex magnetic behavior at lower temperatures. The data indicate canted anti-ferromagnetic order below 57 K in Mn₅(VO₄)₂(OH)₄ and below 45 K in Mn₆O­(VO₄)₂(OH). Members of another class of compounds, K₂M₃(VO₄)₂(OH)₂ (M = Mn, Co), also containing a honeycomb-type sublattice, were also synthesized to allow a comparison of the structural features across all three structure types and to demonstrate extension to other transition metals