Honeycomb-like S = 5/2 Spin–Lattices in Manganese(II) Vanadates
We synthesized new complex manganese vanadate materials as high-quality single crystals in multi-millimeter lengths using a high-temperature, high-pressure hydrothermal method. We grew one compound, Mn 5 (VO 4 ) 2 (OH) 4 , from Mn 2 O 3 and V 2 O 5 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, Mn 6 O(VO 4 ) 2 (OH). Both compounds were structurally characterized by single-crystal X-ray diffraction (Mn 5 (VO 4 ) 2 (OH) 4 : C2/m, Z = 2, a = 9.6568(9) Å, b = 9.5627(9) Å, c = 5.4139(6) Å, β = 98.529(8)°; Mn 6 O(VO 4 ) 2 (OH): P21/m, Z = 2, a = 8.9363(12) Å, b = 6.4678(8) Å, c = 10.4478(13) Å, β = 99.798(3)°), revealing interesting low-dimensional transition-metal features. Mn 5 (VO 4 ) 2 (OH) 4 possesses complex honeycomb-type Mn–O layers, built from edge-sharing [MnO 6 ] octahedra in the bc plane, with bridging vanadate groups connecting these layers along the a-axis. Mn 6 O(VO 4 ) 2 (OH) presents a more complicated structure with both octahedral [MnO 6 ] and trigonal bipyramidal [MnO 5 ] 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 [MnO 5 ] 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. Furthermore, 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 Mn 5 (VO 4 ) 2 (OH) 4 and below 45 K in Mn 6 O(VO 4 ) 2 (OH). Members of another class of compounds, K 2 M 3 (VO 4 ) 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.