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, Mn-5(VO4)(2)(OH)(4), was grown from Mn2O3 and V2O5 in 3 M CsOH at 580 degrees 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 (Mn-5(VO4)(2)(OH)(4): C2/m, Z = 2, a = 9.6568(9) angstrom, b = 9.5627(9) angstrom, c = 5.4139(6) angstrom, beta = 98.529(8)degrees; Mn6O(VO4)(2)(OH): P2(1)/m, Z = 2, a = 8.9363(12) angstrom, b = 6.4678(8) angstrom, c = 10.4478(13) angstrom, beta = 99.798(3)degrees), revealing interesting low-dimensional transition-metal features. Mns(VO4)(2)(OH)(4) possesses complex honeycomb-type Mn-O layers, built from edge-sharing [MnO6] octahedra in the be plane, with bridging vanadate groups connecting these layers along the a-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 Mns(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.