K₂Mn₃O(OH)(VO₄)(V₂O₇) with Sawtooth Chains of Multivalent Manganese Triangular Trimer Units: Magnetic Susceptibility...

ortho-Pyrovanadate (or ortho-diorthovanadate) K₂Mn₂³⁺Mn²⁺O(OH)(VO₄)(V₂O₇) synthesized hydrothermally crystallizes in the orthorhombic space group Pnma with a = 17.9155(5), b = 5.8940(2), c = 10.9971(3) Å, V = 1161.23(6) ų, and Z = 4. Its crystal structure features linear chains of edge-sharing Mn³⁺O₆ octahedra with every second pair of Mn³⁺O₆ octahedra condensed with a Mn²⁺O₆ octahedron on one side of a chain in a sawtooth pattern so that each sawtooth chain consists of a triangular trimer. These sawtooth chains, running parallel to the b axis and linked by the VO₄ and V₂O₇ groups, form a framework with channels populated by K atoms. The new compound is a structural analogue of the mineral zoisite Ca₂Al₃O(OH)(SiO₄)(Si₂O₇), showing a striking example of very different chemical compositions. K₂Mn₃O(OH)(VO₄)(V₂O₇) undergoes a phase transition into an ordered antiferromagnetic (AFM) state at T_N = 14.4 K, which was detected by high-frequency electron spin resonance as well as by both specific heat C_p and Fisher's specific heat d(χT)/dT measurements. However, this phase transition was not detected by magnetic susceptibility measurements. The origin of this puzzling observation was resolved by evaluating the spin exchanges of K₂Mn₃O(OH)(VO₄)(V₂O₇), which revealed that each triangular trimer is a ferromagnetically coupled cluster, and the observed ordering involves an AFM ordering between the ferromagnetic (FM) clusters. This ordering is shrouded in magnetic susceptibility measurements due to the susceptibility contributions from the individual FM triangular trimers even below T_N. We showed that the magnetic susceptibility of K₂Mn₃O(OH)(VO₄)(V₂O₇) between ∼30 K and room temperature is satisfactorily described by an AFM chain made up of ferromagnetically coupled triangular clusters, as described by a few spin-exchange parameters.