K 2 Mn 3 O(OH)(VO 4 )(V 2 O 7 ) with Sawtooth Chains of Multivalent Manganese Triangular Trimer Units: Magnetic Susceptibility Shrouding a Long-Range Antiferromagnetic Order of Ferromagnetic Triangles.

ortho -Pyrovanadate (or ortho -diorthovanadate) K 2 Mn 2 3+ Mn 2+ O(OH)(VO 4 )(V 2 O 7 ) 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) Å 3 , and Z = 4. Its crystal structure features linear chains of edge-sharing Mn 3+ O 6 octahedra with every second pair of Mn 3+ O 6 octahedra condensed with a Mn 2+ O 6 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 VO 4 and V 2 O 7 groups, form a framework with channels populated by K atoms. The new compound is a structural analogue of the mineral zoisite Ca 2 Al 3 O(OH)(SiO 4 )(Si 2 O 7 ), showing a striking example of very different chemical compositions. K 2 Mn 3 O(OH)(VO 4 )(V 2 O 7 ) 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(χ T )/d T 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 K 2 Mn 3 O(OH)(VO 4 )(V 2 O 7 ), 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 K 2 Mn 3 O(OH)(VO 4 )(V 2 O 7 ) 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.