K2Mn3O(OH)(VO4)(V2O7) with Sawtooth Chains of Multivalent Manganese Triangular Trimer Units: Magnetic Susceptibility Shrouding a Long-Range Antiferromagnetic Order of Ferromagnetic Triangles

Multivalentmanganese ortho-pyrovanadate,K2Mn2 3+Mn2+O(OH)(VO4)(V2O7), was prepared and was foundto have chains of edge-sharing Mn3+O6 octahedracondensed with Mn2+O6 octahedra in a sawtoothpattern. Its long-range antiferromagnetic ordering detected by specificheat measurements is shrouded in magnetic susceptibility measurementsbecause these orders are clusters of ferromagnetically coupled magneticions. ortho-Pyrovanadate(or ortho-diorthovanadate)K2Mn2 3+Mn2+O(OH)(VO4)(V2O7) synthesized hydrothermally crystallizesin the orthorhombic space group Pnma with a = 17.9155(5), b = 5.8940(2), c = 10.9971(3) & ANGS;, V = 1161.23(6) & ANGS;(3), and Z = 4. Its crystal structure featureslinear chains of edge-sharing Mn3+O6 octahedrawith every second pair of Mn3+O6 octahedra condensedwith a Mn2+O6 octahedron on one side of a chainin a sawtooth pattern so that each sawtooth chain consists of a triangulartrimer. 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. Thenew 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 wasdetected by high-frequency electron spin resonance as well as by bothspecific heat C (p) and Fisher's specificheat d(& chi;T)/dT measurements.However, this phase transition was not detected by magnetic susceptibilitymeasurements. The origin of this puzzling observation was resolvedby evaluating the spin exchanges of K2Mn3O(OH)(VO4)(V2O7), which revealed that each triangulartrimer is a ferromagnetically coupled cluster, and the observed orderinginvolves an AFM ordering between the ferromagnetic (FM) clusters.This ordering is shrouded in magnetic susceptibility measurementsdue to the susceptibility contributions from the individual FM triangulartrimers even below T (N). We showed thatthe magnetic susceptibility of K2Mn3O(OH)(VO4)(V2O7) between & SIM;30 K and roomtemperature is satisfactorily described by an AFM chain made up offerromagnetically coupled triangular clusters, as described by a fewspin-exchange parameters.