RbMn4(AsO4)3: Molten-Salt Synthesis, Structure, and Magnetic Properties of a New Manganese(II) Arsenate

Abstract

Crystals of RbMn4(AsO4)3have been grown in an eutectic flux of CsCl/RbCl. The X-ray single-crystal structure analysis shows that the rubidium manganese(II) arsenate crystallizes in an orthorhombic lattice witha= 10.109(1) Å,b= 17.408(1) Å,c= 6.552(1) Å, andV= 1153.0(2) Å3;Pnnm(No. 58);Z= 4. The structure is refined by the least-squares method to a final solution withR/Rw/GOF = 0.027/0.032/2.09. The compound is isostructural with KFe4(PO4)3. The new structure description based on the heteropoly [As2Mn3O16] structural units and the detailed account of extended framework formation based on Mn–O linkages are given. The Rb cations reside in the channels and coordinate with [6 + 2] oxygen atoms in an unusual geometry of bicapped trigonal prism where two capping O atoms are directed out beyond the opposite triangular faces of the prism. The observed Mn–O bond distances, BVS calculations, and magnetic susceptibility analysis suggest that the title compound contains high-spin, divalent (d5) manganese cations. An antiferromagnetic transition is observed at ca. 7.5 K. Structure and bonding of RbMn4(AsO4)3vs KFe4(PO4)3, with respect to the transition-metal-oxide framework, are discussed. Comparison to the structurally distinct phases ofAMn4(AsO4)3(A= Na, K) is briefly presented.