RBMN4(ASO4)(3) - MOLTEN-SALT SYNTHESIS, STRUCTURE, AND MAGNETIC-PROPERTIES OF A NEW MANGANESE(II) ARSENATE

Crystals of RbMn4(AsO4)(3) have been grown in an eutectic flux of CsCl /RbCl. The X-ray single-crystal structure analysis shows that the rubi dium manganese(II) arsenate crystallizes in an orthorhombic lattice wi th a = 10.109(1) Angstrom, b = 17.408(1) Angstrom, c = 6.552(1) Angstr om, and V = 1153.0(2) Angstrom(3); Pnnm (No. 58); Z = 4. The structure is refined by the least-squares method to a final solution with R/R(w )/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 forma tion based on Mn-O linkages are given. The Rb cations reside in the ch annels 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 (d(5)) m anganese cations. An antiferromagnetic transition is observed at ca. 7 .5 K. Structure and bonding of RbMn4(AsO4)(3) vs KFe4(PO4)(3), with re spect to the transition-metal-oxide framework, are discussed. Comparis on to the structurally distinct phases of AMn(4)(AsO4)(3), (A = Na, K) is briefly presented. (C) 1996 Academic Press,Inc.