STUDY OF MNXO4.NH2O (X = P, AS) PHASES AND SYNTHESIS AND STRUCTURE OF THE SIMPLE, NOVEL SALT MNASO4

MnIIIXO4.nH2O (X = P, As; n = 1.0-1.5) phases have been characterized through electron microscopy, density measurements, thermal analysis, and X-ray powder diffraction. Samples with n > 1 behave as intergrowths of stoichiometric, crystalline MnXO4.H2O and amorphous MnXO4.mH2O with m approximately 4. A detailed study of the thermal behavior of these systems has revealed that the presence of the amorphous phase strongly affects the dehydration and reduction temperatures. The phosphate and arsenate always decompose through different pathways, as the phosphate undergoes reduction of Mn3+ to Mn2+ before water loss commences, whereas the arsenate shows a reversible dehydration before being irreversibly reduced. This has enabled the simple new salt MnAsO4 to be prepared by careful decomposition of MnAsO4.nH2O, and the crystal structure (monoclinic, space group P2(1)/n, a = 6.679(3) angstrom, b = 8.940(3) angstrom, c = 4.791(2) angstrom, beta = 93.76(4)-degrees) has been determined from laboratory X-ray powder diffraction data using the Rietveld method (R(WP) = 7.4%, R(P) = 5.5%, and R(F) = 2.7%). This arrangement is a unique example of a monoclinically-distorted CuSO4 type structure. Chains of edge-sharing MnO6 octahedra containing two different Mn3+ sites that display cooperative [4 + 2] and [2 + 2 + 2] Jahn-Teller distortions are linked through distorted AsO4 groups.