Study of manganese phosphate or arsenate hydrates, MnXO4.nH2O (X = P, As), phases and synthesis and structure of the simple, novel salt MnAsO4

Abstract

Mn111XOcnH20 (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 MnXOcH20 and amorphous MnX04.mH20 with m - 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 MnAs04 to be prepared by careful decomposition of MnAsOcnH20, and the crystal structure (monoclinic, space group P2'/n, a = 6.679(3) A, b = 8.940(3) A, c = 4.791(2) A, j3 = 93.76(4)O) has been determined from laboratory X-ray powder diffraction data using the Rietveld method (RwP = 7.4%, Rp = 5.5%, and RF = 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 + 21 and [2 + 2 + 21 Jahn-Teller distortions are linked through distorted AS04 groups.