A NEW FERRIC ORTHOARSENATE HYDRATE - STRUCTURE AND MAGNETIC-ORDERING OF FEASO4.3/4H2O

A new hydrated form of FeAsO4 has been synthesized and its crystal structure determined. FeAsO4.3/4H2O crystallizes in a triclinic cell, P1BAR, with cell constants a = 6.600 (2) angstrom, b = 9.015 (2) angstrom, c = 6.539 (1) angstrom, alpha = 104.39 (2)-degrees, beta = 104.40 (2)-degrees, gamma = 84.25 (2)-degrees, V = 364.7 angstrom-3, and Z = 4. It exhibits a novel tetrameric unit of iron octahedra, two FeO6 and two FeO5(H2O), that share edges. These units are interconnected by AsO4 tetrahedra to create a network structure having open channels containing the water molecules bonded to iron as well as water of hydration. Mossbauer spectroscopy measurements over the range T = 298-4.2 K reveal a transition to a three-dimensional magnetically ordered state at approximately 47.5 K. The Zeeman-split spectra allow for resolution of the two inequivalent iron sites for which internal hyperfine fields of H(n) = 535 and 526 kG have been determined at 4.2 K. Magnetic susceptibility measurements indicate antiferromagnetic order below approximately 49 K in agreement with the Mossbauer spectroscopy results, with the paramagnetic Curie temperature-THETA approximately -128 K. The magnetic moment calculated from the Curie-Weiss fit in the paramagnetic region is 5.68-mu-beta. The magnitude of T(Neel) for FeAsO4.3/4H2O (= Fe4(AsO4)4.3H2O) is compared to T(Neel) for other antiferromagnetically ordered iron(III) arsenate compounds.