STRUCTURAL AND MAGNETIC-PROPERTIES OF TRANSITION-METAL ARSENATES, AAS(2)O(6), A=MN, CO, AND NI

The divalent transition metal arsenates with the general formula AAs(2)O(6), where A is Mn, Co, Ni, were prepared. Their crystal structures were refined using the Rietveld profile method with neutron powder diffraction data in the space group P-31m. NiAs2O6 has the unit cell dimensions (Angstrom) a = 4.7585(7), c = 4.4349(4), Z = 1, and V = 86.97 Angstrom(3); for CoAs2O6, a = 4.7768(12), c = 4.4968(8), and V = 88.09 Angstrom(3); and for MnAs2O6 a = 4.7956(9), c = 4.6923(6), and V = 93.45 Angstrom(3). Magnetic properties were examined down to 5 K. Magnetic susceptibility data reveal the presence of long-range order in these materials, while higher temperature data resemble those of a simple antiferromagnet and no indication of the presence of any short-range order was observed except in the case of NiAs2O6, in which rather weak ferromagnetic correlations were detected. The magnetically ordered state was investigated by the use of the low-temperature neutron diffraction. NiAs2O6 and CoAs2O6 have the same propagation vector, k = (00 1/2), while an incommensurate magnetic structure was observed for MnAs2O6, in which the magnetic superlattice reflections were indexed with the propagation vector k = (0.055, 0.389, 0.136). These results are compared with the phase diagrams of Reimers and Dahn for a triangular lattice with ..AAA.. stacking. (C) 1995 Academic Press, Ice.