Hydrothermal synthesis, thermal, structural, spectroscopic and magnetic studies of the Mn5-xCox(HPO4)(2)(PO4)(2)(H2O)(4) (x=1.25, 2, 2.5 and 3) finite solid solution

摘要

The Mn5-xCox(HPO4)(2)(PO4)(2)(H2O)(4) (x = 1.25, 2, 2.5, 3) finite solid solution has been synthesized by mild hydrothermal conditions under autogeneous pressure. The phases crystallize in the C2/e space group with Z = 4, belonging to the monoclinic system. The unit-cell parameters obtained from single crystal X-ray diffraction are: a = 1 7.525(1), b 9.0535(6), c 9.4517(7) angstrom, beta = 96.633(5)degrees being R1 = 0.0436, wR2 = 0.0454 for Mn75Co25; a = 17.444(2), b = 9.0093(9), c = 9.400(1)angstrom, beta = 96.76(1)degrees being R1 = 0.0381, wR2 = 0.0490 for Mn60Co40; a = 17.433(2), b = 8.9989(9), c = 9.405(1) angstrom, beta = 96.662(9)degrees being R1 = 0.0438, wR2 = 0.0515 for Mn50Co50 and a = 17.4257(9), b = 8.9869(5), c = 9.3935(5) angstrom, = 96.685(4)degrees being R1 = 0.0296, wR2 = 0.0460 for Mn40Co6O. The structure consists of a three dimensional network formed by octahedral pentameric entities (Mn,CO)(5)O-16(H2O)(6) sharing vertices with the (PO4)(3-) and (HPO4)(2-) tetrahedra. The limit of thermal stability of these Compounds is, approximately, 165 degrees C, near to this mean temperature the phases loose their water content in two successive steps. IR spectra show the characteristic bands of the water molecules and the phosphate and hydrogen-phosphate oxoanions. The diffuse reflectance spectra are consistent with the presence Of MO6 octahedra environments in slightly distorted octahedral geometry, except for the M(3)O-6 octahedron which presents a remarkable distortion and so a higher Dq parameter. The mean value for the Dq and B-Racah parameter for the M(1),(2)O-6 octahedra is 685 and 850 cm(-1), respectively. These parameters for the most distorted M(3)O-6 polyhedron are 825 and 880 cm(-1) respectively. The four phases exhibit antiferromagnetic couplings as the major magnetic interactions. However, a small spin canting phenomenon is observed at low temperatures for the two phases with major content in the anisotropic-Co(II) cation. (c) 2007 Published by Elsevier Inc.