Variation of structural and magnetic properties with composition in the (Co_xNi_1-x)1.5[Fe(CN)₆]·zH₂O series

Abstract

Structural and magnetic properties of the compounds (Co_xNi_1-x)1.5[Fe(CN)₆]·zH₂O (0⩽x⩽1) have been studied by x-ray and neutron-diffraction, dc magnetization, infrared (IR), and Mössbauer spectroscopic techniques. All studied compounds of the series crystallize in the fcc structure with Fm3m space group. Detailed structural studies by x-ray diffraction followed by Mössbauer and IR studies reveal disordered structures with the presence of Co/Ni−N C−Fe−C N−Co/Ni- and Co/Ni−C N−Fe−N C−Co/Ni-type linear chains along the edges of the unit-cell cube. Neutron-diffraction patterns of all the members of the series have diffuse scattering and a broad peaklike feature at the (100) position which is forbidden under the Fm3m space group. The reverse Monte Carlo simulations of this diffuse scattering, present due to structural disorders, indicate a correlated substitution of water molecules at the empty Fe(CN)₆ sites. The magnetic ordering temperature decreases with increasing amount of the Co substitution and it is minimum for the x = 0.75 compound. Field-dependent magnetization measurements at 1.5 K and up to a maximum field of 70kOe reveal that all compounds (except x = 1) of the series are ferromagnetic with positive exchange interactions between the nearest-neighbor low-spin Co²⁺(S = 1/2)/Ni²⁺ (S = 1) and the mixed-spin Fe³⁺ [low-spin (S = 1/2) and high-spin (S = 5/2)] ions. However, the end compound Co_1.5[Fe(CN)₆]·zH₂O (x = 1) of the series is a ferrimagnet with negative exchange interactions between the high-spin Co²⁺ (S = 3/2) and the low-spin Fe³⁺ (S = 1/2) and also between the low-spin Co²⁺ (S = 1/2) and the high-spin Fe³⁺ (S = 5/2) ions. The observed dip in the values of the lattice parameter, transition temperature, saturation magnetization, coercive field, and remanence around x = 0.75 is ascribed due to the presence of the largest amount of structural disorder around that composition. Neutron-diffraction patterns of the compounds, down to 1.5 K, do show negligibly small magnetic contribution to the fundamental (nuclear) Bragg reflections, however, these contributions are more than one order of magnitude smaller than the modulated diffuse scattering contributions. The effects of structural disorder, due to the presence of vacancies of the Fe(CN)₆ and uncoordinated water molecules, and the quenched disorder, due to random substitution of Ni by Co at the (0, 0, 0) site, on the magnetic ordering in these molecular magnetic compounds have been brought out.