Neutron diffraction study of molecular magnetic compound Ni_1.125Co_0.375[Fe(CN)₆]·6.4H₂O

Structural and magnetic properties of Prussian Blue analogue Ni_1.125Co_0.375[Fe(CN)₆]·6.4H₂O have been studied using neutron diffraction (ND) as a main probe. It crystallizes under Fm3m space group as observed for the Ni_1·5−xCo_x[Fe(CN)₆]·zH₂O series. Structure, derived from ND patterns, confirms two different sites for each Fe, Co and Ni ions, consistent with the results of our Mössbauer and IR studies. Two atomic sequences of the type Fe–C–N–Co/Ni–N–C–Fe (91%) and Fe–N–C–Co/Ni–C–N–Fe (9%) are found in the unit cell. The oxygen ions of ∼63% of the water molecules are coordinated to Co/Ni atoms to fill the deficiency of [Fe(CN)₆] units in the lattice. A paramagnetic to ferromagnetic transition at 19.6 K is found in magnetization study. However, no observable magnetic Bragg intensity is found in the ND patterns down to 1.5 K. Strong diffuse scattering (temperature independent) is observed. It appears below the Bragg peaks as well as a broad hump centered around the (1 0 0) position where a Bragg peak is forbidden under Fm3m space group. The diffuse scattering has been explained due to inherent structural disorder. The role of the observed structural disorder on its magnetically ordered state has been brought out