Cyanide-bridged RuxNi3-3x/2[Cr(CN)6]2•zH2O molecular magnets: Controlling structural disorder and magnetic properties by a 4d ion (ruthenium) substitution

We report the effect of a 4d ion (Ru3+) substitution on structural disorder and magnetic ordering in the cyanide-bridged molecular magnets, RuxNi3-3x/2[Cr(CN)(6)](2)center dot zH(2)O (0 <= x <= 0.5). Structural analysis, by employing Rietveld refinement of x-ray diffraction patterns, reveals a face centered cubic structure for all these compounds. In the present study, we have controlled the [Cr(CN)(6)] vacancies by substituting Ru at the Ni site and then the role of varying [Cr(CN)(6)] vacancies on the structural correlations and consequently on the magnetic properties has been investigated. We have succeeded in increasing the magnetic ordering temperature (T-C) with increasing Ru concentration up to x=0.2. The IR spectra of the Ru doped compounds indicate the presence of Cr2+-C N-Ru4+ type sequences amounting to an additional structural disorder. The magnetic Bragg scattering in the neutron diffraction patterns is masked by a large diffuse scattering arising due to the structural disorder. The reverse Monte Carlo calculations quantify the total structural disorder, which enhances with increasing x. With x>0.2, a combined effect of the stronger magnetic interaction between the 3d/4d-3d spin carriers (due to reduction in vacancies) and the reduction in the average spin of the Ru/Ni site leads to a decrease in the TC. An antiferromagnetic coupling of Ru3+ (low spin, S=1/2) moments with both Ni2+ (S=1) and Cr3+ (S=3/2) moments is found. The present study thus gives a pathway for designing new molecular magnets by controlling vacancies, which leads to a tuning of the intertwined structural and magnetic properties. (C) 2010 American Institute of Physics. [doi:10.1063/1.3311966]