Cyanide-bridged Ru_xNi_3-3x/2[Cr(CN)₆]₂·zH₂O molecular magnets: controlling structural disorder and magnetic properties by a 4d ion (ruthenium) substitution

Abstract

We report the effect of a 4d ion (Ru³⁺) substitution on structural disorder and magnetic ordering in the cyanide-bridged molecular magnets, Ru_xNi_3-3x/2[Cr(CN)₆]₂·zH₂O (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 T_C. 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.