Spin frustration in MII[C(CN)3]2 (M = V, Cr).: A magnetism and neutron diffraction study

Three-dimensional coordination network solids of M-II[C(CN)(3)](2) (M = V, Cr) composition possess interpenetrating rutile-like network structures. Each [C(CN)(3)](-) bonds to three different metal ions in a triangular array, affording a geometrical topology akin to a Kagome lattice leading to competing spin exchange interactions and spin frustration. The crystal and magnetic structure of Cr-II[C(CN)(3)] was determined by Rietveld refinement of the powder neutron diffraction data at 2 and 15 K and belongs to the orthorhombic space group Pmna [a = 7.313(1) Angstrom, b = 5.453(1) Angstrom, c = 10.640(1) Angstrom, Z = 2, T = 15 K]. Each Cr-II has a tetragonally elongated-octahedral structure with four Cr-N(1) distances of 2.077(2) Angstrom and two significantly longer axial Cr-N(2) distances of 2.452(2) Angstrom. Magnetic susceptibility measurements between 1.7 and 300 K reveal strong antiferromagnetic interactions for both V- and Cr[C(CN)(3)](2) with theta = -67 and -46 K, respectively, from a fit to the Curie-Weiss law. Long-range magnetic ordering does not occur for M = V above 1.7 K, in contrast to M = Cr, which antiferromagnetically orders at low temperature. This is attributed to Jahn-Teller distorted Cr-II sites relieving frustration in one dimension, leading to 2-D Ising antiferromagnetism, as observed by both magnetic susceptibility and specific heat studies. Neutron diffraction experiments at 2 K for Cr[C(CN)(3)](2) yielded additional Bragg reflections as a result of antiferromagnetic ordering with the moments on the Cr-II atoms aligned parallel to c and 4.7(1) mu(B). Fitting of the magnetic order parameter to a power law yielded T-N = 6.12(4) K and beta = 0.18(1) consistent with 2-D Ising behavior. A T-N Of 6.13 K is also observed from the specific heat data.