Tricyanomethane Magnets
Spin Frustration in MII[C(CN)3]2 (M = V, Cr). A Magnetism and Neutron Diffraction Study
, J. L. Manson, E. Ressouche, J. S. Miller, Inorg. Chem. 39, 1135-1141 (2000).Abstract
Three-dimensional coordination network solids of MII[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 Kagomé lattice leading to competing spin exchange interactions and spin frustration. The crystal and magnetic structure of CrII[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), b = 5.453(1), c = 10.640(1) Å, Z = 2, T = 15 K]. Each CrII has a tetragonally-elongated octahedral structure with four Cr-N(1) distances of 2.077 (2) Å and two significantly longer axial Cr-N(2) distances of 2.452 (2) Å. Magnetic susceptibility measurements between 1.7 and 300 K reveal strong antiferromagnetic interactions for both V- and Cr[C(CN)3]2 with
q = -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 CrII 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 CrII atoms aligned parallel to c and 4.7 (1) mB. Fitting of the magnetic order parameter to a power law yielded TN = 6.12 (4) K and b = 0.18 (1) consistent with 2-D Ising behavior. A Tc of 6.13 K is observed from the specific heat data.
Illustration of the magnetic structure of a single lattice of Cr[C(CN)3]
2 at 2 K. The arrows delineate the spin orientation of the Cr ordered magnetic moment. The unit cell is outlined and only one interpenetrating network has been shown for clarity.
Variation of the Cr magnetic moment of Cr[C(CN)3]2 as a function of temperature. The solid line represents the best of the data to a power law giving TN = 6.12 (4) K and
b = 0.18 (1)
Specific heat of Cr[C(CN)3]2 taken between 2 and 20 K. Temperature dependence of the first derivative of the specific heat, Cp(T)/T, of Cr[C(CN)3]2 (inset).