Spin frustration in M-II[C(CN)(3)](2) (M = V, Cr). A magnetism and neutrondiffraction study

Three-dimensional coordination network solids of M-II[C(CN)(3)](2) (M = V, Cr) composition possess interpenetrating rutile-like network structures. Ea ch [C(CN)(3)](-) bonds to three different metal ions in a triangular array, affording a geometrical topology akin to a Kagome lattice leading to compe ting spin exchange interactions and spin frustration. The crystal and magne tic structure of Cr-II[C(CN)(3)] was determined by Rietveld refinement of t he powder neutron diffraction data at 2 and 15 K and belongs to the orthorh ombic space group Pmna [a = 7.313(1) Angstrom, b = 5.453(1) Angstrom, c = 1 0.640(1) Angstrom, Z = 2, T = 15 K]. Each Cr-II has a tetragonally elongate d-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. Mag netic susceptibility measurements between 1.7 and 300 K reveal strong antif erromagnetic 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 magn etic 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 t o Jahn-Teller distorted Cr-II sites relieving frustration in one dimension, leading to 2-D Ising antiferromagnetism, as observed by both magnetic susc eptibility and specific heat studies. Neutron diffraction experiments at 2 K for Cr[C(CN)(3)](2) yielded additional Bragg reflections as a result of a ntiferromagnetic ordering with the moments on the Cr-II atoms aligned paral lel to c and 4.7(1) mu(B). Fitting of the magnetic order parameter to a pow er 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.