Spin Frustration in M^II[C(CN)₃]₂ (M = V, Cr). A Magnetism and Neutron Diffraction Study

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) A, b = 5.453(1) A, c = 10.640(1) A, Z = 2, T = 15 K]. Each CrII has a tetragonally elongated octahedral structure with four Cr-N(1) distances of 2.077(2) A and two significantly longer axial Cr-N(2) distances of 2.452(2) A. 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 CrII site 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) microB. Fitting of the magnetic order parameter to a power law yielded TN = 6.12(4) K and beta = 0.18(1) consistent with 2-D Ising behavior. A TN of 6.13 K is also observed from the specific heat data.

Reminder: Magnetic structures description and determination by neutron diffraction

“…The positions and the intensities of the Bragg peaks for the whole scattering pattern are calculated from a parametrized model assuming a certain shape of the peaks and a width of these peaks 02001-p. 13 Collection SFN varying according to the experimental resolution. The parameters of the model are least-squares fitted so that the calculated pattern reproduces the observed one, allowing to determine separately very close Bragg peaks.…”

“…Such a task requires three steps, which are (i) the identification of the propagation vectors k, (ii) the determination of the coupling between the Fourier components m ν,k and (iii) the determination of the direction and amplitudes of these Fourier components. To illustrate these different points, we will take a real example of an investigation in a molecular compound, the Cr[C(CN) 3 ] 2 complex [13].…”

Design of Spin-Frustrated Monomer-Type C60•− Mott Insulator

“…However, QSL systems are scarce in materials with spin quantum numbers S > 1/2, even in the triangular spin lattices and kagome lattices with f ≥ 10 2 [30][31][32]. Even for the triangular S = 1/2 spin systems with large |Θ CW | and f, no QSL materials have been prepared due to the difficulty in maintaining the precise geometry of spin-frustrated lattices at low temperatures [33][34][35].…”

Unusual magnetic-field dependence of partially frustrated triangular ordering in manganese tricyanomethanide

“…The tricyanomethanide anion, C(CN) 3 -, has three-fold symmetry and therefore appears to be an ideal candidate. Recently, the synthesis and magnetic properties of the series M[C(CN) 3 ] 2 , with M = transition metal, has been reported [7][8][9][10][11]. As described in detail below, these compounds may be regarded as examples for partially frustrated triangular lattices represented by the "row model" introduced by Kawamura [12] and Zhang et al [13].…”

“…The critical behavior of Cr[C(CN) 3 ] 2 is consistent with the 2D Ising model. No ordering above 2 K was observed in V[C(CN) 3 ] 2 [10] as well as the Cu, Ni and Co compounds [9,11]. Here we describe heat capacity and neutron diffraction studies of the magnetic ordering below 1.18 K in Mn[C(CN) 3 ] 2 .…”