Spin fluctuations and orbital ordering in quasi-one-dimensional a-Cu(dca) 2 (pyz) {dca = dicyanamide = N(CN) 2  ; pyz = pyrazine}, a molecular analogue of KCuF 3 Jamie L. Manson a, * , Tom Lancaster b , Stephen J. Blundell b , Yiming Qiu c,d , John Singleton e , Pinaki Sengupta e,f , Francis L. Pratt g , Jinhee Kang h , Changhoon Lee h , Myung-Hwan Whangbo h a Department of Chemistry and Biochemistry, Eastern Washington University, Cheney, WA 99004, USA b Clarendon Laboratory, Department of Physics, Oxford University, Oxford, OX1 3PU, UK c NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA d Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742, USA e National High Magnetic Field Laboratory, Los Alamos National Laboratory, Los Alamos, NM 87545, USA f Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA g ISIS Pulsed Muon Facility, Rutherford Appleton Laboratory, Chilton, Oxfordshire, OX11 0QX, UK h Department of Chemistry, North Carolina State University, Raleigh, NC 27695, USA article info Article history: Available online 1 July 2009 Keywords: Low-dimensional Copper Dicyanamide Pyrazine Molecular magnet abstract The magnetic properties of a-Cu(dca) 2 (pyz) were examined by magnetic susceptibility, magnetization, inelastic neutron scattering (INS), muon-spin relaxation (lSR) measurements and by first-principles den- sity functional theoretical (DFT) calculations and quantum Monte Carlo (QMC) simulations. The v versus T curve shows a broad maximum at 3.5 K, and the data between 2 and 300 K is well described by an S = 1/ 2 Heisenberg uniform chain model with g = 2.152(1) and J/k B = 5.4(1) K. lSR measurements, conducted down to 0.02 K and as a function of longitudinal magnetic field, show no oscillations in the muon asym- metry function A(t). This evidence, together with the lack of spin wave formation as gleaned from INS data, suggests that no long-range magnetic order takes place in a-Cu(dca) 2 (pyz) down to the lowest mea- sured temperatures. Electronic structure calculations further show that the spin exchange is significant only along the Cu–pyz–Cu chains, such that a-Cu(dca) 2 (pyz) can be described by a Heisenberg antiferro- magnetic chain model. Further support for this comes from the M versus B curve, which is strongly con- cave owing to the reduced spin dimensionality. a-Cu(dca) 2 (pyz) is a molecular analogue of KCuF 3 owing to d x 2 y 2 orbital ordering where nearest-neighbor magnetic orbital planes of the Cu 2+ sites are orthogonal in the planes perpendicular to the Cu–pyz–Cu chains. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Over the past several years, we and others have been interested in the magnetic properties of dicyanamide (dca) containing coordi- nation polymers. Much of the interest in these materials can be attributed to the coordinative versatility of the dca ligand and its subsequent role as a superexchange mediator [1]. To date, several modes have been identified (Scheme 1) although the vast majority of the reported polymeric coordination compounds consist of l 1,5 -dca bridges [2]. Three-coordinate M–dca complexes are exem- plified by rutile-like a-M(dca) 2 (M = V, Cr, Mn, Fe, Co and Ni) in which the M ion is divalent [1]. Combining dca bridges with auxil- iary organic ligands of varying denticity can lead to zero-, one-, two- and three-dimensional (0D, 1D, 2D and 3D, respectively) spin dimensionality. The 3D polymeric structures exhibited by a-M(dca) 2 (pyz) (M = Mn [3,4], Fe [4], Co [4], Ni [4], Cu [5], and Zn [4]; pyz = pyra- zine) are particularly interesting in that they possess 2-fold inter- penetrating networks. Two-dimensional (2D) M(dca) 2 layers are linked via M–dca–M bridges that are further connected by l-pyra- zine ligands to yield a rigid 3D lattice. In essence the general struc- ture of the a-phase can be viewed as 1D M–pyz–M chains that are joined to form a 3D framework via l 1,5 -dca ligands. It should be noted that b-phases of the same chemical composition can be pro- duced for M = Co, Ni, Cu, and Zn although this structure type con- sists of 2D sheets of 1D bi-bridged M–(dca) 2 –M ribbons that are linked via l-pyz ligands [4,5]. Of particular interest to us is a-Cu(dca) 2 (pyz) because it can be described as a molecular analogue of KCuF 3 . While a low-temper- ature crystal structure and variable-temperature magnetic 0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2009.06.063 * Corresponding author. Tel.: +1 (509)359 2878; fax: +1 (509)359 6973. E-mail address: jmanson@ewu.edu (J.L. Manson). Polyhedron 29 (2010) 514–520 Contents lists available at ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly