ИСТИНА

Magnetically active transition-metal carbodiimides have been known for less than a decade. Among these, the phase CuNCN, which may be looked upon as the nitrogen analogue of cupric oxide, CuO, was first synthesized and structurally characterized (space group Cmcm) in 2005 [1]. Since then its somewhat mysterious magnetic properties have been under investigation but are far from being fully understood. For example, while the first-order Jahn–Teller distortion found for the nitrogen-coordinated Cu2+ ion as well as the magnetic susceptibility allude to antiferromagnetic order between the spin-½ sites, the first neutron-diffraction studies carried out with the SV7 diffractometer at the DIDO reactor did not show any sign of a magnetic ordering [2]. Likewise, diffraction studies using polarized neutrons by use of the DNS machine at FRM II did not evidence any magnetic Bragg peaks [3]. Herein, we focus on the results of a high-resolution temperature-dependent structural study of CuNCN carried out at the POWGEN instrument at the SNS. Puzzingly enough, the diffraction data clearly manifest a surprising non-monotonous behavior of the a lattice parameter as a function of the temperature: its decrease while the temperature lowers turns to a distinguishable increase below 100 K. This effect in the copper spin-½ system was related to the activated behavior of ESR- and SQUID-measured spin susceptibilities. In addition, an adequate, yet approximate, picture of a two-dimensional resonating valence-bond state has been proposed for CuNCN [4]. References [1] X. Liu, M. A. Wankeu, H. Lueken, R. Dronskowski, Z. Naturforsch. 60b, 593 (2005). [2] X. Liu, R. Dronskowski, R. K. Kremer, M. Ahrens, C. Lee, M.-H. Whangbo, J. Phys. Chem. C 112, 11013 (2008). [3] H. Xiang, X. Liu, R. Dronskowski, J. Phys. Chem. C 113, 18891 (2009). [4] A. Zorko, P. Jeglič, A. Potočnik, D. Arčon, A. Balčytis, Z. Jagličič, X. Liu, A. L. Tchougréeff, R. Dronskowski, Phys. Rev. Lett. 107, 047208 (2011).