ИСТИНА

CuNCN is a nitrogen-based analog of copper oxide [1]. Although stoichiometrically and structurally it is quite similar to the other members of the transition-metal carbodiimide family, its magnetic behavior strikingly differs. While all others are more or less standard antiferromagnets, the nonmetallic CuNCN exhibits no magnetic scattering of polarized neutrons down to several Kelvin [1, 2]. At intermediate temperatures (between 80 and 350 K) CuNCN shows temperature-independent paramagnetism, which changes to a fair Arrhenius decay of the susceptibility below 80 K [3]. We have related [3] the above features to the formation of spin-liquid (resonating valence bond - RVB) phases of unpaired electrons residing on Cu2+ ions similar to tentative ground states of the high-Tc superconducting cuprates [4]. In [5] we have suggested tentative structural manifestations of the 1D- to 2D-RVB transition confirmed by a recent synchrotron and neutron studies [6]. Moreover, we could extract the electronic heat capacity of CuNCN and attribute it to the spin-liquid [7]. Very remarkably the spin-liquid states cannot be reproduced by any available solid state quantum chemistry software which is absolutely and alternativelessly dominated by the Hartree-Fock approximation. The very possibility of such a ground state is not programmed. The situation is in a way scandalous in a view of importance of the RVB states for the high-Tc behavior, but also due to its possible wider occurrence in practice as proven by our CuNCN experience, which remains elusive due to restrictions of the software. References [1] X. Liu, R. Dronskowski, R. K. Kremer, M. Ahrens, C. Lee, M.-H. Whangbo, J. Phys. Chem. C 2008, 112, 1101. [2] H. Xiang, X. Liu, R. Dronskowski, J. Phys. Chem. C 2009, 113, 18891. [3] A. Zorko, P. Jeglič, A. Potočnik, D. Arčon, A. Balčytis, Z. Jagličić, X. Liu, A. L. Tchougréeff, R. Dronskowski, Phys. Rev. Lett. 2011, 107, 047208. [4] P. W. Anderson, Science 1987, 235, 1196. [5] A.L. Tchougréeff, R. Dronskowski. J. Phys.: Cond. Matt. 2013, 25, 435602. [6] A. L. Tchougréeff, X. Liu, P. Müller, W. van Beek, U. Ruschewitz, R. Dronskowski, J. Phys. Chem. Lett. 2012, 3, 3360; P. Jacobs, A. Houben, A.L. Tchougréeff, R. Dronskowski, J. Chem. Phys. 2013, 139, 224707. [7] A.L. Tchougréeff, R.P. Stoffel, A. Houben, P. Jacobs, R. Dronskowski, M. Pregelj, A. Zorko, D. Arčon, O. Zaharko. J. Phys.: Cond. Matt., 2017, 29, 235701.