Chemistry and structure by design: ordered CuNi(CN)₄ sheets with copper(ii) in a square-planar environment

Abstract

Layered copper-nickel cyanide, CuNi(CN)4, a 2-D negative thermal expansion material, is one of a series of copper(ii)-containing cyanides derived from Ni(CN)2. In CuNi(CN)4, unlike in Ni(CN)2, the cyanide groups are ordered generating square-planar Ni(CN)4 and Cu(NC)4 units. The adoption of square-planar geometry by Cu(ii) in an extended solid is very unusual.

Phonon dynamics in the layered negative thermal expansion compounds CuxNi2−x(CN)d'Ambrumenil1, Zbiri2, Chippindale3 et al. 2019Phys. Rev. B11070View full textAdd to dashboardCite

“…In CuNi(CN)4, the change is probably occurring in the local stacking order, due to the layers sliding over one another more easily as temperature and interlayer separation increases. These changes were not detected in diffraction experiments [11], and were likely masked by the high level of stacking disorder inherent in this material. The linear thermal expansion coefficients of the solid-solution compounds, CuxNi2-x(CN)4 (x = 0.5, 0.33, 0.25.…”

“…The αc and αV coefficients are also correspondingly larger compared to those of Ni(CN)2, and their values increases with temperature [11]. The metal atoms alternate within the sheets as in a checkerboard and, unlike in Ni(CN)2, in which the cyanide ligands show 'head-to-tail' disorder, the cyanide ligands in CuNi(CN)4 are completely ordered with the carbon end bonding to Ni [11]. A single sheet of CuNi(CN)4 is shown in Figure 1.…”

“…The CuxNi2-x(CN)4 solid solution compounds were synthesized from their hydrates, as described previously [11]. Aqueous solutions of CuCl2·2H2O and NiCl2·6H2O in the correct molar ratios were quickly and simultaneously added to an aqueous solution of KCN.…”

“…Key: Cu, blue, Ni, silver, C, brown, N, grey. The unit cell outlined has lattice parameters a = b = 6.99 Å at 15 K [11]. Refined values of the lattice parameters from x-ray diffraction (XRD) and neutron pair distribution function (PDF) analysis, along with relaxed values using different DFT schemes are shown in Figure 7.…”

“…We have also extended our INS measurements to probe the phonon spectra of CuxNi2-x(CN)4 with compositions other than x = 1, namely x = 0.1, 0.25, 0.33 and 0.5, which from part of a solid solution (when 0 ≤ x ≤ 0.5). These materials also have layered structures [11], although it is unlikely that the layers are planar. Here, we demonstrate that they also show 2D NTE.…”