Relationship between phonons and thermal expansion in Zn(CN)2and Ni(CN)2from inelastic neutron scattering andab initiocalculations

Abstract

). We have measured the temperature dependence of phonon spectra in these compounds and analyzed them using ab-initio calculations. The spectra of the two compounds show large differences that cannot be explained by simple mass renormalization of the modes involving Zn (65.38 amu) and Ni (58.69 amu) atoms. This reflects the fact that the structure and bonding are quite different in the two compounds. The calculated pressure dependence of the phonon modes and of the thermal expansion coefficient, α V , are used to understand the anomalous behavior in these compounds. Our ab-initio calculations indicate that it is the low-energy rotational modes in Zn(CN) 2 , which are shifted to higher energies in Ni(CN) 2 , that are responsible for the large negative thermal expansion. The measured temperature dependence of the phonon spectra has been used to estimate the total anharmonicity of both compounds. For Zn(CN) 2 , the temperature-dependent measurements (total anharmonicity), along with our previously reported pressure dependence of the phonon spectra (quasiharmonic), is used to separate the explicit temperature effect at constant volume (intrinsic anharmonicity).

CuNi(CN)4: a two-dimensional cyanide framework containing CuIIin a square-planar environment

“…The overall appearance of the powder X-ray diffraction pattern, including the unusual peak shapes of the observed Bragg reflections, has been successfully explained using models incorporating stacking disorder between next nearest neighbour layers. CuNi(CN) 4 shows similar thermal expansion behaviour to that observed previously for Ni(CN) 2 [1,4] with negative thermal expansion within the layers (α a = -9.7 MK -1 ) and positive thermal expansion between the layers (α c = +89 MK The stability of Cu(II) atoms in a cyanideonly environment has been investigated by varying the ratio of the Cu 2+ and Ni 2+ ions used in the synthesis. Using a Cu:Ni ratio of 1:1, the anhydrous phase, CuNi(CN) 4 , is precipitated directly.…”

“…CuNi(CN) 4 shows similar thermal expansion behaviour to that observed previously for Ni(CN) 2 [1,4] with negative thermal expansion within the layers (α a = -9.7 MK -1 ) and positive thermal expansion between the layers (α c = +89 MK The stability of Cu(II) atoms in a cyanideonly environment has been investigated by varying the ratio of the Cu 2+ and Ni 2+ ions used in the synthesis. Using a Cu:Ni ratio of 1:1, the anhydrous phase, CuNi(CN) 4 , is precipitated directly. For Cu:Ni ratios less than one, hydrates of the form Cu 1-x Ni 1+x (CN) 4 ·yH 2 O (½ ≤ x < 1; y ≤ 6) are produced which can be dehydrated to form the corresponding anhydrous compounds, Cu 1-x Ni 1+x (CN) 4 4 , which contains such units, is not found in a hydrated form.…”

“…The structure consists of flat layers of perfectly square-planar [Ni(CN) 4 ] and [Cu(NC) 4 ] units linked by shared cyanide groups i.e. both the metal and cyanide groups are perfectly ordered with Cu(II) coordinated to the nitrogen end of the cyanide group and Ni(II) to the carbon end.…”

“…However, copper(II) can be stabilised in a cyanide-only environment in the stoichiometric, mixed copper-nickel cyanide, CuNi(CN) 4 , and in the solid-solution, Cu 1-x Ni 1+x (CN) 4 (½ ≤ x < 1). The atomic structure of the layers in CuNi(CN) 4 and the stacking relationship between nearestneighbour layers have been determined from total neutron diffraction studies at 10 and 300 K. The structure consists of flat layers of perfectly square-planar [Ni(CN) 4 ] and [Cu(NC) 4 ] units linked by shared cyanide groups i.e.…”

“…Using a Cu:Ni ratio of 1:1, the anhydrous phase, CuNi(CN) 4 , is precipitated directly. For Cu:Ni ratios less than one, hydrates of the form Cu 1-x Ni 1+x (CN) 4 ·yH 2 O (½ ≤ x < 1; y ≤ 6) are produced which can be dehydrated to form the corresponding anhydrous compounds, Cu 1-x Ni 1+x (CN) 4 4 , which contains such units, is not found in a hydrated form. With Cu:Ni ratios above one, partial reduction of the Cu(II) occurs to form LT-CuCN [6], in addition to CuNi(CN) 4 .…”