Local and Average Structure in Zinc Cyanide: Toward an Understanding of the Atomistic Origin of Negative Thermal Expansion

Abstract

Neutron diffraction at 11.4 and 295 K and solid-state (67)Zn NMR are used to determine both the local and the average structures in the disordered, negative thermal expansion (NTE) material, Zn(CN)2. Solid-state NMR not only confirms that there is head-to-tail disorder of the C≡N groups present in the solid, but yields information about the relative abundances of the different Zn(CN)4–n(NC)n tetrahedral species, which do not follow a simple binomial distribution. The Zn(CN)4 and Zn(NC)4 species occur with much lower probabilities than are predicted by binomial theory, supporting the conclusion that they are of higher energy than the other local arrangements. The lowest energy arrangement is Zn(CN)2(NC)2. The use of total neutron diffraction at 11.4 K, with analysis of both the Bragg diffraction and the derived total correlation function, yields the first experimental determination of the individual Zn–N and Zn–C bond lengths as 1.969(2) and 2.030(2) Å, respectively. The very small difference in bond lengths, of ~0.06 Å, means that it is impossible to obtain these bond lengths using Bragg diffraction in isolation. Total neutron diffraction also provides information on both the average and the local atomic displacements responsible for NTE in Zn(CN)2. The principal motions giving rise to NTE are shown to be those in which the carbon and nitrogen atoms within individual Zn–C≡N–Zn linkages are displaced to the same side of the Zn···Zn axis. Displacements of the carbon and nitrogen atoms to opposite sides of the Zn···Zn axis, suggested previously in X-ray studies as being responsible for NTE behavior, in fact make negligible contributions at temperatures up to 295 K.

On the Scope of XPS as Sensor in Coordination Chemistry of Transition Metal Hexacyanometallates

“…In the hydrated phase, Zn atom are found with octahedral coordination, and it crystallizes with the cubic unit cell typical for PBAs, but when it is dehydrated under moderated heating (> 60°), a structural transition takes place to a solid that crystallizes with a hexagonal unit cell, R 3 c space group, where the Zn atom is found with a tetrahedral coordination. Zn(CN) 2 has a diamond‐like crystal structure with the Zn atom in tetrahedral coordination …”

A Material Showing Colossal Positive and Negative Volumetric Thermal Expansion with Hysteretic Magnetic Transition

“…[13,14] In 1,t he rotation of the [Fe2Tp-(CN) 3 ] À units induce the enhancements of the interlayer p···p interactions between the two interlayer pyrazolyl rings of the [Fe2Tp(CN) 3 ] À units,w ith the p···p interaction distances decrease from 3.769(7) at 300 Kt o3 .595(9) at 350 K (Supporting Information, Figure S11). [13,14] In 1,t he rotation of the [Fe2Tp-(CN) 3 ] À units induce the enhancements of the interlayer p···p interactions between the two interlayer pyrazolyl rings of the [Fe2Tp(CN) 3 ] À units,w ith the p···p interaction distances decrease from 3.769(7) at 300 Kt o3 .595(9) at 350 K (Supporting Information, Figure S11).…”

“…Moreover,t he NTE arises not from ad ecrease of interatomic distances between bonded atoms with increasing temperature,b ut from ad ecrease in the distance between non-bonded atoms. [13,14] In 1,t he rotation of the [Fe2Tp-(CN) 3 ] À units induce the enhancements of the interlayer p···p interactions between the two interlayer pyrazolyl rings of the [Fe2Tp(CN) 3 ] À units,w ith the p···p interaction distances decrease from 3.769 (7) at 300 Kt o3 .595(9) at 350 K (Supporting Information, Figure S11). Thee nhanced p···p interactions induce acloser packing and decrease the distance…”

Quantitative understanding of negative thermal expansion in scandium trifluoride from neutron total scattering measurements

“…This approach is used to refine configurations of atoms so that both their long-range and their local structure are consistent with experimental data. While there have been a few reports of total scattering measurements of NTE materials [15,[17][18][19][20][21][22][23][24][25][26] [19,21], has the method been used to generate an atomic model of the fluctuations associated with NTE to provide a consistent examination of the tension effect. From our analysis of the atomic configurations across a wide range of temperatures generated in this study we show that the fluctuations associated with the tension effect are a mix of whole-body rotations and bond-bending distortions of ScF 6 octahedra.…”