Local and Average Structure in Zinc Cyanide: Toward an Understanding of the Atomistic Origin of Negative Thermal Expansion
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- 作者：Simon J. Hibble ; Ann M. Chippindale ; Elena Marelli ; Scott Kroeker ; Vladimir K. Michaelis ; Brandon J. Greer ; Pedro M. Aguiar ; Edward J. Bilb茅 ; Emma R. Barney ; Alex C. Hannon
- 刊名：Journal of the American Chemical Society
- 出版年：2013
- 出版时间：November 6, 2013
- 年：2013
- 卷：135
- 期：44
- 页码：16478-16489
- 全文大小：485K
- 年卷期：v.135,no.44(November 6, 2013)
- ISSN：1520-5126
文摘
Neutron diffraction at 11.4 and 295 K and solid-state 67Zn 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鈮 groups present in the solid, but yields information about the relative abundances of the different Zn(CN)4鈥?i>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鈥揘 and Zn鈥揅 bond lengths as 1.969(2) and 2.030(2) 脜, respectively. The very small difference in bond lengths, of 0.06 脜, 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鈥揅鈮鈥揨n linkages are displaced to the same side of the Zn路路路Zn axis. Displacements of the carbon and nitrogen atoms to opposite sides of the Zn路路路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.