Negative linear compressibility and complex high-Pstructure in Zn[Au(CN)2]2

Abstract

The structure of the framework material zinc dicyanoaurate is built up of tetrahedrally-coordinated Zn cations linked by linear [NC-Au-CN]- complex anions, which form a six-fold interpenetrated -quartz-type network. This material exhibits strong negative linear thermal expansion along the c direction due to framework flexibility [1]. In situ, single-crystal and powder x-ray diffraction under high pressure in diamond anvil cells indicate that the c lattice parameter increases by 8% from ambient pressure up to 1.8 GPa, which is the highest negative linear compressibility (NLC) known for any material. Structural data obtained from single crystal x-ray diffraction indicate that the NLC mechanism is linked to the closing of N-Zn-N angles and the bending of Au-C-N-Zn linkages. A large number of superlattice reflections are observed above 2 GPa, indicating that the unit cell is doubled in all three directions. This phase transition is accompanied by a strong decrease in NLC. Systematic absences indicate that the space group changes from P6222 to P6422 or vice versa depending on the absolute configuration of the initial crystal corresponding to an instability at one of the L points in the Brillouin zone. The resulting unit cell has the following lattice parameters at 3.6 GPa: a=14.485(2)A, c=45.796(6)A with 528 atoms per unit cell. The ZnAu2 sublattice of the new, high-pressure form was solved by direct methods and followed by the use of difference Fourier maps to locate the C and N atoms. A significant change occurs in the Au sublattice with the Au-Au interatomic vector becoming canted with respect to c. This provides an additional compression mechanism, which reduces the need for the structure to expand along c. These results provide a detailed structural mechanism for the exceptional and unusual mechanical properties of this material. [1] Goodwin, A. L., Kennedy, B. J.. & Kepert, C. J. (2009). J. Am. Chem. Soc. 131, 6334-6335. Keywords: structural phase transitions; high-pressure structure determination; mechanical properties of crystals