A High-pressure Raman Spectroscopic Study of the Negative Thermal Expansion (NTE) Behaviour of Some Cadmium(II) Cyanide Materials

The structure of dual-network, cadmium cyanide, Cd(CN)2, is that of an infinite cuprite lattice, usually consisting of two interlocking tetrahedral frameworks with Cd atoms at each intersection bridged linearly by four CN groups. This material crystallizes in the cubic Pn 3 m space group, indicating a disordered orientation within the lattice, as also shown earlier by Cd-NMR spectroscopy. Single-network cadmium cyanide, which consists of only one of the interlocking lattices, is a negative thermal expansion (NTE) material. Such materials have promising applications in the construction of new composite materials which can withstand changes in temperature for high-precision applications. The NTE behaviour in Cd(CN)2 is due to the cyanide bridges between the Cd atoms having out-of-plane transverse vibrational modes that constrict the crystal lattice. These vibrational modes become more populated at high temperatures, bringing the Cd atoms closer together and decreasing the size of the unit cell. The presence of a diatomic bridge in the cyanide molecule permits more transverse vibrational modes, thereby allowing certain metal cyanides to show NTE behaviour. These transverse modes can simply be considered to be rotations of M(CN)2 rigid tetrahedra, which preserve the coordination geometry of each metal center, although a more detailed analysis has shown that the NTE vibrational modes also involve transverse motions of the metal centers. Metal cyanide NTE materials have much more negative thermal coefficients of expansion than do metal oxides, such as ZrW(2)O(8), which were the first NTE materials to be discovered1. Theoretical calculations have assigned the vibrational modes responsible for negative thermal expansion and evidence from X-ray diffraction data confirms that the transverse vibrational modes do reduce the distance between metal centres in Zn(CN)2.