Structures and negative thermal expansion properties of the one-dimensional cyanides, CuCN, AgCN and AuCN

The behaviour of the lattice parameters of HT-CuCN (high-temperature form), AgCN and AuCN have been investigated as a function of temperature over the temperature range 90-490 K. All materials show one-dimensional negative thermal expansion (NTE) along the -(M-C N)- chain direction c (alpha(c)(HT-CuCN) = -32.1 x 10(-6) K-1, alpha(c)(AgCN) = -23.9 x 10(-6) K-1 and alpha(c)(AuCN) = -9.3 x 10(-6) K-1 over the temperature range 90-490 K). The origin of this behaviour has been studied using RMC modelling of Bragg and total neutron diffraction data from AgCN and AuCN at 10 and 300 K. These analyses yield details of the local motions within the chains responsible for NTE. The low-temperature form of CuCN, LT-CuCN, has been studied using single-crystal X-ray diffraction. In this form of CuCN, wavelike distortions of the -(Cu-C N)-chains occur in the static structure, which are reminiscent of the motions seen in the RMC modelling of AgCN and AuCN, which are responsible for the NTE behaviour.