Negative Thermal Expansion of Cyanides

Thermal expansion Thermal Expansion is the change in volume of a material when heated. Generally, materials increase in volume when heated. A material’s “coefficient of thermal expansion” tells you how much its volume changes with temperature over time. A select group of materials actually decrease in size when heated. This is called Negative Thermal Expansion (NTE).

Uses

Why do cyanides undergo NTE? These cyanide-bridged compounds, such as Zn(CN) 2 and Cd(CN) 2 have tough octahedral units with strong metal– carbon and metal–nitrogen bonds joined by loose cyanide bridges. At a given temperature, the C and N atoms oscillate around the M-M axis. The displacement of these atoms can either be in the same direction or in the opposite direction. Both of these modes of oscillation cause the bonds to shorten as the metal atoms are anchored closer together. This increases as the temperature rises, as the oscillations are greater.

Displacement of C and N atoms (a)single-atom and (b) diatomic linkages * Black circle= heavy atoms White circle=bridging atoms e.g cyanide Displacement same direction as axis Displacement different directions to axis

Computer modelling of the lattice To investigate this behaviour, we used the supercomputer to run simulations of Zinc and Cadmium cyanide crystal structures at different temperatures. The computer simulates the various interactions between the particles in the lattice. Surface effects are avoided by simulating a cube where a particle leaving though one side will emerge through the opposite. We interfaced with the supercomputer using the DL_POLY module through Linux. As well as Zinc and Cadmium cyanides, mixtures of zinc and cadmium cyanides were modelled (Zn x Cd 1-x (CN) 2 ). We used different ratios of Zn and Cd (x = 0.2,0.4,0.6,0.8).

The Results…

Zn40

Zn20 Zn40 Zn60Zn80 600K

Zn20 at 300K

Zn20 at 400K As temperature increases, so does bond energy. The greater the bond energy, the more vibrations so more displacement in either of the modes, the shorter the bond length.

Zn

Zn

Zn

Zn

Calculations 100k  k 

Better Formats

Conclusion Many difficulties – needed to learn Linux quickly However managed to get data despite time pressure. Many areas for further research such as introduction of ‘guests’/different geometries. Interested to see experimental results Many thanks