Negative Thermal Expansion of Cyanides

Negative Thermal Expansion of Cyanides

Thermal expansionThermal Expansion is the change in volume of a material when heated.

Generally, materials increase in volume when heated.

A materials 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 metalcarbon and metalnitrogen 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

single-atom and (b) diatomic linkages *Black circle= heavy atomsWhite circle=bridging atoms e.g cyanideDisplacement same direction as axisDisplacement different directions to axisComputer modelling of the latticeTo 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 (ZnxCd1-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

Zn60

Zn80600KZn20 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.Zn20 1000

Zn40 1000

Zn60 1000

Zn80 1000

Calculations100k -0.02395500k -0.01726Better Formats

ConclusionMany difficulties needed to learn Linux quicklyHowever managed to get data despite time pressure. Many areas for further research such as introduction of guests/different geometries.Interested to see experimental resultsMany thanks