Martin, G. J.; (2003) Computational and Experimental Studies of Ternary Intercalation Compounds of Graphite. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The structure and dynamics of potassium-ammonia ternary graphite intercalation compounds of type KC24(NH3)x, where x < 4, have been studied using a combination of total energy pseudopotential calculations, and neutron scattering techniques. The main aim of this research is to obtain a qualitative understanding of the interlayer structure and dynamics, and electronic properties. Computational studies at x = 2 and x = 4 show a displacement of the potassium ion from the mid-plane, dependent on c-axis spacing and x. The displacement at x = 2, with a layer spacing of 6.3 A, is 1.09 A, while 6.65 A spacings result in the potassium moving about 1.25 A for x = 2 and 1.17 A in the case of x = 4. The calculations show that ammonia situates itself such that the dipole is pointing away from the potassium ion with the C3-axis tilted to about 10 - 20° dependent on ammonia content. Hirshfeld analysis of charge densities shows that transfer from the guest to the host layer is 0.50 electrons per unit cell for x = 2 and 0.48 electrons for x = 4 compared with the calculated transfer in the potassium GIC of 0.56 and 0.58 using the same atomic positions as the ternary compounds. The potassium ion displacement is found to be responsible for additional back-transfer of electrons into the mid-plane region: for x = 2, the displacement results in an additional back-transfer of 0.02 electrons. Quasi elastic neutron scattering (QENS) was used to examine the diffusional and rotational dynamics of ammonia for 0 < x < 4 over a temperature range 2.5 - 280 K. Models of diffusion and rotation were fitted to the Q-dependence of the QENS peak-widths and elastic incoherent scattering functions, in order to determine the nature of the motion. This shows that a transition in the ammonia dynamics occurs at around 190 K. Above the transition, the motion is characteristic of a hexagonal in-plane jump diffusion, with a characteristic jump length of 4.9±0.2 A and diffusion coefficient of 5.2±0.4 10-5 cm2s-1 at 260K. Below the transition, the motion of the ammonia is found to be spatially restricted to a path around the potassium ion, at a radius of approximately 2.46 A. At 28 K and x= 1.5, the axis of rotation of ammonia about its C3-direction was oriented at 24 ± 17° relative to the graphene planes. This rotation disappears near 15 K.

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