Short-Range Order in Ferroelectric Triglycine Sulphate
Jessica Hudspeth, now working hard at the ESRF, completed her PhD last year, and got it passed earlier this year.
In summary, she used diffuse x-ray and neutron scattering to look at the local ordering in triglycine sulphate, and learned some new and interesting information about the phase transition. The work shows that the use of diffuse scattering can reveal important new information even for materials that have been heavily studied using other techniques.
This is the abstract:
The short-range order in triglycine sulphate (TGS) was investigated using x-ray and neutron diffraction techniques. Complete deuteration of TGS was required for the neutron diffraction experiments and a new method was developed to grow single crystals of fully deuterated TGS by vapour diffusion crystallisation. The long-range structure of fully deuterated TGS was refined at several temperatures from single crystal neutron diffraction data and found to be consistent with the published structure of hydrogenous TGS. The phase transition temperature was
found to increase from about 322 K to about 334 K with complete deuteration.
The evolution of the long-range structure with temperature was investigated using x-ray and neutron powder diffraction. All of the lattice parameters hada single cusp at the phase transition, except for the b lattice parameter, which also had a second cusp about 34 K below TC. In contrast to the lattice parameter behaviour, the unit cell volume was found to increase monotonically with temperature. The length of the hydrogen bonds between the disordered N atom on glycine 1 (G1) and the surrounding molecules was found to increase with temperature, whereas the length of the short hydrogen bond between G2 and G3 decreased slightly with temperature. This supports the suggestion that weakening of the hydrogen bonds decouples G1 from G2 and G3, allowing the system to become disordered. Except around the ferroelectric to paraelectric phase transition temperature, no abnormalities in the behaviour of any of the refined parameters were observed, suggesting that TGS only has a single phase transition.
The short-range order in TGS was investigated by collecting single crystal x-ray and neutron diffuse scattering at several temperatures from well below to well above TC. Well below TC, the diffuse scattering was purely thermal diffuse scattering due to correlations of the atomic displacements. Close to the phase transition, diffuse streaks perpendicular to b∗ were also present in the diffuse scattering patterns, which were due to short-range order of the G1 orientations parallel to the ferroelectric b axis. The onset of significant short-range order appears to occur about 40 K below TC. The correlations are strongest at the phase transition and then decrease with temperature above TC.
The short-range order was modelled using a combination of a displacive disorder and an orientational disorder Monte Carlo simulation. The intermolecular interactions that give rise to correlated atomic displacements were modelled by treating them like Hooke’s law springs. The force constants for the interactions were parameterised in a number of ways, the most successful of which was an empirical formula developed by Chan et al.
The short-range order of the G1 orientations was modelled using an Ising-type model. The G1 interactions that lead to short-range order along the ferroelectric b-axis appear to be mediated by the short hydrogen bond between G2 and G3. This suggests that the hydrogen bonding, rather than the dipole-dipole interactions, plays the dominant role in the ferroelectric ordering of TGS. While the hydrogen bonding gives rise to strongly correlated chains of G1 molecules along b, it is likely that there are also weaker correlations between the chains due to dipole-dipole
interactions. This provides a mechanism for TGS to go from short-range ordered in 1-dimension, to long-range ordered in 3-dimensions as it is cooled through TC.
And here’s a picture (HTGS means hydrogenous, as distinct from deuterated):