Neutron diffuse scattering and its application to studies of disorder in solids

J. Kulda

Institut Laue-Langevin, BP 156, 380042 Grenoble Cedex, France

kulda@ill.eu

Similarly to its X-ray analogue the diffuse intensity of neutrons observed between the Bragg spots carries information on local departures from perfect periodicity of the crystal lattice. Due to the general drawback of the neutron scattering techniques - the lack of neutron sources with brightness comparable at least to present-day X-ray laboratory sources - it can only be observed with relatively bulky samples and relaxed beam collimations, which prevent to achieve high momentum resolution. As a result it is virtually impossible to collect reliable diffuse scattering intensities closer than 1-10 x 10^-2 Å^-1 to a Bragg spot. This limits the field of view to correlations ranging over just a few unit cells and pushes the focus of the technique from investigations of subtle effects related to lattice defects, as is often the case with X-rays, to studies of short-range order and of the ordering processes.

On the other hand, in these domains the general features of neutron scattering like

• low absorption

comparable contributions from both light and heavy atoms

straightforward separation of the elastic and inelastic response

• magnetic scattering channel, which can be separated by polarization analysis

can represent substantial advantages.

In recent years diffuse scattering studies [1] of magnetic systems exhibiting novel types of magnetic order have produced a considerable impact uncovering new types of nanoscale topological order in systems like frustrated antiferromagnets, considered as disordered to classical criteria [2].

Another focus of interest in the last decade represent the investigations of relaxor ferroelectric, where “chemical frustration” (impossibility to regularly distribute atoms of solution components onto a perovskite lattice) prevents the long-range ferroelectric ordering [3]. Here effects of true chemical (dis)order superpose with strong distortions of the initially cubic perovskite lattice and call for complex molecular dynamics simmulations to reproduce the extremely rich diffuse scattering response [4].

References

1. T. Fennell, P.P. Deen, A.R. Wildes, K. Schmalzl, D. Prabhakaran, A.T. Boothroyd, R.J. Aldus, D.F. McMorrow, S.T. Bramwell, Science, 326, (2009), 415.

2. H.B. Braun, Advances in Physics, 61, (2012), 1.

3. R.A. Cowley, S.N. Gvasaliya, S.G. Lushnikov, B. Roessli, G.M. Rotaru, Advances in Physics 60, (2011), 229.

4. M. Pasciak, T.R. Welberry, J. Kulda, M. Kempa, J. Hlinka, Phys. Rev. B, (2012), accepted.