Neutron diffraction camera method applied in the study of phase transitions

Abstract

The neutron diffraction camera method is used for studying the crystallographic and magnetic phase transitions. Since this method shows a two-dimensional view of the reciprocal space at the same time, it is very useful for observing the pattern of satellite reflections or diffuse scatterings related to any kind of structural phase transitions. And also, since the instrument used is a Laue camera, an oscillating camera and a Weissenberg camera constructed for neutron diffraction, it is possible to determine crystallographic and magnetic structures as by an X-ray diffraction camera method. Neutron diffraction camera D12 at ILL was improved by Wright (1981) to remove a parasitic background, which comes from powder diffractions and from incoherent scatterings of the specimen-holder, radiation shields, and the specimen. The present experiments are done using D12-A and D12-B for studying the low-dimensionality of the crystallographic or magnetic phase transitions by observing the pattern of diffuse scatterings. The following themes are selected in order to test the application of the neutron diffraction camera method: (1) Ferroelectric transition: Since NaNO 2 has two structural transitions at 163 and 164° above room temperature, neutron photographs are taken around the principal axes at each phase by means of a Laue method. It is found that there are two kinds of diffuse scatterings above about 100°C. This fact suggests that the structural transition may be micro-structurally induced from about 100°C. Also, one of the diffuse scatterings is plane-like, in the (010) plane, and the other is rod-like, along [100]. The fine structure of these diffuse scatterings may be related to the two structural transitions by changing the pattern. (2) Low-dimensionality of magnetic short-range ordering: Although there is a short-range ordering of spin correlation above any kind of magnetic transitions, a uniaxial or layer structural compound often shows the low-dimensional one. Several experiments have been done for the specimens K 2CuF 4, K 2Cu x Zn 1− x F 4, KFeF 4, CsFeF 4, BaMnF 4 and TMMC by means of an oscillating camera and a Weissenberg camera. These compounds show a particular pattern, which is related to the short-range correlation among spins.