X-ray and neutron investigations of the P1--I1- transition in pure anorthite

The temperature dependence of sharp and diffuse c and d reflections in pure anorthite was investigated by X-ray and neutron measurements from room temperature up to 700 K. At 513 +- 4 K a complete, reversible and continuous phase transition P1--I1- can be observed. Here the diffuse scattering reaches a maximum. It has a remarkable anisotropy along the b* and (a* - b*) directions. All experimental findings as well as the formation of an antiphase domain structure can be explained by a simple dynamical model. (Auth.)