F. Rustichelli

Evidence by X-ray diffraction for hexatic B and crystal B structures in a pure compound

Abstract This letter presents an X-ray experiment which provides direct evidence of the existence in a pure compound of both B modifications, namely the crystal B and the hexatic B phases. In the hexatic B phase a hexagonal 1 0 0 diffuse peak was observed for which a satisfactory fit was obtained by assuming an in-plane structure factor of Lorentzian shape in agreement with theoretical predictions, leading to an in-plane positional correlation length ξ ∣ = 60 ± 6 A .

Neutron diffraction study of the smectic C-smectic a phase transition of TBBA

Abstract The smectic C-smectic A transition of terephtal bis butyl aniline (TBBA) is investigated using neutron diffraction. The critical exponent relating the tilt angle to the reduced temperature is found to be β = 0.44 ± 0.04. Comparison with results obtained by other methods and with theoretical predictions is presented.

Simple dynamical theory approach to neutron diffraction by mosaic crystals

Abstract A simple model based on dynamical theory of neutron diffraction by perfect crystals was used for the description of neutron diffraction by non-absorbing mosaic crystals. The model predictions were compared with the theory of Bacon and Lowde for mosaic crystals. From the satisfactory agreement it appears that no substantial difference exists between primary and secondary extinctions. In addition this work opens up the possibility to derive, in the near future, theoretical predictions for neutron diffraction by curved mosaic crystals.

Overcoming of the 12 reflectivity limit in Laue neutron diffraction

Abstract Experimental evidence for the overcoming of the 50% peak reflectivity limit in nuetron diffraction in the Laue geometry is presented. Peak reflectivity up to 75%, in spite of the neutron absorption, has been obtained with a bent germanium crystal. This effect has been explained by a simple model based on the dynamical theory of neutron diffraction and appears as a consequence of the elimination, due to a proper curvature, of the Pendellosung effect.