Ref.: EmaFsu05-001

Analysis of the influence of the asymmetry factor on the diffraction profiles by the dynamic theory of x-ray diffraction

Apresentador: Getulio Marcos Silva

Autores (Instituição): Silva, G.M.(UNIVERSIDADE FEDERAL DO CEARÁ); SASAKI, J.M.(UNIVERSIDADE FEDERAL DO CEARÁ);

Resumo:
The dynamic theory of X-ray diffraction is a broader and more comprehensive theory than the kinematic theory. This considers all interactions between electromagnetic waves within the crystal, consequently all physical properties of the medium. These interactions bring about effects that are neglected in the kinematic theory, a more traditional and commonly used theory when it comes to fine crystal refinement. In X-ray diffraction, the width at half height (FWHM), intensity and angular position (2?) are important parameters in the analysis of diffraction profiles, these are directly related to the size of the crystallite, microdeformation, among others. In this work, we will present the theoretical study of how the asymmetry factor (b_ass) and the crystal thickness (t_0) influence the shape of the diffraction profile. Depending on the size of the crystal and the value assigned to the asymmetry factor, the diffraction profiles undergo significant changes in width at half height (FWHM) and intensity. We present diffraction profiles for the nickel and copper crystal referring to reflections (111), (020) and (022) using the fundamental equation of dynamic theory for the Laue case. Initially, we will use as sample crystals of the type whiskers of nickel and copper grown from an alumina and carbon membrane with a diameter from 250 nm was modeled to analyze how the intensity and width at half height vary as t_0 and b_ass vary . Next, we use some distribution functions to represent the asymmetry factor and simulate a randomly distributed polycrystalline system. To analyze how these distributions affected the diffraction profile, we used different distribution widths and confounded with the fundamental dynamical theory equation for the Laue case. Our results show that for thickness values around hundreds of nanometers for the purely symmetric Laue case, the profiles obtained present a diffraction profile that corroborates the two theories of X-ray diffraction. When we vary b_ass and t_0 , it is noticed that the diffraction profile is directly affected. The results obtained showed that for a fixed thickness of 100nm the intensity decreases exponentially as the b_ass value approaches zero. For the widths (FWHM), the exact opposite happens, when b_ass approaches zero, the width (FWHM) grows exponentially.