DEFORMATION INDUCED TEXTURE IN MÖSSBAUER ABSORBERS

Résumé
Les anomalies observées dans les intensités des raies d'absorption Mössbauer pour des échantillons exposés à des pressions non hydrostatiques, sont attribuées à des effets de texture. Nous calculons l'influence sur la texture à la déformation de l'échantillon et de la forme des cristallites. De la fonction de texture nous déterminons l'asymétrie d'une doublet quadrupole de 1/2→3/2. Nous discutons les possibilités d'éviter les effets de texture en spectroscopie Mössbauer et nous proposons une méthode pour préparer des absorbants exempts de texture.

Abstract
Mössbauer absorbers subjected to (often unintentional) non-hydrostatic pressure may show pronounced texture. This is a geometrical consequence of deformation. The texture function of a polycrystalline sample after deformation from the original height of unity to p was calculated supposing that the shape of the crystallites can be approximated by a cylinder of radius r and height h. From the texture function the asymmetry R=I_π/I_α of a 1/2→3/2 quadrupole doublet was calculated for the case of axially symmetric EFG and thin absorber : Rq = 3(1 +α) / (5 - 3α) where [MATH] Here s is a shape parameter of the cylinder : s = (π r-2h) / (π r+2h) being -1 for a needle, +1 for a flake and 0 for a "cubic cylinder" (having cross-sections of the same area in two perpendicular directions like a cube). For small deformations δ = 1-p << 1 the asymmetry is : R_q = 1+(2/5)sδ+... From the deformation dependence of R_q an effective shape parameter can be determined which characterizes the deformation-sensitivity of the investigated system. The evaluation of Ericsson and Wäppling's data for chlorite /1/ yielded s = 0.12. For the systems FeCO₃ and FeSiF₆.6H₂O (both mixed with active carbon) the respective values s = 0.21 and s = 0.00 were found. In the light of the above considerations the possibilities of avoiding texture effects in Mössbauer spectroscopy are discussed and a method is suggested for preparing texture-free absorbers.