Shoko Yamazaki1, H. Toraya1
1Ceramics Research Laboratory, Nagoya Institute of Technology, Asahigaoka, Tajima 507-0071, Japan
Tóm tắt
The crystal structures of olivine-type Mg_{2-x}MnxSiO4(x = 0, 1, 1.4 and 2) were refined using high-resolution synchrotron radiation data and the Rietveld method, adopting the new weight function with the form 1/Y_o^e\;(e\simeq 2) (Yois the observed profile intensity) [Toraya (1998).J. Appl. Cryst.31, 333–343]. Positional parameters were accurately determined by optimizingein 1/Y_o^e. Derived structures were in accordance with observations on olivine structures, having a constant tetrahedral Si–O distance and octahedral metal–oxygen distances which increase linearly on substitution of octahedral atoms with those having larger atomic radii. Site-occupancy parameters of octahedral atoms were refined under various refinement conditions and could be determined within ±1.2% under the constraint of chemical composition. Deviations ofxfrom assumed chemical compositions, when the site-occupancy parameters were refined without the constraint, were 1.0–3.5% using the new weight function; they are comparable with values obtained in single-crystal studies on Mg–Fe olivine (0.5–2.7%). The use of atomic scattering factors for fully ionized atoms gave better site-occupancy parameters, by 0.1–2.5%, than those for neutral atoms in the present analysis.
