Site occupancy of Fe~(2+), Fe~(3+) and Ti~(4+) in titanomagnetite determined by valence-difference contrast in synchrotron X-ray resonant scattering

摘要

A synchrotron X-ray diffraction study of a single crystal of titanomagnetite shows that the cation distribution of Fe~(2+), Fe~(3+) and Ti~(4+) is of the inverse-spinel type. The valence-difference contrast (VDC) method of resonant scattering was applied at a wavelength of λ = 1.7441 ?(E = 7.1085 keV) within the pre-edge of the Fe K absorption spectrum, utilizing the large difference in the real part of anomalous scattering factors, between -7.45 and -6.50, for Fe~(2+) and Fe~(3+), respectively. The most plausible atomic arrangement in Ti_(0.31)Fe_(2.69)O_4 obtained from our analysis is [Fe_(1.00)~(3+) ]~A[Fe_(0.38)~(3+) Fe_(1.31)~(2+) Ti_(0.31)~(4+) ]~BO_4, where A and B in an AB_2O_4- type structure correspond to the tetrahedral and octahedral sites, respectively. This result suggests that titanomagnetite has the complete inverse-spinel structure continuously from the end-member of magnetite, even in the case of relatively high Ti content. The physical properties may be described by the Né el model, which claims that Fe~(3+) preferentially occupies the tetrahedral site, within a Ti-poor half-region of the solid solution. Based on the ordering scheme the magnetic structure of titanomagnetite is considered to be analogous to that of magnetite. The combination of circularly polarized X-rays and a horizontal-type four-circle diffractometer used in this VDC technique has the advantage of increasing the experimental accuracy and freedom with the simultaneous reduction of experimental noise.