Site occupancy of Fe²⁺, Fe³⁺and Ti⁴⁺in titanomagnetite determined by valence-difference contrast in synchrotron X-ray resonant scattering

Abstract

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 = 1.7441 Å (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 À7.45 and À6.50, 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 3+ 1.00 ] A [Fe 3+ 0.38 Fe 2+ 1.31 Ti 4+ 0.31 ] B O 4 , where A and B in an AB 2 O 4type 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. research papers J. Synchrotron Rad. (2018). 25, 1694-1702 Maki Okube et al. Fe 2+ , Fe 3+ and Ti 4+ in titanomagnetite 1695 Figure 1 Crystal structure of titanomagnetite, showing A and B sites and oxygen atoms.

Hidden chemical order in disordered Ba7Nb4MoO20 revealed by resonant X-ray diffraction and solid-state NMR

“…Thus, the chemical order is an important unresolved issue with numerous materials (Supplementary Table 1 ). Herein, we propose a technique to elucidate the chemical order, which is a combination of resonant X-ray powder diffraction (RXRD) 17 – 21 and solid-state nuclear magnetic resonance (NMR) 22 – 25 assisted by density functional theory (DFT) calculations 26 – 33 . Most materials are polycrystalline or powdered.…”

Empirical and experimental constraints on Fe-Ti oxide-melt titanium isotope fractionation factors

“…Titanium (as Ti 4+ ) is thought to almost exclusively occupy the octahedral site (Fujino, 1974;O'Neill and Navrotsky, 1983;Trestman-Matts et al, 1983;, although some studies have postulated that minor Ti 4+ also exists in the tetrahedral site (Stout and Bayliss, 1980;Sedler et al, 1994). As the Ti content of titanomagnetite increases, the proportion of the ulvöspinel endmember increases which results in a higher proportion of Ti 4+ in the octahedral site at the expense of Fe 3+ , and a concurrent increase in Fe 2+ in both the tetrahedral and octahedral sites to maintain charge balance (Akimoto et al, 1957;Bosi et al, 2009;Pearce et al, 2010;Okube et al, 2018). The ionic radii of Ti 4+ and Fe 3+ occupying VI-fold coordination are relatively similar, at 0.0605 and 0.0645 nm respectively (Shannon, 1976;O'Neill and Navrotsky, 1983).…”

Resolution of the Cationic Distribution in Synthetic Germanite Cu22Fe8Ge4S32 by an Experimental Combinatorial Approach Based on Synchrotron Resonant Powder Diffraction Data: A Case Study and Guidelines for Analogous Compounds

“…Thanks to such simplicity, this principle is at the base of one of the most diffused methods for ab initio structure determination in biological crystallography as the multi-wavelength anomalous dispersion (MAD). − However, although quite a few experiments were performed, ,,− its use is more limited in solid state chemistry. This is probably related to the constraints induced by the coexistence of different “heavy” atoms, the concurrence of other available methods, its relative experimental complexity ( i.e.…”