SITE-SELECTIVE DOPING OF THE CHARGE ORDER IN MAGNETITE

X-ray diffraction was used to show that one of the eight distinct Fe2+ sites within the complex Fe2+/Fe3+ ordered structure is selectively oxidised in a 2%-doped magnetite. This charge order within a charge order effect reflects the instability of the surrounding trimeron and could be used in selective redox reactions or for information storage.

STRUCTURE OF MATERIALS

130 ESRF

The electronic state of magnetite (Fe3O4) at low temperatures has been investigated since the Verwey transition at 125 K was reported in 1939 [1]. A full structure determination in 2012 revealed a complex ordering pattern of Fe2+/Fe3+ charge states, Fe2+ d-orbitals, and weak Fe-Fe bonding effects within three-Fe-atom trimeron units, as shown in Figure 112 [2].

Previous work has shown that magnetite can be hole-doped to an upper limit near 3.5% [3]. The structural effects of doping have been investigated here by comparing the structure of the undoped sample (#1) measured previously [2] with those of an Fe-deficient sample (#2) with 1.16% hole-doping, and of a Zn-doped material (#3) at 2.28%. Crystal structures were determined by microcrystal X-ray diffraction [2] using data collected at beamline ID11.

Doped structures #2 and #3 both show the same qualitative charge ordering arrangement as undoped sample #1. Three local distortion mode parameters were used to detect any small changes in the charge localisation. Qrad measuring radial expansion is sensitive to the size difference between charge-ordered Fe2+ and Fe3+ cations. Tetragonal Jahn-Teller distortion due to Fe2+ orbital order is given by QJT, and shortening of Fe-Fe distances in the trimeron around each Fe2+ is quantified by the Qtri mode. Oxidation of an Fe2+ cation within the charge ordered array is expected to decrease Qrad and increase QJT and Qtri. Hence the differences ∆Q = −∆Qrad, ∆QJT, and ∆Qtri, between values for undoped sample #1 and for the doped samples #2 and #3 are used to compare changes for the eight distinct Fe2+ sites in the bar chart of Figure 113.

Almost all of the ∆Q distortion mode changes in Figure 113 are positive, demonstrating that the refinements are sensitive to the tiny structural changes from 1-2% hole doping, and their distribution across the Fe2+ sites tends towards uniform as the defects used for chemical doping are located randomly with respect to the charge ordering. However, it is striking that site B42 in crystal #3 has exceptionally large changes in all three modes. This reveals a site- selectivity which is remarkable for being seen at such a low hole-doping. The 2.28% doping of sample #3 is only around one-fifth of the 12.5% level needed to fully oxidise one of the eight distinct Fe2+ sites in the charge ordered superstructure.

The low temperature structure of magnetite was previously shown to consist of a network of corner sharing trimerons [2], and seven of the eight structurally distinct trimerons have

Fig. 112: Orders of the charge states (with Fe2+/Fe3+ shown as

blue/yellow spheres) and trimerons (green ellipsoids) in the low

temperature unit cell of magnetite. The

selectively oxidised B42 site is labelled and the

unusual termination of its trimeron by another

Fe2+ is marked by a square.

Fig. 113: Changes of the three Fe2+ site distortion amplitudes ∆Q with hole doping of magnetite. Changes for each distinct Fe2+ site between the

undoped sample #1 and doped samples #2 and #3 are shown by the left and right columns respectively. Negative values are shown below the axis. Fe2+ site

labels are taken from [2].