Strong local lattice instability in hexagonal ferritesRFe2O4(R= Lu,Y,Yb) revealed by x-ray absorption spe

Abstract

We present here an x-ray absorption study of the RFe 2 O 4 series (LuFe 2 O 4 , YFe 2 O 4 , YbFe 2 O 4 , and LuCoFeO 4 ) at the Fe K-edge. Extended x-ray absorption fine structure (EXAFS) and x-ray absorption near-edge structure (XANES) spectra were measured at temperatures ranging from 100 K to 390 K crossing the charge ordering (CO) transition temperatures on both isotropic and oriented powder samples. Unpolarized and polarized x-ray absorption spectra with the x-ray polarization parallel and perpendicular to the hexagonal c axis were obtained separately. The XANES spectra show almost no dependence with either polarization or temperature. This indicates that, in contrast to its average crystallographic structure, the local electronic and geometrical state of the Fe atom is barely anisotropic, and it remains the same above and below the proposed CO. Moreover, the linear combination of two spectra corresponding to the pure ionic states Fe 2+ and Fe 3+ does not fit to the experimental XANES of either LuFe 2 O 4 , YFe 2 O 4 , or YbFe 2 O 4 , discarding total ionic segregation in favor of an intermediate mixed valence state. The maximum charge disproportionation compatible with the XANES spectra is 0.5 ± 0.1 electrons in the whole temperature range. The polarized EXAFS spectra have allowed us to analyze the local structure separating the different contributions mixed up in the polycrystal. Best-fit results show that the five oxygen atoms of the first coordination shell are at nearly the same distance but with a large Debye-Waller factor. Neither the interatomic distances nor the Debye-Waller factors of the first oxygen coordination shell change with temperature, indicating that the dynamical structural distortion of the high-temperature symmetric hexagonal phase freezes upon cooling down. Thus, the local structure instability of the mixed valence is in the origin of the structural transitions.

Structural properties inRFe2O4compounds (R=Tm, Yb, and Lu)

“…First, the refined temperature factors are very high, especially for oxygen atoms, indicating a strong disorder. This agrees with the dynamic disorder deduced around the Fe atoms in a previous x-ray absorption investigation of these ferrites [32]. Second, the BVS analysis shows that R atoms are overbonded, whereas Fe atoms are underbonded, and the deviation from the valence sum rule increases with increasing the R size.…”

“…Table I shows the refined parameters obtained at 400, 300, and 80 K. The high temperature factor observed for all atoms is noticeable, especially for oxygen atoms. This result is likely related to the observation of local distortions by means of x-ray absorption spectroscopy [32], and it may be also related to the incommensurate oxygen displacement patterns outlined for the YbFe 2 O 4 compound [20].…”

Stoichiometric YFe2O4−δ single crystals grown by the optical floating zone method

“…The experiment was performed at the P09 beamline [50,51] at PETRA III. The resonant features in the spectrum in Figure 7 suggest some involvement of Fe-ions in the ordering, which would be consistent with charge ordering of Fe 2.5±x , where the amount of charge separation is still in debate in the related LuFe 2 O 4−δ [52][53][54] and other ferrites [55]. In the trigonal bipyramidal crystal field the Fe 3d orbitals are split to (d xy ,d x 2 −y 2 ) and (d yz ,d xz ) doublets and a (d z 2 ) singlet [56].…”

P-T Phase Diagram of LuFe2O4

“…Low pressure spectrum is consistent with those previously reported in the literature [13], Figure 13. High quality, high-pressure X-Ray absorption spectra data of LuFe 2 O 4 at the Fe K edge were normalized to the jump at the absorption edge and a clear change is observed at about 16 GPa, Figure 13.…”