Sprecher
Beschreibung
Rhombohedral rare earth ferrites $R$Fe$_2$O$_4$ have a strong tendency for coupled charge (Fe$^{2+/3+}$) and spin order, despite the strong geometrical frustration of the interactions [1]. For $R$=Lu or Yb corresponding superstructure reflections or diffuse scattering appear at ($\frac{1}{3}\frac{1}{3}\ell$). However, the size of Y$^{3+}$ is considerably larger, with calculations [2] suggesting smaller in-plane propagation such as ($\frac{1}{4}\frac{1}{4}$).
Here, we focus on single-crystal x-ray diffraction performed on highly stoichiometric single crystals [3] of YFe$_2$O$_4$. Magnetization vs $T$ indicates two subsequent highly hysteretic magnetic transitions, separating a low-temperature (LT), an intermediate (IT), and a high-temperature (paramagnetic, HT) phase. XRD shows that these transitions are structural as well. In the HT phase, diffuse scattering at ($\frac{1}{3}\frac{1}{3}\ell$) is similar to other rare earth ferrites. However, cooling into the magnetic phases, sharp superstructure reflections at different positions appear, with propagation ($\frac{2}{7}\frac{2}{7}\frac{3}{7}$) in IT and ($\frac{1}{4}\frac{1}{4}\frac{3}{4}$) in LT. We refined the superstructure in both phases and applied bond-valence-sum (BVS) analysis to probe the charge order [4].
In the LT phase, the structure is refined in P-1, with the inversion center between the two Fe layers. The BVS indicates a full charge order, with two of the Fe sites having valence close to 2+ and the other two close to 3+. Although of ($\frac{1}{4}\frac{1}{4}$)-type, the arrangement of the valences differs from the prediction in [2]. Structural distortions also suggest (ferro) orbital order of the Fe$^{2+}$ sites.
The IT phase is also refined in $P\overline{1}$. A clear splitting of one Y indicates the presence of disorder, which is likely connected to a fragile nature of this phase, which was not theoretically predicted. Of the 7 Fe sites, BVS shows 3 to have valences very close to 2+, indicating a strong localization of 3 electrons on these sites. However, the BVS of one of the sites is about 2.5, and for the remaining sites also further away from the ideal 3 (BVS~2.8). Thus the localization of electrons in the IT phase is incomplete, and it must be a dynamic situation with electron hopping occurring.
[1] M. Angst, Physica Status Solidi RRL, 7, 383 (2013).
[2] M. Naka, A. Nagano, and S. Ishihara, Phys. Rev. B, 77, 224441 (2008).
[3] Th. Mueller, J. de Groot, J. Strempfer, and M. Angst, J. Crystal Growth, 428, 40 (2015).
[4] Th. Mueller and M. Angst, submitted to PRB.
