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@INPROCEEDINGS{Angst:1047034,
      author       = {Angst, M. and Müller, Thomas},
      title        = {{C}oupled charge, orbital, and spin degrees of freedom in
                      geometricallyfrustrated {YF}e2{O}4},
      reportid     = {FZJ-2025-04085},
      year         = {2025},
      abstract     = {Rhombohedral rare earth ferrites RFe2O4 have a strong
                      tendency for coupled charge (Fe2+/3+) andspin order, despite
                      the strong geometrical frustration of the interactions [1].
                      For R=Lu or Yb correspondingsuperstructure reflections or
                      diffuse scattering appear at ( 1313 ℓ). However, the size
                      of Y3+is considerably larger, with calculations [2]
                      suggesting smaller in-plane propagation such as ( 1414
                      ).Here, we focus on single-crystal x-ray diffraction
                      performed on highly stoichiometric single crystals[3] of
                      YFe2O4. 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
                      scatteringat ( 1313 ℓ) is similar to other rare earth
                      ferrites. However, cooling into the magnetic phases,
                      sharpsuperstructure reflections at different positions
                      appear, with propagation ( 272737 ) in IT and ( 141434 )
                      inLT. We refined the superstructure in both phases and
                      applied bond-valence-sum (BVS) analysis toprobe 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 theother two close to 3+.
                      Although of ( 1414 )-type, the arrangement of the valences
                      differs from the predictionin [2]. Structural distortions
                      also suggest (ferro) orbital order of the Fe2+ sites.The IT
                      phase is also refined in P1. 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. Ofthe 7 Fe sites, BVS shows 3 to
                      have valences very close to 2+, indicating a strong
                      localization of 3electrons on these sites. However, the BVS
                      of one of the sites is about 2.5, and for the remainingsites
                      also further away from the ideal 3 (BVS~2.8). Thus the
                      localization of electrons in the IT phaseis 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.},
      month         = {Oct},
      date          = {2025-10-07},
      organization  = {JCNS Workshop 2025, Trends and
                       Perspectives in Neutron Scattering.
                       Quantum Materials: Theory and
                       Experiments, Evangelische Akademie
                       Tutzing (Germany), 7 Oct 2025 - 9 Oct
                       2025},
      subtyp        = {Invited},
      cin          = {JCNS-2 / JARA-FIT / JCNS-4},
      cid          = {I:(DE-Juel1)JCNS-2-20110106 / $I:(DE-82)080009_20140620$ /
                      I:(DE-Juel1)JCNS-4-20201012},
      pnm          = {632 - Materials – Quantum, Complex and Functional
                      Materials (POF4-632) / 6G4 - Jülich Centre for Neutron
                      Research (JCNS) (FZJ) (POF4-6G4)},
      pid          = {G:(DE-HGF)POF4-632 / G:(DE-HGF)POF4-6G4},
      typ          = {PUB:(DE-HGF)6},
      url          = {https://juser.fz-juelich.de/record/1047034},
}