Both aliovalent doping and the charge state of multivalent lattice ions determine the oxygen non-stoichiometry ( δ ) of mixed ionic and electronic conductors (MIECs). Unfortunately, it has been challenging for both modeling and experiments to determine the multivalent ion charge states in MIECs. Here, the Fe charge state distribution was determined for various compositions and phases of the MIEC La 1−x Sr x FeO 3−δ (LSF) using the spin-polarized density functional theory (DFT)-predicted magnetic moments on Fe. It was found that electron occupancy and crystal-field-splitting-induced differences between the Fe 3d-orbitals of the square pyramidally coordinated, oxygen-vacancy-adjacent Fe atoms and the octahedrally-coordinated, oxygen-vacancy-distant-Fe atoms determined whether the excess electrons produced during oxygen vacancy formation remained localized at the first nearest neighbor Fe atoms (resulting in small oxygen vacancy polarons, as in LaFeO 3 ) or were distributed to the second-nearest-neighbor Fe atoms (resulting in large oxygen vacancy polarons, as in SrFeO 3 ). The progressively larger polaron size and anisotropic shape changes with increasing Sr resulted in increasing oxygen vacancy interactions, as indicated by an increase in the oxygen vacancy formation energy above a critical δ threshold. This was consistent with experimental results showing that Sr-rich LSF and highly oxygen deficient compositions are prone to oxygen-vacancy-ordering-induced phase transformations, while Sr-poor and oxygen-rich LSF compositions are not. Since oxygen vacancy induced phase transformations cause a decrease in the mobile oxygen vacancy site fraction ( X ), both δ and X were predicted as a function of temperature and oxygen partial pressure, for multiple LSF compositions and phases using a combined thermodynamics and DFT approach.
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This content will become publicly available on December 1, 2024
Interplay between Kondo and magnetic interactions in Pr0.75Gd0.25ScGeH
Combined experimental and density functional theory (DFT) study of Pr0.75Gd0.25ScGe and its hydride
(Pr0.75Gd0.25ScGeH) reveals intricacies of composition-structure-property relationships in those distinctly layered
compounds. Hydrogenation of the intermetallic parent, crystalizing in a tetragonal CeScSi-type structure, leads to
an anisotropic volume expansion, that is, a(=b) lattice parameter decreases while the lattice expands along the c
direction, yielding a net increase of cell volume. DFT calculations predict an antiparallel coupling of localized Gd
and Pr magnetic moments in both materials at the ground state. While experiments corroborate this for the
parent compound, there is no conclusive experimental proof for the hydride, where Pr moments do not order
down to 3 K. DFT results also reveal that rare-earth – hydrogen interactions reduce spin-polarization of the Pr
and Gd 5d and Sc 3d states at the Fermi energy, disrupt indirect exchange interactions mediated by conduction
electrons, dramatically reduce the magnetic ordering temperature, and open a pseudo-gap in the majority-spin
channel. Both experiments and theory show evidence of Kondo-like behavior in the hydride in the absence of an
applied magnetic field, whereas increasing the field promotes magnetic ordering and suppresses Kondo-like
behavior.
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- Award ID(s):
- 1944551
- NSF-PAR ID:
- 10487536
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Journal of Alloys and Compounds
- Volume:
- 966
- Issue:
- C
- ISSN:
- 0925-8388
- Page Range / eLocation ID:
- 171351
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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