We present an exploration of a family of compositionally complex cubic spinel ferrites featuring combinations of Mg, Fe, Co, Ni, Cu, Mn, and Zn cations, systematically investigating the average and local atomic structures, chemical short-range order, magnetic spin configurations, and magnetic properties. All compositions result in ferrimagnetic average structures with extremely similar local bonding environments; however, the samples display varying degrees of cation inversion and, therefore, differing apparent bulk magnetization. Additionally, first-order reversal curve analysis of the magnetic reversal behavior indicates varying degrees of magnetic ordering and interactions, including potentially local frustration. Finally, reverse Monte Carlo modeling of the spin orientation demonstrates a relationship between the degree of cation inversion and the spin collinearity. Collectively, these observations correlate with differences in synthesis procedures. This work provides a framework for understanding magnetic behavior reported for “high-entropy spinels,” revealing many are likely compositionally complex oxides with differing degrees of chemical short-range order—not meeting the community established criteria for high or medium entropy compounds. Moreover, this work highlights the importance of reporting complete sample processing histories and investigating local to long-range atomic arrangements when evaluating potential entropic mixing effects and assumed property correlations in high entropy materials.
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Elucidating the Factors That Cause Cation Diffusion Shutdown in Spinel-Based Electrodes
We report on a systematic study of guest cation (i.e., Li, Na, or Mg) diffusion within spinel intercalation compounds, a promising class of materials for Li-, Na-, and Mg-ion batteries. Using kinetic Monte Carlo simulations, we identify factors that are responsible for a strong concentration dependence of the cation diffusion coefficient. We focus on spinels in which the guest cations prefer the octahedral sites and where diffusion is mediated by vacancy clusters. Starting with MgyTiS2, we predict an abrupt drop in the Mg diffusion coefficient that spans several orders of magnitude around y ≈ 0.5 due to the onset of highly correlated Mg diffusion. The prediction is consistent with previous experimental studies that are only able to achieve half the theoretical capacity of MgyTiS2. We next perform a parametric study of diffusion in spinels using kinetic Monte Carlo simulations applied to lattice model Hamiltonians and identify a critical topological weakness of the spinel crystal structure that makes it prone to highly correlated cation diffusion at intermediate-to-high guest cation concentrations. We find that the onset of this highly correlated diffusion becomes more pronounced as the nearest-neighbor repulsion between pairs of guest cations becomes stronger, since this increases the dependence of long-range cation diffusion on triple-vacancy clusters. The results of this study provide guidance with which the concentration dependence of cation diffusion coefficients in spinel can be tailored to reduce the onset of sluggish diffusion at high cation concentrations. The conclusions drawn from this study also apply to other close-packed anion hosts such as disordered rocksalt electrodes and partially ordered spinels.
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- Award ID(s):
- 2004693
- NSF-PAR ID:
- 10323250
- Date Published:
- Journal Name:
- Chemistry of materials
- Volume:
- 33
- Issue:
- 16
- ISSN:
- 0897-4756
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/acs.chemmater.1c01668
