Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2021 Sep 1;2(1):8-22.
doi: 10.1021/acsorginorgau.1c00019. eCollection 2022 Feb 2.

Expanding the Ambient-Pressure Phase Space of CaFe2O4-Type Sodium Postspinel Host-Guest Compounds

Justin C Hancock  1   2 , Kent J Griffith  1   2 , Yunyeong Choi  2   3 , Christopher J Bartel  2   3 , Saul H Lapidus  2   4 , John T Vaughey  2   5 , Gerbrand Ceder  2   3   6 , Kenneth R Poeppelmeier  1   2
Affiliations

Expanding the Ambient-Pressure Phase Space of CaFe2O4-Type Sodium Postspinel Host-Guest Compounds

Justin C Hancock et al. ACS Org Inorg Au. .

Abstract

CaFe2O4-type sodium postspinels (Na-CFs), with Na+ occupying tunnel sites, are of interest as prospective battery electrodes. While many compounds of this structure type require high-pressure synthesis, several compounds are known to form at ambient pressure. Here we report a large expansion of the known Na-CF phase space at ambient pressure, having successfully synthesized NaCrTiO4, NaRhTiO4, NaCrSnO4, NaInSnO4, NaMg0.5Ti1.5O4, NaFe0.5Ti1.5O4, NaMg0.5Sn1.5O4, NaMn0.5Sn1.5O4, NaFe0.5Sn1.5O4, NaCo0.5Sn1.5O4, NaNi0.5Sn1.5O4, NaCu0.5Sn1.5O4, NaZn0.5Sn1.5O4, NaCd0.5Sn1.5O4, NaSc1.5Sb0.5O4, Na1.16In1.18Sb0.66O4, and several solid solutions. In contrast to earlier reports, even cations that are strongly Jahn-Teller active (e.g., Mn3+ and Cu2+) can form Na-CFs at ambient pressure when combined with Sn4+ rather than with the smaller Ti4+. Order and disorder are probed at the average and local length-scales with synchrotron powder X-ray diffraction and solid-state NMR spectroscopy. Strong ordering of framework cations between the two framework sites is not observed, except in the case of Na1.16In1.18Sb0.66O4. This compound is the first example of an Na-CF that contains Na+ in both the tunnel and framework sites, reminiscent of Li-rich spinels. Trends in the thermodynamic stability of the new compounds are explained on the basis of crystal-chemistry and density functional theory (DFT). Further DFT calculations examine the relative stability of the CF versus spinel structures at various degrees of sodium extraction in the context of electrochemical battery reactions.

PubMed Disclaimer

Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
CaFe2O4 structure. Gray spheres represent Na+. Green and blue octahedra represent the two symmetrically independent framework sites comprising the two “double rutile” chains.
Figure 2
Figure 2
Schematic summarizing the synthetic approach and results presented in this paper. The ion combinations in green were successfully synthesized in the postspinel structure, whereas the ion combinations in red formed alternative phases.
Figure 3
Figure 3
Rietveld refinement using synchrotron data for NaMg0.5Ti1.5O4. Blue crosses are the observed intensities, the green curve is the fitted pattern, the black curve is the difference pattern, and the red tick marks indicate the location of the CF-NaMg0.5Ti1.5O4 peaks.
Figure 4
Figure 4
Powder XRD patterns for the 1:1:1 and 0.99:0.99:1.01 Na/Cr/Ti samples compared to a calculated pattern for the CF phase. Asterisks highlight peaks from the secondary phase Na0.6Cr0.6Ti0.4O2 in the 1:1:1 Na/Cr/Ti sample.
Figure 5
Figure 5
A combinatorial representation of the Na-CF phase space. The phase-space shown covers all Na-CF compounds and combinations known to have been attempted in previous studies or this work. Conceivable but unexplored combinations (e.g., Sc3+ and Mo4+) are not depicted.
Figure 6
Figure 6
23Na solid-state NMR spectra at 12.5 kHz MAS and 9.4 T: (a) Na1.16In1.18Sb0.66O4, (b) Na0.96In0.96Sn1.04O4, and (c) “NaSc1.5Sb0.5O4” central transition resonances.
Figure 7
Figure 7
Unit cell volume of the new Na-CFs versus the weighted-average effective ionic radius of the framework cations in octahedral coordination.
Figure 8
Figure 8
(a) Computed stability of NaxA3+B4+O4 and NaxA2+0.5B4+1.5O4 CF phases with three degrees of sodiation (x = 0, 0.5, 1.0). (b) Energy above hull of each xNa = 1 compound after including ideal mixing entropy for the octahedral cations at 1200 K. Green and red boxes show successful/failed syntheses at ambient pressure. (c) Energy difference between the CF and spinel structure. Note that the empty boxes correspond to In-containing compounds that failed to converge in either the CF or spinel structures and were consequently excluded from this analysis.
See this image and copyright information in PMC

References

    1. Bertraut E.-F.; Blum P.; Magnano G. Structure des vanadite, chromite et ferrite monocalciques. Bull. Mineral. 1956, 79, 536–561.
    1. Hill P. M.; Peiser H. S.; Rait J. R. The crystal structure of calcium ferrite and β calcium chromite. Acta Crystallogr. 1956, 9, 981–986. 10.1107/S0365110X56002862. - DOI
    1. Irifune T.; Fujino K.; Ohtani E. A new high-pressure form of MgAl2O4. Nature 1991, 349, 409–411. 10.1038/349409a0. - DOI
    1. Funamori N.; Jeanloz R.; Nguyen J. H.; Kavner A.; Caldwell W. A.; Fujino K.; Miyajima N.; Shinmei T.; Tomioka N. High-pressure transformations in MgAl2O4. J. Geophys. Res. Solid Earth 1998, 103, 20813–20818. 10.1029/98JB01575. - DOI
    1. Liu L. High pressure NaAlSiO4: the first silicate calcium ferrite isotype. Geophys. Res. Lett. 1977, 4, 183–186. 10.1029/GL004i005p00183. - DOI