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

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.


Figure S1
Rietveld refinement for NaCrTiO4. 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-NaCrTiO4 peaks.

Figure S2
Rietveld refinement for NaRhTiO4. 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-NaRhTiO4 peaks.

Figure S3
Rietveld refinement for NaCrSnO4. Blue crosses are the observed intensities, the green curve is the fitted pattern, the black curve is the difference pattern, the red tick marks indicate the location of the CF-NaCrSnO4 peaks, and the magenta tick marks indicate the location of NaCrO2 peaks.

Figure S4
Rietveld refinement for NaMnSnO4. 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-NaMnSnO4 peaks. The inset is included to show that the peak at 3.7° is an unknown impurity and too far away from the calculated angle of the (1 0 1) peak. Thus, cation site preference is either very weak or nonexistent in NaMnSnO4.

Figure S5
Rietveld refinement for NaInSnO4. 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-NaInSnO4 peaks.

Figure S6
Rietveld refinement for NaMg0.5Sn1.5O4. 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-NaMg0.5Sn1.5O4 peaks.

Figure S7
Rietveld refinement for NaCo0.5Sn1.5O4. Blue crosses are the observed intensities, the green curve is the fitted pattern, the black curve is the difference pattern, the red tick marks indicate the location of the CF-NaCo0.5Sn1.5O4 peaks, and the magenta tick marks indicate the location of SnO2 peaks.

Figure S8
Rietveld refinement for NaNi0.5Sn1.5O4. Blue crosses are the observed intensities, the green curve is the fitted pattern, the black curve is the difference pattern, the red tick marks indicate the location of the CF-NaNi0.5Sn1.5O4 peaks, and the magenta tick marks indicate the location of SnO2 peaks.

Figure S9
Rietveld refinement for NaCu0.5Sn1.5O4. Blue crosses are the observed intensities, the green curve is the fitted pattern, the black curve is the difference pattern, the red tick marks indicate the location of the CF-NaCu0.5Sn1.5O4 peaks, and the magenta tick marks indicate the location of SnO2 peaks.

Figure S10
Rietveld refinement for NaZn0.5Sn1.5O4. 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-NaZn0.5Sn1.5O4 peaks.

Figure S11
Rietveld refinement for "NaSc1.5Sb0.5O4." Blue crosses are the observed intensities, the green curve is the fitted pattern, the black curve is the difference pattern, the red tick marks indicate the location of the CF-NaSc1.5Sb0.5O4 peaks, and the magenta tick marks indicate the location of Sc2O3 peaks.

Figure S12
Rietveld refinement for Na1.16In1.18Sb0.66O4. 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-Na1.16In1.18Sb0.66O4 peaks.

Figure S13
Rietveld refinement for NaFe0.5Ti1.5O4. 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-NaFe0.5Ti1.5O4 peaks.

Figure S14
Rietveld refinement for NaMn0.5Sn1.5O4. 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-NaMn0.5Sn1.5O4 peaks.

Figure S15
Rietveld refinement for NaFe0.5Sn1.5O4. Blue crosses are the observed intensities, the green curve is the fitted pattern, the black curve is the difference pattern, the red tick marks indicate the location of the CF-NaFe0.5Sn1.5O4 peaks, and the magenta tick marks indicate the location of SnO2 peaks.

Figure S16
Rietveld refinement for NaCd0.5Sn1.5O4. Blue crosses are the observed intensities, the green curve is the fitted pattern, the black curve is the difference pattern, the red tick marks indicate the location of the CF-NaCd0.5Sn1.5O4 peaks, and the magenta tick marks indicate the location of SnO2 peaks.