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DOE OSTI2020

Materials Data on NaMg by Materials Project

NaMg is beta-prime cadmium gold structured and crystallizes in the orthorhombic Pmma space group. The structure is three-dimensional. Na is bonded to four equivalent Na and eight equivalent Mg atoms to form NaNa4Mg8 cuboctahedra that share corners with eight equivalent MgNa8Mg4 cuboctahedra, corners with ten equivalent NaNa4Mg8 cuboctahedra, edges with six equivalent NaNa4Mg8 cuboctahedra, edges with twelve equivalent MgNa8Mg4 cuboctahedra, faces with eight equivalent MgNa8Mg4 cuboctahedra, and faces with twelve equivalent NaNa4Mg8 cuboctahedra. There are two shorter (3.29 Å) and two longer (3.34 Å) Na–Na bond lengths. There are a spread of Na–Mg bond distances ranging from 3.42–3.60 Å. Mg is bonded to eight equivalent Na and four equivalent Mg atoms to form MgNa8Mg4 cuboctahedra that share corners with eight equivalent NaNa4Mg8 cuboctahedra, corners with ten equivalent MgNa8Mg4 cuboctahedra, edges with six equivalent MgNa8Mg4 cuboctahedra, edges with twelve equivalent NaNa4Mg8 cuboctahedra, faces with eight equivalent NaNa4Mg8 cuboctahedra, and faces with twelve equivalent MgNa8Mg4 cuboctahedra. There are two shorter (3.23 Å) and two longer (3.29 Å) Mg–Mg bond lengths.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Expanding the Ambient-Pressure Phase Space of CaFe 2 O 4 -Type Sodium Postspinel Host–Guest Compounds

CaFe 2 O 4 -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 NaCrTiO 4 , NaRhTiO 4 , NaCrSnO 4 , NaInSnO 4 , NaMg 0.5 Ti 1.5 O 4 , NaFe 0.5 Ti 1.5 O 4 , NaMg 0.5 Sn 1.5 O 4 , NaMn 0.5 Sn 1.5 O 4 , NaFe 0.5 Sn 1.5 O 4 , NaCo 0.5 Sn 1.5 O 4 , NaNi 0.5 Sn 1.5 O 4 , NaCu 0.5 Sn 1.5 O 4 , NaZn 0.5 Sn 1.5 O 4 , NaCd 0.5 Sn 1.5 O 4 , NaSc 1.5 Sb 0.5 O 4 , Na 1.16 In 1.18 Sb 0.66 O 4 , and several solid solutions. In contrast to earlier reports, even cations that are strongly Jahn–Teller active (e.g., Mn 3+ and Cu 2+ ) can form Na-CFs at ambient pressure when combined with Sn 4+ rather than with the smaller Ti 4+ . 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 Na 1.16 In 1.18 Sb 0.66 O 4 . 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.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2020

Materials Data on NaMgSO4F by Materials Project

NaMg(SO4)F crystallizes in the monoclinic C2/c space group. The structure is three-dimensional. Na1+ is bonded in a 7-coordinate geometry to six O2- and one F1- atom. There are a spread of Na–O bond distances ranging from 2.44–2.75 Å. The Na–F bond length is 2.19 Å. Mg2+ is bonded to four O2- and two equivalent F1- atoms to form MgO4F2 octahedra that share corners with two equivalent MgO4F2 octahedra and corners with four equivalent SO4 tetrahedra. The corner-sharing octahedral tilt angles are 41°. There are two shorter (2.14 Å) and two longer (2.16 Å) Mg–O bond lengths. Both Mg–F bond lengths are 1.92 Å. S6+ is bonded to four O2- atoms to form SO4 tetrahedra that share corners with four equivalent MgO4F2 octahedra. The corner-sharing octahedra tilt angles range from 35–49°. There is two shorter (1.49 Å) and two longer (1.50 Å) S–O bond length. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded in a 4-coordinate geometry to two equivalent Na1+, one Mg2+, and one S6+ atom. In the second O2- site, O2- is bonded in a 3-coordinate geometry to one Na1+, one Mg2+, and one S6+ atom. F1- is bonded in a distorted trigonal planar geometry to one Na1+ and two equivalent Mg2+ atoms.

36 MATERIALS SCIENCE↗