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FeCl2 is trigonal omega-like structured and crystallizes in the trigonal R-3m space group. The structure is two-dimensional and consists of three FeCl2 sheets oriented in the (0, 0, 1) direction. Fe2+ is bonded to six equivalent Cl1- atoms to form edge-sharing FeCl6 octahedra. All Fe–Cl bond lengths are 2.47 Å. Cl1- is bonded in a distorted T-shaped geometry to three equivalent Fe2+ atoms.
Here, we report for the first time the formation of a HP-PdF 2 -type FeCl 2 phase (space group Pa$\bar{3}$), through high pressure-temperature (P-T) reactions in the hydrous systems (Mg 0.6 Fe 0.4 )SiO 3 –H 2 O–NaCl and FeO2H–NaCl in a laser-heated diamond-anvil cell up to 108 GPa and 2000 K. Applying single-crystal X-ray diffraction (XRD) analysis to individual submicrometer-sized grains, we have successfully determined the crystal structure of the as-synthesized FeCl 2 phase, in agreement with our theoretical structure search results. In situ high P-T XRD data revealed the substitution of Cl for OH(O) in such a cubic Pa$\bar{3}$ structure, demonstrating that this topology is a potential host for both H and Cl in the deep Earth. The chemical analysis of the recovered sample showed that the post-perovskite phase contains considerable amounts of Na 2 O and Fe 2 O 3 . The coexistence of the cubic FeCl 2 phase and post-perovskite suggests that the lowermost mantle could be a potential reservoir of Cl. The possible presence of volatiles such as H and Cl in the deep lower mantle would impact the composition and iron valence state of the post-perovskite phase.
FeCl2(O)4Sb(OCl3)2O2 crystallizes in the triclinic P1 space group. The structure is one-dimensional and consists of one hydrogen peroxide molecule; one FeCl2(O)4 cluster; and one Sb(OCl3)2 ribbon oriented in the (1, 0, 0) direction. In the FeCl2(O)4 cluster, Fe is bonded in a 6-coordinate geometry to four O and two Cl atoms. There are a spread of Fe–O bond distances ranging from 1.83–2.14 Å. There are one shorter (2.28 Å) and one longer (2.29 Å) Fe–Cl bond lengths. There are four inequivalent O sites. In the first O site, O is bonded in a single-bond geometry to one Fe atom. In the second O site, O is bonded in a 2-coordinate geometry to one Fe and one O atom. The O–O bond length is 1.28 Å. In the third O site, O is bonded in a single-bond geometry to one Fe atom. In the fourth O site, O is bonded in a 2-coordinate geometry to one Fe and one O atom. There are two inequivalent Cl sites. In the first Cl site, Cl is bonded in a single-bond geometry to one Fe atom. In the second Cl site, Cl is bonded in a single-bond geometry to one Fe atom. In the Sb(OCl3)2 ribbon, Sb is bonded in an octahedral geometry to six Cl atoms. There are a spread of Sb–Cl bond distances ranging from 2.36–2.51 Å. There are two inequivalent O sites. In the first O site, O is bonded in a 3-coordinate geometry to one O and two Cl atoms. The O–O bond length is 2.01 Å. There are one shorter (2.41 Å) and one longer (2.50 Å) O–Cl bond lengths. In the second O site, O is bonded in a distorted L-shaped geometry to one O and one Cl atom. The O–Cl bond length is 2.45 Å. There are six inequivalent Cl sites. In the first Cl site, Cl is bonded in a single-bond geometry to one Sb atom. In the second Cl site, Cl is bonded in a single-bond geometry to one Sb atom. In the third Cl site, Cl is bonded in a single-bond geometry to one Sb and one O atom. In the fourth Cl site, Cl is bonded in a single-bond geometry to one Sb and one O atom. In the fifth Cl site, Cl is bonded in a single-bond geometry to one Sb atom. In the sixth Cl site, Cl is bonded in a distorted single-bond geometry to one Sb and one O atom.
Transitioning from fossil-fuels to renewable energy sources is a critical goal of greenhouse gas re-duction and climate control. Major improvements have made wind and solar power increasingly cost-competitive with fossil fuels. However, the inherent intermittency of renewable power sources such motivates pairing these resources with energy storage. Electrochemical energy storage in batteries is widely used in many fields and increasingly for grid-level storage, but current battery technologies still fall short of performance, safety, and cost. This review focuses on sodium metal halide (Na-MH) batteries, such as well-known Na-NiCl2 batteries, as a promising solution to safe and economical grid-level energy storage. Important features of conventional Na-MH batteries are discussed, and recent literature on the development of intermediate temperature, low-cost cathodes for Na-MH batteries is highlighted. By employing lower cost metal halides (e.g. FeCl2, ZnCl2, and CuCl2, etc.) in the cathode and operating at lower temperatures (e.g. 190 °C vs. 280 °C), new Na-MH batteries have the potential to offer comparable performance at much lower overall costs, providing an exciting alternative technology to enable widespread adoption of renewables plus storage for the grid.
FeCl2(H2O)4 crystallizes in the monoclinic P2_1/c space group. The structure is three-dimensional. Fe2+ is bonded in a distorted octahedral geometry to four O2- and two equivalent Cl1- atoms. There are two shorter (2.12 Å) and two longer (2.21 Å) Fe–O bond lengths. Both Fe–Cl bond lengths are 2.53 Å. There are four inequivalent H1+ sites. In the first H1+ site, H1+ is bonded in a single-bond geometry to one O2- atom. The H–O bond length is 0.98 Å. In the second H1+ site, H1+ is bonded in a single-bond geometry to one O2- atom. The H–O bond length is 0.98 Å. In the third H1+ site, H1+ is bonded in a single-bond geometry to one O2- and one Cl1- atom. The H–O bond length is 1.00 Å. The H–Cl bond length is 2.10 Å. In the fourth H1+ site, H1+ is bonded in a single-bond geometry to one O2- and one Cl1- atom. The H–O bond length is 0.99 Å. The H–Cl bond length is 2.14 Å. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted water-like geometry to one Fe2+ and two H1+ atoms. In the second O2- site, O2- is bonded in a distorted water-like geometry to one Fe2+ and two H1+ atoms. Cl1- is bonded in a 1-coordinate geometry to one Fe2+ and two H1+ atoms.
FeCl2(H2O)4 crystallizes in the monoclinic P2_1/c space group. The structure is zero-dimensional and consists of two 13478-10-9 molecules. Fe2+ is bonded in an octahedral geometry to four O2- and two equivalent Cl1- atoms. There are two shorter (2.11 Å) and two longer (2.13 Å) Fe–O bond lengths. Both Fe–Cl bond lengths are 2.55 Å. There are four inequivalent H1+ sites. In the first H1+ site, H1+ is bonded in a single-bond geometry to one O2- atom. The H–O bond length is 0.99 Å. In the second H1+ site, H1+ is bonded in a single-bond geometry to one O2- atom. The H–O bond length is 0.99 Å. In the third H1+ site, H1+ is bonded in a single-bond geometry to one O2- atom. The H–O bond length is 0.99 Å. In the fourth H1+ site, H1+ is bonded in a single-bond geometry to one O2- atom. The H–O bond length is 0.99 Å. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted water-like geometry to one Fe2+ and two H1+ atoms. In the second O2- site, O2- is bonded in a distorted water-like geometry to one Fe2+ and two H1+ atoms. Cl1- is bonded in a single-bond geometry to one Fe2+ atom.
FeCl2(O)4SbCl6(O2)2 crystallizes in the tetragonal P4/mmm space group. The structure is zero-dimensional and consists of one dichloroiron;tetrahydrate molecule, four water molecules, and one SbCl6 cluster. In the SbCl6 cluster, Sb is bonded in an octahedral geometry to six Cl atoms. There are two shorter (2.36 Å) and four longer (2.44 Å) Sb–Cl bond lengths. There are two inequivalent Cl sites. In the first Cl site, Cl is bonded in a single-bond geometry to one Sb atom. In the second Cl site, Cl is bonded in a single-bond geometry to one Sb atom.
FeCl2(O)4 crystallizes in the monoclinic P2_1/c space group. The structure is three-dimensional. Fe is bonded in a distorted square co-planar geometry to four O and two equivalent Cl atoms. There are two shorter (2.03 Å) and two longer (2.09 Å) Fe–O bond lengths. Both Fe–Cl bond lengths are 2.88 Å. There are two inequivalent O sites. In the first O site, O is bonded in a bent 120 degrees geometry to one Fe and one Cl atom. The O–Cl bond length is 1.58 Å. In the second O site, O is bonded in a bent 120 degrees geometry to one Fe and one Cl atom. The O–Cl bond length is 1.58 Å. Cl is bonded in a distorted trigonal non-coplanar geometry to one Fe and two O atoms.