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All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Solid State Solar Thermochemical Fuel (SoFuel) for Long Duration Storage

Efficient thermal storage systems, when coupled with renewable energy, enable the decarbonization of numerous industrial processes requiring high temperature steam or air, and provide a path for seasonal building heating, especially for colder climates. Existing thermal storage systems face a significant challenge due to losses inherent to all high temperature systems. A viable route to long-term storage is to use thermochemical reactions to convert concentrated solar energy to a fuel that is shelf-stable and can be stored at room temperature, thus eliminating losses associated with high temperature storage. The Solid-State Solar Thermochemical Fuel (SoFuel) technology developed by Michigan State University, Oregon State University, and Mississippi State University provides reactors and processes with minimal sensible heat losses and allows storing solar energy as a solid-state fuel at room temperature for long duration. The production of SoFuel occurs within a cylindrical cavity reduction chemical reactor that captures concentrated solar radiation from a solar field. Reactive magnesium manganese oxide (Mg-Mn-O) resides within the cylindrical cavity chemical reactor and undergoes thermal reduction as the temperature exceeds 1350°C. The thermally reduced Mg-Mn-O pellets (the SoFuel) are cooled down through a recuperative process and stored within a bin until used. The SoFuel can directly supply up to 1100°C heat to an adjacent power plant for electricity generation or industrial heating. Oxidation of SoFuel pellets occurs in a counter flow reactor and supplies heat to the user for electricity generation or industrial processing, after which the fuel is returned to the concentrating solar field where it is regenerated for re-use. Both reactors can be controlled well using a variety of strategies. With the low cost of the material, its cyclability, and the possibility of using the pelletized with on-sun reactors, or with electricity that would be curtailed, this project offers a viable option of medium- and long-term thermal energy storage.

14 SOLAR ENERGY↗
DOE OSTI2025

Solid State Solar Thermochemical Fuel (SoFuel) for Long Duration Storage

Efficient thermal storage systems, when coupled with renewable energy, enable the decarbonization of numerous industrial processes requiring high temperature steam or air, and provide a path for seasonal building heating, especially for colder climates. Existing thermal storage systems face a significant challenge due to losses inherent to all high temperature systems. A viable route to long-term storage is to use thermochemical reactions to convert concentrated solar energy to a fuel that is shelf-stable and can be stored at room temperature, thus eliminating losses associated with high temperature storage. The Solid-State Solar Thermochemical Fuel (SoFuel) technology developed by Michigan State University, Oregon State University, and Mississippi State University provides reactors and processes with minimal sensible heat losses and allows storing solar energy as a solid-state fuel at room temperature for long duration. The production of SoFuel occurs within a cylindrical cavity reduction chemical reactor that captures concentrated solar radiation from a solar field. Reactive magnesium manganese oxide (Mg-Mn-O) resides within the cylindrical cavity chemical reactor and undergoes thermal reduction as the temperature exceeds 1350°C. The thermally reduced Mg-Mn-O pellets (the SoFuel) are cooled down through a recuperative process and stored within a bin until used. The SoFuel can directly supply up to 1100C heat to an adjacent power plant for electricity generation or industrial heating. Oxidation of SoFuel pellets occurs in a counter flow reactor and supplies heat to the user for electricity generation or industrial processing, after which the fuel is returned to the concentrating solar field where it is regenerated for re-use. Both reactors can be controlled well using a variety of strategies. With the low cost of the material, its cyclability, and the possibility of using the pelletized with on-sun reactors, or with electricity that would be curtailed, this project offers a viable option of medium- and long-term thermal energy storage.

25 ENERGY STORAGE↗
DOE OSTI2021

Thermochemical reduction modeling in a high-temperature moving-bed reactor for energy storage: 1D model

The design of robust and efficient high-temperature thermochemical reactors and determination of operating conditions are critical steps toward enabling high-efficiency long-duration solar energy storage. This work presents a computational model for the thermal reduction of a metal oxide material (Mg-Mn-O) up to 1450 °C and the coupled complex transport phenomena in a novel tubular thermal reactor design that features the capability for a high extent-of-reduction (high energy storage density) and inherent heat recuperation. A one-dimensional model coupling counter-current gas–solid flow, two-phase heat transfer, thermochemical redox reactions, and species transport in a moving-bed reactor is developed. Simplified versions of the model are validated with published results in the literature for packed beds with both inert and reactive particles; the fully coupled model is also validated with experimental measurements of a moving-bed reactor in terms of local temperatures and oxygen release at the exit. Detailed comparisons on the effects of different boundary conditions in the reaction zone (prescribed wall temperature vs. heat flux conditions) and formulations based on a simple uniform flow assumption vs. plug flow using Ergun equation for gas flow are investigated. The results are compared with experimental measurements, and for all cases, the energy flow components in the reactor system and the thermal to chemical conversion efficiency and overall system efficiency are computed. Finally, the predicted high thermal-to-chemical efficiency ~95% and system efficiency ~30% agree with experimental measurements.

25 ENERGY STORAGE↗
DOE OSTI2021

Ultra-High Temperature Thermal Conductivity Measurements of a Reactive Magnesium Manganese Oxide Porous Bed Using a Transient Hot Wire Method

Pelletized magnesium manganese oxide shows promise for high temperature thermochemical energy storage. It can be thermally reduced in the temperature range between 1250 °C and 1500 °C and re-oxidized with air at typical gas-turbine inlet pressures (1–25 bar) in the temperature range between 600 °C and 1500 °C. The combined thermal and chemical volumetric energy density is approximately 2300 MJ/m3. The rate at which a thermochemical storage module can be charged is limited by heat transfer inside the solid packed bed. Hence, the effective thermal conductivity of packed beds of magnesium-manganese oxide pellets is a crucial parameter for engineering Mg-Mn-O redox storage devices. We have measured the effective thermal conductivity of a packed bed of 3.66 ± 0.516 mm sized magnesium manganese oxide (Mn to Mg molar ratio of 1:1) pellets in the temperature range of 300–1400 °C. Since the material is electrically conductive at temperatures above 600 °C, the sheathed transient hot wire method is used for measurements. Raw data is analyzed using the Blackwell solution to extract the bed thermal conductivity. The effective thermal conductivity standard deviation is less than 10% for a minimum of three repeat measurements at each temperature. Experimental results show an increase in the effective thermal conductivity with temperature from 0.50 W/m °C around 300 °C to 1.81 W/m °C close to 1400 °C. We propose a dual porosity model to express the effective thermal conductivity as a function of temperature. This model also considers the effect of radiation within the bed, as this is the dominant heat transfer mode at high temperatures. The proposed model accounts for microscale pellet porosity, macroscale bed porosity, pellet size, solid thermal conductivity (phonon transport), and radiation (photon transport). The coefficient of determination between the proposed model and the experimental results is greater than 0.90.

Engineering↗
DOE OSTI2020

Materials Data on Mg6MnO8 by Materials Project

Mg6MnO8 crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. Mg2+ is bonded to six O2- atoms to form MgO6 octahedra that share corners with six equivalent MgO6 octahedra, edges with two equivalent MnO6 octahedra, and edges with eight equivalent MgO6 octahedra. The corner-sharing octahedra tilt angles range from 0–9°. There are two shorter (2.12 Å) and four longer (2.13 Å) Mg–O bond lengths. Mn4+ is bonded to six equivalent O2- atoms to form MnO6 octahedra that share edges with twelve equivalent MgO6 octahedra. All Mn–O bond lengths are 1.94 Å. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded to six equivalent Mg2+ atoms to form OMg6 octahedra that share corners with six equivalent OMg6 octahedra and edges with twelve equivalent OMg4Mn square pyramids. The corner-sharing octahedral tilt angles are 0°. In the second O2- site, O2- is bonded to four equivalent Mg2+ and one Mn4+ atom to form OMg4Mn square pyramids that share corners with nine equivalent OMg4Mn square pyramids, edges with four equivalent OMg6 octahedra, and edges with four equivalent OMg4Mn square pyramids.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Mg2Mn3O8 by Materials Project

Mg2Mn3O8 crystallizes in the orthorhombic Cmce space group. The structure is three-dimensional. Mg2+ is bonded to four O2- atoms to form MgO4 tetrahedra that share corners with nine MnO6 octahedra. The corner-sharing octahedra tilt angles range from 52–64°. There are a spread of Mg–O bond distances ranging from 1.95–2.12 Å. There are two inequivalent Mn4+ sites. In the first Mn4+ site, Mn4+ is bonded to six O2- atoms to form MnO6 octahedra that share corners with six equivalent MgO4 tetrahedra and edges with four MnO6 octahedra. There are a spread of Mn–O bond distances ranging from 1.89–2.02 Å. In the second Mn4+ site, Mn4+ is bonded to six O2- atoms to form MnO6 octahedra that share corners with six equivalent MgO4 tetrahedra and edges with four equivalent MnO6 octahedra. There is four shorter (1.90 Å) and two longer (1.98 Å) Mn–O bond length. There are three inequivalent O2- sites. In the first O2- site, O2- is bonded in a trigonal planar geometry to one Mg2+ and two Mn4+ atoms. In the second O2- site, O2- is bonded in a rectangular see-saw-like geometry to one Mg2+ and three Mn4+ atoms. In the third O2- site, O2- is bonded in a trigonal non-coplanar geometry to one Mg2+ and two equivalent Mn4+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Mg2Mn3O8 by Materials Project

Mg2Mn3O8 crystallizes in the orthorhombic Pmn2_1 space group. The structure is three-dimensional. there are two inequivalent Mg2+ sites. In the first Mg2+ site, Mg2+ is bonded to six O2- atoms to form MgO6 octahedra that share corners with seven MnO6 octahedra, corners with two equivalent MgO4 tetrahedra, and edges with three MnO6 octahedra. The corner-sharing octahedra tilt angles range from 15–56°. There are a spread of Mg–O bond distances ranging from 2.06–2.28 Å. In the second Mg2+ site, Mg2+ is bonded to four O2- atoms to form MgO4 tetrahedra that share corners with two equivalent MgO6 octahedra and corners with nine MnO6 octahedra. The corner-sharing octahedra tilt angles range from 58–63°. There are a spread of Mg–O bond distances ranging from 1.97–2.01 Å. There are two inequivalent Mn4+ sites. In the first Mn4+ site, Mn4+ is bonded to six O2- atoms to form MnO6 octahedra that share corners with three equivalent MgO6 octahedra, corners with three equivalent MgO4 tetrahedra, an edgeedge with one MgO6 octahedra, and edges with four equivalent MnO6 octahedra. The corner-sharing octahedra tilt angles range from 15–49°. There are a spread of Mn–O bond distances ranging from 1.91–1.96 Å. In the second Mn4+ site, Mn4+ is bonded to six O2- atoms to form MnO6 octahedra that share corners with two equivalent MgO6 octahedra, corners with three equivalent MgO4 tetrahedra, an edgeedge with one MgO6 octahedra, and edges with four MnO6 octahedra. The corner-sharing octahedra tilt angles range from 53–56°. There are a spread of Mn–O bond distances ranging from 1.89–2.02 Å. There are six inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted trigonal planar geometry to one Mg2+ and two equivalent Mn4+ atoms. In the second O2- site, O2- is bonded to one Mg2+ and three Mn4+ atoms to form a mixture of distorted edge and corner-sharing OMgMn3 trigonal pyramids. In the third O2- site, O2- is bonded in a distorted rectangular see-saw-like geometry to one Mg2+ and three Mn4+ atoms. In the fourth O2- site, O2- is bonded in a distorted trigonal planar geometry to one Mg2+ and two equivalent Mn4+ atoms. In the fifth O2- site, O2- is bonded to two Mg2+ and two Mn4+ atoms to form distorted OMg2Mn2 tetrahedra that share a cornercorner with one OMg2Mn2 tetrahedra, a cornercorner with one OMgMn3 trigonal pyramid, an edgeedge with one OMg2Mn2 tetrahedra, and an edgeedge with one OMgMn3 trigonal pyramid. In the sixth O2- site, O2- is bonded in a distorted trigonal planar geometry to one Mg2+ and two Mn4+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on MgMn2O4 by Materials Project

Computed materials data using density functional theory calculations. These calculations determine the electronic structure of bulk materials by solving approximations to the Schrodinger equation. For more information, see https://materialsproject.org/docs/calculations

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on MgMn4O8 by Materials Project

MgMn4O8 is beta indium sulfide-derived structured and crystallizes in the trigonal R-3m space group. The structure is three-dimensional. Mg2+ is bonded to six equivalent O2- atoms to form distorted MgO6 octahedra that share corners with six equivalent MnO6 octahedra and edges with six equivalent MnO6 octahedra. The corner-sharing octahedral tilt angles are 14°. All Mg–O bond lengths are 2.12 Å. There are two inequivalent Mn+3.50+ sites. In the first Mn+3.50+ site, Mn+3.50+ is bonded to six O2- atoms to form MnO6 octahedra that share edges with two equivalent MgO6 octahedra and edges with six MnO6 octahedra. There is four shorter (1.94 Å) and two longer (1.98 Å) Mn–O bond length. In the second Mn+3.50+ site, Mn+3.50+ is bonded to six equivalent O2- atoms to form MnO6 octahedra that share corners with six equivalent MgO6 octahedra and edges with six equivalent MnO6 octahedra. The corner-sharing octahedral tilt angles are 14°. All Mn–O bond lengths are 2.16 Å. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded in a rectangular see-saw-like geometry to one Mg2+ and three Mn+3.50+ atoms. In the second O2- site, O2- is bonded in a distorted trigonal non-coplanar geometry to three equivalent Mn+3.50+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on MgMn2O4 by Materials Project

MgMn2O4 crystallizes in the monoclinic P2_1/m space group. The structure is three-dimensional. there are two inequivalent Mg2+ sites. In the first Mg2+ site, Mg2+ is bonded to six O2- atoms to form distorted MgO6 pentagonal pyramids that share corners with six equivalent MnO6 octahedra, edges with six MnO6 octahedra, and edges with two equivalent MgO6 pentagonal pyramids. The corner-sharing octahedra tilt angles range from 5–20°. There are a spread of Mg–O bond distances ranging from 2.13–2.29 Å. In the second Mg2+ site, Mg2+ is bonded to six O2- atoms to form distorted MgO6 pentagonal pyramids that share corners with twelve MnO6 octahedra, edges with two equivalent MgO6 pentagonal pyramids, and faces with two equivalent MnO6 octahedra. The corner-sharing octahedra tilt angles range from 41–55°. There are a spread of Mg–O bond distances ranging from 2.08–2.29 Å. There are three inequivalent Mn3+ sites. In the first Mn3+ site, Mn3+ is bonded to six O2- atoms to form distorted MnO6 octahedra that share corners with five MgO6 pentagonal pyramids, edges with six MnO6 octahedra, an edgeedge with one MgO6 pentagonal pyramid, and a faceface with one MgO6 pentagonal pyramid. There are a spread of Mn–O bond distances ranging from 1.88–2.34 Å. In the second Mn3+ site, Mn3+ is bonded to six O2- atoms to form MnO6 octahedra that share corners with four equivalent MgO6 pentagonal pyramids, edges with six MnO6 octahedra, and edges with two equivalent MgO6 pentagonal pyramids. There are a spread of Mn–O bond distances ranging from 1.98–2.18 Å. In the third Mn3+ site, Mn3+ is bonded to six O2- atoms to form MnO6 octahedra that share corners with four equivalent MgO6 pentagonal pyramids, edges with six MnO6 octahedra, and edges with two equivalent MgO6 pentagonal pyramids. There are a spread of Mn–O bond distances ranging from 1.97–2.24 Å. There are four inequivalent O2- sites. In the first O2- site, O2- is bonded to one Mg2+ and three Mn3+ atoms to form distorted OMgMn3 trigonal pyramids that share corners with four OMg2Mn3 trigonal bipyramids, corners with six OMgMn3 trigonal pyramids, edges with four OMg2Mn3 trigonal bipyramids, and an edgeedge with one OMgMn3 trigonal pyramid. In the second O2- site, O2- is bonded to one Mg2+ and three Mn3+ atoms to form distorted OMgMn3 trigonal pyramids that share corners with four OMg2Mn3 trigonal bipyramids, corners with six OMgMn3 trigonal pyramids, edges with four OMg2Mn3 trigonal bipyramids, and an edgeedge with one OMgMn3 trigonal pyramid. In the third O2- site, O2- is bonded to two Mg2+ and three Mn3+ atoms to form distorted OMg2Mn3 trigonal bipyramids that share corners with five OMg2Mn3 trigonal bipyramids, corners with four OMgMn3 trigonal pyramids, edges with four OMg2Mn3 trigonal bipyramids, and edges with four OMgMn3 trigonal pyramids. In the fourth O2- site, O2- is bonded to two Mg2+ and three Mn3+ atoms to form OMg2Mn3 trigonal bipyramids that share corners with five OMg2Mn3 trigonal bipyramids, corners with four OMgMn3 trigonal pyramids, edges with four OMg2Mn3 trigonal bipyramids, and edges with four OMgMn3 trigonal pyramids.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on MgMn2O4 by Materials Project

MgMn2O4 is Spinel structured and crystallizes in the cubic Fd-3m space group. The structure is three-dimensional. Mg2+ is bonded to four equivalent O2- atoms to form MgO4 tetrahedra that share corners with twelve equivalent MnO6 octahedra. The corner-sharing octahedral tilt angles are 58°. All Mg–O bond lengths are 2.02 Å. Mn3+ is bonded to six equivalent O2- atoms to form MnO6 octahedra that share corners with six equivalent MgO4 tetrahedra and edges with six equivalent MnO6 octahedra. All Mn–O bond lengths are 2.06 Å. O2- is bonded to one Mg2+ and three equivalent Mn3+ atoms to form a mixture of distorted edge and corner-sharing OMgMn3 trigonal pyramids.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Mg2Mn2O5 by Materials Project

Mg2Mn2O5 is Aluminum carbonitride-like structured and crystallizes in the orthorhombic Pbam space group. The structure is three-dimensional. Mg2+ is bonded in a 6-coordinate geometry to six O2- atoms. There are a spread of Mg–O bond distances ranging from 2.05–2.48 Å. Mn3+ is bonded to five O2- atoms to form corner-sharing MnO5 trigonal bipyramids. There are a spread of Mn–O bond distances ranging from 1.88–2.05 Å. There are three inequivalent O2- sites. In the first O2- site, O2- is bonded to four equivalent Mg2+ and two equivalent Mn3+ atoms to form distorted OMg4Mn2 octahedra that share corners with eight equivalent OMg2Mn2 tetrahedra, corners with four equivalent OMg2Mn2 trigonal pyramids, edges with two equivalent OMg4Mn2 octahedra, edges with two equivalent OMg2Mn2 tetrahedra, and edges with four equivalent OMg2Mn2 trigonal pyramids. In the second O2- site, O2- is bonded to two equivalent Mg2+ and two equivalent Mn3+ atoms to form OMg2Mn2 tetrahedra that share corners with four equivalent OMg4Mn2 octahedra, corners with four equivalent OMg2Mn2 tetrahedra, corners with four equivalent OMg2Mn2 trigonal pyramids, an edgeedge with one OMg4Mn2 octahedra, and edges with two equivalent OMg2Mn2 trigonal pyramids. The corner-sharing octahedra tilt angles range from 18–65°. In the third O2- site, O2- is bonded to two equivalent Mg2+ and two equivalent Mn3+ atoms to form distorted OMg2Mn2 trigonal pyramids that share corners with two equivalent OMg4Mn2 octahedra, corners with four equivalent OMg2Mn2 tetrahedra, corners with two equivalent OMg2Mn2 trigonal pyramids, edges with two equivalent OMg4Mn2 octahedra, edges with two equivalent OMg2Mn2 tetrahedra, and an edgeedge with one OMg2Mn2 trigonal pyramid. The corner-sharing octahedral tilt angles are 57°.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on MgMn2O4 by Materials Project

MgMn2O4 is Spinel-like structured and crystallizes in the triclinic P1 space group. The structure is three-dimensional. there are six inequivalent Mg2+ sites. In the first Mg2+ site, Mg2+ is bonded to four O2- atoms to form MgO4 tetrahedra that share corners with three MgO6 octahedra and corners with nine MnO6 octahedra. The corner-sharing octahedra tilt angles range from 56–61°. There are three shorter (2.02 Å) and one longer (2.05 Å) Mg–O bond lengths. In the second Mg2+ site, Mg2+ is bonded to six O2- atoms to form MgO6 octahedra that share corners with six MnO4 tetrahedra, edges with two MgO6 octahedra, and edges with four MnO6 octahedra. There are a spread of Mg–O bond distances ranging from 2.06–2.14 Å. In the third Mg2+ site, Mg2+ is bonded to six O2- atoms to form MgO6 octahedra that share corners with two equivalent MgO4 tetrahedra, corners with four MnO4 tetrahedra, an edgeedge with one MgO6 octahedra, and edges with five MnO6 octahedra. There are a spread of Mg–O bond distances ranging from 2.06–2.11 Å. In the fourth Mg2+ site, Mg2+ is bonded to six O2- atoms to form MgO6 octahedra that share corners with six MnO4 tetrahedra, edges with two MgO6 octahedra, and edges with four MnO6 octahedra. There are a spread of Mg–O bond distances ranging from 2.06–2.14 Å. In the fifth Mg2+ site, Mg2+ is bonded to six O2- atoms to form MgO6 octahedra that share corners with six MnO4 tetrahedra, edges with two MgO6 octahedra, and edges with four MnO6 octahedra. There are four shorter (2.07 Å) and two longer (2.11 Å) Mg–O bond lengths. In the sixth Mg2+ site, Mg2+ is bonded to six O2- atoms to form MgO6 octahedra that share a cornercorner with one MgO4 tetrahedra, corners with five MnO4 tetrahedra, an edgeedge with one MgO6 octahedra, and edges with five MnO6 octahedra. There are a spread of Mg–O bond distances ranging from 2.06–2.11 Å. There are twelve inequivalent Mn3+ sites. In the first Mn3+ site, Mn3+ is bonded to six O2- atoms to form MnO6 octahedra that share corners with two equivalent MgO4 tetrahedra, corners with four MnO4 tetrahedra, edges with three MgO6 octahedra, and edges with three MnO6 octahedra. There are a spread of Mn–O bond distances ranging from 1.92–2.07 Å. In the second Mn3+ site, Mn3+ is bonded to four O2- atoms to form MnO4 tetrahedra that share corners with six MgO6 octahedra and corners with six MnO6 octahedra. The corner-sharing octahedra tilt angles range from 58–61°. There are a spread of Mn–O bond distances ranging from 2.01–2.10 Å. In the third Mn3+ site, Mn3+ is bonded to six O2- atoms to form MnO6 octahedra that share corners with three equivalent MgO4 tetrahedra, corners with three equivalent MnO4 tetrahedra, edges with two MgO6 octahedra, and edges with four MnO6 octahedra. There are a spread of Mn–O bond distances ranging from 1.98–2.07 Å. In the fourth Mn3+ site, Mn3+ is bonded to six O2- atoms to form MnO6 octahedra that share corners with two equivalent MgO4 tetrahedra, corners with four MnO4 tetrahedra, edges with three MgO6 octahedra, and edges with three MnO6 octahedra. There are a spread of Mn–O bond distances ranging from 1.92–2.07 Å. In the fifth Mn3+ site, Mn3+ is bonded to six O2- atoms to form MnO6 octahedra that share corners with six MnO4 tetrahedra, edges with two equivalent MnO6 octahedra, and edges with four MgO6 octahedra. There is two shorter (1.94 Å) and four longer (2.01 Å) Mn–O bond length. In the sixth Mn3+ site, Mn3+ is bonded to four O2- atoms to form MnO4 tetrahedra that share corners with six MgO6 octahedra and corners with six MnO6 octahedra. The corner-sharing octahedra tilt angles range from 55–63°. There are two shorter (2.02 Å) and two longer (2.05 Å) Mn–O bond lengths. In the seventh Mn3+ site, Mn3+ is bonded to six O2- atoms to form MnO6 octahedra that share corners with six MnO4 tetrahedra, edges with two equivalent MnO6 octahedra, and edges with four MgO6 octahedra. There is two shorter (1.94 Å) and four longer (2.01 Å) Mn–O bond length. In the eighth Mn3+ site, Mn3+ is bonded to six O2- atoms to form MnO6 octahedra that share a cornercorner with one MgO4 tetrahedra, corners with five MnO4 tetrahedra, edges with three MgO6 octahedra, and edges with three MnO6 octahedra. There are a spread of Mn–O bond distances ranging from 1.92–2.08 Å. In the ninth Mn3+ site, Mn3+ is bonded to four O2- atoms to form MnO4 tetrahedra that share corners with six MgO6 octahedra and corners with six MnO6 octahedra. The corner-sharing octahedra tilt angles range from 56–62°. There are two shorter (2.02 Å) and two longer (2.05 Å) Mn–O bond lengths. In the tenth Mn3+ site, Mn3+ is bonded to six O2- atoms to form MnO6 octahedra that share a cornercorner with one MgO4 tetrahedra, corners with five MnO4 tetrahedra, edges with three MgO6 octahedra, and edges with three MnO6 octahedra. There are a spread of Mn–O bond distances ranging from 1.92–2.06 Å. In the eleventh Mn3+ site, Mn3+ is bonded to four O2- atoms to form MnO4 tetrahedra that share corners with six MgO6 octahedra and corners with six MnO6 octahedra. The corner-sharing octahedra tilt angles range from 58–61°. There are a spread of Mn–O bond distances ranging from 2.01–2.10 Å. In the twelfth Mn3+ site, Mn3+ is bonded to four O2- atoms to form MnO4 tetrahedra that share corners with three MgO6 octahedra and corners with nine MnO6 octahedra. The corner-sharing octahedra tilt angles range from 57–63°. There are one shorter (2.04 Å) and three longer (2.05 Å) Mn–O bond lengths. There are twenty-four inequivalent O2- sites. In the first O2- site, O2- is bonded to two Mg2+ and two Mn3+ atoms to form distorted OMg2Mn2 trigonal pyramids that share corners with two OMgMn3 tetrahedra, corners with two OMn4 trigonal pyramids, and edges with two OMgMn3 trigonal pyramids. In the second O2- site, O2- is bonded to one Mg2+ and three Mn3+ atoms to form a mixture of distorted edge and corner-sharing OMgMn3 trigonal pyramids. In the third O2- site, O2- is bonded to two Mg2+ and two Mn3+ atoms to form distorted OMg2Mn2 trigonal pyramids that share corners with two OMgMn3 tetrahedra, corners with two OMn4 trigonal pyramids, and edges with two OMg2Mn2 trigonal pyramids. In the fourth O2- site, O2- is bonded in a distorted rectangular see-saw-like geometry to one Mg2+ and three Mn3+ atoms. In the fifth O2- site, O2- is bonded to two Mg2+ and two Mn3+ atoms to form a mixture of distorted edge and corner-sharing OMg2Mn2 trigonal pyramids. In the sixth O2- site, O2- is bonded in a rectangular see-saw-like geometry to one Mg2+ and three Mn3+ atoms. In the seventh O2- site, O2- is bonded in a distorted rectangular see-saw-like geometry to one Mg2+ and three Mn3+ atoms. In the eighth O2- site, O2- is bonded in a distorted rectangular see-saw-like geometry to two Mg2+ and two Mn3+ atoms. In the ninth O2- site, O2- is bonded to two Mg2+ and two Mn3+ atoms to form a mixture of distorted edge and corner-sharing OMg2Mn2 trigonal pyramids. In the tenth O2- site, O2- is bonded in a distorted rectangular see-saw-like geometry to one Mg2+ and three Mn3+ atoms. In the eleventh O2- site, O2- is bonded in a distorted rectangular see-saw-like geometry to one Mg2+ and three Mn3+ atoms. In the twelfth O2- site, O2- is bonded in a distorted rectangular see-saw-like geometry to two Mg2+ and two Mn3+ atoms. In the thirteenth O2- site, O2- is bonded in a distorted rectangular see-saw-like geometry to two Mg2+ and two Mn3+ atoms. In the fourteenth O2- site, O2- is bonded in a rectangular see-saw-like geometry to one Mg2+ and three Mn3+ atoms. In the fifteenth O2- site, O2- is bonded to two Mg2+ and two Mn3+ atoms to form a mixture of distorted edge and corner-sharing OMg2Mn2 trigonal pyramids. In the sixteenth O2- site, O2- is bonded in a distorted rectangular see-saw-like geometry to two Mg2+ and two Mn3+ atoms. In the seventeenth O2- site, O2- is bonded in a rectangular see-saw-like geometry to two Mg2+ and two Mn3+ atoms. In the eighteenth O2- site, O2- is bonded in a distorted rectangular see-saw-like geometry to one Mg2+ and three Mn3+ atoms. In the nineteenth O2- site, O2- is bonded in a rectangular see-saw-like geometry to one Mg2+ and three Mn3+ atoms. In the twentieth O2- site, O2- is bonded to two Mg2+ and two Mn3+ atoms to form a mixture of distorted edge and corner-sharing OMg2Mn2 trigonal pyramids. In the twenty-first O2- site, O2- is bonded to four Mn3+ atoms to form distorted OMn4 trigonal pyramids that share corners with two OMgMn3 tetrahedra, corners with two OMg2Mn2 trigonal pyramids, edges with two OMgMn3 tetrahedra, and an edgeedge with one OMgMn3 trigonal pyramid. In the twenty-second O2- site, O2- is bonded to one Mg2+ and three Mn3+ atoms to form distorted OMgMn3 tetrahedra that share a cornercorner with one OMgMn3 tetrahedra, corners with three OMn4 trigonal pyramids, an edgeedge with one OMgMn3 tetrahedra, and edges with two OMgMn3 trigonal pyramids. In the twenty-third O2- site, O2- is bonded to one Mg2+ and three Mn3+ atoms to form distorted OMgMn3 trigonal pyramids that share corners with two OMg2Mn2 trigonal pyramids, edges with two OMgMn3 tetrahedra, and an edgeedge with one OMn4 trigonal pyramid. In the twenty-fourth O2- site, O2- is bonded to one Mg2+ and three Mn3+ atoms to form distorted OMgMn3 tetrahedra that share a cornercorner with one OMgMn3 tetrahedra, corners with three OMn4 trigonal pyramids, an edgeedge with one OMgMn3 tetrahedra, and edges with two OMgMn3 trigonal pyramids.

36 MATERIALS SCIENCE↗