MnO6 Octahedra Research Articles

Layered manganese oxides structures: Micro‐Raman and selected mobile Raman spectroscopic studies

AbstractThe main structural building blocks that form manganese oxides are MnO6 octahedra; these share corners and edges to construct specific structures, which can either be tunneled or layered. In the layered structures, that is, phyllomanganates, the MnO6 octahedra form sheets, which, in turn, alternate with sheets of metal oxides and H2O. These metal ions can vary (Zn, Co, Ni, Al, Li, …) and give rise to an entire range of different metal oxides. The characterization of these layered materials is important as they have various economical/industrial applications. Birnessite‐type materials, a specific type of layered manganese oxides, are widely studied for their use as cathode materials in alkali‐ion batteries. Phyllomanganates are also commonly found as constituents in sediments and soils or as coatings on rock surfaces. Their natural occurrence as black colored components have ensured that these minerals were also applied as pigments in archaeological and historical contexts. They are, for example, often found in rock art paintings and on pottery. As the oxides are used in unique archaeological objects, Raman spectroscopy is an evident choice for characterization due to its non‐destructive nature of analysis. In the current study, five mineral samples of (layered) manganese oxides are analyzed with different Raman instrumentations, including mobile systems and a benchtop micro‐Raman setup. The characterization of each selected manganese oxide and their comparison with literature data is discussed for the micro‐Raman instrumentation. In addition, the ability of identifying and characterizing layered manganese oxides and the possible challenges when using mobile instrumentation are discussed as well.

Tuning the Composition, Crystal Structure and Morphology of Manganese(III/IV) Oxide for High-Power Storage Applications

If inherently intermittent renewable energy technologies are to offer a practical alternative to conventional heat engines, it is critically important that efficient, cost-effective and sustainable energy storage solutions are developed to decouple supply and demand profiles. A promising strategy is to pair the competitive energy densities of batteries with the superior cycling lifetimes and power densities of supercapacitors,[1] employing an appropriate maximum power point tracking (MPPT) system to optimise energy exchange between these complementary devices.[2] Mixed-valence manganese(III/IV) oxides (MnOz, where z ranges between 1.5 and 2.0) have emerged as foremost candidates for the electroactive material in supercapacitor cathodes due to their low toxicity, minimal cost,[3] ready availability (manganese appears at an average concentration of ca. 1,000 ppm in the Earth’s crust[4]), wide electrochemical stability window (ca. 1.0 V in aqueous electrolytes[5]), and high maximum theoretical specific capacitance (ca. 1,370 F g-1, based on the reduction of Mn4+ ions by a single electron[6]). Charge storage in MnOz is predominantly mediated by the pseudocapacitive intercalation of cations from the surrounding electrolyte in each discharge phase, followed by the release of these guest ions during charging.Of the various crystallographic phases of MnOz, cryptomelane (α-MnOz) and birnessite (δ-MnOz) polymorphs are deemed two of the most promising candidates for storage applications because they possess characteristically wide channels for intercalation/de-intercalation:[8,9] α-MnOz consists of ca. 4.6 Å diameter one-dimensional tunnels enclosed between corner-sharing MnO6 octahedra, whereas δ-MnOz comprises two-dimensional layers of MnO6 octahedra separated by ca. 7.0 Å. Herein, a scalable, low-cost synthesis procedure is discussed that enables the production of highly pseudocapacitive birnessite and cryptomelane electrodes with tuneable material characteristics, including particle morphology, crystallographic structure, average Mn oxidation state, and the level of pre-intercalation by Na+ and K+ ions.[10] In addition to comparing the electrochemical properties of these NaxKyMnOz products in aqueous conditions, their application within an asymmetric supercapacitor is showcased: by optimising the relative masses of a NaxKyMnOz cathode and activated carbon anode in a 100 cm2 aqueous full-cell configuration, discharge capacitances in the range 10-30 F g-1 can be achieved over a voltage span of 1.8 V at current densities as high as 10 A g-1, with Coulombic efficiency exceeding 80% up to ca. 20 A g-1. With these competitive performance characteristics, the developed NaxKyMnOz materials are a promising option for high-power supercapacitor cathodes, helping to pave the way towards more efficient and sustainable energy storage.

Tuning strain-stress and electron-lattice effects to enhance electrical transport properties of La0.7Ca0.25K0.05MnO3 films via optimizing sintering temperatures

Facile-prepared films with a high-temperature coefficient of resistivity (TCR) are crucial for application in high-sensitivity bolometers. In this study, La0.7Ca0.25K0.05MnO3 (LCKMO) films were grown on (001)-oriented La0.3Sr0.7Al0.65Ta0.35O3 (LSAT) substrates using the spin-coating method. Various sintering temperatures (900 °C ≤ Ts ≤ 1050 °C) were utilized in the film preparation, causing the lattice mismatch between films and substrate degraded. The effect of Ts on the strain state of films was investigated through X-ray diffraction (XRD) analysis. The distortion of MnO6 octahedra and the Mn–O bond were assessed using Raman and FTIR spectra, respectively. The structural perfection and elemental valence of films were adjusted via optimizing Ts. Meanwhile, the strain state and defect structure were effectively improved with Ts increasing. These results indicate that the LCKMO films with high crystalline quality can be achieved by optimizing the sintering process. The TCR of LCKMO films was modified. When Ts at 1000 °C, the LCKMO films exhibit the remarkable TCR of 16.29% K-1 at 269.54 K, indicating the substantial potential of LCKMO films in infrared bolometers.

Engineering dual-crystal configurations in perovskite oxides boosts electrocatalysis of lithium–oxygen batteries

Sculpting crystal configurations can vastly affect the charge and orbital states of electrocatalysts, fundamentally determining the catalytic activity of lithium-oxygen (Li–O2) batteries. However, the crucial role of crystal configurations in determining the electronic states has usually been neglected and needs to be further examined. Herein, we introduce orthorhombic and trigonal system into 0.5La0.6Sr0.4MnO3-0.5LaMn0.6Co0.4O3 (LSMCO) by selectively incorporating Sr and Co cations into the LaMnO3 framework during the sol-gel process, which is used to explore the relationship among crystal structure, electronic states and catalytic performance. Based on both experimental and theoretical calculations, the dual-crystal configurations induce strong lattice distortion, which promotes MnO6 octahedra vibration and shortened MnO bonds. Furthermore, the suppressed Jahn-Teller distortion weakens the orbital arrangement and accelerates the charge delocalization, leading to the conversion of Mn3+ to Mn4+ and optimized electronic states. Ultimately, this resulted in optimized Mn 3d and O 2p orbital hybridization and activated lattice oxygen function, leading to a significant improvement in electrocatalytic activity. The LSMCO catalyzed Li–O2 battery achieves enhanced discharge capacity of 14498.7 mAh/g and cycling stability of 258 cycles. This work highlights the significance of inner structure and presents a feasible strategy for engineering crystal configurations to boost electrocatalysis of Li–O2 batteries.

Complex dielectric and impedance analysis in Dy1-XPrXMnO3 compounds: Partial substitution effects

Synthesis and characterization of single phasic Dy1-xPrxMnO3 (DPMO) ceramic samples through solid-state reaction (SSR) method are reported in present study. XRD analysis confirmed the structural formation of DPMO and revealed lattice changes induced by Pr substitution that affect JT-distortion and tilting of MnO6 octahedra. Fine-grained microstructures with a micrometre-sized grain size were observed using FESEM images, which increased with increasing Pr concentration. The variation of activation energy in DPMO was found to be induced by modification in Mn–O bond distances. The dielectric behaviour of DPMO was significantly enhanced, which was explained using the UDR model fitting. Impedance analysis confirmed the existence of non-Debye type relaxation and negligible electronic polarization mechanisms in all DPMO samples. With the increase in temperature, a reduction in the magnitude of Z' was observed, indicating a decrease in the resistance of contributing electroactive components such as grains, grain boundaries, and electrode interfaces. Electric modulus studies revealed the existence of grain and grain boundary relaxation processes throughout the studied frequency and temperature range in DPMO compounds. Overall, the study provides deep insights into the effect of Pr substitution on the structural, electrical, and dielectric properties of DPMO and highlights the potential of DPMO for future electronic and energy device applications.

Stabilizing the Deep Sodiation Process in Layered Sodium Manganese Cathodes by Anchoring Boron Ions.

Advanced high-energy-density sodium-ion batteries (SIBs) are inseparable from cathode materials with high specific capacities. Layered manganese-rich oxides (NaxMnO2, 0.6≤x≤1) are promising cathode materials owing to their ease of intercalation and extraction of a considerable amount of sodium ions. However, lattice interactions, especially electrostatic repulsive forces and anisotropic stresses, are usually caused by deep desodiatin/sodiation process, resulting in intragranular cracks and capacity degradation in SIBs. Here, boron ions are introduced into the layered structure to build up B─O─Mn bonds. The regulated electronic structure in Na0.637B0.038MnO2 (B-NMO) materials inhibits the deformation of MnO6 octahedra, which finally achieves a gentle structural transition during the deep sodiation process. B-NMO electrode exhibits a high capacity (141mAhg-1) at 1C with a capacity retention of 81% after 100 cycles. Therefore, anchoring boron to manganese-rich materials inhibits the detrimental structural evolution of deep sodiation and can be used to obtain excellent cathode materials for SIBs.

Calculation of the Phonon Spectrum of PbMnBO4 Crystal Using Density Functional Theory

The phonon dispersion and Raman spectrum of the PbMnBO4 ferromagnetic crystal have been calculated within the density functional theory. Imaginary phonon branches have been observed at the points Y, Z, and Г and along the X–S direction of the Brillouin zone, which indicates structural instability and a possible phase transition with variation in external factors (temperature and pressure). The shapes of vibrations and symmetry types of the normal modes of the crystal at the center of the Brillouin zone have been determined. The calculation results are compared with the experimental and theoretical spectra from other studies. It is shown that the vibrational mode of highest intensity at 692.5 cm–1 in the spectrum and the mode at 272.3 cm–1, corresponding to the experimental modes at 690.5 and 224.7 cm–1, are bending vibrations of oxygen atoms in distorted MnO6 octahedra.

Local Structural Investigation of Near-Infrared-Reflective Black Ca2(Mn,Ti)O4 Pigments Using Synchrotron Radiation X-Ray and Density Functional Theory Calculations.

Layered perovskite black Ca2(Mn,Ti)O4 ceramics were studied by using synchrotron radiation X-rays (SRX) and density functional theory (DFT) calculations to investigate the valence states of the cations, average/local structures, and electronic states. The crystallographic data were obtained by the Rietveld refinement of the obtained synchrotron radiation X-ray powder diffraction patterns. X-ray absorption near edge structure (XANES) spectroscopy measurements revealed that the ratios of Mn4+ and Ti4+ were about 82 and 95%, respectively, in all samples, and the Mn/Ti valences were not affected by the introduction of Ti4+. In addition, two pre-edge peaks were observed in the Mn-XANES spectra, but their peak positions and intensities were affected by doping with Ti4+, indicating that the symmetry of the MnO6 octahedra was changed. A comparison of the atomic distances estimated from the Rietveld analysis and radial distribution function (RDF) revealed that there were large differences between the M-M distances (M = Mn, Ti). Therefore, XANES simulations were carried out to obtain models of the local structure. The experimental and theoretical data indicate that the Mn atoms were configured in a zigzagging arrangement, and the distortion of the MnO6 octahedra increased with the increase in the degree of Ti4+ doping. The origin of the changes to the pre-edge peaks was not only the crystal field strength around Mn but also the symmetry of the MnO6 octahedra.

Whole-Voltage-Range Solid-Solution Reaction in Layered Oxide Cathode of Sodium-Ion Batteries.

Layered manganese-based oxides (LMOs) are promising cathode materials for sodium-ion batteries (SIBs) due to their versatile structures. However, the Jahn-Teller effect of Mn3+ induces severe distortion of MnO6 octahedra, and the resultant low symmetry is responsible for the gliding of MnO2 layers and then inferior multiple-phase transitions upon Na+ extraction/insertion. Here, hexagonal P2-Na0.643 Li0.078 Mn0.827 Ti0.095 O2 is synthesized through the incorporation of Li and Ti into the distorted orthorhombic P'2-Na0.67 MnO2 to function as a phase-transition-free oxide cathode. It is revealed that Li in both the transition-metal and Na layers enhances the covalency of Mn-O bonds and allows degeneracy of Mn 3d eg orbitals to favor the formation of hexagonal phase, and the high strength of Ti-O bonds reduces the electrostatic interaction between Na and O for suppressed Na+ /vacancy rearrangements. These collectively lead to a whole-voltage-range solid-solution reaction between 1.8 and 4.3V with a small volume variation of 1.49%. This rewards its excellent cycling stability (capacity retention of 90% after 500 cycles) and rate capability (89 mAh g-1 at 2000mA g-1 ).

Ligand field regulation of δ-MnO2 by polyanion modification enables extended potential window in supercapacitors

Extending the potential range of MnO2 is imperative to improve power density and capacity density for supercapacitors. However, MnO2 usually suffers from severe Mn dissolution, irreversible phase transition, and poor cycling performance under an extended potential range. Herein, a ligand field regulation strategy is rationally designed to enhance the structural stability of δ-MnO2 to extend the potential window to 0–1.3 V (vs. Ag/AgCl). As revealed, the generated strong covalent B–O plane triangle or P–O tetrahedron configuration effectively decreases the lattice O activity of δ-MnO2. The B–O–Mn and P–O–Mn bonding interactions enhance Mn ion crystal field stabilization energy in the MnO6 octahedra. Therefore, the lattice O loss, Mn dissolution, and irreversible phase transition are well suppressed. The PO43− doped δ-MnO2 electrodes (with a high mass loading of ∼10 mg cm−2) can deliver a large specific capacitance (268.3 F g−1 at 0.5 A g−1) and excellent cycling performance (83.5% after 10000 cycles) in the extended potential window of 0–1.3 V (vs. Ag/AgCl). Our ligand field regulation strategy affords opportunities for developing simultaneously high-capacity and high-voltage electrodes for supercapacitors.

Structural features and thermodynamic properties of Pr0.5Ba0.25Sr0.25MnO3–δ

The structure, oxygen non-stoichiometry, defect equilibrium, and thermodynamic properties of perovskite-type Pr0.5Ba0.25Sr0.25MnO3–δ manganite were studied. It was shown that the manganite can be air synthesized in one step at 1450 °C. Though having a disordered arrangement of Pr, Ba, and Sr atoms, the compound crystallizes with a tetragonal structure (S.G. I4/mcm) due to the regular tilts of MnO6 octahedra. The oxygen content in oxide as a function of oxygen partial pressure and temperature was measured by coulometric titration, and the data were used for the modeling of defect equilibrium. The oxygen content measurements show stability of the manganite at 973–1223 K and oxygen pressure decrease down to 10−16–10−12 atm. The equilibration of defects in Pr0.5Ba0.25Sr0.25MnO3-δ involves oxygen exchange with the gaseous ambient, intrinsic electron excitation, and oxygen disordering reactions. According to thermodynamic calculations, oxygen sites available for oxygen exchange are characterized by praseodymium-rich coordination. The disordering caused by the half substitution of barium by strontium leads to a lower energy of chemical bonding of the labile oxygen in Pr0.5Ba0.25Sr0.25MnO3–δ compared to the parent Pr0.5Ba0.5MnO3–δ manganite.

High-Entropy Na-Deficient Layered Oxides for Sodium-Ion Batteries.

Sodium layered oxides always suffer from sluggish kinetics and deleterious phase transformations at deep-desodiation state (i.e., >4.0 V) in O3 structure, incurring inferior rate capability and grievous capacity degradation. To tackle these handicaps, here, a configurational entropy tuning protocol through manipulating the stoichiometric ratios of inactive cations is proposed to elaborately design Na-deficient, O3-type NaxTmO2 cathodes. It is found that the electrons surrounding the oxygen of the TmO6 octahedron are rearranged by the introduction of MnO6 and TiO6 octahedra in Na-deficient O3-type Na0.83Li0.1Ni0.25Co0.2Mn0.15Ti0.15Sn0.15O2-δ (MTS15) with expanded O-Na-O slab spacing, giving enhanced Na+ diffusion kinetics and structural stability, as disclosed by theoretical calculations and electrochemical measurements. Concomitantly, the entropy effect contributes to the improved reversibility of Co redox and phase-transition behaviors between O3 and P3, as clearly revealed by ex situ synchrotron X-ray absorption spectra and in situ X-ray diffraction. Notably, the prepared entropy-tuned MTS15 cathode exhibits impressive rate capability (76.7% capacity retention at 10 C), cycling stability (87.2% capacity retention after 200 cycles) with a reversible capacity of 109.4 mAh g-1, good full-cell performance (84.3% capacity retention after 100 cycles), and exceptional air stability. This work provides an idea for how to design high-entropy sodium layered oxides for high-power density storage systems.

Ionic-Liquid-Assisted Synthesis of Mixed-Phase Manganese Oxide Nanorods for a High-Performance Aqueous Zinc-Ion Battery

Aqueous zinc-ion batteries (ZIBs) provide a safer and cost-effective energy storage solution by utilizing nonflammable water-based electrolytes. Although many research efforts are focused on optimizing zinc anode materials, developing suitable cathode materials is still challenging. In this study, one-dimensional, mixed-phase MnO2 nanorods are synthesized using ionic liquid (IL). Here, the IL acts as a structure-directing agent that modifies MnO2 morphology and introduces mixed phases, as confirmed by morphological, structural, and X-ray photoelectron spectroscopy (XPS) studies. The MnO2 nanorods developed by this method are utilized as a cathode material for ZIB application in the coin-cell configuration. As expected, Zn//MnO2 nanorods show a significant increase in their capacity to 347 Wh kg-1 at 100 mA g-1, which is better than bare MnO2 nanowires (207.1 Wh kg-1) synthesized by the chemical precipitation method. The battery is highly rechargeable and maintains good retention of 86% of the initial capacity and 99% Coulombic efficiency after 800 cycles at 1000 mA g-1. The ex situ XPS, X-ray diffraction, and in-depth electrochemical analysis confirm that MnO6 octahedra experience insertion/extraction of Zn2+ with high reversibility. This study suggests the potential use of MnO2 nanorods to develop high-performance and durable battery electrode materials suitable for large-scale applications.

Surface Energies of LaMnO3 High-Index Surfaces Obtained from Density-Functional Theory

The first six cubic LaMnO3 high-index surfaces, (210), (211), (221), (310), (311), and (320), are examined for the first time. The unrelaxed and relaxed surface energies of the surfaces are computed as a function of surface model thickness using unrelaxed and relaxed surface models, respectively. The (210), (320), (211), and (221) surfaces are found to be more stable than the low-index (011) and (111) surfaces. Helping to explain this result, the surface terminations of the (210), (320), (211), or (221) surface are seen to exhibit a rotational relaxation of the MnO6 or oxygen octahedra extending from the surface into the bulk. By contrast, the relaxed surface energies of the (310) and (311) surfaces are concluded to be difficult to determine due to a structural transformation or phase change in the surface models of the surfaces away from cubic LaMnO3. The finding of a phase change is important. It indicates that a phase change in the surface models of a surface for another material could occur if the surface is relatively open and a different phase of the material is stable at low temperature. When modeling such a surface, two steps are suggested to be taken.

Layered K0.37MnO2·0.25H2O as cathode material for potassium ion batteries

Lithium supply shortages have prompted the search for alternatives to widespread grid system applications. Potassium-ion batteries (PIBs) have emerged to promising candidates for this purpose. Nonetheless, the large radius of K+ (1.38 Å) impedes the march of satisfactory cathode materials. Here, we used solid-phase synthesis to prepare a layered K0.37MnO2·0.25H2O (KMO) cathode, comprising alternately connected MnO6 octahedra with a large interlayer spacing (0.71 nm) to accommodate the migration and transport of K+ ions. The cathode material achieved initial specific capacities of 102.3 and 88.1 mA h g−1 at current densities of 60 mA g−1 and 1 A g−1, respectively. The storage mechanism of K+ ions in PIBs was demonstrated ex situ using x-ray diffraction, x-ray photoelectron spectroscopy, and Raman spectroscopy measurements. Overall, our proposed KMO was confirmed as an auspicious cathode material for potential use in PIBs.

The Crystal Structure of Manganotychite, Na6Mn2(CO3)4(SO4), and Structural Relations in the Northupite Group

The crystal structure of manganotychite has been refined using the holotype specimen from the Alluaiv Mountain, Lovozero massif, Kola peninsula, Russia. The mineral is cubic, Fd3¯, a = 14.0015(3) Å, V = 2744.88(18) Å3, Z = 8, R1 = 0.020 for 388 independently observed reflections. Manganotychite is isotypic to tychite and ferrotychite. Its crystal structure is based upon a three-dimensional infinite framework formed by condensation of MnO6 octahedra and CO3 groups by sharing common O atoms. The sulfate groups and Na+ cations reside in the cavities of the octahedral-triangular metal-carbonate framework. In terms of symmetry and basic construction of the octahedral-triangular framework, the crystal structure of manganotychite is identical to that of northupite, Na3Mg(CO3)2Cl. The transition northupite → tychite can be described as a result of the multiatomic 2Cl− → (SO4)2− substitution, where both chlorine and sulfate ions are the extra-framework constituents. However, the positions occupied by sulfate groups and chlorine ions correspond to different octahedral cavities within the skeletons of Na atoms. The crystal structure of northupite can be considered as an interpenetration of two frameworks: anionic [Mg(CO3)2]2− octahedral-triangular framework and cationic [ClNa3]2− framework with the antipyrochlore topology. Both manganotychite and northupite structure types can be described as a modification of the crystal structure of diamond (or the dia net) via the following steps: (i) replacement of a vertex of the dia net by an M4 tetrahedron (no symmetry reduction); (ii) attachment of (CO3) triangles to the triangular faces of the M4 tetrahedra (accompanied by the Fd3¯m → Fd3¯ symmetry reduction); (iii) filling voids of the resulting framework by Na+ cations (no symmetry reduction); and (iv) filling voids of the Na skeleton by either sulfate groups (in tychite-type structures) or chlorine atoms (in northupite). As a result, the information-based structural complexity of manganotychite and northupite exceeds that of the dia net.

Harnessing Pseudo-Jahn-Teller Disordering of Monoclinic Birnessite for Excited Interfacial Polarization and Local Magnetic Domains.

The symmetry in a polymorph is one of the most important elements for determining the inherent lattice nature. The MnO2 host tends to high-symmetry MnO6 octahedra as a result of the electronic structure t2g 3 eg 0 of Mn4+ ions, displaying an ordered structure accompanying with poor polarization loss and limiting its application toward high-performance microwave absorbers. Here, a pseudo-Jahn-Teller (PJT) distortion and PJT disordering design with abundant self-forming interfaces and local magnetic domains in the monoclinic birnessite-MnO2 host is first reported. The PJT distortion can give rise to asymmetric MnO6 octahedra, inducing the formation of interfaces and increased electron spin magnetic moment in the lattice. The resultant birnessite with PJT distortions and PJT disordering delivers an outstanding reflection loss value of -42.5dB at an ultralow thickness of 1.7mm, mainly derived from the excited interfacial polarization and magnetic loss. This work demonstrates an effective approach in regulating the lattice structure of birnessite for boosting microwave absorption performance.

Raman study of pressure-induced phase transitions in imidazolium manganese- hypophosphite hybrid perovskite

By using Raman spectroscopy, we demonstrate that [IM]Mn(H2POO)3 is a highly compressible material that undergoes three pressure-induced phase transitions. Using a diamond anvil cell we performed high-pressure experiments up to 7.1 GPa, using paraffin oil as the compression medium. The first phase transition, which occurs near 2.9 GPa, leads to very pronounced changes in the Raman spectra. This behavior indicates that this transition is associated with very large reconstruction of the inorganic framework and collapse of the perovskite cages. The second phase transition, which occurs near 4.9 GPa, is associated with subtle structural changes. The last transition takes place near 5.9 GPa and it leads to further significant distortion of the anionic framework. In contrast to the anionic framework, the phase transitions have weak impact on the imidazolium cation. Pressure dependence of Raman modes proves that compressibility of the high-pressure phases is significantly lower compared to the ambient pressure phase. It also indicates that the contraction of the MnO6 octahedra prevails over that of the imidazolium cations and hypophosphite linkers. However, compressibility of MnO6 strongly decreases in the highest pressure phase. Pressure-induced phase transitions are reversible.

Structural and Thermoelectric Properties of Gd2−2xSr1+2xMn2O7 Double-Layered Manganites

Double-layered manganites are natural superlattices with low thermal conductivity, which is of importance for potential thermoelectric applications. The Gd2-2xSr1+2xMn2O7 (x = 0.5; 0.625; 0.75) were prepared by the solid-state reaction method. All the samples crystallize in the tetragonal I4/mmm Sr3Ti2O7 type structure. The unit cell volume and the distortion in the MnO6 octahedra increase with increasing Gd content. Their thermoelectric properties were investigated between 300 and 1200 K. All exhibit an n-type semiconducting behavior. The electrical conductivity (σ) increases while the absolute value of the Seebeck coefficient (|S|) decreases with increasing Gd content. Simultaneous increases in σ and |S| with increasing temperature are observed at temperatures approximately higher than 600 K, and the power factor reaches a maximum value of 18.36 μW/(m K²) for x = 0.75 at 1200 K. The thermal conductivity (κ) is lower than 2 W/(m K) over the temperature range of 300-1000 K for all the samples and a maximum dimensionless figure of merit ZT of 0.01 is obtained for x = 0.75 at 1000 K.

Efficient utilization of biogenic manganese oxides in bioaugmentation columns for remediation of thallium(I) contaminated groundwater

Little attention has been paid to the in situ-generated biogenic manganese oxides (BMnOx) for practical implementation in continuous groundwater remediation systems. The enrichment effects of manganese oxidizing bacteria (MOB) in bioaugmentation columns and the in situ-generated BMnOx for continuous thallium(I) (Tl(I)) removal from groundwater were investigated. Results indicated that Pseudomonas Putida MnB1 (strain MnB1) attached on the groundwater sediments (GS) can achieve a maximum of 97.37 % Mn(II) oxidation and generate 29.6 mg/L BMnOx, which was superior than that of traditional quartz sand (QS). The in situ-generated BMnOx in MOB_GS column effectively removed 10–100 μg/L Tl(I) under the interference of high concentrations of Fe(II) and Mn(II) in groundwater. Distinctive microbial enrichment effects occurred in the bioaugmentation columns under the competition of indigenous microbes in groundwater. The release of Mn(II) from the BMnOx inhibited with the decrease in Tl(I) removal efficiency. XAFS analysis revealed Tl(I) was effectively adsorbed by BMnOx and Mn-O octahedra with Tl-O tetrahedral coordination existed in BMnOx. This study provides an in-depth understanding of the in situ-generated BMnOx for the Tl(I) removal and contributes to the application of BMnOx in groundwater remediation.