The effect of the synthesis temperature on the chemical composition of "Li 1.20 Mn 0.54 Co 0.13 Ni 0.13 O 2 " was considered using thermogravimetric analyses (TGA) and in situ X-ray diffraction (XRD) during thermal treatment. A continuous and small weight loss is observed above 800 °C because of Li evaporation, and the lamellar phase disappears to the benefit of a spinel-type phase formed above 940 °C. The layered structure is recovered upon cooling under air. "Li 1.20 Mn 0.54 Co 0.13 Ni 0.13 O 2 " materials synthesized at 800, 900, and 1000 °C show very similar compositions, structures, and electrochemical properties despite very different crystallization states. Their average structure is α-NaFeO 2 -type and described in the R3̅ m space group, with less than 0.02 Ni 2+ ions in the Li site. This peculiar composition "Li 1.20 Mn 0.54 Co 0.13 Ni 0.13 O 2 ", with one-third of large cations (Li + , Ni 2+ ) and two-thirds of small cations (Mn 4+ , Co 3+ ) promotes the extension of the cation ordering in the slabs as revealed by the √3a hex. × √3a hex. superstructure, but without full correlation between the ordered slabs along the c hex. stacking axis. Neutron and electron diffraction associated with NMR and Raman spectroscopies are shown to be efficient tools to get more insights into the average and local structures of these complex layered materials.

Doping is generally used to tune and enhance the properties of metal oxides. However, their chemical composition cannot be readily modified beyond low dopant amounts without disrupting the crystalline atomic structure. In the case of anatase TiO 2 , we introduce a new solution-based chemical route allowing the composition to be significantly modified, substituting the divalent O 2− anions by monovalent F − and OH − anions resulting in the formation of cationic Ti 4+ vacancies (□) whose concentration can be controlled by the reaction temperature. The resulting polyanionic anatase has the general composition Ti 1−x−y □ x+y O 2−4(x+y) F 4x (OH) 4y , reaching vacancy concentrations of up to 22%, i.e., Ti 0.78 □ 0.22 O 1.12 F 0.4 (OH) 0.48 . Solid-state 19 F NMR spectroscopy reveals that fluoride ions can accommodate up to three different environments, depending on Ti and vacancies (i.e. Ti 3 -F, Ti 2 □ 1 -F, and Ti 1 □ 2 -F), with a preferential location close to vacancies. DFT calculations further confirm the fluoride/vacancy ordering. When its characteristics were evaluated as an electrode for reversible Li-ion storage, the material shows a modified lithium reaction mechanism, which has been rationalized by the occurrence of cationic vacancies acting as additional lithium hosting sites within the anatase framework. Finally, the material shows a fast discharging/charging behavior, compared to TiO 2 , highlighting the benefits of the structural modifications and paving the way for the design of advanced electrode materials, based on a defect mediated mechanism. ■ INTRODUCTIONTransition-metal oxides are an important class of materials, whose properties are dependent on many factors, including their composition, structure, and morphology. Among them, titanium dioxide (TiO 2 ) is a multifunctional material used for a broad range of applications. The low toxicity and the abundance of titanium have favored the emergence of Tibased compounds for photocatalytic hydrogen production by water splitting, rechargeable batteries/supercapacitors, dyesensitized solar cells, sensors, and biomedical devices. 1−6 Over the years, several approaches have been developed to improve its properties. For instance, crystal facets engineering and structural modifications have been widely employed. 7−9 The latter approach comprises the introduction of reduced titanium or heteroatoms within the lattice. Although the use of dopants has been proved as an effective way to tune this material's properties, changing the chemical composition of TiO 2 with a degree of substitution that goes beyond the doping level appears as a challenging and promising way to tailor its properties. On more general grounds, knowing how strongly a metal oxide can be modified while maintaining its original network is of fundamental interest.With the goal of modifying the composition of anatase, which is one of the polymorphs of TiO 2 , we propose an approach where divalent oxide anions are substituted by monovalent ones such as fluoride and hydroxide. The anatase structure...

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Identifying and characterizing defects in crystalline solids is a challenging problem, particularly for lithium-ion intercalation materials, which often exhibit multiple stable oxidation and spin states as well as local ordering of lithium and charges. Here, we reveal the existence of characteristic lithium defect environments in the crystalline lithium-ion battery electrode LiVPO 4 F and establish the relative sub-nanometer-scale proximities between them. Well-crystallized LiVPO 4 F samples were synthesized with the expected tavorite-like structure, as established by X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) measurements. Solidstate 7 Li nuclear magnetic resonance (NMR) spectra reveal unexpected paramagnetic 7 Li environments that can account for up to 20% of the total lithium content. Multi-dimensional and site-selective solid-state 7 Li NMR experiments using finite-pulse radio-frequency-driven recoupling (fp-RFDR) establish unambiguously that the unexpected lithium environments are associated with defects within the LiVPO 4 F crystal structure, revealing the existence of dipole-dipole-coupled defect pairs. The lithium defects exhibit local electronic environments that are distinct from lithium ions in the crystallographic LiVPO 4 F site, which result from altered oxidation and/or spin states of nearby paramagnetic vanadium atoms. The results provide a general strategy for identifying and characterizing lithium defect environments in crystalline solids, including paramagnetic materials with short 7

In this work, we investigate the crystal chemistry of Fe/V-mixed NASICON [sodium (Na) Super Ionic CONductor] compositions Na 3 FeV(PO 4 ) 3 and Na 4 FeV(PO 4 ) 3 that are structurally related to Na 3 V 2 (PO 4 ) 3 , a positive electrode for Na-ion batteries. To synthesize Na 4 FeV(PO 4 ) 3 , various synthesis routes (solidstate, sol−gel-assisted, and electrochemical syntheses) were investigated. Direct syntheses resulted in the formation of a NASICON-type phase in the presence of NaFePO 4 and Na 3 PO 4 impurities. The successful preparation of pure Na 4 FeV(PO 4 ) 3 has been achieved by the electrochemical sodiation of Na 3 FeV(PO 4 ) 3 . Both synchrotron X-ray absorption and Mossbauer spectroscopy allowed probing the local V and Fe environments and their oxidation states in Na 3 FeV(PO 4 ) 3 and Na 4 FeV(PO 4 ) 3 . Na 3 FeV(PO 4 ) 3 crystallizes in the space group C2/c (a = 15.1394(2) Å; b = 8.72550(12) Å; c = 21.6142(3) Å; β = 90.1744(9)°; and Z = 12), and it is isostructural to an ordered αform of Na 3 M 2 (PO 4 ) 3 (M = Fe, V). It presents a superstructure due to Na + ordering, as confirmed by differential scanning calorimetry and in situ temperature X-ray diffraction. The electrochemically sodiated Na 4 FeV(PO 4 ) 3 powder crystallizes in the space group R3̅ c (a = 8.94656(8) Å, c = 21.3054(3) Å, and Z = 6) within which the two sodium sites, Na(1) and Na(2), are almost fully occupied. Na 4 FeV(PO 4 ) 3 allows the electrochemical extraction of 2.76 Na + per formula unit within the voltage range of 1.3−4.3 V versus Na + /Na through the Fe III/II , V IV/III , and V V/IV redox couples. This identifies an interesting material for Na-ion batteries.

Dynamic spin interchange where crystals explode with preservation of magnetic memory is observed for a mononuclear hysteretic Fe(iii) Schiff-base compound.

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The two room-temperature NaNbO3 polymorphs crystallizing with orthorhombic symmetry have been successfully isolated at using a new preparative method. The pure polar phase, annealed at 600 °C under air after hydrothermal treatment at 200 °C, adopts the P21ma space group, whereas the well-known and thermodynamically stable form with the Pbma structure is obtained at higher temperatures under air (T = 950 °C). Thanks to the combination of powder-XRD Rietveld analysis, 23 Na solid-state NMR spectroscopy and second harmonic generation studies, structural features of both these polymorphs reveal clear structural differences. The stability of each atom site is investigated by mean of bond distances, Madelung potentials and DFT calculations. The key role of Na atomic positions is highlighted, especially how they influence the [NbO6] octahedron distortion. In the polar P21ma-phase, the higher distortion both along the apical axis and the equatorial plane is consecutive to the relaxation of the overall network with the stabilization of Na atoms in two sites sharing the same symmetry. Focusing on oxygen mobility, both polymorphs show distinct reactivities toward reductive heat treatments: as characterized by thermogravimetric analysis and ESR measurements, the Pbma framework is relatively insensitive, while the P21ma one yields a non-stoichiometric oxide with a Nb 4+ content of 18 % corresponding to NaNbO2.91 chemical composition. The fine control in phasic purity together with advances on the structural features of room-temperature phases should benefit both non-linear optical applications and photocatalytic performances of sodium niobates.