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The structure of β-Na 0.33 V 2 O 4.67 F 0.33 has been investigated by both theoretical and experimental methods. It exhibits the same structure as that of the parent bronze β-Na 0.33 V 2 O 5 . The partial substitution of oxygen by fluorine has little effect on the average structure and cell parameters, but the sodium environment changes significantly. Using DFT calculations, we determined the most stable positions of fluorine atoms in the unit cell. It was found that the partial replacement of oxide by fluoride takes mainly place in the coordination sphere of Na producing a shortening of the Na-anion bond lengths. We also analyzed the electronic properties based on density of states and Bader charge distribution. The crystallochemical situation of sodium ions in β-Na 0.33 V 2 O 4.67 F 0.33 oxyfluoride, detected by both experimental and computational methods, affects its mobility with respect to the parent oxide. The higher ionicity in the Na coordination sphere of β-Na 0.33 V 2 O 4.67 F 0.33 is related to a sodium ion diffusion coefficient, D Na+ , that is 1 order of magnitude lower (1.24 × 10 -13 cm 2 s -1 ) than in the case of β-Na 0.33 V 2 O 5 (1.13 × 10 -12 cm 2 s -1 ). Electrochemical sodium insertion/deinsertion properties of the oxyfluoride have been also investigated and are compared to the oxide. Insertion/deinsertion equilibrium potential for the same formal oxidation state of vanadium increases due to fluorination (for instance reduction of V +4.3 occurs at 1.5 V in the oxide and at 1.75 V in the oxyfluoride). However, the capacity of Na 0.33 V 2 F 4.67 F 0.33 at constant current is lower than in the case of β-Na 0.33 V 2 O 5 , and a lower oxidation state of vanadium owing to the aliovalent O/F substitution.D Na+ , and a lower oxidation state of vanadium owing to the aliovalent O/F substitution.

Metal oxyfluorides are currently attracting much attention for next-generation rechargeable batteries because of their high theoretical capacity and resulting high energy density. Rhombohedral VO 2 F is promising because it allows two-electron transfer during electrochemical lithium cycling, with a theoretical capacity of 526 mAh g -1 . However, the chemical changes it undergoes during operation are not clearly understood. In this work, a combination of synchrotron X-ray and neutron diffraction was employed to accurately describe the crystal structure of both pristine and lithiated VO 2 F, using samples with high crystallinity to overcome challenges in previous studies. The mechanism and reversibility of the lithium insertion was monitored in real time by high angular synchrotron diffraction measurements, performed in operando on a lithium battery in the high-voltage range: 3.9-2.3 V vs Li + /Li. Insertion of up to one lithium ion proceeds through a solid-solution reaction, while Rietveld refinements of neutron powder diffraction data revealed that the lithiated states adopt the noncentrosymmetric R 3 c framework, uncovering an octahedral Li-(O/F) 6 coordination with reasonable Li-O/F bond lengths. This work further evaluates the redox changes of VO 2 F upon Li intercalation. By a comparison of changes in electronic states of all the elements in the compound, it clarifies the critical role of both anions, O and F, in the charge compensation through their covalent interactions with the 3d states of V. The clear evidence of participation of F challenges existing assumptions that its high electronegativity renders this anion largely a spectator in the redox reaction.

The symmetry of the room-temperature (RT) structure of title compounds La 2-x Sr x CoTiO 6-δ changes with x, from P2 1 /n (0 ≤ x ≤ 0.2) to Pnma (0.3 ≤ x ≤ 0.5) and to R3̅c (0.6 ≤ x ≤ 1). For x = 1 the three pseudocubic cell parameters become very close suggesting a transition to a cubic structure for higher Sr contents. Similar phase transitions were expected to occur on heating, paralleling the effect of internal pressure induced by substitution of La 3+ by Sr 2+ . However, only some of these aforementioned transitions have been thermally induced. The symmetry-adapted modes formalism is used in the structural refinements and fitting of neutron diffraction data recorded from RT to 1273 K. Thus, for x = 1, the out-of-phase tilting of the BO 6 octahedra vanishes progressively on heating, and a cubic structure with Pm3̅m symmetry is found at 1073 K. For lower Sr contents this transition is predicted to occur far above the temperature limit of common experimental setups. The analysis of the evolution of the perovskite tolerance factor, t-factor, with both Sr content and temperature indicates that temperature has a limited ability to release structural stress and thus to enable transitions to more symmetric phases. This is particularly true when compared to the effect of internal pressure induced by substitution of La by Sr. The existence of phase transitions in materials for solid oxide fuel cells that are usually exposed to heating-cooling cycles may have a detrimental effect. This work suggests strategies to stabilize the high-symmetry high-temperature phase of perovskite oxides through internal-pressure chemically induced.

A new hydrated sodium nickel fluoride with nominal composition NaNiF3·3H2O was synthesized using an aqueous solution route. Its structure was solved by means of ab initio methods from powder X-ray diffraction and neutron diffraction data. NaNiF3·3H2O crystallizes in the cubic crystal system, space group Pn3̅ with a = 7.91968(4) Å. The framework, derived from the ReO3 structure type, is built from NaX6 and NiX6 (X = O, F) corner-shared octahedra, in which F and O atoms are randomly distributed on a single anion site. The 2a × 2a × 2a superstructure arises from the strict alternate three-dimensional linking of NaX6 and NiX6 octahedra together with the simultaneous tilts of the octahedra from the cube axis (φ = 31.1°), with a significant participation of hydrogen bonding. NaNiF3·3H2O corresponds to a fully cation-ordered variant of the In(OH)3 structure, easily recognizable when formulated as NaNi(XH)6 (X = O, F). It constitutes one of the rare examples for the a(+)a(+)a(+) tilting scheme with 1:1 cation ordering in perovskite-related compounds. The Curie-like magnetic behavior well-reflects the isolated paramagnetic Ni(2+) centers without worth mentioning interactions. While X-ray and neutron diffraction data evidence Na/Ni order in combination with O/F disorder as a main feature of this fluoride, results from Raman and magic-angle spinning NMR spectroscopies support the existence of specific anion arrangements in isolated square windows identified in structural refinements. In particular, formation of water molecules derives from unfavorable FH bond formation.