Na 3 V 2 (PO 4 ) 2 F 3 is a positive electrode material for Na-ion batteries which is attracting strong interest due to its high capacity, rate capability and long-term cycling stability. The sodium extraction mechanism from this material has been always described in the literature as a straightforward solid solution, but several hints point towards a more complicated phase diagram. In this work we performed high angular resolution synchrotron radiation diffraction measurements, realized operando on sodium batteries upon charge. We reveal an extremely interesting phase diagram, created by the successive crystallization of four intermediate phases before the end composition NaV 2 (PO 4 ) 2 F 3 is reached. Only one of these phases undergoes a solid solution reaction, in the interval between 1.8 and 1.3 Na per formula unit. The ability to resolve weak Bragg reflections allowed us to reveal differences in terms of symmetry amongst the phases, to determine their previously unknown space groups and to correlate them with sodium (dis)ordering in the structure. Rietveld refinements enabled us to follow fine structural modifications in great detail. Intermediate identified phases are not simply described by their unit cell parameters but bond-lengths variations can be tracked, as well as polyhedral distortions and site occupancy factors for mobile sodium ions. For NaV 2 (PO 4 ) 2 F 3 a full crystal structure determination was also carried out for the first time directly from operando measurements, assigning it to the Cmc2 1 space group and revealing two vanadium environments: V 3+ and V 5+ . Our study demonstrates that improved angular resolution and high intensity diffraction data are key parameters for direct observation of fine reaction pathways in electrode materials, and that the obtained insight is crucial for the understanding of (de)intercalation mechanisms in Na-ion batteries.

Several “Beyond Li-Ion Battery” concepts such as all solid-state batteries and hybrid liquid/solid systems envision the use of a solid electrolyte to protect Li-metal anodes. These configurations are very attractive due to the possibility of exceptionally high energy densities and high (dis)charge rates, but they are far from being realized practically due to a number of issues including high interfacial resistance and difficulties associated with fabrication. One of the most promising solid electrolyte systems for these applications is Al or Ga stabilized Li7La3Zr2O12 (LLZO) based on high ionic conductivities and apparent stability against reduction by Li metal. Nevertheless, the fabrication of dense LLZO membranes with high ionic conductivity and low interfacial resistances remains challenging; it definitely requires a better understanding of the structural and electrochemical properties. In this study, the phase transition from garnet (Ia3̅d, No. 230) to “non-garnet” (I4̅3d, No. 220) space group as a function of composition and the different sintering behavior of Ga and Al stabilized LLZO are identified as important factors in determining the electrochemical properties. The phase transition was located at an Al:Ga substitution ratio of 0.05:0.15 and is accompanied by a significant lowering of the activation energy for Li-ion transport to 0.26 eV. The phase transition combined with microstructural changes concomitant with an increase of the Ga/Al ratio continuously improves the Li-ion conductivity from 2.6 × 10–4 S cm–1 to 1.2 × 10–3 S cm–1, which is close to the calculated maximum for garnet-type materials. The increase in Ga content is also associated with better densification and smaller grains and is accompanied by a change in the area specific resistance (ASR) from 78 to 24 Ω cm2, the lowest reported value for LLZO so far. These results illustrate that understanding the structure–properties relationships in this class of materials allows practical obstacles to its utilization to be readily overcome.

Three new perovskites in the LaBaMn 2 O 6-x family have been synthesized by controlling the oxygen pressure, during both synthesis and postannealing. Structural determination from powder neutron diffraction (PND) data shows that one form of LaBaMn 2 O 6 is cubic (a ) 3.906 Å), with a disordered distribution of La 3+ and Ba 2+ cations, whereas a second form of LaBaMn 2 O 6 is tetragonal (a ) 3.916 Å; c ) 7.805 Å), with an alternate stacking of lanthanum and barium layers along c. The same La/Ba cation order is observed for the ordered, oxygen-deficient perovskite LaBaMn 2 O 5 , which is also tetragonal (a ) 5.650 Å; c ) 7.808 Å) and adopts a YBaCuFeO 5 -related structure. Elucidation of the magnetic structure of LaBaMn 2 O 5 , from low-temperature PND data, leads to a G-type antiferromagnetic model; the superimposed Mn 2+ /Mn 3+ charge order results in ferrimagnetic behavior for this phase and explains its magnetic properties, as obtained from susceptibility measurements. In both forms of LaBaMn 2 O 6 , the PND data show a ferromagnetic contribution. The CMR properties of the "O 6 " forms exhibit a remarkable feature: T C is increased from 270 K for the disordered phase to 335 K for the ordered one, probably owing to the La/Ba ordering.

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The structures and phase transitions of AgNbO 3 were investigated using neutron powder diffraction and restricted single-crystal x-ray diffraction. Both methods have revealed the high temperature M 3 -O 1 , O 2 -T and T-C phase transitions but have not given any significant evidence of low temperature M 1 -M 2 and M 2 -M 3 ones. The refinements of neutron diffraction patterns allowed us to determine the symmetry, space group and crystal structure for all phases except the O 1 one. The existence of structural disorder in the T and probably O 2 phases was found. The high temperature paraelectric phase transitions can be interpreted on the basis of consecutive condensation of oxygen octahedron tilts around the main axis. The ferroelectric and antiferroelectric behaviour has been associated with Ag and Nb cations. The reason why phase transitions between low temperature ferroelectric and antiferroelectric phases are not detectable by diffraction methods is discussed. The sequence of phase transitions in AgNbO 3 can then be understood in the framework of a long range and/or local orderdisorder type arrangement.