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We present a complete structural study of the successive phase transitions observed in the YBaMn2O6compound with the layered ordering of cations on the perovskite A-site. We have combined synchrotron radiation X-ray powder diffraction and symmetry-adapted mode analysis to describe the distorted structures as pseudosymmetric with respect to the parent tetragonal structure. The YBaMn2O6compound shows three consecutive phase transitions on cooling from 603 K down to 100 K. It undergoes a first-order structural transition atT1≈ 512 K from aC2/mcell with a single Mn site to aP21/ccell with two nonequivalent Mn sites. No checkerboard ordering of the two types of MnO6octahedra is revealed, and there is no significant charge segregation. A second transition is observed belowT2≈ 460 K giving rise to a duplication of thec-axis and the occurrence of four nonequivalent Mn sites. These sites are grouped in two pairs, producing, in this case, a checkerboard arrangement in theab-plane with an average charge segregation of Δq≈ 0.4 e-. The observed distortions in this phase disagree with the formation of an orbital-ordered phase. Finally, another structural transition is observed coupled to the magnetic transition atTN≈ 200 K and thec-axis is no longer duplicated. The low-temperature phase is polar with SGP21. It also contains four nonequivalent Mn sites grouped in two pairs. The charge difference between these pairs is increased, achieving a value of Δq≈ 0.7 e-. In this phase, an asymmetric stretching mode favors a Jahn-Teller-like distortion in the expanded MnO6octahedra that could be associated with an ordering ofeg(3dx2-z2/3dy2-z2) orbitals. Our refinements disclose that this phase is ferroelectric with significant polar displacements of the Mn and Obasalatoms along theb-axis. The simultaneous occurrence of ferroelectricity and magnetic ordering indicates that YBaMn2O6can be considered as a type II multiferroic compound and can present magnetoelectric coupling. ; The authors would like to acknowledge the use of Servicio General de Apoyo a la Investigación from Universidad de Zaragoza. Granted beam time at ALBA synchrotron is appreciated (Proposal No. 2018093038). For financial support, the authors thank the Spanish Ministerio de Ciencia, Innovación y Universidades (Project Nos. RTI2018-098537-B-C22 and -C21 co-funded by ERDF from EU, and Severo Ochoa FUNFUTURE, CEX2019-000917-S), and Diputación General de Aragón (Project E12-20R). ; With funding from the Spanish government through the 'Severo Ochoa Centre of Excellence' accreditation (CEX2019-000917-S). ; Peer reviewed

ε-Fe2O3 has gained considerable interest due to its intriguing properties and great application potentials. In this work ε-(Fe1−xCrx)2O3 (x = 0, 0.03, 0.075) nanoparticles encapsulated in silica have been successfully prepared by sol–gel chemistry. Synchrotron diffraction confirms that the polar Pna21 structure of ε-Fe2O3 is robust against the substitution of Fe by Cr. The incorporation of chromium is accompanied by a softer magnetic behaviour, and also a gradual decrease in the ferromagnetic signal associated to the super-hard FM2 ferrimagnetic phase. While the Cr substitution has little impact on the low temperature incommensurate transition, it is found that Cr substitution shifts the super-hard and soft ferrimagnetic transitions (respectively, TN2 and TN1) to notably lower temperatures. ; This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No. [819623]). We also acknowledge financial support from the Spanish Ministerio de Ciencia, Innovación y Universidades (MINCIU), through Project No. RTI2018-098537-B-C21, cofunded by ERDF from EU, and "Severo Ochoa" Programme for Centres of Excellence in R&D (Grant No. SEV-2015-0496) and from Generalitat de Catalunya through the projects 2017SGR00765 and 2017SGR1632. Z.M. was financially supported by China Scholarship Council (CSC) with No. 201606070094. Z.M's work was done as a part of the Ph.D program in Materials Science at Universitat Autònoma de Barcelona. We also acknowledge ALBA synchrotron (exp. No. 2018093113) for provision of beam time. ; Peer reviewed

AbstractVanadium pentoxide polymorphs are attracting much interest due to their remarkable electrochemical performance as positive electrodes for rechargeable metal‐ion batteries. High‐pressure β‐V2O5 is an interesting positive electrode material for Na‐ion batteries able to intercalate 1 Na+/formula unit delivering a capacity of 147 mAh g−1. The sodium intercalation of 0.4 Na+ has been described in the literature as a two‐phase domain involving an important structural rearrangement yielding NaxV2O5, but details about the structural changes upon sodium intercalation and structure of intercalated Na0.4V2O5 have not been elucidated so far. In this work, we performed operando synchrotron X‐ray diffraction measurements on sodium batteries on discharge. A full crystal structure determination for such phase, which we have called κ‐Na0.4V2O5, was carried out for the first time directly from operando measurements in monoclinic space group C2/m. Structural analysis was completed with bond valence sum calculations revealing the sites of intercalated sodium in κ‐NaxV2O5 and the structure was further optimized by means of density functional theory calculations. Operando X‐ray absorption spectroscopy and ex situ X‐ray photoelectron spectroscopy allowed to unveil the redox changes upon the phase transformation from β‐V2O5 into κ‐NaxV2O5, providing information about the reduction of vanadium from V5+ to V4+. The phase transformation also produced enhanced charge transfer and sodium diffusion processes, as deduced from both impedance and kinetic study.

Synchrotron X-ray diffraction and inelastic neutron scattering measurements have been applied to evaluate the breathing phenomena in small nanocrystals of ZIF-7 upon gas adsorption. The experimental results show that an extended solvent exchange process with methanol is crucial to get a solvent-free narrow pore structure. Under these conditions, nano-ZIF-7 is indeed able to adsorb N2 with a total BET surface area of around 380 m2 g−1, in close agreement with theoretical predictions. The breathing phenomenon upon nitrogen adsorption is accompanied by a phase-to-phase transition, from a narrow-pore (phase II) to a large-pore (phase I) structure and a suppression of the cooperative deformation of the framework involving mainly the flapping motion of the benzimidazolate (bIm) ligand with the 4- and 6-membered rings. Whereas nitrogen requires temperature and pressure conditions close to condensation (close to 1 bar and 77 K) to induce the breathing in ZIF-7, CO2 can do it under milder conditions (at room temperature and low relative pressures). These results reflect that the nature of the adsorptive probe and the gas–framework interactions, rather than the molecular diameter and/or shape, play a crucial role in defining the pressure and temperature conditions required to induce the breathing. The presence of two different cavities in ZIF-7 as suggested by theoretical predictions, one with a window diameter of below 0.4 nm (cavity A) and the other with a pore size of around 0.6 nm (cavity B), has been confirmed experimentally using immersion calorimetry. ; J. S. A. and C. C. C. acknowledge financial support from the University of Alicante (ACIE16-04) to cover all the expenses for the INS measurements at ORNL. J. S. A. gratefully acknowledges financial support from MINECO (MAT2016-80285-p), European Union H2020 (MSCA-RISE-2016/NanoMed Project) and Generalitat Valenciana (PROMETEOII/2014/004), Spanish ALBA synchrotron for beam time availability (Project ID: 2016021724) and Oak Ridge beam time availability (Project ID: IPTS-16291.1).

Ca2MnO3.5 and CaV2O4 were found to be potentially interesting as positive electrode materials for calcium metal-based high-energy density batteries with DFT-predicted average voltages of 3.7 V and 2.5 V and energy barriers for Ca migration of 1.1 eV and 0.6 eV, respectively. Both compounds were prepared by solid state reaction under reducing atmospheres. Optimum conditions to achieve Ca2MnO3.5 comprised the reduction of Ca2MnO4 under NH3 gas at 420⁰C with a flow rate of 1200 ml/min while CaV2O4 was achieved by reduction of CaV2O6 at 700 ⁰C under H2 flow. Electrochemical oxidation of Ca2MnO3.5 in lithium or calcium cells resulted in the formation of an orthorhombic phase with cell parameter (a= 5.2891(1), b= 10.551(2), c= 12.1422(1)). Operando synchrotron radiation diffraction experiments indicate that the charge compensation mechanism is not related to Ca2+ extraction but to intercalation of F− (originated from electrolyte salt decomposition) into the anion vacancy position, as confirmed by EELS and EDS. This process was found to be irreversible. In the case of CaV2O4, oxidation induces the electrochemical extraction of calcium with the formation of an orthorhombic phase (space group Pbnm) with cell parameters a = 10.72008(9) Å, b = 9.20213(2) Å and c = 2.89418(3) Å. The process was also investigated via operando synchrotron radiation diffraction, with the oxidized phase being found to reintercalate Ca2+ ions upon reduction, with the formation of a solid solution. Preliminary cycling tests reveals a decrease in the polarization after the first cycle and call for further investigation of this system. ; This research was funded by the European Union's Horizon 2020 research and innovation programme H2020 FETOPEN-1- 2016-2017 (CARBAT, grant agreement no. 766617). The authors are grateful for access to the computational facilities from Universidad de Oviedo (MALTA-Consolider cluster) and to ALBA synchrotron for beamtime (proposal 2021024935). ICMAB-CSIC members thank the Spanish Agencia Estatal de Investigación ...