Rotational Dynamics of Organic Cations in Formamidinium Lead Iodide Perovskites

We report results from quasi-elastic neutron scattering studies on the rotational dynamics of formamidinium (HC[NH2]2+, FA) and methylammonium (CH3NH3+, MA) cations in FA1-xMAxPbI3 with x = 0 and 0.4 and compare it to the dynamics in MAPbI3. For FAPbI3, the FA cation dynamics evolve from nearly isotropic rotations in the high-temperature (T > 285 K) cubic phase through reorientations between preferred orientations in the intermediate-temperature tetragonal phase (140 K < T ⩽ 285 K) to an even more complex dynamics, due to a disordered arrangement of the FA cations, in the low-temperature tetragonal phase (T ⩽ 140 K). For FA0.6MA0.4PbI3, the dynamics of the respective organic cations evolve from a relatively similar behavior to FAPbI3 and MAPbI3 at room temperature to a different behavior in the lower-temperature phases where the MA cation dynamics are a factor of 50 faster as compared to those of MAPbI3. This insight suggests that tuning the MA/FA cation ratio may be a promising approach to tailoring the dynamics and, in effect, optical properties of FA1-xMAxPbI

Local structure and vibrational dynamics of proton conducting Ba2In2O5(H2O)x

We study the local structure and vibrational dynamics of the brownmillerite-based proton conductors Ba2In2O5(H2O)x, with x = 0.30, 0.76, and 0.92, using infrared spectroscopy, inelastic neutron scattering and ab initio molecular dynamics simulations. Ba2In2O5(H2O)x is found to exhibit two main types of proton sites, H(1) and H(2). The H(1) site is characterised by the coexistence of two intra-octahedral hydrogen-bond geometries, whereas the H(2) site is characterised by inter-octahedral hydrogen bonding. While the strength of the hydrogen bonding is similar for the majority of protons in the two proton sites, ≈10% of the H(2) protons forms unusually strong hydrogen bonds due to local proton environments characterised by an unusually short oxygen-oxygen separation distance of ≈2.6 \uc5. These local proton environments are manifested as two O-H stretch bands in the infrared absorbance spectra, at 255 and 290 meV, respectively. These O-H stretch bands are as well observed in the related class of In-doped perovskite-type oxides, BaInyZr1-yO3-y/2 (0.25 ≤ y ≤ 0.75), suggesting that these perovskites may display brownmillerite-like distortions on a local length scale. In effect, these results point towards a clustering of the In atoms in these perovskite materials. Further, the infrared spectra of Ba2In2O5(H2O)x show a minor evolution as a function of x, because the protons tend to segregate into oxygen-rich hydrogen-rich domains upon dehydration. This points towards a highly anisotropic proton conduction mechanism in partially hydrated phases. This insight motivates efforts to identify ways to avoid phase separation, perhaps by suitable cation substitutions, as a route to accommodate high proton conductivity

Proton Diffusion Mechanism in Hydrated Barium Indate Oxides

We report on quasielastic neutron scattering (QENS) andab initiomolecular dynamics (AIMD) simulations of the mechanism of proton diffusionin the partially and fully hydrated barium indate oxide proton conductorsBa(2)In(2)O(5)(H2O)( x ) (x = 0.30 and 0.92). Structurally,these materials are featured by an intergrowth of cubic and "pseudo-cubic"layers of InO6 octahedra, wherein two distinct proton sites,H(1) and H(2), are present. We show that the main localized dynamicsof these protons can be described as rotational diffusion of O-H(1)species and H(2) proton transfers between neighboring oxygen atoms.The mean residence times of both processes are in the order of picosecondsin the two studied materials. For the fully hydrated material, Ba2In2O5(H2O)(0.92), we also reveal the presence of a third proton site, H(3), whichbecomes occupied upon increasing the temperature and serves as a saddlestate for the interexchange between H(1) and H(2) protons. Crucially,the occupation of the H(3) site enables long-range diffusion of protons,which is highly anisotropic in nature and occurs through a two-dimensionalpathway. For the partially hydrated material, Ba2In2O5(H2O)(0.30), the occupationof the H(3) site and subsequent long-range diffusion are not observed,which is rationalized by hindered dynamics of H(2) protons in thevicinity of oxygen vacancies. A comparison to state-of-the-art proton-conductingoxides, such as barium zirconate-based materials, suggests that thegenerally lower proton conductivity in Ba2In2O5(H2O)( x ) is dueto a large occupation of the H(1) and H(2) sites, which, in turn,means that there are few sites available for proton diffusion. Thisinsight suggests that the chemical substitution of indium by cationswith higher oxidation states offers a novel route toward higher protonconductivity because it reduces the proton site occupancy while preservingan oxygen-vacancy-free structure

Diffusional Dynamics of Hydride Ions in the Layered Oxyhydride SrVO2H

Perovskite-type oxyhydrides are hydride-ion-conducting materials of promise for several types of technological applications; however, the conductivity is often too low for practical use and, on a fundamental level, the mechanism of hydride-ion diffusion remains unclear. Here, we, with the use of neutron scattering techniques, investigate the diffusional dynamics of hydride ions in the layered perovskite-type oxyhydride SrVO2H. By monitoring the intensity of the elastically scattered neutrons upon heating the sample from 100 to 430 K, we establish an onset temperature for diffusional hydride-ion dynamics at about 250 K. Above this temperature, the hydride ions are shown to exhibit two-dimensional diffusion restricted to the hydride-ion sublattice of SrVO2H and that occurs as a series of jumps of a hydride ion to a neighboring hydride-ion vacancy, with an enhanced rate for backward jumps due to correlation effects. Analysis of the temperature dependence of the neutron scattering data shows that the localized jumps of hydride ions are featured by a mean residence time of the order of 10 ps with an activation energy of 0.1 eV. The long-range diffusion of hydride ions occurs on the timescale of 1 ns and with an activation energy of 0.2 eV. The hydride-ion diffusion coefficient is found to be of the order of 1 7 10-6 cm2 s-1 in the temperature range of 300-430 K, which is similar to other oxyhydrides but higher than for proton-conducting perovskite analogues. Tuning of the hydride-ion vacancy concentration in SrVO2H thus represents a promising gateway to improve the ionic conductivity of this already highly hydride-ion-conducting material

Resonant enhancement of grazing incidence neutron scattering for the characterization of thin films

We use signal enhancement in a quantum resonator for the characterization of a thin layer of vanadium hydride using neutron reflectometry and demonstrate that pressure-concentration isotherms and expansion coefficients can be extracted from the measurement of totally externally reflected neutrons only. Moreover, a consistent data analysis of the attenuation cross section allows us to detect and quantify off-specular and small angle scattering. As our experiments are effective direct beam measurements, combined with resonant signal enhancement, counting times become considerably reduced. This allows us to overcome the challenges resulting from the comparatively low brilliance of neutron beams for grazing incidence scattering experiments. Further, we discuss the potential of resonant enhancement to increase any scattering, which is of particular interest for grazing incidence small angle neutron scattering and spectroscopy

Unraveling the ground-state structure of BaZrO3 by neutron scattering experiments and first-principle calculations

The all-inorganic perovskite barium zirconate, BaZrO3, is a widely used material in a range of different technological applications. However, fundamental questions surrounding the crystal structure of BaZrO3, especially in regard to its ground-state structure, remain. While diffraction techniques indicate a cubic structure all the way down to T = 0 K, several first-principles phonon calculation studies based on density functional theory indicate an imaginary (unstable) phonon mode due to the appearance of an antiferrodistortive transition associated with rigid rotations of ZrO6 octahedra. The first-principles calculations are highly sensitive to the choice of exchange-correlation functional and, using six well-established functional approximations, we show that a correct description about the ground-state structure of BaZrO3 requires the use of hybrid functionals. The ground-state structure of BaZrO3 is found to be cubic, which is corroborated by experimental results obtained from neutron powder diffraction, inelastic neutron scattering, and neutron Compton scattering experiments

Local Coordination Environments and Vibrational Dynamics of Protons in Hexagonal and Cubic Sc-Doped BaTiO3 Proton-Conducting Oxides

The proton local coordination environments and vibrational dynamics associated with the two order of magnitude change in proton conductivity in hydrated forms of hexagonal and cubic structured BaTi1-xScxO3Hx (0.16 < x < 0.7) were investigated using optical spectroscopy, neutron spectroscopy, and first-principles calculations. Whereas the cubic structure compositions display a single proton site, we show that protons occupy three distinct sites in compositions exhibiting the hexagonal structure. The principal site is characterized by interoctahedral hydrogen bonds, while two additional low occupancy sites are similar to those in the cubic structure, with classic intraoctahedral geometry. Furthermore, the proton hydrogen bond strength increases with decreasing scandium doping level. We infer from this that the stronger, more energetic hydrogen bonds in the hexagonal structure, resulting from proton sites with lower symmetry (lower multiplicity), are predominantly responsible for the significant reduction in macroscopic conductivity between cubic and hexagonal BaTi1-xScxO3Hx materials, rather than simply the absolute number of protons. Our findings are highly relevant to the field, clarifying the advantages of high-symmetry structures with high-multiplicity proton sites to favorable properties in ceramic proton-conducting oxides

Local structure and vibrational dynamics in indium-doped barium zirconate

Barium zirconate (BaZrO3), when substituted with trivalent acceptor ions to replace Zr4+, is a proton conducting material of interest for several electrochemical applications. The local coordination environments, and vibrational dynamics, of the protons are known to critically influence the material\u27s proton conducting properties, however, the nature of the static and dynamic structure around the protons and, especially, how it is affected by the dopant atoms for high doping concentrations, remains to be elucidated. Here we report results from X-ray powder diffraction, infrared (IR) spectroscopy, inelastic neutron scattering (INS) and ab initio molecular dynamics (AIMD) simulations on a hydrated sample of BaZrO3 substituted with 50% In3+. The investigation of the momentum-transfer (Q) dependence of the INS spectrum is used to aid the analysis of the spectra and the assignment of the spectral components to fundamental O-H bend and O-H stretch modes and higher-order transitions. The AIMD simulations show that the INS spectrum is constituted of the overlapping spectra of protons in several different local structural environments, whereas the local proton environments for specific protons are found to vary with time as a result of thermally activated vibrations of the perovskite lattice. It is argued that, converse to more weakly doped systems, such as 20% Y-doped BaZrO3, the dopant-proton association effect does not hinder the diffusion of protons due to the presence of percolation paths of dopant atoms throughout the perovskite lattice

Local structure and vibrational dynamics in indium-doped barium zirconate

Barium zirconate (BaZrO3), when substituted with trivalent acceptor ions to replace Zr4+, is a proton conducting material of interest for several electrochemical applications. The local coordination environments, and vibrational dynamics, of the protons are known to critically influence the material\u27s proton conducting properties, however, the nature of the static and dynamic structure around the protons and, especially, how it is affected by the dopant atoms for high doping concentrations, remains to be elucidated. Here we report results from X-ray powder diffraction, infrared (IR) spectroscopy, inelastic neutron scattering (INS) and ab initio molecular dynamics (AIMD) simulations on a hydrated sample of BaZrO3 substituted with 50% In3+. The investigation of the momentum-transfer (Q) dependence of the INS spectrum is used to aid the analysis of the spectra and the assignment of the spectral components to fundamental O-H bend and O-H stretch modes and higher-order transitions. The AIMD simulations show that the INS spectrum is constituted of the overlapping spectra of protons in several different local structural environments, whereas the local proton environments for specific protons are found to vary with time as a result of thermally activated vibrations of the perovskite lattice. It is argued that, converse to more weakly doped systems, such as 20% Y-doped BaZrO3, the dopant-proton association effect does not hinder the diffusion of protons due to the presence of percolation paths of dopant atoms throughout the perovskite lattice

Etude des mécanismes de diffusion des ions oxydes dans Nd2NiO4+d

The development of devices for energy conversion, such as solid state fuel cells, depends on the availability of materials showing high oxygen ion conduction together with low operating temperatures. Moreover, adequate structural and thermochemical stability of the pure ionic conductor membrane and the mixed ionic electronic electrodes as well as their matching at the interface are essential for the durability of the device. In this regard, the Nd2NiO4+d system proved to be a good candidate as a stable electrode for intermediate temperature solid state fuel cells.More intriguing is the fact that Nd2NiO4+d, like a few other nonstoichiometric oxides derived from the perovskite framework like the Brownmillerite-type Sr(Fe,Co)O2.5 and K2NiF4-type RE2MO4+d (RE = La, Pr and M = Ni, Cu, Co), shows oxygen ionic mobility in an electrochemical reaction at room temperature. This surprising behavior raises questions about the real microscopic transport mechanisms in these classes of materials when the temperature is as low as T=300K.A « phonon assisted diffusion » mechanism, based on the presence of a low-lying phonon modes, has been developed in Brownmillerites to describe on an atomic scale how oxygen ion diffusion can be triggered in solid oxides down to ambient temperature. Concerning the RE2MO4+d systems, oxygen conductors La2CuO4.07, Pr2NiO4.25, and Nd2NiO4.25 have been reported to show a dynamical delocalization of apical oxygen atoms of the MO6 octahedra on a circle of at least 1Å diameter. This structural instability, activated by the presence of excess oxygen, implies important shifts of apical oxygen atoms closer to vacant interstitial sites, and is thus believed to play a major role on the non-classical oxygen mobility at ambient temperature.Through this work, we have investigated, in the Nd2NiO4+d phases, the correlations between structural instabilies induced by the oxygen hyper-stoichiometry and their subsequent effects on the lattice dynamics and role in promoting oxygen diffusion in the moderate-temperature regime. We have, in particular, developed innovative approaches to investigate lattice dynamics of highly-correlated and disordered systems. Results from single-crystal diffraction, inelastic neutron scattering and first-principle simulations have evidenced that indeed the non-classical oxygen diffusion at ambient temperature can be depicted as an interplay of specific lattice-activated and single-particle motions, both directly correlated to the oxygen hyperstoichiometry. The subsequent mechanism, closely related to the “phonon assisted diffusion mechanism”, provides a comprehensive framework to describe on an atomic scale the nonclassical oxygen diffusion in non-stoichiometric oxides.Le développement de systèmes pour la conversion d’énergie, comme les piles à combustibles à oxydes solides, est lié à la disponibilité de matériaux montrant une forte conductivité ionique à des températures de fonctionnement réduites. De plus, pour assurer la durabilité des dispositifs, ces matériaux, qu’ils soient purs conducteurs ioniques ou conducteurs mixtes ioniques-électroniques, doivent présenter de bonnes propriétés structurales et thermochimiques en terme de stabilité et d’interface. A cet égard, le système Nd2NiO4+d est un bon candidat en tant qu’électrode stable à température modérée pour piles à combustibles à oxydes solides.De façon surprenante, le système Nd2NiO4+d, ainsi que quelques autres oxydes nonstoechiométriques comme les Brownmillerites Sr(Fe,Co)O2.5 ou les phases RE2MO4+d type K2NiF4 (avec RE = La, Pr et M = Ni, Cu, Co), montrent une mobilité ionique à température ambiante sous potentiel électrochimique. Ce comportement soulève des questions quant au mécanisme microscopique réel de transport ionique dans ces familles de matériaux pour destempératures aussi basses que T=300K.Un mécanisme de « diffusion assistée par les phonons », impliquant la présence d’excitations de réseau à basse énergie, a été développé dans les phases Brownmillerites pour décrire, à l’échelle atomique, comment la diffusion des ions oxydes peut être déclenchée à température ambiante. Dans le cas des phases RE2MO4+d, les conducteurs ioniques La2CuO4.07, Pr2NiO4.25, et Nd2NiO4.25 montrent une délocalisation dynamique des oxygènes apicaux des octaèdres MO6, sur un cercle de 1Å de diamètre. Cette instabilité structurale, activée par la présence d’oxygènes excédentaires, induit des déplacements importants des oxygènes apicaux vers les sites interstitiels vacants, et donc est supposée jouer un rôle majeur sur la mobilité nonclassique de l’oxygène à température ambiante.A travers l’examen des phases Nd2NiO4+d, nous avons étudié les corrélations entre les instabilités structurales induites par l’hyper-stoechiométrie en oxygène et ses effets sur la dynamique de réseau et leur rôle dans la diffusion des ions oxydes à tempérure modérée. En particulier, nous avons développé des approches innovantes pour l’étude de la dynamique de réseau dans les systèmes désordonnés et fortement corrélés. Les résultats issus de diffractionsur monocrystal, de diffusion inélastique des neutrons et des simulations ab initio, ont montré que la diffusion non-classique des ions oxydes à température ambiante est liée à la fois à des mouvements spécifiques activés par le réseau et des mouvements de particules isolées, tout deux directement induits par la présence d’oxygènes excédentaires. Le mécanisme proposé, étroitement associé à celui de « diffusion assistée par les phonons », propose un cadre conceptuel permettant la description à l’échelle atomique de la diffusion non-classique des oxygènes dans les oxydes non-stoechiométriques