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-xMAxPbI3.

Promoting Persistent Superionic Conductivity in Sodium Monocarba-closo-dodecaborate NaCB11H12 via Confinement within Nanoporous Silica.

Superionic phases of bulk anhydrous salts based on large cluster-like polyhedral (carba)borate anions are generally stable only well above room temperature, rendering them unsuitable as solid-state electrolytes in energy-storage devices that typically operate at close to room temperature. To unlock their technological potential, strategies are needed to stabilize these superionic properties down to subambient temperatures. One such strategy involves altering the bulk properties by confinement within nanoporous insulators. In the current study, the unique structural and ion dynamical properties of an exemplary salt, NaCB11H12, nanodispersed within porous, high-surface-area silica via salt-solution infiltration were studied by differential scanning calorimetry, X-ray powder diffraction, neutron vibrational spectroscopy, nuclear magnetic resonance, quasielastic neutron scattering, and impedance spectroscopy. Combined results hint at the formation of a nanoconfined phase that is reminiscent of the high-temperature superionic phase of bulk NaCB11H12, with dynamically disordered CB11H12 - anions exhibiting liquid-like reorientational mobilities. However, in contrast to this high-temperature bulk phase, the nanoconfined NaCB11H12 phase with rotationally fluid anions persists down to cryogenic temperatures. Moreover, the high anion mobilities promoted fast-cation diffusion, yielding Na+ superionic conductivities of ∼0.3 mS/cm at room temperature, with higher values likely attainable via future optimization. It is expected that this successful strategy for conductivity enhancement could be applied as well to other related polyhedral (carba)borate-based salts. Thus, these results present a new route to effectively utilize these types of superionic salts as solid-state electrolytes in future battery applications.

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 × 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.

Cation Dynamics and Structural Stabilization in Formamidinium Lead Iodide Perovskites.

The vibrational dynamics of pure and methylammonium-doped formamidinium lead iodide perovskites (FAPbI3) has been investigated by high-resolution neutron spectroscopy. For the first time, we provide an exhaustive and accurate analysis of the cation vibrations and underlying local structure around the organic moiety in these materials using first-principles electronic-structure calculations validated by the neutron data. Inelastic neutron scattering experiments on FAPbI3 provide direct evidence of the formation of a low-temperature orientational glass, unveiling the physicochemical origin of phase metastability in the tetragonal structure. Further analysis of these data provides a suitable starting point to explore and understand the stabilization of the perovskite framework via doping with small amounts of organic cations. In particular, we find that the hydrogen-bonding interactions around the formamidinium cations are strengthened as a result of cage deformation. This synergistic effect across perovskite cages is accompanied by a concomitant weakening of the methylammonium interactions with the surrounding framework.

Neutron Scattering Investigations of the Global and Local Structures of Ammine Yttrium Borohydrides.

Complex metal hydrides are a fascinating and continuously expanding class of materials with many properties relevant for solid-state hydrogen and ammonia storage and solid-state electrolytes. The crystal structures are often investigated using powder X-ray diffraction (PXD), which can be ambiguous. Here, we revisit the crystal structure of Y(11BD4)3·3ND3 with the use of neutron diffraction, which, in comparison to previous PXD studies, provides accurate information about the D positions in the compound. Upon cooling to 10 K, the compound underwent a polymorphic transition, and a new monoclinic low-temperature polymorph denoted as α-Y(11BD4)3·3ND3 was discovered. Furthermore, the series of Y(11BH4)3·xNH3 (x = 0, 3, and 7) were also investigated with inelastic neutron scattering and infrared spectroscopy techniques, which provided information of the local coordination environment of the 11BH4- and NH3 groups and unique insights into the hydrogen dynamics. Partial deuteration using ND3 in Y(11BH4)3·xND3 (x = 3 and 7) allowed for an unambiguous assignment of the vibrational bands corresponding to the NH3 and 11BH4- in Y(11BH4)3·xNH3, due to the much larger neutron scattering cross section of H compared to D. The vibrational spectra of Y(11BH4)3·xNH3 could roughly be divided into three regions: (i) below 55 meV, containing mainly 11BH4- librational motions, (ii) 55-130 meV, containing mainly NH3 librational motions, and (iii) above 130 meV, containing 11B-H and N-H bending and stretching motions.

Phonon spectra of pure and acceptor doped BaZrO3 investigated with visible and UV Raman spectroscopy.

We report results from visible and UV Raman spectroscopy studies of the phonon spectra of a polycrystalline sample of the prototypical perovskite type oxide BaZrO3 and a 500 nm thick film of its Y-doped, proton conducting, counterpart BaZr0.8Y0.2O2.9. Analysis of the Raman spectra measured using different excitation energies (between 3.44 eV and 5.17 eV) reveals the activation of strong resonance Raman effects involving all lattice vibrational modes. Specifically, two characteristic energies were identified for BaZrO3, one around 5 eV and one at higher energy, respectively, and one for BaZr0.8Y0.2O2.9, above 5 eV. Apart from the large difference in spectral intensity between the non-resonant and resonant conditions, the spectra are overall similar to each other, suggesting that the vibrational spectra of the perovskites are stable when investigated using an UV laser as excitation source. These results encourage further use of UV Raman spectroscopy as a novel approach for the study of lattice vibrational dynamics and local structure in proton conducting perovskites, and open up for, e.g., time-resolved experiments on thin films targeted at understanding the role of lattice vibrations in proton transport in these kinds of materials.

Role of the doping level in localized proton motions in acceptor-doped barium zirconate proton conductors.

Acceptor-doped barium zirconates are currently receiving considerable interest because of their high proton conductivity at intermediate temperatures, making them applicable as electrolytes in various electrochemical devices, but the mechanism of proton conduction is unclear. Here, we investigate the role of the acceptor-dopant level in the localized proton motions, i.e. proton transfers between oxygens and O-H reorientations, in hydrated samples of the proton conducting, acceptor-doped, perovskites BaZr1-xInxO3-x/2 with x = 0.10 and 0.20, using quasielastic neutron scattering (QENS). Analysis of the QENS spectra reveals that several proton transfer and O-H reorientational motions contribute to the QENS signal, as a consequence of the locally disordered nature of the structure due to the In doping of these materials, and establishes a generic and complex picture of localized proton dynamics in acceptor-doped barium zirconate based proton conductors. A comparison of the QENS results with vibrational spectroscopy data of the same materials, as reported in the literature, suggests a predominance of O-H reorientational motions in the observed dynamics. The highest doping level corresponds to a more distorted structure and faster dynamics, which thus indicates that some degree of structural disorder is favourable for high local proton mobility.

Hydride Reduction of BaTiO3 - Oxyhydride Versus O Vacancy Formation.

We investigated the hydride reduction of tetragonal BaTiO3 using the metal hydrides CaH2, NaH, MgH2, NaBH4, and NaAlH4. The reactions employed molar BaTiO3/H ratios of up to 1.8 and temperatures near 600 °C. The air-stable reduced products were characterized by powder X-ray diffraction (PXRD), transmission electron microscopy, thermogravimetric analysis (TGA), and 1H magic angle spinning (MAS) NMR spectroscopy. PXRD showed the formation of cubic products-indicative of the formation of BaTiO3-x H x -except for NaH. Lattice parameters were in a range between 4.005 Å (for NaBH4-reduced samples) and 4.033 Å (for MgH2-reduced samples). With increasing H/BaTiO3 ratio, CaH2-, NaAlH4-, and MgH2-reduced samples were afforded as two-phase mixtures. TGA in air flow showed significant weight increases of up to 3.5% for reduced BaTiO3, suggesting that metal hydride reduction yielded oxyhydrides BaTiO3-x H x with x values larger than 0.5. 1H MAS NMR spectroscopy, however, revealed rather low concentrations of H and thus a simultaneous presence of O vacancies in reduced BaTiO3. It has to be concluded that hydride reduction of BaTiO3 yields complex disordered materials BaTiO3-x H y □(x-y) with x up to 0.6 and y in a range 0.04-0.25, rather than homogeneous solid solutions BaTiO3-x H x . Resonances of (hydridic) H substituting O in the cubic perovskite structure appear in the -2 to -60 ppm spectral region. The large range of negative chemical shifts and breadth of the signals signifies metallic conductivity and structural disorder in BaTiO3-x H y □(x-y). Sintering of BaTiO3-x H y □(x-y) in a gaseous H2 atmosphere resulted in more ordered materials, as indicated by considerably sharper 1H resonances.

Inorganic Phosphor Materials for Lighting.

This chapter addresses the development of inorganic phosphor materials capable of converting the near UV or blue radiation emitted by a light emitting diode to visible radiation that can be suitably combined to yield white light. These materials are at the core of the new generation of solid-state lighting devices that are emerging as a crucial clean and energy saving technology. The chapter introduces the problem of white light generation using inorganic phosphors and the structure-property relationships in the broad class of phosphor materials, normally containing lanthanide or transition metal ions as dopants. Radiative and non-radiative relaxation mechanisms are briefly described. Phosphors emitting light of different colors (yellow, blue, green, and red) are described and reviewed, classifying them in different chemical families of the host (silicates, phosphates, aluminates, borates, and non-oxide hosts). This research field has grown rapidly and is still growing, but the discovery of new phosphor materials with optimized properties (in terms of emission efficiency, chemical and thermal stability, color, purity, and cost of fabrication) would still be of the utmost importance.

The role of the ionic liquid C6C1ImTFSI in the sol-gel synthesis of silica studied using in situ SAXS and Raman spectroscopy.

The sol-gel synthesis of a silica based ionogel using the ionic liquid 1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (C6C1ImTFSI) as the solvent has been followed in situ by combined µ-focused X-ray scattering and µ-Raman spectroscopy. By covering the momentum transfer range 0.2 < q < 30 nm(-1) we probe the evolution of the characteristic peaks of the ionic liquid, associated with the existence of polar and non-polar domains, as a function of reaction time. Our detailed analysis of the small angle X-ray scattered (SAXS) pattern reveals that the nano-structure of the ionic liquid is partially retained during the sol-gel synthesis, as indicated by the broader yet distinguishable SAXS signatures. We also observe that the signature associated with the non-polar and polar domains shift to higher and lower q-values, respectively. Interestingly, this behavior correlates with the evolution of the chemical composition of the sol as probed by Raman spectroscopy. More precisely, we observe that both the nano-structural changes and the production of polar molecules arrest at the point of gelation. This is rationalized by the tendency of the reagents and products of the sol-gel reaction to locate in different portions of the nano-structure of the ionic liquid.

Structural and vibrational properties of silyl (SiH3(-)) anions in KSiH3 and RbSiH3: new insight into Si-H interactions.

The alkali metal silyl hydrides ASiH3 (A = K, Rb) and their deuteride analogues were prepared from the Zintl phases ASi. The crystal structures of ASiH3 consist of metal cations and pyramidal SiH3(-) ions. At room temperature SiH3(-) moieties are randomly oriented (α modifications). At temperatures below 200 K ASiH3 exist as ordered low-temperature (ß) modifications. Structural and vibrational properties of SiH3(-) in ASiH3 were characterized by a combination of neutron total scattering experiments, infrared and Raman spectroscopy, as well as density functional theory calculations. In disordered α-ASiH3 SiH3(-) ions relate closely to freely rotating moieties with C3v symmetry (Si-H bond length = 1.52 Å; H-Si-H angle 92.2 °). Observed stretches and bends are at 1909/1903 cm(-1) (ν1, A1), 1883/1872 cm(-1) (ν3, E), 988/986 cm(-1) (ν4, E), and 897/894 cm(-1) (ν2, A1) for A = K/Rb. In ordered ß-ASiH3 silyl anions are slightly distorted with respect to their ideal C3v symmetry. Compared to α-ASiH3 the molar volume is by about 15% smaller and the Si-H stretching force constant is reduced by 4%. These peculiarities are attributed to reorientational dynamics of SiH3(-) anions in α-ASiH3. Si-H stretching force constants for SiH3(-) moieties in various environments fall in a range from 1.9 to 2.05 N cm(-1). These values are considerably smaller compared to silane, SiH4 (2.77 N cm(-1)). The reason for the drastic reduction of bond strength in SiH3(-) remains to be explored.

Proton dynamics in oxides: insight into the mechanics of proton conduction from quasielastic neutron scattering.

This article is concerned with the use of quasielastic neutron scattering as a technique for investigation of the dynamical properties of proton conducting oxides. Currently, the main interest in these materials comes from their promise as electrolytes in future electrochemical devices and particularly through their use as electrolytes in next-generation, intermediate-temperature, fuel cells. However, the realization of such devices depends critically on the development of new, more highly proton conducting oxides. Such a development depends on increasing the current understanding of proton conduction in oxides and for this purpose quasielastic neutron scattering is an important mean. The aim of this article is to introduce the non-specialist reader to the basic principles of quasielastic neutron scattering, its advantages and disadvantages, to summarize the work that has been done on proton conducting oxides using this technique, as well as to discuss future opportunities within this field of research.

Perspectives of neutron scattering on proton conducting oxides.

Understanding the fundamental properties of materials of relevance for alternative energy technologies is crucial in addressing the global challenge of cleaner sources of energy. This Perspective article aims to demonstrate the important role that neutron scattering now plays in advancing the state of the art of the basic understanding of proton conducting oxides, which show potential as electrolytes in next-generation intermediate temperature fuel cells. In particular, the breadth of neutron scattering work on perovskite structured oxides, which continue to be the most promising class of electrolytes for intermediate-temperature applications, is reviewed. Key fundamental properties that are addressed include structures, proton sites, hydrogen-bonding interactions, proton dynamics, and concentration of protons in materials. Furthermore, the perspectives for future neutron studies within this field of research are discussed.