Neutron spectroscopy studies of vibrational and diffusional dynamics in organometal halide and oxyhydride perovskites

This thesis deals with inelastic and quasielastic neutron scattering studies on the\ua0dynamical properties of the organometal halide perovskite system MA1-xFAxPbI3\ua0(MA = methylammonium; FA = formamidinium) and of the layered perovskite-type\ua0oxyhydride SrVO2H. These materials systems are of high interest for their excellent\ua0photovoltaic performance (MA1-xFAxPbI3) and hydride-ion conductivity (SrVO2H)\ua0and concomitant promise for various technological devices; however, the local structure\ua0and dynamics underpinning these materials properties remain unclear.\ua0\ua0 \ua0With regards to the MA1-xFAxPbI3 system, the studies focused on the nature\ua0of the organic cation dynamics. For the parent compound, FAPbI3 (x = 1), the\ua0results showed that, in the cubic phase, the FA cations undergo nearly isotropic rotations\ua0whilst in the lower temperature tetragonal phases, the FA cation rotations\ua0are anisotropic and more complex. For the solid solutions, MA1-xFAxPbI3 (x = 0.6,\ua00.9), it was found that the hydrogen-bonding interactions around the FA cations are\ua0strengthened in the MA-doped materials, which provides an atomistic understanding\ua0of the improved stability of the perovskite structure of FAPbI3 with doping of MA.\ua0\ua0 \ua0For SrVO2H, the studies focused on the dynamical properties of the hydride-ions,\ua0and the results unraveled the nature of both the diffusional and vibrational dynamics.\ua0It was found that the hydride-ion diffusion can be described in terms of a correlated\ua0vacancy-assisted diffusion mechanism in the ab-plane of the crystal structure, with\ua0an enhanced rate for backward jumps. Interestingly, the vibrational modes of the\ua0hydride-ions were found to be split due to the antiferromagnetism. Analysis of the\ua0neutron scattering data in combination with density functional theory calculations\ua0reveal unusually large spin-phonon coupling, which highlights the interesting couplings\ua0between magnetism and the hydrogen dynamics that occurs in SrVO2H.\ua0\ua0 \ua0These new insights adds significantly to the current understanding of the dynamical\ua0properties in these materials and may be important for the development of\ua0materials with properties optimized towards their application in technological devices,\ua0such as solar cells, and energy storage and conversion devices

Dynamical properties of metal halide and oxyhydride perovskites

This thesis concerns investigations on the dynamical properties of two classes of energy-relevant materials, namely metal halide and oxyhydride perovskites, using neutron scattering techniques. These two classes of materials share the same aspect of a perovskite-type crystal structure but are different in terms of their functional properties and concomitant promise for various technological applications.\ua0\ua0 Regarding the metal halide perovskites, these materials are of large interest for use in, e.g., next-generation solar cells and light-emitting diodes. In this thesis, the studies on these types of materials focused on elucidating the nature of organic cation and lattice dynamics in the hybrid organic-inorganic systems FA1-xMAxPbI3 (MA = methylammonium and FA = formamidinium), BA2PbI4 and PEA2PbI4 (BA = butylammonium and PEA = phenethylammonium), and the all-inorganic perovskite CsPbI3, by using a wide variety of quasielastic and inelastic neutron scattering techniques. For FA1-xMAxPbI3, a key result is that MA-doping of FAPbI3 leads to significantly different cation dynamics, which is directly related to the stabilizationof the perovskite crystal structure. Overall, the results showcase the importance of organic cation and lattice dynamics and the couplings between these types of dynamics in this class of materials and highlight the importance of dynamics for an understanding of the properties of metal halide perovskites.\ua0\ua0Regarding the oxyhydride perovskites, these materials are of interest because of their hydride ion conductivity. Here, the studies focused on elucidating the dynamical properties of hydride ions in the perovskite materials SrVO2H and BaTiO3-xHy, using quasielastic neutron scattering techniques. For SrVO2H, the results showed the presence of a correlated jump diffusion mechanism, with an enhanced jump rate for backward jumps, which slows down the long-range diffusion, and localizes hydride ions in the vicinity of a particular vacancy. For BaTiO3-xHy, a faster diffusion process was observed, which can be explained by the relatively larger amount of anion vacancies in this material, which promotes diffusion. The vibrational dynamics of hydride ions were further studied in SrVO2H. Interestingly, it was found that the V−H- and V−O-based phonons are largely influenced by the antiferromagnetic ordering of the material. These results showcase the important couplings between the magnetism and vibrational dynamics which can occur in magnetic oxyhydrides

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

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

Vibrational properties of SrVO2 H with large spin-phonon coupling

The antiferromagnetic transition metal oxyhydride SrVO2H is distinguished by its stoichiometric composition and an ordered arrangement of H atoms. The tetragonal structure is related to the cubic perovskite and consists of alternating layers of VO2 and SrH. d2 V(III) attains a sixfold coordination by four O and two H atoms. The latter are arranged in a trans fashion, which produces H-V-H chains along the tetragonal axis. Here, we investigate the vibrational properties of SrVO2H by inelastic neutron scattering and infrared spectroscopy combined with phonon calculations based on density functional theory. The H-based vibrational modes divide into a degenerate bending motion perpendicular to the H-V-H chain direction and a highly dispersed stretching motion along the H-V-H chain direction. The bending motion, with a vibrational frequency of approximately 800 cm-1, is split into two components separated by about 50 cm-1, owing to the doubled unit cell from the antiferromagnetic structure. Interestingly, spin-phonon coupling stiffens the H-based modes by 50-100cm-1 although super-exchange coupling via H is very small. Frequency shifts of the same order of magnitude also occur for V-O modes. It is inferred that SrVO2H displays the hitherto largest recognized coupling between magnetism and phonons in a 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