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

Licentiate thesis, 2021

This thesis deals with inelastic and quasielastic neutron scattering studies on the dynamical properties of the organometal halide perovskite system MA1-xFAxPbI3 (MA = methylammonium; FA = formamidinium) and of the layered perovskite-type oxyhydride SrVO2H. These materials systems are of high interest for their excellent photovoltaic performance (MA1-xFAxPbI3) and hydride-ion conductivity (SrVO2H) and concomitant promise for various technological devices; however, the local structure and dynamics underpinning these materials properties remain unclear. 
   With regards to the MA1-xFAxPbI3 system, the studies focused on the nature of the organic cation dynamics. For the parent compound, FAPbI3 (x = 1), the results showed that, in the cubic phase, the FA cations undergo nearly isotropic rotations whilst in the lower temperature tetragonal phases, the FA cation rotations are anisotropic and more complex. For the solid solutions, MA1-xFAxPbI3 (x = 0.6, 0.9), it was found that the hydrogen-bonding interactions around the FA cations are strengthened in the MA-doped materials, which provides an atomistic understanding of the improved stability of the perovskite structure of FAPbI3 with doping of MA. 
   For SrVO2H, the studies focused on the dynamical properties of the hydride-ions, and the results unraveled the nature of both the diffusional and vibrational dynamics. It was found that the hydride-ion diffusion can be described in terms of a correlated vacancy-assisted diffusion mechanism in the ab-plane of the crystal structure, with an enhanced rate for backward jumps. Interestingly, the vibrational modes of the hydride-ions were found to be split due to the antiferromagnetism. Analysis of the neutron scattering data in combination with density functional theory calculations reveal unusually large spin-phonon coupling, which highlights the interesting couplings between magnetism and the hydrogen dynamics that occurs in SrVO2H. 
   These new insights adds significantly to the current understanding of the dynamical properties in these materials and may be important for the development of materials with properties optimized towards their application in technological devices, such as solar cells, and energy storage and conversion devices.