Cation Dynamics and Structural Stabilization in Formamidinium Lead Iodide Perovskites

Abstract

<div>The vibrational dynamics of pure and methylammonium-doped formamidinium lead iodide perovskites</div><div>(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</div><div>calculations validated by the neutron data. Inelastic Neutron Scattering experiments on FAPbI3</div><div>provide direct evidence of the formation of a low-temperature orientational glass, unveiling the</div><div>physico-chemical origin of phase metastability in the tetragonal structure. Further analysis of these</div><div>data provides a suitable starting point to understand and explore the stabilization of the perovskite</div><div>framework via doping with small amounts of organic cations. In particular, we ?find that hydrogen</div><div>bonds in FAPbI3 are strengthened in a synergistic manner as a result of cage deformation induced</div><div>by the dynamics of the neighbouring methylammonium cations.</div>

Interplay between Local Structure and Nuclear Dynamics in Tungstic Acid: A Neutron Scattering Study

“…Both of them allow for the presence of a colored noise reproducing the power spectral density of a quantum harmonic oscillator, and both result in INS spectra of very similar quality. 78,79 We have considered two temperature conditions, the lowest-accessible temperature for the simulations, 50 K, and 300 K. Inspection of Figure 3c confirms that earlier-discussed broadening of the band (iii) is an artifact stemming from the use of the same effective temperature for all degrees of freedom. with the (iii) band observed at 95 meV, that is in perfect agreement with our semiclassical simulations at the room temperature.…”

“…Convergence criteria were chosen to be consistent with the accuracy of the parallel calculations using CASTEP, showing that this approach provides a similar level of accuracy for both computational codes in line with our previous findings. 78,79 Finally, the CP2K simulations were extended by imposing Quantum-Thermostatting (QT-NVT) following a generalized Langevin equation (GLE) for two temperatures (T = 50 and 300 K; stage ii). 80,81 The system was equilibrated for 30 ps at target temperatures, providing the initial conditions for subsequent NVE trajectories.…”

“…However, in order to account for the combined effect of anharmonicity and nuclear quantum nature of the hydrogen manifested in the form of the zero-point energy, one has to apply in AIMD simulations an effective temperature which is a function of the mode frequency, a technique referred to as quantum thermostatting. 78,79 Thus, in our AIMD simulations, we have augmented the classical AIMD simulation by applying two different quantum thermostatting schemes, hereinafter referred to as semiclassical simulations. Both of them allow for the presence of a colored noise reproducing the power spectral density of a quantum harmonic oscillator, and both result in INS spectra of very similar quality.…”

Unraveling the Ordered Phase of the Quintessential Hybrid Perovskite MAPbI3─Thermophysics to the Rescue

“…To set the scene, Figure 1 provides a summary of the current state of affairs concerning the ordered structure of MAPbI 3 , based on new computational predictions using harmonic lattice dynamics (HLD) calculations and extensive benchmarking with several DFAs: PBE-TS, 24 PBE-D3(BJ), 25,26 PBEsol, 27 and rSCAN. 28,29 These calculations capitalize from our previous works 21,22 and those of Bokdam et al 30,31 As shown by the INS data in Figure 1a, sharp spectral features reflect a welldefined local structure, with no discernible inhomogeneous broadenings that would otherwise signal the emergence of static disorder or the presence of distinct domains. Below 20 meV, these features are associated with librational modes of MA + , and they reflect a tendency for it to arrange itself in preferential directions.…”

“…In this case, the disruption of a fully fledged hydrogen-bonding network is accompanied by short-range octahedral-tilting distortions around the cations. In subsequent studies, 22,23 this improved model (hereafter denoted P1) was used with success in the study of both MA + and formamidinium (FA + ) cations in more complex MA 1−x FA x PbI 3 solid solutions. In particular, it was possible to quantify the extent of disorder as well as to shed new light onto the mechanism of physical stabilization of the perovskite framework in these materials.…”