Cation Dynamics and Structural Stabilization in Formamidinium Lead Iodide Perovskites

Phase Transitions and Dynamics in Mixed Three- and Low-Dimensional Lead Halide Perovskites

Lead halide perovskites are extensively investigated as efficient solution-processable materials for photovoltaic applications. The greatest stability and performance of these compounds are achieved by mixing different ions at all three sites of the APbX3 structure. Despite the extensive use of mixed lead halide perovskites in photovoltaic devices, a detailed and systematic understanding of the mixing-induced effects on the structural and dynamic aspects of these materials is still lacking. The goal of this review is to summarize the current state of knowledge on mixing effects on the structural phase transitions, crystal symmetry, cation and lattice dynamics, and phase diagrams of three- and low-dimensional lead halide perovskites. This review analyzes different mixing recipes and ingredients providing a comprehensive picture of mixing effects and their relation to the attractive properties of these materials.

Atomic Structure and Dynamics of Organic-Inorganic Hybrid Perovskite Formamidinium Lead Iodide

The atomic dynamic behaviors of formamidinium lead iodide [HC(NH2)2PbI3] are critical for understanding and improving photovoltaic performances. However, they remain unclear. Here, we investigate the structural phase transitions and the reorientation dynamics of the formamidinium cation [HC(NH2)2+, FA+] of FAPbI3 using neutron scattering techniques. Two structural phase transitions occur with decreasing temperature, from cubic to tetragonal phase at 285 K and then to another tetragonal at 140 K, accompanied by gradually frozen reorientation of FA cations. The nearly isotropic reorientation in the cubic phase is suppressed to reorientation motions involving a two-fold (C2) rotation along the N···N axis and a four-fold (C4) rotation along the C-H axis in the tetragonal phase, and eventually to local disordered motion as a partial C4 along the C-H axis in another tetragonal phase, thereby indicating an intimate interplay between lattice and orientation degrees of freedom in the hybrid perovskite materials. The present complete atomic structure and dynamics provide a solid standing point to understand and then improve photovoltaic properties of organic-inorganic hybrid perovskites in the future.

Cation Dynamics as Structure Explorer in Hybrid Perovskites-The Case of MAPbI3

Hybrid organic-inorganic perovskites exhibit remarkable potential as cost-effective and high-efficiency materials for photovoltaic applications. Their exceptional chemical tunability opens further routes for optimizing their optical and electronic properties through structural engineering. Nevertheless, the extraordinary softness of the lattice, stemming from its interconnected organic-inorganic composition, unveils formidable challenges in structural characterization. Here, by focusing on the quintessential methylammonium lead triiodide, MAPbI3, we combine first-principles modeling with high-resolution neutron scattering data to identify the key stationary points on its shallow potential energy landscape. This combined experimental and computational approach enables us to benchmark the performance of a collection of semilocal exchange-correlation functionals and to track the local distortions of the perovskite framework, hallmarked by the inelastic neutron scattering response of the organic cation. By conducting a thorough examination of structural distortions, we introduce the IKUR-PVP-1 structural data set. This data set contains nine mechanically stable structural models, each manifesting a distinct vibrational response. IKUR-PVP-1 constitutes a valuable resource for assessing thermal behavior in the low-temperature perovskite phase. In addition, it paves the way for the development of accurate force fields, enabling a comprehensive understanding of the interplay between the structure and dynamics in MAPbI3 and related hybrid perovskites.

Stabilizing Polar Domains in MAPbBr3 via the Hydrostatic Pressure-Induced Liquid Crystal-like Transition

Pressure-induced phases of MAPbBr₃ were investigated at room temperature in the range of 0-2.8 GPa by ab initio molecular dynamics. Two structural transitions at 0.7 GPa (cubic → cubic) and 1.1 GPa (cubic → tetragonal) involved both the inorganic host (lead bromide) and the organic guest (MA). MA dipoles behave like a liquid crystal undergoing isotropic → isotropic and isotropic → oblate nematic transitions as pressure confines their orientational fluctuations to a crystal plane. Beyond 1.1 GPa, the MA ions lie alternately along two orthogonal directions in the plane forming stacks perpendicular to it. However, the molecular dipoles are statically disordered, leading to stable polar and antipolar MA domains in each stack. H-Bond interactions, which primarily mediate host-guest coupling, facilitate the static disordering of MA dipoles. Interestingly, high pressures suppress CH₃ torsional motion, emphasizing the role of C-H···Br bonds in the transitions.

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 FA_1-xMA_xPbI₃ 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 FA_0.6MA_0.4PbI₃, 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 FA_1-xMA_xPbI₃.

Static and Dynamic Disorder in Formamidinium Lead Bromide Single Crystals

We show that formamidinium-based crystals are distinct from methylammonium-based halide perovskite crystals because their inorganic sublattice exhibits intrinsic local static disorder that coexists with a well-defined average crystal structure. Our study combines terahertz-range Raman scattering with single-crystal X-ray diffraction and first-principles calculations to probe the evolution of inorganic sublattice dynamics with temperature in the range of 10-300 K. The temperature evolution of the Raman spectra shows that low-temperature, local static disorder strongly affects the crystal structural dynamics and phase transitions at higher temperatures.

A Complete Picture of Cation Dynamics in Hybrid Perovskite Materials from Solid-State NMR Spectroscopy

The organic cations in hybrid organic-inorganic perovskites rotate rapidly inside the cuboctahedral cavities formed by the inorganic lattice, influencing optoelectronic properties. Here, we provide a complete quantitative picture of cation dynamics for formamidinium-based perovskites and mixed-cation compositions, which are the most widely used and promising absorber layers for perovskite solar cells today. We use ²H and ¹⁴N quadrupolar solid-state NMR relaxometry under magic-angle spinning to determine the activation energy (E_a) and correlation time (τ_c) at room temperature for rotation about each principal axis of a series of organic cations. Specifically, we investigate methylammonium (MA⁺), formamidinium (FA⁺), and guanidinium (GUA⁺) cations in current state-of-the-art single- and multi-cation perovskite compositions. We find that MA⁺, FA⁺, and GUA⁺ all have at least one component of rotation that occurs on the picosecond timescale at room temperature, with MA⁺ and GUA⁺ also exhibiting faster and slower components, respectively. The cation dynamics depend on the symmetry of the inorganic lattice but are found to be insensitive to the degree of cation substitution. In particular, the FA⁺ rotation is invariant across all compositions studied here, when sufficiently above the phase transition temperature. We further identify an unusual relaxation mechanism for the ²H of MA⁺ in mechanosynthesized FA_xMA_1-xPbI₃, which was found to result from physical diffusion to paramagnetic defects. This precise picture of cation dynamics will enable better understanding of the relationship between the organic cations and the optoelectronic properties of perovskites, guiding the design principles for more efficient perovskite solar cells in the future.

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

Hybrid perovskites continue to attract an enormous amount of attention, yet a robust microscopic picture of their different phases as well as the extent and nature of the disorder present remains elusive. Using specific-heat data along with high-resolution inelastic neutron scattering and ab initio modeling, we address this ongoing challenge for the case of the ordered phase of the quintessential hybrid-perovskite MAPbI₃. At low temperatures, the specific heat of MAPbI₃ reveals strong deviations from the Debye limit, a common feature of pure hybrid perovskites and their mixtures. Our thermophysical analysis demonstrates that the (otherwise ordered) structure around the organic moiety is characterized by a substantial lowering of the local symmetry relative to what can be inferred from crystallographic studies. The physical origin of the observed thermophysical anomalies is unequivocally linked to excitations of sub-terahertz optical phonons responsible for translational-librational distortions of the octahedral units.

Changes in the hydrogen nuclear kinetic energy across the several phases of methylammonium lead tribromide

We present an experimental investigation of methylammonium lead tribromide single crystals in the orthorhombic, tetragonal, and cubic phases based on inelastic and deep inelastic neutron scattering experiments. We show how the average hydrogen nuclear kinetic energy, mainly affected by zero-point vibrational energies, shows differences larger compared to the changes simply related to temperature effects when moving from one phase to another. In particular, the Gaussian contribution to the average nuclear kinetic energy is larger in the tetragonal phase compared to the cubic and orthorhombic ones. Moreover, we find that the vibrational densities of states of MAPbBr3 single crystals in the orthorhombic phase are compatible with previously reported results on powder samples, and that the only vibrational modes that show slightly different frequencies compared to MAPbI3 are those in the energy range between 100 and 300 cm-1, related to librational/rotational modes. As these shifts are of about 10 cm-1, and do not affect any higher-energy vibrational mode, we conclude that the zero-point energies and average nuclear kinetic energies in the two-hybrid organic/inorganic perovskites are expected to be approximately the same within a harmonic framework.

(C5H6N)Pb2X5 (X = Cl, Br): Hybrid Lead Halides Based on Seven-Coordinate Pb(II)

Hybrid lead halides are a diverse family of compounds, of interest for their optoelectronic properties, that vary in the dimensionality and connectivity of their inorganic substructures. The great majority of these compounds are based on lead-centered octahedra, with few examples featuring inorganic architectures containing higher coordination numbers. Here, we report the synthesis and characterization of a pyridinium lead bromide phase that is based on seven-coordinate Pb(II) centers. Through edge- and face-sharing, the polyhedra form a corrugated, two-dimensional inorganic substructure. Electronic structure calculations were used to examine the band structure and the role of the stereoactive lone pair in the inherently asymmetric, seven-coordinate Pb(II) geometry. For reference, we have visualized the role of the lone pair in the binary halide PbBr₂, which also has a seven-coordinate inner ligand sphere. A comparison of the new structure with the limited number of existing hybrid lead halides with similar inorganic architectures highlights the templating role of the organic cation for these compounds. We also contribute characterization and discussion of isomorphic pyridinium lead chloride, which had been deposited in the Cambridge Structural Database but never, to our knowledge, addressed in the literature. The compounds were synthesized using solution conditions and structures determined with single-crystal X-ray diffraction. The materials were also characterized via powder X-ray diffraction, combustion elemental analysis, and diffuse reflectance UV-vis spectroscopy. While the structures reported here are centrosymmetric, the seven-coordinate, capped trigonal prismatic geometry that we have identified is a source of local asymmetry that could be used as a component in designing globally noncentrosymmetric structures.

ICE-MAN the Integrated Computational Environment for Modeling and Analysis for Neutrons at ORNL

ICE-MAN is a modeling and analysis workbench for multi-modal studies, designed with neutron science in mind. It streamlines the workflow between different experimental techniques, computer modeling, and databases and reduces the time and learning curve needed to access them thus making a holistic approach to data interpretation more amenable and efficient.

Dynamics & Spectroscopy with Neutrons-Recent Developments & Emerging Opportunities

This work provides an up-to-date overview of recent developments in neutron spectroscopic techniques and associated computational tools to interrogate the structural properties and dynamical behavior of complex and disordered materials, with a focus on those of a soft and polymeric nature. These have and continue to pave the way for new scientific opportunities simply thought unthinkable not so long ago, and have particularly benefited from advances in high-resolution, broadband techniques spanning energy transfers from the meV to the eV. Topical areas include the identification and robust assignment of low-energy modes underpinning functionality in soft solids and supramolecular frameworks, or the quantification in the laboratory of hitherto unexplored nuclear quantum effects dictating thermodynamic properties. In addition to novel classes of materials, we also discuss recent discoveries around water and its phase diagram, which continue to surprise us. All throughout, emphasis is placed on linking these ongoing and exciting experimental and computational developments to specific scientific questions in the context of the discovery of new materials for sustainable technologies.