Session A59: Halide Perovskites II: Structure and Lattice Dynamics

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Lead halide perovskites exhibit structural instabilities and large atomic fluctuations thought to impact their optical and thermal properties, yet detailed structural and temporal correlations of their atomic motions remain poorly understood. Here, these correlations are resolved in CsPbBr₃ crystals using momentum-resolved neutron and x-ray scattering measurements as a function of temperature, complemented with first-principles simulations. We uncover a striking network of diffuse scattering rods, arising from the liquid-like damping of low-energy Br-dominated phonons, reproduced in our simulations of the anharmonic phonon self-energy. These overdamped modes cover a continuum of wave vectors along the edges of the cubic Brillouin zone, corresponding to two-dimensional (2D) sheets of correlated rotations in real space, and could represent precursors to proposed 2D polarons. Further, these motions directly impact the electronic gap edge states, linking soft anharmonic lattice dynamics and optoelectronic properties. These results provide new insights into the highly unusual atomic dynamics of halide perovskites, relevant to further optimize their optical and thermal properties.   

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Local stress may develop in organic metal-halide perovskites which affects different optoelectronic properties and can play a major role in their performance and stability. Previously we have studied the high-temperature pseudo-cubic phase of CH₃NH₃PbI₃ [arXiV:1907.03673]. In this work, we analyzed the vibrations of low-temperature orthorhombic and room-temperature tetragonal hybrid perovskite phases under uniaxial strain using density functional perturbation theory. We have identified specific IR- and Raman-active modes with large linear shifts that may be useful to measure strain within these perovskite materials using spectroscopy. Considering exact and approximate symmetries, we have analyzed changes in the structural parameters, phonon displacement patterns, and dynamical matrices, to understand changes in frequency from a perturbative approach. We also identify which atomic interactions play a significant role in change of vibrational modes under strain. Additionally, we calculate the full stiffness tensor for both the phases to understand their mechanical properties. Our study gives insight into the interaction between strain, structural changes and vibrational modes which may help to understand photovoltaic performance and degradation.   

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The halide double perovskite Cs₂AgBiBr₆ has emerged as a promising nontoxic alternative to the lead halide perovskites APbX₃ (A = organic cation or Cs; X = I or Br). Here, we perform high-pressure synchrotron X-ray total scattering on Cs₂AgBiBr₆ and discover local disorder that is hidden from conventional Bragg analysis. While our powder diffraction data show that the average structure remains cubic up to 2.1 GPa, analysis of the X-ray pair distribution function reveals that the local structure is better described by a monoclinic space group, with significant distortion within the Ag-Br and Bi-Br octahedra and off-centering of the Cs atoms. By tracking the distribution of interatomic Cs-Br distances, we find the local disorder is enhanced upon compression and we corroborate these results with molecular dynamics simulations. The observed local disorder affords new understanding of this promising material and potentially offers a new parameter to tune in halide perovskite lattices.   

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Organic-inorganic hybrid metal-halide perovskites provide a tunable platform for engineering their optoelectronic properties. By combined photoluminescence (PL), synchrotron-based XRD, and Raman scattering studies as a function of pressure from methylammonium lead bromide (MAPbBr₃), we shed light on an isostructural phase transition due to the coupling of the CH₃NH₃ (MA) cation and the PbBr₆ lattice through hydrogen bonding. The PL peak position, intensity and width of the excitonic peak show significant changes at 2 GPa, which corroborate the changes observed in high-pressure Raman scattering studies. The frequencies of the lattice modes and the C–H/N–H bending and stretching modes of the MA cation show anomalous changes at 2 GPa. The suppression of rotational and orientational disorder of the organic moiety is initiated at 2 GPa and the ordering is completed by 3.0 GPa, leading to an order-disorder type cubic II (Im3) to orthorhombic (Pnma) phase transition. Along with the revelation of an isostructural transformation at 2 GPa, this work highlights the impact of molecular vibrations on the electronic properties of MAPbBr₃ under pressure.   

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Hybrid organic-inorganic perovskites are one of the most promising solar cell materials for space applications due to their light weight and exceptionally high radiation resistance. Yet, perovskite based solar cells have a relevant instrinsic instability related to the often occurring migration of the halides. Here we present a virtual screening study searching for perovskite structures that show strong halide bonding to prevent their migration. The non-covalent bond strength, being mainly hydrogen bonds between the organic cations and the halides, is assessed via a QT-AIM[1] approach which provides a connection between the topology of the electron density and the non-covalent bond strengths.
Finally, using a compressed sensing technique (SISSO)[2] we present new topological descriptors that predict the structural stability, providing new directions towards the design of perovskite solar cells with a long lifetime.

[1] P. Kumar, Shyam Vinod, V. Raghavendra and V. Subramanian, J. of Chem. Sciences, 128 (10), 1527 (2016).
[2] R. Ouyang, S. Curtarolo, E. Ahmetcik, M. Scheffler and L. M. Ghiringhelli, Phys. Rev. Mater. 2, 083802 (2018).   

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Recently, development of lead free perovskites as a potential alternative material and environment friendly candidate has received critical attention. Here, we report the synthesis of all inorganic millimeter size lead-free bismuth based halide perovskites Cs₃Bi₂X₉ (X= Cl, Br, I) single crystals and their structural disorderness, optical, and spin relaxation properties in details. Higher Urbach energy in Cs₃Bi₂X₉ single crystals reveals high degree of local structural disorderness and short range of crystallinity. We show structural disorder not only affect the optical properties but also impacts the magnetic and spin relaxation properties. We observe that increased structural disorder leads to enhanced smearing of local energy bands and high spin orbit coupling. The spin relaxation time is determined in the picoseconds time scale, which corresponds to fast charge carrier dynamics. Our work provides a new design strategy and in-depth understanding to develop environmental friendly lead free and stable perovskite based optoelectronics and spintronic devices.   

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Photobleaching, combined with several microscopy and spectroscopy approaches, was used in order to examine degradation and healing in the bulk and surface of three APbBr₃ single crystals (A=methylammonium (MA); formamidinium (FA); or Cs). Significant differences were observed, depending on the location and the type of A cation: at the surface, self-healing was impeded by the escape of vaporization products. A passivation effect largely compensated for this in the case of the Cs system, but not for the other cations. In the bulk, the FA cation led to the fastest post-damage healing process whereas the Cs system showed the slowest healing rate. The MA system in some cases showed improvement under bleaching. In order to explain the observed bulk phenomena, DFT calculations were performed. The difference in the rate of self-healing in each of the systems was correlated to the A-cation’s effect on Br₃^--defect stability, whereas the unique attributes of the MA system were associated with a possible defect passivation effect of photo-dissociated methylamine. These results rationalize the use of mixed A-cation materials for optimizing both solar cell stability and overall performance of HaP-based devices.   

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Lead halide perovskites have garnered a lot of attention both in displays and photovoltaics. Along with their remarkable linear optical properties, these materials provide a testbed for exploring nonlinear optical properties. As in the case with several of the 3D lead halide perovskite systems, the underlying centrosymmetric crystal structure precludes the phenomenon of second harmonic generation. However, the third and higher-order harmonic generation are allowed. In this work, we probe the third harmonic generation (THG) from CsPbBr₃ nanocrystals (NCs), synthesized via hot injection method, and compare it to the THG from CsPbBr₃ NCs with Ruddlesden-Popper planar faults (RP-CsPbBr₃), formed via post-synthetic fusion-growth. The THG from CsPbBr₃ NCs is negligible compared with that of RP-CsPbBr₃ NCs within a wide range of femtosecond excitation wavelengths: 1100–1400 nm. The THG efficiency of a thin film of RP-CsPbBr₃ is found to be at least three times larger than the value from a single crystal of methylammonium lead bromide (MAPbBr₃). By comparing with MAPbBr₃, we obtain χ³_eff for a thin film of RP-CsPbBr₃ to be of the order of 10⁻¹⁷m²V⁻².   

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The ABX₃ halide perovskites that have attracted so much renewed attention usually have the B site occupied by divalent Sn or Pb, with ns² lone pairs of electrons. These lone pairs are frequently hidden, in the sense that the crystal structures are not consistent with the presence of a stereochemically active lone pair, that would normally be associated with certain characteristic distortions of the BX₆ polyhedra in the structure. Such hidden lone pairs can be seen in many of the perovskites including the hybrids with methylammonium and formamidinium ions on the A site. These lone pairs are associated with proximal instabilities that can profoundly influence material properties. We will discuss the understanding that we have developed from extensive real and k-space studies of local and average structure in these materials using synchrotron and neutron scattering. Density Functional Theory-based electronic structure calculations and Nuclear Magnetic Resonance studies complement the structural studies to obtain a coherent picture of what is happening, structurally and electronically, in these materials.   

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Translation of structural patterns between different atomic-scale building blocks plays a fundamental role in nature, enabling unique functionalities in contexts ranging from biological systems to synthetic materials. In this work, we demonstrate the transfer of structural asymmetry from layers of chiral molecules to an originally achiral inorganic component in a group of two-dimensional hybrid perovskites with chiral organic cations. Asymmetric hydrogen-bonding interactions, caused by the orientation preference of R-(+)- or S-(−)-1-(1-naphthyl)ethylammonium organic spacer cations, account for the asymmetric distortion pattern of Pb-Br 2D inorganic framework. Spin-orbit coupled hybrid density-functional theory band structure calculations reveal substantial Rashba-Dresselhaus spin-splitting and spin textures of frontier bands associated with the distorted structures, potentially useful for control of spin properties of carriers in devices.   

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Halide perovskites undergo structural expansion when exposed to various stimuli. This expansion affects their electronic properties and charge carrier dynamics. It is essential to atomistically model how geometric changes modify the electronic characteristics important for applications such as light harvesting. We use ab initio simulations to investigate the optical and electronic properties of FAPbI₃ under tensile strain. The applied strain leads to elongation of the Pb–I bonds and a decrease in PbI₆ octahedral tilting, which manifests as a blue-shift in the band gap. Nonadiabatic molecular dynamics simulations reveal that charge carrier recombination rates are moderately decreased in these expanded lattices. The influence of lattice dynamics on electron–phonon scattering results in a longer carrier lifetime, which is advantageous for efficient solar cells. By providing detailed information about structure–property relationships, this work emphasizes the role of controlled lattice expansion in enhancing the electronic functionalities of halide perovskites.   

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Mechanical compression provides an effective way to adjust the structures and properties in hybrid organic inorganic perovskites (HOIPs) without changing the chemical composition. An in situ high-pressure femtosecond transient absorption (TA) spectroscopic has been studied here in the prototype MAPbI₃ perovskite thin films, and the pressure response of TA spectrum has been characterized by carrier lifetime, carrier temperature, and hot carrier cooling rate. Both band-gap narrowing and carrier lifetime prolongation has been realized at 1.0 GPa, which may significantly increase the photovoltaic performance. A long carrier building up process due to the extraordinary electron-phonon interaction has been observed at the phase transition critical point. High excitation study show that the applied pressure can significantly reduce the hot carrier cooling rate, while offer higher initial carrier temperature. This study of the mechanisms of compression on carrier dynamics provides insights for the optimal structure design for HOIPs solar cells.   

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Perovskite photovoltaic systems are being studied due to their high energy conversion efficiencies with current emphasis on pure inorganic systems such as CsPbBr₃. In light of the inconsistency of existing space group assignments with recent experiments on this perovskite, high-resolution in-situ synchrotron single-crystal X-ray diffraction and local structure measurements complemented by optical and calorimetric measurements are used to explore the changes in atomic structure for temperatures between 100 and 500 K. The currently accepted space group assignments for CsPbBr₃ are found to be incorrect in a manner that profoundly impacts physical properties. The newly observed structural distortions of the bulk structure are consistent with the expectation of previous photoluminescence and Raman measurements. The new derived orthorhombic structure supports can support a ferroelectric state below room temperature. Multiple low-pressure phases are found, one of which exists as a metastable phase at ambient pressure. This work should help guide research in the perovskite photovoltaic CsPbX₃ community to better control the structure under operational conditions.