Raman Scattering Studies of the Structural Phase Transitions in Single-Crystalline CH3NH3PbCl3

Influence of Halides on Elastic and Vibrational Properties of Mixed-Halide Perovskite Systems Studied by Brillouin and Raman Scattering

The relationship between halogen content and the elastic/vibrational properties of MAPbBr_3-xCl_x mixed crystals (x = 1.5, 2, 2.5, and 3) with MA = CH₃NH₃⁺ has been studied using Brillouin and Raman spectroscopy at room temperature. The longitudinal and transverse sound velocities, the absorption coefficients and the two elastic constants C₁₁ and C₄₄ could be obtained and compared for the four mixed-halide perovskites. In particular, the elastic constants of the mixed crystals have been determined for the first time. A quasi-linear increase in the sound velocity and the elastic constant C₁₁ with increasing chlorine content was observed for the longitudinal acoustic waves. C₄₄ was insensitive to the Cl content and very low, indicating a low elasticity to shear stress in mixed perovskites regardless of the Cl content. The acoustic absorption of the LA mode increased with increasing heterogeneity in the mixed system, especially for the intermediate composition where the Br and Cl ratio was 1:1. In addition, a significant decrease in the Raman-mode frequency of the low-frequency lattice modes and the rotational and torsional modes of the MA cations was observed with decreasing Cl content. It clearly showed that the changes in the elastic properties as the halide composition changes were correlated with the lattice vibrations. The present findings may facilitate a deeper understanding of the complex interplay between halogen substitution, vibrational spectra and elastic properties, and may also pave the way for optimizing the operation of perovskite-based photovoltaic and optoelectronic devices by tailoring their chemical composition.

Polarized Raman Study of Internal Vibrations of the Organic Cation in 3-Cyanopyridinium Lead Tribromide Post-perovskite

In this work, we apply polarized Raman spectroscopy for study of internal vibrations of the 3-cyanopyridinium cation in the halide post-perovskite (3cp)PbBr₃ (3cp = 3-CN-C₅H₅NH⁺). For a single cation, the vibrational frequencies and intensities of the Raman signal were calculated using the density functional theory. Selection rules were established for vibrations of cations in the crystal. These rules together with modeling results were used to identify the internal vibrations of the cation in the Raman spectrum of the crystal. Narrow and isolated internal vibrations of cations could be used as spectators of the crystalline environment.

Recruiting Unicellular Algae for the Mass Production of Nanostructured Perovskites

Functional capacities of lead halide perovskites are strongly dependent on their morphology, crystallographic texture, and internal ultrastructure on the nano- and the meso-scale. In the last decade, significant efforts are directed towards the development of novel synthesis routes that would overcome the morphological constraints provided by the physical and crystallographic properties of these materials. In contrast, various living organisms, such as unicellular algae, have the ability to mold biogenic crystals into a vast variety of intricate nano-architectured shapes while keeping their single crystalline nature. Here, using the cell wall of the dinoflagellate L. granifera as a model, sustainably harvested biogenic calcite is successfully transformed into nano-structured perovskites. Three variants of lead halide perovskites CH₃ NH₃ PbX₃ are generated with X = Cl^- , Br^- and I^- ; exhibiting emission peak-wavelength ranging from blue, to green, to near-infrared, respectively. The approach can be used for the mass production of nano-architectured perovskites with desired morphological, textural and, consequently, physical properties exploiting the numerous templates provided by calcite forming unicellular organisms.

Structural Phase Transitions and Thermal Degradation Process of MAPbCl3 Single Crystals Studied by Raman and Brillouin Scattering

Raman spectroscopy was applied to MAPbCl₃ single crystals in a wide frequency range from 10 to 3500 cm^-1 over a broad temperature range from -196 °C to 200 °C including both two structural phase transitions and a thermal degradation range. Low-frequency lattice modes of MAPbCl₃ were revealed for the first time, which showed discontinuous anomalies along with the change in the number of Raman modes at the transition points of -114 °C and -110 °C. Several Raman modes related to the C-N stretching and MA rocking modes in addition to the lattice modes displayed temperature dependences similar to those of MAPbBr₃ in both Raman shifts and half widths, indicating that the MA cation arrangement and H-halide bond interactions behave similarly in both systems during the phase transition. The substantial increase in the half widths of nearly all Raman modes especially suggests that the dynamic disorder caused by the free rotational motions of MA cations induces significant anharmonicity in the lattice and thus, reduces the phonon lifetimes. High-temperature Raman and Brillouin scattering measurements showed that the spectral features changed drastically at ~200 °C where the thermal decomposition of MAPbCl₃ into PbCl₂ began. This result exhibits that combined Raman and Brillouin spectroscopic techniques can be a useful tool in monitoring temperature-induced or temporal changes in lead-based halide perovskite materials.

Anharmonic IR spectra of solvated ammonium and aminium ions: resemblance between water and bisulfate solvations

In this work, we analyze the vibrational spectra of ammonium, methylammonium, and dimethylammonium ions solvated by either water molecules or bisulfate anions using anharmonic vibrational algorithms. Rich and complicated spectral features in the 2700-3200 cm^-1 region of the experimental spectra of these clusters are attributed to originate from strong Fermi resonance between hydrogen-bonded NH stretching fundamentals and NH bending overtones. Additional weaker bands around 2500-2600 cm^-1 in solvated aminium ions are assigned to the combination tones involving the CH-NH (methyl-amino) rocking modes. Furthermore, the qualitative resemblance in band positions and spectral patterns between two-water-solvated and two-bisulfate-solvated cations suggest a common vibrational coupling scheme beneath the two seemingly different micro-solvation environments.

Acoustic Anomalies and the Critical Slowing-Down Behavior of MAPbCl3 Single Crystals Studied by Brillouin Light Scattering

Inelastic light scattering spectra of organic–inorganic halide perovskite MAPbCl3 single crystals were investigated by using Brillouin spectroscopy. Sound velocities and acoustic absorption coefficients of longitudinal and transverse acoustic modes propagating along the cubic [100] direction were determined in a wide temperature range. The sound velocities exhibited softening upon cooling in the cubic phase, which was accompanied by the increasing acoustic damping. The obtained relaxation time showed a critical slowing-down behavior, revealing the order–disorder nature of the phase transition, which is consistent with the growth of strong central peaks upon cooling toward the phase transition point. The temperature dependences of the two elastic constants C11 and C44 were obtained in the cubic phase for the first time. The comparison of C11 and C44 with those of other halide perovskites showed that C11 of MAPbCl3 is larger and C44 is slightly smaller compared to the values of MAPbBr3 and MAPbI3. It suggests that MAPbCl3 has a more compact structure (smaller lattice constant) along with stronger binding forces, causing larger C11 and bulk modulus in this compound, and that the shear rigidity is exceedingly small similar to other halide perovskites. The reported elastic constants in this study may serve as a testbed for theoretical and calculational approaches for MAPbCl3.

An ab initio anharmonic approach to IR, Raman and SFG spectra of the solvated methylammonium ion

The methylammonium ion (CH₃NH₃⁺, or noted as MA-H⁺) is one of the smallest organic ammonium ions that play important roles in organic-inorganic halide perovskites. Despite the simple structure, the vibrational spectra of MA-H⁺ exhibit complicated features in the 3 μm region which are sensitive to the solvation environment. In the present work, we have applied the ab initio anharmonic algorithm at the CCSD/aug-cc-pVDZ level to simulate the IR and Raman spectra of the solvated methylammonium ion, MA-H⁺⋯X₃, where X denotes the solvent molecules, to understand the Fermi resonance mechanism in which the overtones of NH bending modes are coupled with the fundamentals of NH stretching modes. The spectral features of the solvated clusters with proper solvent species resemble those observed in the perovskite crystal, indicating that they have similar solvation environments and hydrogen bond interactions. Therefore, a linkage between the gas-phase cluster models and the condensed-phase materials can be established, and our simulations and anharmonic analyses help in interpreting the spectral assignments of the observed IR and Raman spectra of perovskites reliably. Furthermore, we have extended this approach to the SFG spectra to demonstrate the selective appearance of bands depending on both the beam polarization configurations and the symmetry of vibrational modes.

Room-temperature synthesis, growth mechanisms and opto-electronic properties of organic–inorganic halide perovskite CH3NH3PbX3 (X = I, Br, and Cl) single crystals

Growth of MAPbX₃ (X = I, Br, and Cl) single crystals by room temperature crystallization (RTC) method, and the crystallization pathway illustrated by the solubility curve of MAPbCl₃ in DMSO, compared with inverse temperature crystallization (ITC) method.

Magnetic hybrid organic–inorganic perovskite (CH3NH3)2XCl4 (X = Mn, Cu, Co) crystals

We reported three hybrid organic–inorganic perovskite single crystals of (CH₃NH₃)₂MnCl₄, (CH₃NH₃)₂CuCl₄ and (CH₃NH₃)₂CoCl₄. Magnetic measurements show that all the single crystals display magnetic ordering at low temperature.