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The rapidly emerging perovskites hold immense potential for revolutionizing optoelectronic and photovoltaic applications. However, understanding the complex structural dynamics and their response at extreme conditions in robust environments is crucial for optimizing the practical performance of perovskite‐based devices. Previous X‐ray diffraction studies have shown great potential for studying the average structures of these materials, but for more complex analysis at the local atomic level, Raman spectroscopy is a powerful tool. In this study, temperature‐ and pressure‐dependent Raman spectroscopy was used to elucidate the intricate lattice dynamics, phase transitions, and amorphization of FAPbBr3 single crystals at extreme conditions such as low temperature or high pressure. Temperature‐dependent Raman spectra unveiled significant variations in wavenumbers, full width at half maximum, and the vanishing of specific modes at higher temperatures. These were ascribed to symmetry changes caused by two crystallographic phase transitions at −127°C from orthorhombic to tetragonal and at −33°C from tetragonal to cubic phase. In addition, some of the Raman modes disappeared upon heating at −90°C, which was associated with a crystallographically unresolved transition. Pressure‐dependent Raman scattering revealed two phase transitions at 0.3 GPa and 2.2 GPa, which corresponded to the contraction of the PbBr6 octahedra and their tilting distortion, respectively. Further compression uncovered amorphization at 3.6 GPa, characterized by the vanishing of crystalline Raman modes together with the emergence of broad new modes due to the disorder within the crystal structure. Upon pressure release, the original Raman modes were recovered, indicating the reversibility of the pressure‐induced phase transitions. The changes in the Raman spectra were the most significant, especially in the low‐wavenumber lattice modes, when the FAPbBr3 underwent orthorhombic phase transition. It indicates that the inorganic sublattice is affected by the structural change into the orthorhombic phase caused either by temperature or pressure variation. [ABSTRACT FROM AUTHOR]

In recent years, there have been intense studies on hybrid organic–inorganic compounds (HOIPs) due to their tunable and adaptable features. This present study reports the vibrational, structural, and elastic properties of mixed halide single crystals of MAxFA1-xPbCl3 at room temperature by introducing the FA cation at the A-site of the perovskite crystal structure. Powder X-ray diffraction analysis confirmed that its cubic crystal symmetry is similar to that of MAPbCl3 and FAPbCl3 with no secondary phases, indicating a successful synthesis of the MAxFA1-xPbCl3 mixed halide single crystals. Structural analysis confirmed that the FA substitution increases the lattice constant with increasing FA concentration. Raman spectroscopy provided insight into the vibrational modes, revealing the successful incorporation of the FA cation into the system. Brillouin spectroscopy was used to investigate the changes in the elastic properties induced via the FA substitution. A monotonic decrease in the sound velocity and the elastic constant suggests that the incorporation of large FA cations causes distortion within the inorganic framework, altering bond lengths and angles and ultimately resulting in decreased elastic constants. An analysis of the absorption coefficient revealed lower attenuation coefficients as the FA content increased, indicating reduced damping effects and internal friction. The current findings can facilitate the fundamental understanding of mixed lead chloride perovskite materials and pave the way for future investigations to exploit the unique properties of mixed halide perovskites for advanced optoelectronic applications. [ABSTRACT FROM AUTHOR]

The acoustic properties of tetragonal PbxBa1−xNb2O6 (PBN-x) single crystals with x = 0.5 were studied by Brillouin spectroscopy over a wide temperature range from room temperature to 600 °C. Anomalous changes in the Brillouin frequency shift and the half width of acoustic waves were observed at ∼350 °C that corresponds to the ferroelectric–paraelectric phase transition temperature of PBN-0.50. The longitudinal sound velocity and its associated elastic constant (C33) showed substantial softening in the paraelectric phase when the phonon propagated along the polar c axis, while their degree of softening for the phonon along the a axis was small, revealing clear acoustic anisotropy. It was associated with the strong polarization dependence of the quasi-elastic central peak. The relaxation time derived from the central peak seemed to follow the critical slowing-down behavior indicating order–disorder nature of the phase transition. Among C44 and C66, the former displayed substantial softening in the ferroelectric phase resembling the order parameter behavior. The longitudinal sound velocity and the related acoustic damping showed significant anomalies in the ferroelectric phase of unpoled PBN-0.5, which disappeared completely under poled condition. The possible origin for these unexpected acoustic anomalies was suggested. [ABSTRACT FROM AUTHOR]

The acoustic properties of unpoled (Na1/2Bi1/2)TiO3-5%BaTiO3 (NBT-5%BT) lead-free single crystals were investigated over a wide temperature range from 740 °C to room temperature. Temperature dependences of sound velocities, acoustic attenuation coefficients, and independent elastic constants were determined by using Brillouin spectroscopy. Among the three symmetrized eigen elastic constants C44, (C11 − C12)/2, and C11 + 2C12, (C11 − C12)/2 was the smallest and showed the largest softening at high temperatures above the dielectric maximum temperature (Tm). The softening of (C11 − C12)/2 resulted in the substantial increase of the elastic anisotropy parameter defined as 2C44/(C11 − C12). This suggested that the local symmetry and the associated structural instability of local polar distortions were tetragonal. The onset temperature of quasistatic dynamics of polar nanoregions was ∼500 °C at which all four attenuation coefficients began to increase noticeably. The broad and diffused acoustic anomalies indicated that the low-temperature phase is a typical relaxor state without any long-range order. However, the noncubic local distortions at low temperatures seem to drive the average symmetry of NBT-5%BT to deviate from the cubic phase because the premise of three independent elastic constants for the ideal cubic phase was no longer valid below Tm. [ABSTRACT FROM AUTHOR]