The application of high pressure allows tuning physicochemical properties of materials by changing interatomic distances. Pressure may also induce structural phase transitions into new phases with enhanced or novel functional properties. Here, we report complementary high-pressure single-crystal X-ray diffraction, Raman spectroscopy, and optical studies of a two-dimensional (2D) perovskite, MHy 2 PbBr 4 , comprising a very small spacer cation (methylhydrazinium, MHy + ). This crystal exhibits highly desired ferroelectric and extraordinary multiple linear and nonlinear optical (NLO) properties. Single-crystal X-ray diffraction shows that MHy 2 PbBr 4 undergoes an unusual Pmn2 1 → P2 1 phase transition near 4 GPa, associated with the extrusion of some MHy + cations from the interlayer space into voids located within the inorganic sheets, not reported for any 2D hybrid perovskite. The transport of counter cations leads to a significant increase of Pb− NH 2 interactions, an unprecedented threefold increase of positive linear compressibility perpendicular to the polyanionic layers and a large negative linear compressibility of −22.39 TPa −1 within the layers. The Raman data confirm the association of the phase transition with strong distortion of the crystal structure and reorganization of the hydrogen bond network, while the absorption spectra of the compressed ambient-pressure Pmn2 1 phase show the band gap narrowing, followed by its widening in the highpressure P2 1 phase. A similar change in the pressure dependence from a red shift to a blue shift is also observed for the free-exciton (FE) photoluminescence (PL). Furthermore, the pressure-induced phase transition leads to a giant enhancement of PL intensity, especially pronounced for the broad-band emission attributed to the self-trapped excitons (STEx). We attribute the effects, observed in absorption and PL spectra, to the shortening of Pb−Br bonds in the ambient pressure phase and increased distortion of the inorganic layers and tilts of PbBr 6 octahedra in the high-pressure phase. Overall, our results for a 2D hybrid compound comprising very small spacer cations extend the understanding of the pressure effect on the properties of 2D hybrid perovskites in general and demonstrate a very different behavior under compression compared to the analogues with large organic cations. They revealed that the structure−strain mechanism can be used for engineering new high-pressure phases with unusual structural, mechanical, and optoelectronic properties.

Perovskite-like [TPrA][Cd(dca)3] undergoes four temperature-induced phase transitions associated with dielectric anomalies and one pressure-induced phase transition into a monoclinic phase.

The [TPrA][Mn(dca)3] perovskite shows highly anisotropic compression and the presence of three pressure-induced phase transitions near 0.4, 3 and 5 GPa into lower symmetry phases.

A series of pressure-induced phase transitions between [MHy]­Mn­(H2POO)3 (MHy+ = methylhydrazinium) phases reveals the structural mechanism behind the elastic properties of this hypophosphite perovskite. In the ambient pressure phase α, NH···O hydrogen bonds to H2POO– linkers stabilize the MHy+ cations outside the perovskite cages. When pressure increases to 1.1 GPa, the MHy+ cations are pushed into the perovskite cages, but the manganese-hypophosphite framework of this new phase β is similar to that of the phase α. This type of phase transition was not reported for related formate perovskites. Another phase transition to phase γ is observed at 1.2 GPa. This phase transition leads to collapse of the perovskite cages, but the conformation and positions of MHy+ cations in the cages hardly change. The phase transitions are equitranslational (zellengleichen), with the symmetry space group changing from Pnma (phase α), Pcmn (phase β), and P1121/n (phase γ). The space group type of phases α and β is the same, but the crystal directions [x] and [z] are exchanged. Owing to the hierarchy of interactions, the sequence of volume drops is rather unusual: it is smaller for the lower-pressure phase transition from phase α to β than for the subsequent phase transition to phase γ. Raman data give evidence for yet another transformation to phase δ between 4.2 and 4.7 GPa. Crystal structure of this phase could not be solved, but very pronounced changes in the Raman spectra indicate that the phase transition to phase δ is associated with very large reconstruction of the manganese-hypophosphite framework.

Understanding the thermodynamic properties of materials is a fundamental issue in physics, and its knowledge is crucial for targeting a specific material for possible applications. In this work, we report a temperature‐ and pressure‐dependent Raman study of bulk GaSe0.5Te0.5 alloy, besides their relevant thermodynamic parameters. Our results show a nonlinear redshift for the A1g and E2g vibrational modes as the temperature increases in the temperature range from 10 to 748 K. Such behavior is well described by considering both thermal expansion and phonon–phonon coupling contributions. By combining density functional theory (DFT) calculations and Raman spectroscopy experiments, the anharmonic constants relative to the three‐ and four‐phonon decay processes, mode‐Grüneisen parameters, Debye temperature, thermal expansion coefficient, and bulk modulus were estimated for GaSe0.5Te0.5 alloy. Furthermore, the high‐pressure measurements and DFT calculations, performed in the pressure range from 0 to 26.4 GPa, show a quadratic trend for the ωA1g and ωE2g modes as a function of pressure, with the A1g modes being more compressible than E2g one, that is, ∂ωA1g∂P>∂ωE2g∂P. No structural phase transition is observed until the maximum pressure reached in the experiment. This study took a step forward in the understanding of mechanical and thermal properties related to GaSe0.5Te0.5 alloy, whose determined parameters are important for designing new applications.