We report the first high-pressure single-crystal structures of hybrid perovskites. The crystalline semiconductors (MA)PbX₃ (MA = CH₃NH₃⁺, X = Br^– or I^–) afford us the rare opportunity of understanding how compression modulates their structures and thereby their optoelectronic properties. Using atomic coordinates obtained from high-pressure single-crystal X-ray diffraction we track the perovskites’ precise structural evolution upon compression. These structural changes correlate well with pressure-dependent single-crystal photoluminescence (PL) spectra and high-pressure bandgaps derived from density functional theory. We further observe dramatic piezochromism where the solids become lighter in color and then transition to opaque black with compression. Indeed, electronic conductivity measurements of (MA)PbI₃ obtained within a diamond-anvil cell show that the material’s resistivity decreases by 3 orders of magnitude between 0 and 51 GPa. The activation energy for conduction at 51 GPa is only 13.2(3) meV, suggesting that the perovskite is approaching a metallic state. Furthermore, the pressure response of mixed-halide perovskites shows new luminescent states that emerge at elevated pressures. We recently reported that the perovskites (MA)Pb(Br_xI_1–x)₃ (0.2 < x < 1) reversibly form light-induced trap states, which pin their PL to a low energy. This may explain the low voltages obtained from solar cells employing these absorbers. Our high-pressure PL data indicate that compression can mitigate this PL redshift and may afford higher steady-state voltages from these absorbers. These studies show that pressure can significantly alter the transport and thermodynamic properties of these technologically important semiconductors.

Functional CsPbI₃ perovskite phases are not stable at ambient conditions and spontaneously convert to a non-perovskite δ phase, limiting their applications as solar cell materials. We demonstrate the preservation of a black CsPbI₃ perovskite structure to room temperature by subjecting the δ phase to pressures of 0.1 – 0.6 GPa followed by heating and rapid cooling. Synchrotron X-ray diffraction and Raman spectroscopy indicate that this perovskite phase is consistent with orthorhombic γ-CsPbI₃. Once formed, γ-CsPbI₃ could be then retained after releasing pressure to ambient conditions and shows substantial stability at 35% relative humidity. First-principles density functional theory calculations indicate that compression directs the out-of-phase and in-phase tilt between the [PbI₆]⁴⁻ octahedra which in turn tune the energy difference between δ- and γ-CsPbI₃, leading to the preservation of γ-CsPbI₃. Here, we present a high-pressure strategy for manipulating the (meta)stability of halide perovskites for the synthesis of desirable phases with enhanced materials functionality.

We investigated high-pressure induced phase transitions in ${\text{Y}}_2{\text{O}}_3$ and Eu-doped ${\text{Y}}_2{\text{O}}_3$ $\left({\text{Y}}_2\text{O}:{\text{Eu}}^{3+}\right)$ using angular dispersive synchrotron x-ray diffraction, Raman spectroscopy, and photoluminescence (PL). With increasing pressure, we observed a series of phase transformations in ${\text{Y}}_2{\text{O}}_3:{\text{Eu}}^{3+}$ , which followed a structure sequence of $\text{cubic}\to \text{monoclinic}\to \text{hexagonal}$ , while ${\text{Y}}_2{\text{O}}_3$ followed a sequence of $\text{cubic}\to \text{hexagonal}$ . During decompression, both hexagonal structured ${\text{Y}}_2{\text{O}}_3$ and ${\text{Y}}_2{\text{O}}_3:{\text{Eu}}^{3+}$ transformed into monoclinic phases which were quenchable back to ambient pressure. Raman and PL measurements shed additional light on the different phase transition behavior in these two samples.

We present a multitechnique approach to experimentally determine the elastic anisotropy of polycrystalline hcp Fe at high pressure. Directional phonon measurements from inelastic X‐ray scattering on a sample with lattice preferred orientation at 52 GPa in a diamond anvil cell were coupled with X‐ray diffraction data to determine the elastic tensor. Comparison of the results from this new method with the elasticity determined by lattice strain analysis of radial X‐ray diffraction measurements showed significant differences, highlighting the importance of strength anisotropy in hcp Fe. At 52 GPa, we found that a method which combines results from inelastic scattering and pressure‐volume measurements gives a shape in the velocity anisotropy close to sigmoidal (with a faster c and slower a axis) a smaller magnitude in the anisotropy and compared to velocities based on the lattice strain method which gives a bell shape velocity distribution with the fast direction between the c and a axes. We used additional results from nuclear resonant inelastic X‐ray scattering to constrain errors and provide additional validation of the accuracy of our results.