Hydrostatic pressure, as an alternative of chemical pressure to tune the crystal structure and physical properties, is a significant technique for novel function material design and fundamental research. In this article, we report the phase stability and visible light response of the organolead bromide perovskite, CH3NH3PbBr3 (MAPbBr3), under hydrostatic pressure up to 34 GPa at room temperature. Two phase transformations below 2 GPa (from Pm3̅m to Im3̅, then to Pnma) and a reversible amorphization starting from about 2 GPa were observed, which could be attributed to the tilting of PbBr6 octahedra and destroying of long-range ordering of MA cations, respectively. The visible light response of MAPbBr3 to pressure was studied by in situ photoluminescence, electric resistance, photocurrent measurements and first-principle simulations. The anomalous band gap evolution during compression with red-shift followed by blue-shift is explained by the competition between compression effect and pressure-induced amorphization. Along with the amorphization process accomplished around 25 GPa, the resistance increased by 5 orders of magnitude while the system still maintains its semiconductor characteristics and considerable response to the visible light irradiation. Our results not only show that hydrostatic pressure may provide an applicable tool for the organohalide perovskites based photovoltaic device functioning as switcher or controller, but also shed light on the exploration of more amorphous organometal composites as potential light absorber.

A pyridinium-carboxylate compound undergoes reversible color change under pressure owing to the formation of radicals via electron transfer; dehydration and hydration can also trigger electron transfer.

Angle-dispersive synchrotron X-ray diffraction measurements were performed on vaterite-type YBO3/Eu3+, GdBO3, and EuBO3, respectively, up to 41 GPa at room temperature using a diamond-anvil cell. Pressure-induced amorphization was observed in hexagonal GdBO3 with a significant compression along the c-axis. Compared to the ions of the distorted GdBO3 phase, its anions may lose their long-range order prior to the cations at high pressures. Based on the experimental pressure-volume data, the obtained bulk moduli of YBO3/Eu3+ and GdBO3 are 329 and 321 GPa, respectively, which are more than 90% larger than that of EuBO3 (167 GPa) and are presumably attributed to Gd3+ and Y3+ with a high density of d valence electrons.

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In this paper, structural evaluations of a room temperature ionic liquid, 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl), were systematically investigated at high pressures. Our Raman spectra, infrared spectra, and synchrotron X-ray diffraction investigations show that crystalline [EMIM]Cl experienced structural instabilities at high pressures and underwent at least four successive structural transitions at around 5.8, 9.3, 15.8, and 19.1 GPa, respectively. Notably, the abrupt emergence of photoluminescence from the sample at around 19.3 GPa, originated from the pressure-induced polymerization of the [EMIM]+ cations, as confirmed by the mass spectrometry experiments. Our results also indicate that high pressure significantly affected the conformational equilibrium of the [EMIM]+ cations. The structural transitions are influenced by the ion stacking modes determined by the hydrogen bonds and possibly by some chemical reactions in addition to the cation conformational isomers.

In this work, we revealed that Sb 2 Se 3 has one second-order isostructural phase transitions before it eventually transformed into a site-disordered alloy during compression. Then, a rare amorphization of alloy was discovered with decreasing pressure in Sb 2 Se 3 by performing in situ high-pressure X-ray diffraction (XRD) and Raman experiments, which may be owed to the significant difference in atomic electronegativity of Sb 2 Se 3 . Compared to the original structure, the amorphous phase of Sb 2 Se 3 is more stable at high pressure. Surprisingly, the structural stability of amorphous Sb 2 Se 3 at varying temperatures was also significantly improved via recompression. Our findings will provide insight into the formation mechanism of the bcc alloys and amorphization in A 2 B 3 (A = Sb, Bi; B = S, Se, Te) compounds and, especially, offer a new way to prepare amorphous materials. ■ INTRODUCTIONGenerally, amorphous materials can be obtained by rapid solidification of vapors and the melts, 1−3 which have been recognized and widely employed in the field of material science. In addition, several alternative routes have been constructed, such as gas absorption of metal alloys, 4−6 mechanical alloying, 7,8 anomalous diffusion, 9 or pulsed laser irradiation, 10 etc. Compared to these traditional methods, pressure-induced amorphization is more complex and unique and has been observed in various elements, 11−15 compounds, 16−22 and alloys. 23−25 However, almost all of these transformations are found in the compression process, so far widely unknown to the crystalline-to-amorphous transition with releasing pressure. Here, we find that a Sb-Se amorphous phase can be synthesized by pressurizing and depressurizing Sb 2 Se 3 .As typical A 2 B 3 -form (A = Sb, Bi; B = S, Se, Te) compounds, Sb 2 Se 3 has attracted a lot of interest due to the interesting properties for several applications, such as thermoelectric devices, 26 solar cells, 27 optical recording material, 28 and hydrogen storage materials, 29 etc. In recent years, Bi 2 Te 3 , Sb 2 Te 3 , and Bi 2 Se 3 have been experimentally observed as the simplest 3D topological insulators, 30,31 which make A 2 B 3 became a hot topic in the field of material research. External pressure can be used to tune the electronic and atomic structures of materials. It has been verified that the thermoelectric properties of Bi 2 Te 3 , Bi 2 Se 3 , and Sb 2 Te 3 can be improved under high pressures. 32−35 Also, pressure-induced electronic topological transitions (ETTs), 36−40 superconductivity, 41−47 and metallization transitions 48−50 of these compounds have also been extensively investigated. However, compared to the sufficient high-pressure studies of rhombohedral forms of A 2 B 3 (Bi 2 Te 3 , Sb 2 Te 3 , and Bi 2 Se 3 ), 39,45,51−59 the Pnma types of A 2 B 3 (Bi 2 S 3 , Sb 2 S 3 , and Sb 2 Se 3 ) have not received similar attention because of the lack of topological properties, and their structural transformations at high pressure are still indistinct. By a combination of high-pressure X...

Organometal halide perovskites offer tremendous potential in developing optoelectronic and photovoltaic devices because of their spectacular band gap properties. Pressure has been demonstrated to be able to modulate their band gap in the energy range of visible spectrum, except in the high-energy region of ∼2.5−3.0 eV. In this work, we present a highpressure study of propylammonium lead bromide perovskite and reveal that the band gap can be tuned between the energy of violet light and yellow light (∼3.0−2.2 eV) by pressure. Upon compression, the band gap of this material is progressively altered from ∼3.0 eV at ambient pressure to 2.28 eV at 9.5 GPa. At a relatively low pressure of 1.3 GPa, a triclinic-to-monoclinic structural transition is also observed with a ∼4.7% band gap reduction. Interestingly, in the pressure range of 9.5−20 GPa, the amorphization of the material leads to an anomalously enlarged band gap as a result of the disorder of organic cations, the slightly distorted [PbBr 6 ] 4− octahedra. The variation of band gap of this perovskite at high pressures is explored to be closely attributed to the lattice density and octahedra distortion of amorphous phase. Findings of this work demonstrate that the band gap of organometal perovskites realizes the first redshift from the violet to visible region through the control of lattice parameter and crystal symmetry at high pressures, providing potential communication and sensing devices ranging from violet to yellow at high pressures. Our results also improve the understanding of the structures and properties of organometal halide perovskites.