Pressure-Driven Polymorphic Transition, Emergent Insulator-To-Metal Transition, and Photoconductivity Switching in Violet Phosphorus.

Polymorphic phase transition is an essential phenomenon in condensed matter that the physical properties of materials may undergo significant changes due to the structural transformation. Phase transition has thus become an important means and dimension for regulating material properties. Herein, this study demonstrates the pressure-induced multi-transition of both structure and physical properties in violet phosphorus, a novel phosphorus allotrope. Under compression, violet phosphorus undergoes sequential polymorphic phase transitions. Concomitant with the first phase transition, violet phosphorus exhibits emergent insulator-metal transition, superconductivity, and dramatic switching from positive to negative photoconductivity. Remarkably, the resistance of violet phosphorus shows a sudden drop of around 107 along with the phase transition. In addition, piezochromism from translucent red to opaque black and suppression of photoluminescence are observed upon compression. Of particular interest is that the sample irreversibly transforms into black phosphorus with a pronounced discrepancy in physical properties from the pristine violet phosphorus after decompression. The abundant polymorphic transitions and property changes in violet phosphorus have significant implications for designing novel pressure-responsive electronic/optoelectronic devices and exploring concealed polymorphic transition materials.

Fe-O4 Motif Activated Graphitic Carbon via Oxo-Bridge for Highly Selective H2O2 Electrosynthesis.

Carbon-based materials have been utilized as effective catalysts for hydrogen peroxide electrosynthesis via two-electron oxygen reduction reaction (2e ORR), however the insufficient selectivity and productivity still hindered the further industrial applications. In this work, we report the Fe-O4 motif activated graphitic carbon material which enabled highly selective H2O2 electrosynthesis operating at high current density with excellent anti-poisoning property. In the bulk production test, the concentration of H2O2 cumulated to 8.6 % in 24 h and the corresponding production rate of 33.5 mol gcat -1 h-1 outperformed all previously reported materials. Theoretical model backed by in situ characterization verified α-C surrounding the Fe-O4 motif as the actual reaction site in terms of thermodynamics and kinetics aspects. The strategy of activating carbon reaction site by metal center via oxo-bridge provides inspiring insights for the rational design of carbon materials for heterogeneous catalysis.

Identification of intermediates of a molecular ruthenium catalyst for water oxidation using in situ electrochemical X-ray absorption spectroscopy.

This is the first study on a Ru(bda) (bda: 2,2'-bipyridine-6,6'-dicarboxylic acid) catalyst in solution using a home-built electrochemical cell, in combination with an energy-dispersive X-ray absorption spectroscopy setup. The oxidation state and coordination number of the catalyst during electrocatalysis could be estimated, while avoiding radiation damage from the X-rays.

High-pressure dielectric behavior of BaMoO4: a combined experimental and theoretical study.

In situ impedance measurements were employed to investigate the electrical transport properties of BaMoO4 under pressures of up to 20.0 GPa. Two anomalous changes in the electrical parameters were found, related to the pressure-induced structural phase transitions. The dielectric performance of BaMoO4 was improved by pressure. The dispersion in the real part of dielectric constant versus frequency weakens with increasing pressure. Based on the first-principles calculations, the increases of resistance with increasing pressure in the tetragonal and monoclinic phases were mainly caused by the increasing defect levels. The decrease of the relative permittivity in the tetragonal phase was attributed to pressure-induced strengthening in electronic localization around Mo atoms, which hindered the polarization of Mo-O electric dipoles.

Lateral Heterostructures Fabricated via Artificial Pressure Gradient.

Hydrostatic conditions are generally pursued in high-pressure research, maintained to prevent the intrinsic pressure gradient on the culets of diamond anvil cells (DACs) from introducing heterogeneity to the structure and physical properties of the regulated materials. Here, a pioneering route to fabricate lateral heterostructures is proposed via artificial pressure gradients intentionally designed in DACs. Under the tailored pressure gradients, different structural phases emerge in distinct parts of the material, resulting in the formation of heterostructures. Harnessing the polymorphic transition nature of violet phosphorus under high pressure, violet/blue and violet/black phosphorus lateral heterostructures with different electrical properties have been successfully prepared by the pressure gradient method. This achievement highlights the potential of artificial pressure gradients as a portable and universal strategy for the fabrication of lateral heterostructures, shedding new light on the preparation and regulation of lateral heterostructures across a wider range of materials.

Pressure Dependence of Mixed Conduction and Photo Responsiveness in Organolead Tribromide Perovskites.

The electrical transport properties of CH3NH3PbBr3 (MAPbBr3) polycrystals were in situ investigated by alternating-current impedance spectroscopy under high pressures up to 5.6 GPa. It is confirmed that ionic and electronic conductions coexist in MAPbBr3. As pressure below 3.3 GPa ions migration is the predominant process, while above 3.3 GPa electronic conduction becomes the main process. An obvious ionic-electronic transition can be observed. The pressure dependent photo responsiveness of MAPbBr3 was also studied by in situ photocurrent measurements up to 3.8 GPa. The mixed conduction can be clearly seen in photocurrent measurement. Additionally, the photocurrents remain robust below 2.4 GPa, while they are suppressed with pressure-induced partial amorphization. Interestingly, the photoelectric response of MAPbBr3 can be enhanced by high pressure, and the strongest photocurrent value appears in the high-pressure phase II at 0.7 GPa, which is similar to previous results in both MAPbI3 and MASnI3.