In situ temperature measurement in the pressure chamber of diamond anvil cell.

The measurements of temperature directly influence the reasonability of experiments at high pressure and high temperature. In this article, we proposed a new integration design, the built-in thermocouple, for in situ temperature measurements in high-pressure-high-temperature experiments by fusing the characteristics of thermocouples and diamond anvil cells together. By integrating an S-type thermocouple inside the gasket of a diamond anvil cell, we successfully measured the temperature of the sample straight inside the pressure chamber at high pressure and high temperature. The setup underwent multiple experimental tests using internal and external heating techniques, the results of which revealed its capability to directly characterize the temperature of the sample with comparable accuracy and reliability to that of the typical external thermocouple setup. The proposed setup has also resolved the issue of the discrepancy of temperatures inside and outside the sample chamber and enormously expedited the temperature measurements by significantly reducing the response time of the thermocouple. In conclusion, the built-in thermocouple is a promising approach toward high-efficiency, in situ temperature measurements under extreme conditions.

Sample heating above 1400 K in a diamond anvil cell.

In high-pressure experimental methods, sample heating in the pressure chamber of a diamond anvil cell is an important topic, and numerous efforts have been made to improve and develop new technologies. In this paper, we propose a new type of internal resistance heating technique, the composite heating gasket, prepared by integrating an annular heater into the sample chamber for direct heating of the sample. As the effective heating area covers the entire pressure chamber wall, a relatively quasi-uniform temperature field is formed within the sample chamber. At the same time, the integration design reduces the risk of diamond oxidation and enables direct measurement of the spectroscopic properties of samples at high temperatures. The preparation of the composite heating gasket is simple and repeatable, and its heating performance is stable at temperatures above 1400 K. When the sample diameter is 210 µm and no thermal insulation is used, the diameter of the temperature zone in which the temperature difference is less than 10 and 20 K exceeds 120 and 170 µm, respectively. The composite heating gasket represents a significant advancement in providing a uniform temperature field for in situ measurements with diamond anvil cells at high pressure and temperature.

Study on pressure-induced isostructural phase transition and electrical transport properties of BiOBr.

In this work, we prepared a BiOBr powder sample by the coprecipitation method for in situ high-pressure AC impedance spectroscopy tests, in situ high-pressure Raman measurements and in situ high-pressure X-ray diffraction experiments to explore its structural properties and electrical transport processes under compression. Two pressure-driven isostructural phase transitions, T-T' and T'-T'' (T - tetragonal, T' - tetragonal 1 and T'' - tetragonal 2), were discovered at around 10.0 and 15.0 GPa, respectively. The pressure-induced changes in the crystal structure and electrical transport of BiOBr can provide a reference for explaining the mechanism of the isostructural phase transition of other similar compounds after compression.

Effects of high pressure on the lattice structure and electrical transport properties of BiOI.

To reveal the pressure effects on BiOX semiconductors, we performed in-situ Raman spectroscopy and electrical transport measurements on BiOI up to 26.1 GPa and 19.2 GPa. BiOI showed good structural stability, while the electron conduction characteristics maintained dominance throughout the pressure range. The influence of grain boundary conduction disappeared at pressures above 9.2 GPa. With pressure elevation, the pressure-induced lattice fragmentation and grain refinement introduced a large number of relevant levels in the energy gap and resulted in a significant increase in the conductivity of BiOI under compression. The conductivity increased by 106 at 19.2 GPa from the initial value and maintained an increase of 102 after depressurization until ambient conditions were attained. At the same time, the space charge polarization of the crystal interface layer became weaker with pressure elevation resulting in a decrease in the relative permittivity of BiOI. The calculation results of the complex permittivity showed that the frequency of orientation polarization response decreases with pressure elevation, and the complex permittivity becomes constant in the high-frequency region. Our work proves that pressure could significantly increase the carrier concentration and mobility, thus effectively improving the conductivity of BiOX semiconductors.

Water-cooling diamond anvil cells: An approach to temperature-pressure relation in heated experiments.

Temperature induced pressure drift in the diamond anvil cell (DAC) is a major issue in high-pressure high-temperature experiments. It is commonly acknowledged that these drifts originate from multiple factors, but no systematic descriptions have been made so far. By introducing an internal water-cooling system in the DAC, we have performed a systematic investigation into temperature induced pressure drifts to reveal the mechanism behind them and to find a proper experimental procedure to achieve minimal pressure variation in DAC's heating experiment. It is revealed in this experiment that pressure variation during heating processes originates from multiple temperature related factors of the DAC. The variation itself can be considered as a rebalancing process of the compression forces on the sample chamber initiated by the disturbance caused by temperature elevation. It is possible to suppress pressure variation by maintaining the temperature of the DAC body at room temperature to ensure the consistency of compression on the sample chamber. At the same time, the best procedure for the heating experiments is to properly pre-heat the sample chamber equipped with the internal water-cooling system before performing the in situ measurements on the temperature-related properties at the pressurized and heated conditions. Our discovery provides a reliable procedure for the sample heating process in the DAC and helps resolve the complex mystery of the influence of the combination of pressure and temperature in high-pressure high-temperature experiments.

Application of impedance spectroscopy in exploring electrical properties of dielectric materials under high pressure.

Impedance spectroscopy (IS) is an indispensable method of exploring electrical properties of materials. In this review, we provide an overview on the specific applications of IS measurement in the investigations of various electrical properties of materials under high pressure, including electric conduction in bulk and grain boundary, dielectric properties, ionic conduction, and electrostrictive effect. Related studies are summarized to demonstrate the method of analyzing different electrical transport processes with various designed equivalent circuits of IS and reveal some interesting phenomena of electrical properties of materials under high pressure.

Pressure effects on the metallization and dielectric properties of GaP.

In situ impedance measurement, resistivity measurements and first-principles calculations have been performed to investigate the effect of high pressure (up to 30.2 GPa) on the metallization and dielectric properties of GaP. It is found that the carrier transport process changes from mixed grain and grain boundary conduction to pure grain conduction at 5.8 GPa, and due to pressure-induced structural phase transition, the resistance drops drastically by three orders of magnitude at 25.5 GPa. Temperature dependence of resistivity measurements and band structure calculations suggest the occurrence of a semiconductor-metal transition. Combining differential charge density and dielectric analysis, it is observed that the electron localization is weakened, which leads to increased polarization and larger relative permittivity in the zb structure. After the phase transition, both the polarization and the relative permittivity decrease. Pressure increases the complex dielectric constant and dielectric loss factor, due to the increase in relaxation polarization and the scattering effect of carriers. Moreover, by comparing the high-pressure behavior of GaP, GaAs and GaSb, the changes in the electronic structure and electric transport process caused by the phase transition can be understood, which can enable us to better understand the metallization behavior and dielectric properties of Ga-based III-V family semiconductors under pressure, and stimulate the design and modification of other related group III-V semiconductors for optoelectronic devices and sensors.

Monoclinic EuSn_{2}As_{2}: A Novel High-Pressure Network Structure.

The layered crystal of EuSn_{2}As_{2} has a Bi_{2}Te_{3}-type structure in rhombohedral (R3[over ¯]m) symmetry and has been confirmed to be an intrinsic magnetic topological insulator at ambient conditions. Combining ab initio calculations and in situ x-ray diffraction measurements, we identify a new monoclinic EuSn_{2}As_{2} structure in C2/m symmetry above ∼14 GPa. It has a three-dimensional network made up of honeycomblike Sn sheets and zigzag As chains, transformed from the layered EuSn_{2}As_{2} via a two-stage reconstruction mechanism with the connecting of Sn-Sn and As-As atoms successively between the buckled SnAs layers. Its dynamic structural stability has been verified by phonon mode analysis. Electrical resistance measurements reveal an insulator-metal-superconductor transition at low temperature around 5 and 15 GPa, respectively, according to the structural conversion, and the superconductivity with a T_{C} value of ∼4 K is observed up to 30.8 GPa. These results establish a high-pressure EuSn_{2}As_{2} phase with intriguing structural and electronic properties and expand our understandings about the layered magnetic topological insulators.

Diamond anvil cell with double coaxial chambers.

In general, pressure calibration in diamond anvil cells (DACs) has been achieved by mixing pressure calibration materials (PCMs) with the sample inside the pressure chamber. However, the chemical reactions between the sample and PCMs are sometimes unavoidable at extreme conditions, such as high pressure and high temperature. These undesired reactions will cause pollution, induce changes in physical properties or phase transformations of PCMs, and result in tremendous error of pressure calibration. In this paper, we report a new design of DAC with double coaxial pressure chambers, sample and PCM chambers, to resolve the challenge by isolating the PCM from the sample. Our test results show that the pressure of the two chambers presents interesting relations with the anvil setup. When the geometric parameters of two anvil sets are the same and the difference of chamber diameters is within a certain range (i.e., below 10 µm), the pressure correlation between the two chambers shows little correlation with the pressure transmitting medium before and after its solidification at both room temperature and high temperatures within the experimental condition range (well below 20 GPa and 634 K). In this case, the pressure of the sample chamber can be well calibrated by the pressure of the PCM chamber. This new DAC setup is thus proved to be effective in calibrating the sample pressure below certain conditions while avoiding undesired sample pollution and pressure induced property changes in PCMs under high pressure and high temperature conditions compared with single-chamber DACs.

Pressure-induced ionic to mixed ionic and electronic conduction transition in solid electrolyte LaF3.

The ionic transport properties of solid electrolyte LaF3 were systematically studied under high pressures up to 30.6 GPa with alternate-current impedance spectra measurements and first-principles calculations. From the impedance spectra measurements, LaF3 was found to transform from pure ionic conduction to mixed ionic and electronic conduction at 15.0 GPa, which results from the pressure-induced structural phase transition from a tysonite-type structure to an anti-Cu3Ti-type structure. F- ion migration can be suppressed by pressure, causing a decrease of the ionic conductivity of LaF3. By first-principles calculations, the pressure-dependent diffusion behaviors of the F- ions can be understood. The increased overlap of electron clouds at the interstitial site between rigid La3+ and liquid F- lattices leads to the appearance of electronic conduction in anti-Cu3Ti-type structured LaF3.

Stimuli-Responsive Luminescent Properties of Tetraphenylethene-Based Strontium and Cobalt Metal-Organic Frameworks.

Herein we report two new TPE-based 3D MOFs, that is, Sr-ETTB and Co-ETTB (TPE=Tetraphenylethylene, H8 ETTB=4',4''',4''''',4'''''''-(ethene-1,1,2,2-tetrayl)tetrakis(([1,1'-biphenyl]-3,5-dicarboxylic acid))). Through tailoring outer shell electron configurations of SrII and CoII cations, the fluorescence intensity of the MOFs is tuned from high emission to complete non-emission. Sr-ETTB with strong blue fluorescence shows reversible fluorescence variations in response to pressure and temperature, which is directly related to the reversible deformation of the crystal structure. In addition, non-emissive Co-ETTB counterpart exhibits a turn-on fluorescent enhancement under the stimulation of analyte histidine. In the process, TPE-cored linkers in the MOFs are released through competitive coordination substitution and subsequently reassembled to perform aggregation-induced luminescence behavior originated from the organic linkers.

Substrate-affected lattice structural evolution in compressed monolayer ReS2.

At ambient conditions, the lattice structure of supported ultrathin transition metal dichalcogenides (TMDs) can be effectively modified by a substrate. When compressed, the effect of substrate is far from settled. In this study, the effects of an Si substrate on the lattice structures of compressed monolayer and multilayer ReS2 were investigated by performing high-pressure Raman measurements and first-principle calculations. Our results revealed substrate-affected strain in compressed monolayer ReS2, which resulted in a distorted unit with S atoms sliding within a single layer. This was evidenced by the split of the Ag-5 mode above 1.7 GPa. However, unlike that of the monolayer ReS2, the Ag-5 mode of multilayer ReS2 remained symmetric up to 4.2 GPa, which can be due to weaker substrate-affected strain in compressed multilayer ReS2 when compared with that in the monolayer ReS2. The noticeably different high-pressure responses between multilayer ReS2 and monolayer ReS2 can be due to the effect of interlayer interactions, and the split of the Ag-5 mode provides a clear indication of the prominent strain in compressed supported ReS2.

Effects of high pressure on the electrical resistivity and dielectric properties of nanocrystalline SnO 2.

The electrical transport and structural properties of tin oxide nanoparticles under compression have been studied by in situ impedance measurements and synchrotron X-ray diffraction (XRD) up to 27.9 GPa. It was found that the conduction of SnO2 can be improved significantly with compression. Abnormal variations in resistivity, relaxation frequency, and relative permittivity were observed at approximately 12.3 and 25.0 GPa, which can be attributed to pressure-induced tetragonal- orthorhombic-cubic structural transitions. The dielectric properties of the SnO2 nanoparticles were found to be a function of pressure, and the dielectric response was dependent on frequency and pressure. The dielectric constant and loss tangent decreased with increasing frequency. Relaxation-type dielectric behaviour dominated at low frequencies. Whereas, modulus spectra indicated that charge carrier short-range motion dominated at high frequencies.

Pressure-induced abnormal ionic-polaronic-ionic transition sequences in AgBr.

The electrical transport behavior of the superionic conductor AgBr was systematically studied under high pressure up to 30.0 GPa with electrochemical impedance spectra measurements and first-principles calculations. From impedance spectra measurements, a pressure-induced abnormal ionic-polaronic-ionic transition was found. Herein, the ionic to polaronic transition at 5.0 GPa occurs with the absence of a structural phase transition. At 8.6 GPa, the ionic state of AgBr can be reactivated after a structural phase transition. Previous structural studies based on X-ray diffraction data cannot provide strong evidence to support the ionic-polaronic transition in AgBr at 5.0 GPa. In this paper, based on first-principles calculations, a localized-electron-soup model was proposed to explain the physical origin of the ionic-polaronic transition. In this model, more localized electrons around the Br atoms are pressed into interstitial spaces and, simultaneously, polarons are formed between Ag+ ions and the localized electron background at 5.0 GPa. Therefore, the diffusion of Ag+ ions is effectively screened by the movement of the localized electron background from its equilibrium position, much like beans completely trapped in a cup of thick soup.

Hydride ion (H-) transport behavior in barium hydride under high pressure.

Hydride ions (H-) have an appropriate size for fast transport, which makes the conduction of H- attractive. In this work, the H- transport properties of BaH2 have been investigated under pressure using in situ impedance spectroscopy measurements up to 11.2 GPa and density functional theoretical calculations. The H- transport properties, including ionic migration resistance, relaxation frequency, and relative permittivity, change significantly with pressure around 2.3 GPa, which can be attributed to the structural phase transition of BaH2. The ionic migration barrier energy of the P63/mmc phase decreases with pressure, which is responsible for the increased ionic conductivity. A huge dielectric constant at low frequencies is observed, which is related to the polarization of the H- dipoles. The current study establishes general guidelines for developing high-energy storage and conversion devices based on hydride ion transportation.

Associated Lattice and Electronic Structural Evolutions in Compressed Multilayer ReS2.

The transition metal dichalcogenide (TMD) ReS2 is a promising material for optoelectronic devices because of its remarkable quantum yield. Pressure can effectively tune the optoelectronic properties of TMDs through control of the atomic displacement. Here, we systematically investigated the lattice and electronic structural evolutions of compressed multilayer ReS2. Both Raman spectra and first-principles calculations suggest the occurrence of an intralayer phase transition followed by an interlayer transition. A transition from one indirect to another indirect bandgap at 2.7 GPa was revealed by both high-pressure photoluminescence (PL) measurements and first-principles calculations, this behavior was elucidated by considering the fundamental relationship between lattice variation and electronic evolution. Moreover, by comparing the high-pressure behavior of MoS2 and ReS2, we demonstrated interlayer coupling plays a critical role in determining the lattice and electronic structures in compressed TMDs. Our findings suggest the potential application of ReS2 in fabricating various stacking devices with tailored properties.

Decompression-Driven Superconductivity Enhancement in In2 Se3.

An unexpected superconductivity enhancement is reported in decompressed In2 Se3 . The onset of superconductivity in In2 Se3 occurs at 41.3 GPa with a critical temperature (Tc ) of 3.7 K, peaking at 47.1 GPa. The striking observation shows that this layered chalcogenide remains superconducting in decompression down to 10.7 GPa. More surprisingly, the highest Tc that occurs at lower decompression pressures is 8.2 K, a twofold increase in the same crystal structure as in compression. It is found that the evolution of Tc is driven by the pressure-induced R-3m to I-43d structural transition and significant softening of phonons and gentle variation of carrier concentration combined in the pressure quench. The novel decompression-induced superconductivity enhancement implies that it is possible to maintain pressure-induced superconductivity at lower or even ambient pressures with better superconducting performance.

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.

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.

Visible light response, electrical transport, and amorphization in compressed organolead iodine perovskites.

Recent scientific advances on organic-inorganic hybrid perovskites are mainly focused on the improvement of power conversion efficiency. So far, how compression tunes their electronic and structural properties remains less understood. By combining in situ photocurrent, impedance spectroscopy, and X-ray diffraction (XRD) measurements, we have studied the electrical transport and structural properties of compressed CH3NH3PbI3 (MAPbI3) nanorods. The visible light response of MAPbI3 remains robust below 3 GPa while it is suppressed when it becomes amorphous. Pressure-induced electrical transport properties of MAPbI3 including resistance, relaxation frequency, and relative permittivity have been investigated under pressure up to 8.5 GPa by in situ impedance spectroscopy measurements. These results indicate that the discontinuous changes of these physical parameters occur around the structural phase transition pressure. The XRD studies of MAPbI3 under high pressure up to 20.9 GPa show that a phase transformation below 0.7 GPa, could be attributed to the tilting and distortion of PbI6 octahedra. And pressure-induced amorphization is reversible at a low density amorphous state but irreversible at a relatively higher density state. Furthermore, the MAPbI3 nanorods crush into nanopieces around 0.9 GPa which helps us to explain why the mixed phase of tetragonal and orthorhombic was observed at 0.5 GPa. The pressure modulated changes of electrical transport and visible light response properties open up a new approach for exploring CH3NH3PbI3-based photo-electronic applications.