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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.

In this paper, both the facet-dependent electrical properties and mechanical stability of submicron-sized polyhedral Cu2O are investigated with in-situ resistivity measurements with pressure up to 25.0 GPa. The pressure-induced morphology changes are characterized by scanning/transmission electron microscopy. X-ray photoelectron spectroscopy measurements are also adopted to rationalize the diversity of the electrical resistivity of polyhedral Cu2O under compression. The electrical properties of cubic, truncated octahedral, and octahedral Cu2O show extremely different pressure dependence, which can be attributed to the preferential adsorption of oxygen on Cu2O (111) facet. The anomalous changes in electrical resistivity at 0.7–2.2, 8.5, 10.3, and 21.6 GPa are due to pressure-induced structural phase transitions of the Cu2O crystals. The dramatic reductions of resistivity in cubes and octahedras at 15.0 GPa and of truncated octahedras at 21.2 GPa are driven by pressure-induced crush and nanocrystallization of Cu2O crystals. The mechanical stability of truncated octahedral Cu2O is better than those of both cubic and octahedral Cu2O. Our results not only elucidate the underlying mechanisms for understanding the two previously reported pressure-dependent electrical resistance change trends difference, but also give a reasonably explanation of the facet-dependent mechanical property of Cu2O.

The electrical transport properties of nanocrystalline tungsten trioxides (WO3) under high pressures have been investigated by various electrical measurements up to 36.5 GPa. The discontinuous changes in direct-current resistivity under high pressures result from two electronic phase transitions at 4.3 and 10.5 GPa and two structural phase transitions at 24.8 and 31.6 GPa. Hall-effect measurement shows that the nanocrystalline WO3 is n-type semiconductor within the whole investigated pressure range. The carrier concentration decreases monotonously with increasing pressure, but mobility increases first and then decreases at 10.4 GPa. Through alternate-current impedance measurement, it can be found that the variation of the ratio of grain boundary resistance to grain resistance synchronizes with that of the mobility under high pressures, indicating that the grain boundary plays more important role in the carrier transport process of nanocrystalline WO3. The discontinuous changes of resistance and relaxation frequency of grain and grain boundary also provide the evidence for electronic phase transitions.

Herein, we report on the intriguing electrical transport properties of compressed Sb 2 Te 3 , a topological insulator. A significant anomaly in Hall coefficient at 3.4 GPa reveals an electronic topological transition deriving from topological morphology changes of Van Hove singularity. Pressure dependent resistivity, Hall coefficient, carrier concentration, and mobility show discontinuous changes at 10.6, 15.2, and 20.8 GPa, i.e., close to the onset of structural phase transitions of Sb 2 Te 3 . Moreover, Hall-effect probing discerns the change of conductive mechanism from the lattice vibration dominated to substitutional alloy conduction guiding at 20.8 GPa. Variable-temperature resistivity measurement reveals the transformation from the semi-conductive to semi-metallic phase around 9.0 GPa despite that Sb 2 Te 3 has the priority to hole conduction till 28.2 GPa, indicating that the topological nontrivial properties of Sb 2 Te 3 will be destroyed via pressure effect beyond 9.0 GPa. Definite conductive phase diagram sheds a new light on building Sb 2 Te 3 -based application of the future.

Pressure-dependent atomic packing structure changes of newly designed sulfur-bearing metallic glass (MG) of Pd37Ni37S26 have been studied by in situ high energy X-ray diffraction and resistivity experiments, and ab initio molecular dynamics simulations from ambient pressure up to 40.9 GPa. Local structural transformation from a low-density amorphous state into high-density one at about 12.8~15.4 GPa has been revealed, which originates from the noticeable kinks during the electron back-donations from S atoms to more localized Ni 3d orbital at about 13~15 GPa and is confirmed by the pressure-dependent resistivity measurements. The origin for the transition is uncovered, i.e., Pd- and Ni-centered clusters first transfer from icosahedral-like structures into higher-coordinated ones, which dominates the initial densification of Pd37Ni37S26 MG during the compression below 13 GPa. In the pressure range above 15 GPa, the low-coordinated S-centered clusters transform into the tri-capped trigonal prism-like structures during the compression of Pd37Ni37S26 MG up to 40.9 GPa. All results obtained here provide insight into the local structural evolution of the sulfur-bearing MGs with pressure.

The in situ electrical conductivity measurements on water in both solid state and liquid state were performed under pressure up to 28 GPa and temperature from 77 K to 300 K using a microcircuit fabricated on a diamond anvil cell (DAC). Water chemically ionization mainly contributes to electrical conduction in liquid state, which is in accord with the results obtained under dynamic pressure. Energy band theory of liquid water was used to understand effect of static pressure on electrical conduction of water. The electric conductivity of H2O decreased discontinuously by four orders of magnitude at 0.7–0.96 GPa, indicating water frozen at this P–T condition. Correspondingly, the conduction of H2O in solid state is determined by arrangement and bending of H-bond in ice VI and ice VII. Based on Jaccard theory, we have concluded that the charge carriers of ice are already existing ions and Bjerrum defects.

Gallium arsenide (GaAs) exhibits outstanding electron transport properties that are useful for many electronics industries. High pressure is a powerful way to induce dramatic electrical conductivity changes in materials. Herein, we investigated the electrical transport properties of GaAs under high pressure up to 25.0 GPa by in situ alternating-current (AC) impedance spectroscopy measurements, temperature dependent electrical resistivity measurements, and first-principles calculations. The experimental results showed that GaAs undergoes a semiconductor to metal transition at approximately 12.0 GPa, which corresponds to the pressure-induced phase transition of GaAs from a zinc-blende to a Cmcm structure. In addition, the dramatic changes in electrical transport parameters, such as the bulk resistance, relaxation frequency, and resistivity, were observed at 12.0 GPa. First-principles calculations show that the energy bands crossed the Fermi level for the Cmcm phase, at the same time electronic localization began to decreases at 12.0 GPa, leading to the metallization of GaAs.

We present a novel technique for controlling oxygen fugacity, which is broadly used to in-situ measure the electrical conductivities in minerals and rocks during diamond anvil cell experiments. The electrical conductivities of olivine are determined under controlled oxygen fugacity conditions (Mo–MoO2) at pressures up to 4.0 GPa and temperatures up to 873 K. The advantages of this new technique enable the measuring of the activation enthalpy, activation energy, and activation bulk volume in the Arrhenius relationship. This provides an improved understanding of the mechanism of conduction in olivine. Electrical conduction in olivine is best explained by small polaron movement, given the oxygen fugacity-dependent variations in conductivity.

The structure and electrical transport properties of molybdenum dioxide (MoO2) under high pressure were studied by X-ray diffraction, in situ Hall effect and variable temperature electrical resistivity measurements. The X-ray diffraction measurements revealed two structural phase transitions at 14.3 GPa and 22.6 GPa, and unusual changes in the electrical parameters with pressure were attributed to these transitions. The internal conduction mechanism of MoO2 under compression was explained by the Hall mobility and charge carrier concentration. The abnormal changes in the temperature coefficient reflected changes in the lattice thermal vibration trend. MoO2 keeps a low temperature coefficient up to 34.4 GPa, showing good stable electrical transport performance. The results of this study will help guide the future development of pressure-modulated devices based on MoO2.

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.

In situ Hall effect measurement of HgSe was conducted up to 31 GPa. It was found that the carrier parameters changed discontinuously at each phase transition. The resistivity variation under compression was described by the carrier parameters. The decrease in cinnabar mobility indicates that the interaction between the helical chains becomes stronger under pressure. Combining the results of experiment and theory calculation, it can be concluded that in the phase transition process from zinc blende through cinnabar to rock salt, if Hg atom is mainly displaced , hole will be generated and its concentration will increase; on the contrary, if Se atom is mainly displaced, electrons will be generated and their concentration will increase.

The structural and electrical transport behaviors of CuInS2 nanocrystals under high pressure have been investigated using angle dispersive X-ray diffraction (ADXRD), in situ Hall effect, and temperature-dependent electrical resistivity measurements. A pressure-induced irreversible structure transformation from tetragonal phase to cubic phase has been confirmed at 10.9 GPa, which has noticeably elevated the transition pressure (1.4 GPa) in nanocrystals compared to the bulk CuInS2. In addition, the pressure-dependent electrical transport parameters, such as Hall coefficient, Hall mobility, carrier concentration, and electrical resistivity, all show dramatic changes around 11 GPa, implying that the pressure-induced structure transformation of CuInS2 can bring about a series of changes in the carrier transport properties. Specifically, the sign of the Hall coefficient is changed from negative to positive around 12 GPa, implying that CuInS2 undergoes a carrier-type inversion from n- to p-type. In addition, the...

With in situ electrical resistivity and Hall effect measurement, electrical transportation property and charge carrier behavior of ZnSe were investigated under high pressure using a diamond anvil cell (DAC). The electrical resistivity changed discontinuously at 7.7 and 11.9 GPa, corresponding to the phase transitions of ZnSe. In the pressure interval of 7.7–11.9 GPa, the electrical resistivity changed continuously, indicating the existence of the intermediate phase between the zinc blende and rock salt phases. The difference of carrier characteristic between the intermediate and rock salt phases can also suggest the existence of the intermediate phase. For the intermediate phase, the increase in electrical resistivity is from the decrease in mobility. While for the rock salt phase, the increase in charge carrier concentration leads to the decrease in electrical resistivity.

High-pressure electrical transport properties of VO2 have been investigated by in situ resistivity, Hall-effect, and temperature dependence of resistivity measurements. The electrical transport parameters including resistivity, Hall coefficient, carrier concentration, and mobility varies significantly around 10.4 GPa, which can be attributed to the isostructural phase transition of VO2. Temperature dependence of resistivity indicates that the phase transition is a semiconductor-to-semiconductor transformation, not the pressure-induced metallization as previously reported by Raman and IR experiment observations. The dramatic increase of activation energy at 10.4 GPa indicates an increasingly insulating behavior of VO2 accompanied with the isostructural phase transition. The electrical transport properties, especially the carries transportation under compression open up a new possible basis for optimizing the performance of VO2 based applications under ambient or extreme conditions.

We report on the intriguing structural and electrical transport properties of compressed InN. Pronounced anomalies of the resistivity, Hall coefficient, electron concentration, and mobility are observed at ∼11.5 GPa, accompanied by a wurtzite-rocksalt structural transition confirmed using high-pressure XRD measurements and first-principles calculations. The pressure-tuned electrical properties of wurtzite and rocksalt InN are also studied, respectively. Particularly, compression pressure significantly decreases the electron concentration of rocksalt InN by two orders of magnitude and increases the mobility by ten fold. The obvious variations in electrical parameters can be rationalized by our band structure simulations, which reveal a direct-indirect energy crossover at 10 GPa, followed by the rapidly increasing patterns of the energy gap with a pressure coefficient of 33 meV GPa-1. Moreover the electron effective mass and energy gap are found to well satisfy with the k·p model. Definite correlation between the structural change and the electrical transport properties should shed a new light on building InN-based applications in the future.

The high-pressure metallization and electrical transport behaviors of GaSb were systematically investigated using in situ temperature-dependent electrical resistivity measurements, Hall effect measurements, transmission electron microscopy analysis, and first-principles calculations. The temperature-dependent resistivity measurements revealed pressure-induced metallization of GaSb at approximately 7.0 GPa, which corresponds to a structural phase transition from F-43m to Imma. In addition, the activation energies for the conductivity and Hall effect measurements indicated that GaSb undergoes a carrier-type inversion (p-type to n-type) at approximately 4.5 GPa before metallization. The first-principles calculations also revealed that GaSb undergoes a phase transition from F-43m to Imma at 7.0 GPa and explained the carrier-type inversion at approximately 4.5 GPa. Finally, transmission electron microscopy analysis revealed the effect of the interface on the electrical transport behavior of a small-resistance GaSb sample and explained the discontinuous change of resistivity after metallization. Under high pressure, GaSb undergoes grain refinement, the number of interfaces increases, and carrier transport becomes more difficult, increasing the electrical resistivity.

The structural and electrical properties of CaB4 under high pressure have been studied by angle dispersive X-ray diffraction, in situ Hall effect measurement, and first-principles calculations. An abnormal change of the c/a ratio was observed around 12 GPa in the experiments and backed by theory calculation, indicating an isostructural phase transition. Unlike other materials, the analysis of the electronic and phonon band structure doesn't reveal an electronic topological transition or phonon softening supporting this phenomenon. While the subtle changes in the electron band dispersions at the Fermi surface shown by the measurements of the resistivity, Hall coefficient, carrier concentration, mobility, and temperature-dependent resistivity can explain the anomaly in the c/a ratio. The study of electrical transport properties provides strong support for the occurrence of the isostructural phase transition in CaB4.

A novel high pressure Seebeck coefficient ( S ) measurement method has been used for bulk-skutterudite CoSb 3 . The Seebeck coefficient and electrical resistivity ( ρ ) of CoSb 3 were measured in the pressure range of 0–25 GPa. The Seebeck coefficient of CoSb 3 increases very fast with the increase of the pressure and reaches a maximum value of 375 μV/K. A considerable pressure induced improvement of the thermoelectric efficiency ( α ) is found at about 10 GPa. A direct band gap change to indirect band gap with increasing pressure was found for CoSb 3 and both S and ρ abruptly rise due to the opening of the energy gap. These results indicate that the thermoelectric properties of CoSb 3 could be effectively improved under certain pressures.

Raman spectroscopy, synchrotron angle-dispersive X-ray diffraction (ADXRD), first-principles calculations, and electrical resistivity measurements were carried out under high pressure to investigate the structural stability and electrical transport properties of metavanadate MgV2O6. The results have revealed the coordination change of vanadium ions (from 5+1 to 6) at around 4 GPa. In addition, a pressure-induced structure transformation from the C2/m phase to the C2 phase in MgV2O6 was detected above 20 GPa, and both phases coexisted up to the highest pressure. This structural phase transition was induced by the enhanced distortions of MgO6 octahedra and VO6 octahedra under high pressure. Furthermore, the electrical resistivity decreased with pressure but exhibited different slope for these two phases, indicating that the pressure-induced structural phase transitions of MgV2O6 was also accompanied by the obvious changes in its electrical transport behavior.

Diamond negatively charged nitrogen-vacancy (NV–) centers provide an opportunity for the measurement of the Meissner effect on extremely small samples in a diamond anvil cell (DAC) due to their high sensitivity in detecting the tiny change of magnetic field. We report on the variation of magnetic field distribution in a DAC as a sample transforms from normal to superconducting state by using finite element analysis. The results show that the magnetic flux density has the largest change on the sidewall of the sample, where NV– centers can detect the strongest signal variation of the magnetic field. In addition, we study the effect of magnetic coil placement on the magnetic field variation. It is found that the optimal position for the coil to generate the greatest change in magnetic field strength is at the place as close to the sample as possible.

The high-pressure electrical transport behavior of microcrystalline tungsten trioxides (WO3) was investigated by direct current electrical resistivity measurement and alternate current impedance spectrum techniques in a diamond anvil cell up to 35.5 GPa. Discontinuous changes of electrical resistivity occurred during the pressure induced structure phase transitions at 1.8, 21.2, and 30.4 GPa. The irreversible resistivity reveals that the structure phase transition is not reversible. In addition, the abnormal changes of bulk resistance and transport activation energy at about 3 and 10 GPa are related to the isostructural phase transition reported by previous Raman study. The temperature induced resistivity change indicates that WO3 is a semiconductor from ambient pressure to 25.3 GPa.

We carried out the accurate in situ Hall-effect measurements, the temperature dependence of electrical resistivity measurements and the first-principles calculations in SnO under high pressure. The...

Carrier behavior of CaB 6 powders was investigated by using in situ Hall effect measurement under high pressure and resistivity property was detected under low temperature with microcircuit fabricated on diamond anvil cell. Carrier behavior was analyzed by carrier concentration and Hall coefficient. The variation of defects in the grain boundary region lead the carrier concentration and the Hall coefficient in the decompression process are much lower than that in the compression process. The resistivity depended temperature at various pressures indicated that CaB 6 has a typical metallic behavior under high pressure.

The resistivity of ZnTe powders has been measured under high pressure up to 29 GPa and temperature ranged from 303.0 to 403.0 K by using a diamond anvil cell. The resistivity changed discontinuously at 9.5 and 13.1 GPa, corresponding to the phase transitions from zinc blende to cinnabar and then to Cmcm structure. The decrease of activation energy by one order of magnitude indicates the deep-to-shallow transition of acceptor levels at 6.7 GPa. The resistivity decreases with temperature increasing indicates the semiconducting character of ZnTe in zinc blende phase. The resistivity of ZnTe in cinnabar phase decreases slightly with temperature increasing, indicating that this phase is a narrow-gap semiconductor. After 19.1 GPa, the resistivity increases with temperature increasing, indicating the metallic character of Cmcm phase. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

An investigation of TiS2 electronic structure was conducted using a first-principles calculation based on density functional theory (DFT). The results showed that there is indirect S3p -Ti3d band overlaps in ground electronic structure of TiS2. The measurements of resistivity and Hall effect were used to characterize the electrical transport behavior of TiS2 under ground state. Both theoretical and experimental results supported that TiS2 within ground state is a semimetal (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The electrical transport properties of indium trisulfide (In2S3) under high pressure were investigated using the in situ Hall-effect and temperature dependent resistivity measurements. Resistivity, Hall coefficient, carrier concentration, and mobility were obtained at pressures up to 41.6 GPa. Pressure induced metallization of In2S3 occurred at approximately 6.8 GPa. This was determined by measuring temperature dependent resistivity. The metallization transition was also determined from compression electrical parameters, and the decompression electrical parameters indicated that the metallization was a reversible transition. The main cause of the sharp decline in resistivity was the increase in carrier concentration at 6.8 GPa. Superconductivity was not observed at the pressures (up to 32.5 GPa) and temperatures (100–300 K) used in the experiment.

A pressure induced semiconductor-semimetal phase transition on tungsten diselenide has been studied using in situ electrical resistivity measurement and first-principles calculation under high pressure. The experimental results indicate that the phase transition takes place at 38.1 GPa. The first-principles calculations performed by CASTEP code based on the density functional theory illustrate that the indirect band gap of WSe2 vanishes at 35 GPa, which results in an isostructural phase transition from semiconductor to semimetal in WSe2. According to the pressure dependence of partial density of states, the semimetallic character of WSe2 is mainly caused by W−Se covalent bonding rather than van der Waals bonding.

A simple method to prepare thermally reduced graphene oxide/polymer composites was developed to enhance the electrical conductivity of the polymer. Graphene oxide sheets were coated onto the surfaces of poly(vinylidene fluoride) powders and then hot pressed at 200 °C to form composites with a segregated structure. After hot-pressing, the thermally reduced graphene oxide sheets were located in the interstices among the polymer domains and formed a two-dimensional conductive network. The resulting composites exhibited excellent electrical conductivity and a low percolation threshold (0.105 vol.%).

A theoretical investigation on the structural stabilities and electronic properties of HgTe under high pressure was conducted using first principles based on density functional theory. Our results demonstrate that the sequence of the pressure-induced phase transitions of HgTe is from the zinc blende, to cinnabar, rocksalt, orthorhombic, and CsCl-type structures. The pressure effects on the optical properties were discussed and compared with previous calculations and experimental data whenever available.

The film-fabricating technology has been used to prepare the microcircuit on diamond anvil for resistivity measurement under extreme conditions. We chose molybdenum as the electrode material and alumina as the insulator and protective material. The sample thickness measurement was conducted and then the resistivity correction was performed under different thickness. The experimental error was proved to be less than 10%. By mounting alumina film between diamond anvil and microcircuit, high-temperature performance of a laser heating diamond anvil cell was improved obviously, which is available for in situ resistivity measurement under high pressure and high temperature. Meanwhile, the impedance spectroscopy of a powdered semiconductor sample was detected. Through the impedance arcs obtained, the grain boundary contribution to the resistivity can be well distinguished.

Using a microcircuit fabricated on a diamond anvil cell, in situ conductivity measurements on nanophase (NP) γ-Fe2O3 are obtained under high pressure. For NP γ-Fe2O3, the abrupt increase in electrical conductivity occurs at a pressure of 21.3 GPa, corresponding to a transition from maghemite to hematite. Above 26.4 GPa, conductivity increases smoothly with increasing pressure. No distinct abnormal change is observed during decompression, indicating that transformation is irreversible. The temperature-dependence of the conductivity of NP γ-Fe2O3 was investigated at several pressures, indicating the electrical conductivity of the sample increases with increasing pressure and temperature, and that a remarkable phenomenon of discontinuity occurs at 400 K. The abnormal change is attributed to the electronic phase transitions of NP γ-Fe2O3 due to the variation of inherent cation vacancies. Besides, the temperature-dependence of the electrical conductivity displays semiconductor-like behavior before 33.0 GPa.

In situ electrical resistivity measurement of powdered Mg2Si has been performed in a diamond anvil cell up to 25.4 GPa. At about 22.2 GPa, Mg2Si underwent a pressure-induced semiconductor–metal phase transition that took place in the Ni2In-type structure rather than the anti-fluorite structure predicted theoretically. The other phases (anti-fluorite and anti-cotunnite) belong to the semiconductor phase.

Ohmic electrodes in the form of n-type (Si-doped) cubic boron nitride (c-BN) bulk crystals were fabricated by utilizing a covering technique, depositing Ti(10 nm)/Mo/(20 nm)/Pt–Au(200 nm) ohmic contact metal on both the sides of the c-BN substrate. The size of the specimen electrode was 100 × 100 μ m2 on one side and 300 × 300 μ m2 on the other side. Measurements on the specimen were made using a specially made device. Linear current–voltage characteristics were obtained. It is considered that the contact between the Ti-and Si-doped c-BN was ohmic.

The four-point method is widely used to determine the electrical resistivity of materials. Using finite element analysis (FEA) to simulate the distribution of the steady current field in sample, we illuminate the electrode factor on accurate resistivity measurement under high pressure in a diamond anvil cell (DAC). The result reveals that the measurement error is close dependent on the size and the resistivity of electrode. In order to minimize the measurement errors and improve the measurement accuracy, reducing the electrode size and increasing the electrode resistivity is a better choice. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The electrical transport properties and phase transitions of CdS were investigated under high pressure and moderate temperature by a microcircuit fabricated on diamond anvil cell. Around 2.0 GPa the resistivity of CdS dropped two orders of magnitude sharply, implying a beginning of a phase transition from wurtzite phase to rock-salt phase. At about 4.0 GPa the resistivity appeared a local minimum indicating an end of the transition to rock-salt phase. A local maximum of the resistivity was observed at about 8.0 GPa, which was attributed to the changeover of indirect gap from ΣνXc to LνXc. Furthermore, two inflexions of resistivity were observed at about 16.0 GPa and 21.0 GPa, which predicting that the indirect band gap of CdS may further change at these pressures. In the range of experimental pressure and temperature CdS still showed the electrical transport property of a semiconductor. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In situ electrical resistivity measurement of CdSe was performed under high pressure and moderate temperature using a diamond anvil cell equipped with a microcircuit. With the pressure increasing, a sharp drop in resistivity of over two orders of magnitude was observed at about 2.6 GPa, it was caused by the transition to the rock-salt CdSe. After that, the resistivity decreased linearly with pressure. However, in different pressure range, the decreasing degree was obviously different. This attributed to the different electron structures. By fitting to the curve of pressure dependence of resistivity in different pressure range, the relationship of the band gap to pressure was given and the metallization pressure was speculated to be in the range of 70–100 GPa. The temperature dependence of resistivity showed that in the experimental temperature and pressure range the resistivity had a positive temperature coefficient.

The phase diagram of water near the ice VI–ice VII–liquid triple point and electrical transport properties of these ices have been studied by in situ electrical conductivity measurements in a diamond anvil cell. The obtained phase boundary between ices VI and VII and the melting curve for these ices are in accord with most previous results. The different properties and amount of orientational defects in ice VI and ice VII are associated with abrupt changes in conductivity when a phase transition from ice VI to ice VII occurs. The electrical transport mechanisms of these two ice polymorphs can be understood in terms of the conduction of the already existing ions and Bjerrum defects.

The thermal stability and electronic transport properties of polyamorphous ${\text{Ce}}_{75}{\text{Al}}_{25}$ metallic glass (MG) have been investigated using in situ high-pressure, high-temperature, energy-dispersive synchrotron x-ray diffraction and in situ high-pressure and low-temperature, four-probe resistance measurements. The results are compared with the properties of ${\text{La}}_{75}{\text{Al}}_{25}$ MG. The pressure dependence of the crystallization temperature and resistance of the ${\text{Ce}}_{75}{\text{Al}}_{25}$ MG exhibited turning points at the polyamorphic transition pressure, 1.5 GPa, and they clearly presented different behaviors below and above 1.5 GPa. In contrast, no turning points were observed in the ${\text{La}}_{75}{\text{Al}}_{25}$ MG (La has no $4f$ electron). Additionally, the pressure-tuned temperature coefficient of resistance of the ${\text{Ce}}_{75}{\text{Al}}_{25}$ MG was observed. These results revealed switchable properties in the polyamorphous ${\text{Ce}}_{75}{\text{Al}}_{25}$ MG that are linked with $4f$ electron delocalization.

The electrical transport properties of Mg2Ge under high pressure were studied with the in situ temperature-dependent resistivity and Hall-effect measurements. The theoretically predicted metallization of Mg2Ge was definitely found around 7.4 GPa by the temperature-dependent resistivity measurement. Other two pressure-induced structural phase transitions were also reflected by the measurements. Hall-effect measurement showed that the dominant charge carrier in the metallic Mg2Ge was hole, indicating the “bad metal” nature of Mg2Ge. The Hall mobility and charge carrier concentration results pointed out that the electrical transport behavior in the antifluorite phase was controlled by the increase quantity of drifting electrons under high pressure, but in both anticotunnite and Ni2In-type phases it was governed by the Hall mobility.

Using a diamond anvil cell, the evolutions of alternate-current impedance spectra and direct- current resistivity in B12As2 have been investigated up to 51.9 GPa. The results provide evidence for the existence of grain and grain boundary effects that are separated in the frequency region. The grain boundary resistance shows a relatively smaller contribution to the total resistance above 16.8 GPa. By using the double-Schottky barrier model, the space charge potential was obtained. A pressure-induced inversion of charge defect concentration in the space charge layer was found at 20.7 GPa. The high-temperature resistivity measurements indicate that the transport activation energy is determined by defect energy levels in the band gap.

Pressure-induced electrical transport properties of Bi2Se3, including Hall coefficient, carrier concentration, mobility, and electrical resistivity, have been investigated under pressure up to 29.8 GPa by in situ Hall-effect measurements. The results indicate that the structural and electronic phase transitions of Bi2Se3 induce discontinuous changes in these electrical parameters. The significant anomaly in Hall coefficient at 5 GPa reveals an electronic topological transition deriving from the topological change of the band extremum (Van Hove singularity). Additionally, electrical resistivity measurements under variable temperatures show that the insulating state of Bi2Se3 becomes increasingly stable with an increase of pressure below 9.7 GPa. But above 9.7 GPa, Bi2Se3 enters into a fully metallic state. As the metallization occurs, the topological property of Bi2Se3 disappears.

The structural stability and electrical properties of AlB2-type MnB2 were studied based on high pressure angle-dispersive x-ray diffraction, in situ electrical resistivity measured in a diamond anvil cell (DAC) and first-principles calculations under high pressure. The x-ray diffraction results show that the structure of AlB2-type MnB2 remains stable up to 42.6 GPa. From the equation of state of MnB2, we obtained a bulk modulus value of 169.9±3.7 GPa with a fixed pressure derivative of 4, which indicates that AlB2-type MnB2 is a hard and incompressible material. The electrical resistance undergoes a transition at about 19.3 GPa, which can be explained by a transition of manganese 3d electrons from localization to delocalization under high pressure.

The in situ electrical resistance and transport activation energies of solid C60 fullerene have been measured under high pressure up to 25 GPa in the temperature range of 300?423 K by using a designed diamond anvil cell. In the experiment, four parts of boron-doped diamond films fabricated on one anvil were used as electrical measurement probes and a W?Ta thin film thermocouple which was integrated on the other diamond anvil was used to measure the temperature. The current results indicate that the measured high-pressure resistances are bigger than those reported before at the same pressure and there is no pressure-independent resistance increase before 8 GPa. From the temperature dependence of the resistivity, the C60 behaviors as a semiconductor and the activation energies of the cubic C60 fullerene are 0.49, 0.43, and 0.36 eV at 13, 15, and 19 GPa, respectively.

The ability to probe charge carriers transport behavior under pressure has the potential to unlock many key questions in photoconduction, resistive switch, solid ion transport, etc. We utilize the Hall-effect measurements and the first-principles calculations on Ag2S to reveal the pressure induced changes in its electrical transport parameters. Beyond 7.8 GPa, the donor level in modified monoclinic phase moves away from the conduction-band minimum as the pressure increases, and then Ag2S changes its dominant majority carriers from electrons to holes around 13.5 GPa. Additionally, increasing the pressure makes the electrical resistivity of Ag2S trend to decrease with temperature. At 15.8 GPa, Ag2S undergoes a transformation from metallic-like conduction to semiconductor conduction. These results are beneficial to achieve unique properties of high-pressure phases and develop Ag2S applications under ambient or extreme conditions.

Accurate high pressure in situ Hall-effect and temperature dependent electrical resistivity measurements have been carried out on Bi2Te3, a topological insulator. The pressure dependent electrical resistivity, Hall coefficient, carrier concentration, and mobility show the abnormal inflection points at 8, 12, and 17.8 GPa, indicating that the pressure-induced structural phase transitions of Bi2Te3 can result in a series of changes in the carrier transport behavior. In addition, the Hall coefficient shows a significant discontinuous change at 4 GPa, which is caused by the electronic topological transition. A sign inversion of Hall coefficient from positive to negative is found around 8 GPa. Furthermore, the temperature dependent electrical resistivity shows that the sample undergoes a semiconductor-to-metal conversion around 9.2 GPa, indicating that the insulating gap of Bi2Te3 becomes closed at this pressure. As the metallization occurs in the sample, the topological property of Bi2Te3 disappears.

We report a quantitative analysis of by-pass current effect on the accuracy of resistivity measurement in a diamond anvil cell. Due to the by-pass current, the sample resistivity calculated by the van der Pauw method is obviously smaller than the actual value and the problem becomes more serious for a high-resistivity sample. For the consideration of high accuracy of resistivity measurement, a method is presented that the inside wall of the sample chamber should be covered by a polymethylmethane layer. With this highly insulating layer, the by-pass current is effectively prevented and the current density distribution inside the sample is very close to the ideal case.

In situ electrical resistivity measurement of graphite compressed in two different pressure cycles in one single experiment at room temperature in the pressure range of 0-34 GPa has been reported. The electrical results indicate that less superhard graphite formed in the second cycle than in the first one. Our experiments identify the phase transition of the graphite at 15.1 and 17.9 GPa for the first and second pressure cycles, respectively, and explain why the superhard post-graphite cannot indent diamond films. The scanning electron microscopy of the graphite powders before and after pressure annealing helps to support the conclusion that both the phase transitions and the formation of superhard post-graphite are sensitive to the grain size of the initial graphite sample under high pressure.

The evolutions of alternate current (AC) impedance spectra, direct current (DC) resistivity and Raman spectra in polycrystalline BaTeO3 have been investigated at high pressures. The abrupt changes observed in electrical transport measurements at 12.73 GPa indicated a kind of phase transition, which was confirmed to be related to the structural phase transition by high-pressure Raman scattering experiments. From the impedance spectra, both grain resistance and grain boundary resistance decreased with increasing pressure, and two clear discontinuities occurred at 12.73 GPa and 17.47 GPa, respectively. After the transition, the contribution to the total resistance by the grain boundary effect dramatically declined and the grain resistance began to play a dominant role. Besides, the electric polarization process was changed due to the phase transition, and it led to a significant rise of the relaxation frequency. From the decompression impedance spectra, it could be clearly seen that the phase transition was irreversible, and it was probably attributed to the irreversible change of the grain boundary for the two different crystal structures. By varying the temperature, the transport behaviour at high pressure was still of semiconductor type but with different slopes for the ambient and high-pressure phases. High-pressure Raman measurements revealed an irreversible structural phase transition occurring above 15 GPa, and the results were consistent with the observed changes of the electrical properties at high pressures.

The electrical transport behavior of SnS under high pressure has been investigated by the temperature dependence of electrical resistivity measurement, the in situ Hall-effect measurement, and the first-principle calculation. The experimental results show that SnS undergoes a semiconductor to semimetal transition at ∼10.3 GPa, and this transition is further substantiated by the band-structure calculation. The total and partial density of states predict that the semimetal character of SnS is attributed to the enhanced coupling of Sn-5s, Sn-5p, and S-3p states with application of pressure. In addition, dramatic changes in electrical transport parameters such as the electrical resistivity, the carrier concentration, and the carrier mobility are observed at 12.6 GPa, which are correlated to the pressure-induced Pnma-Cmcm structural phase transition.

In situ impedance spectroscopy of nanophase and bulk γ-Fe2O3 were performed using a fabricated microcircuit on a diamond anvil cell. The results provide evidence for the existence of grain and grain boundary effects that are separated in the frequency region. The analysis establishes that the grain boundary conductance of nanocrystalline γ-Fe2O3 is higher than that of the bulk material. Both the grain and grain boundary resistances of γ-Fe2O3 smoothly decrease with pressure. A remarkable phenomenon of discontinuity appears at 7.4 GPa for the nanocrystals. This critical value is interpreted by the space charge model and changes in charge densities between the grain boundaries. The impedance analysis further reveals that the relaxation frequency gradually increased with the effect of pressure and the γ-Fe2O3 samples’ impedance imaginary part exhibits a typical capacitive characteristic. Within the given pressure range, the relaxation frequency of γ-Fe2O3 follows the Arrhenius behavior. This result was attri...