Record-High T_{c} and Dome-Shaped Superconductivity in a Medium-Entropy Alloy TaNbHfZr under Pressure up to 160 GPa.

Superconductivity has been one of the focal points in medium and high-entropy alloys (MEAs-HEAs) since the discovery of the body-centered cubic (bcc) HEA superconductor in 2014. Until now, the superconducting transition temperature (T_{c}) of most MEA and HEA superconductors has not exceeded 10 K. Here, we report a TaNbHfZr bulk MEA superconductor crystallized in the BCC structure with a T_{c} of 15.3 K which set a new record. During compression, T_{c} follows a dome-shaped curve. It reaches a broad maximum of roughly 15 K at around 70 GPa before decreasing to 9.3 K at 157.2 GPa. First-principles calculations attribute the dome-shaped curve to two competing effects, that is, the enhancement of the logarithmically averaged characteristic phonon frequency ω_{log} and the simultaneous suppression of the electron-phonon coupling constant λ. Thus, TaNbHfZr MEA may have a promising future for studying the underlying quantum physics, as well as developing new applications under extreme conditions.

Rediscovering the intrinsic mechanical properties of bulk nanocrystalline indium arsenide.

Is the inverse Hall-Petch relation in ceramic systems the same as that in metal systems? The premise to explore this subject is the synthesis of a dense bulk nanocrystalline material with clean grain boundaries. By using the reciprocating pressure-induced phase transition (RPPT) technique, compact bulk nanocrystalline indium arsenide (InAs) has been synthesized from a single crystal in a single step, while its grain size is controlled by thermal annealing. The influence of macroscopic stress or surface states on the mechanical characterization has been successfully excluded by combining first-principles calculations and experiments. Unexpectedly, nanoindentation tests show a potential inverse Hall-Petch relation in the bulk InAs with a critical grain size (Dcri) of 35.93 nm in the experimental scope. Further molecular dynamics investigation confirms the existence of the inverse Hall-Petch relation in the bulk nanocrystalline InAs with Dcri = 20.14 nm for the defective polycrystalline structure, with its Dcri significantly affected by the intragranular-defect density. The experimental and theoretical conclusions comprehensively reveal the great potential of RPPT in the synthesis and characterization of compact bulk nanocrystalline materials, which provides a novel window to rediscover their intrinsic mechanical properties, for instance, the inverse Hall-Petch relation of bulk nanocrystalline InAs.

Squeezing indium arsenide single crystal to ultrafine nanostructured compact bulk.

Reciprocating pressure-induced phase transition (RPPT) has been proposed as a new approach to synthesize nanostructured bulk materials with clean grain boundary interfaces for structures that undergo reversible pressure-induced phase transitions. The modulation effects on grain size under different cycle numbers of RPPT for InAs were investigated and the initial single-crystal bulk, with a dimensional size of about 30 µm, was transformed into a nanostructure with an average grain size of 7 nm by the utilization of the in situ high-pressure diamond anvil cell (DAC) technique. To verify the DAC findings, compact nanostructured bulk InAs with grain sizes ranging from 2-20 nm (average = 8 nm) and large dimensions (3.2 mm × 3.2 mm × 0.5 mm) was successfully synthesized from single-crystal InAs using a large volume press (LVP). The smaller work function (3.86 eV) and larger bandgap energy (2.64 eV) of the compact nanostructured bulk InAs phase compared to those of single-crystal InAs demonstrated that the nanostructure affected the macroscopic properties of InAs. The findings confirm the feasibility of synthesizing nanostructured bulk materials via RPPT.

Pressure-Induced Phase Transition and Compression Properties of HfO2 Nanocrystals.

Nanoparticles exhibit unique properties due to their surface effects and small size, and their behavior at high pressures has attracted widespread attention in recent years. Herein, a series of in situ high-pressure X-ray diffraction measurements with a synchrotron radiation source and Raman scattering have been performed on HfO2 nanocrystals (NC-HfO2) with different grain sizes using a symmetric diamond anvil cell at ambient temperature. The experimental data reveal that the structural stability, phase transition behavior, and equation of state for HfO2 have an interesting size effect under high pressure. NC-HfO2 quenched to normal pressure is characterized by transmission electron microscopy to determine the changing behavior of grain size during phase transition. We found that the rotation of the nanocrystalline HfO2 grains causes a large strain, resulting in the retention of part of an orthorhombic I (OI) phase in the sample quenched to atmospheric pressure. Furthermore, the physical mechanism of the phase transition of NC-HfO2 under high pressure can be well explained by the first-principles calculations. The calculations demonstrate that NC-HfO2 has a strong surface effect, that is, the surface energy and surface stress can stabilize the structures. These studies may offer new insights into the understanding of the physical behavior of nanocrystal materials under high pressure and provide practical guidance for their realization in industrial applications.

Powder conductor for pressure calibration applied to large volume press under high pressure.

Pressure is the core of high-pressure science and technology, and the accuracy of pressure calibration is of much importance for high-pressure experiments and production. Although the pressure limit of the large volume press (LVP) continues to increase, there are no well solutions for in situ pressure calibration. In this study, using in situ high-pressure electrical performance measurement technology, two ideal calibration standard materials in powder conductors, cadmium phosphide (Cd3P2) and zinc telluride (ZnTe) with stable physical and chemical properties and obvious resistance change, are applied to pressure calibration in the LVP. In situ high-pressure synchrotron radiation x-ray diffraction was used to verify the phase transition pressure point of Cd3P2. The introduction of powder conductors for pressure calibration commits to establish a pressure system, which is safer, more stable to operate, and more accurate in experimental measurements for the LVP.

Elucidating the Phase Transformation and Metallization Behavior of Zinc Phosphide under High Pressure.

Among the family of II3V2-type compounds, zinc phosphide (Zn3P2) occupies a unique position. As one of the most promising semiconductors well-suited for photovoltaic applications, Zn3P2 has attracted considerable attention. The stability of its structure and properties are of great interest and importance for science and technology. Here, we systematically investigate the pressurized behavior of Zn3P2 using in situ synchrotron radiation angle-dispersive X-ray diffraction (ADXRD) and in situ electrical resistance measurement under high pressure. The ADXRD experiment shows that Zn3P2 undergoes an irreversible structural phase transition under high pressure, beginning at 11.0 GPa and being completed at ∼17.7 GPa. Consistently, the high-pressure electrical resistance measurement reveals a pressure-induced semiconductor-metal transition for Zn3P2 near 11.0 GPa. The kinetics of the phase transition is also studied using in situ electrical resistance measurement and can be well described by the classical Avrami model. What's more, the new high-pressure structure of Zn3P2 is refined to be orthorhombic with space group Pmmn; the lattice parameters and bulk modulus of this high-pressure phase are determined as a = 3.546 Å, b = 5.004 Å, c = 3.167 Å, and B0 = 126.3 GPa. Interestingly, we also predict a possible structural phase transformation of orthorhombic phase (Pmmn) to cubic phase (P4232) during the decompression process; this cubic Zn3P2 is metastable at ambient conditions. These experimental results reveal the unexpected high-pressure structural behaviors and electrical properties of Zn3P2, which could help to promote the further understanding and the future applications of Zn3P2 as well as other II3V2 compounds.

Synthesis and Phase Stability of the High-Entropy Carbide (Ti0.2Zr0.2Nb0.2Ta0.2Mo0.2)C under Extreme Conditions.

As a novel ultrahigh temperature ceramic, the stability of a high-entropy transition metal carbide under extreme conditions is of great concern to its application. Despite the intense research, the available high-pressure experimental results are few so far. Here, we synthesized the nanocrystalline (Ti0.2Zr0.2Nb0.2Ta0.2Mo0.2)C by a high-pressure solid-state reaction successfully. Meanwhile, synchrotron radiation X-ray diffraction experiments were carried out to explore the phase stability and mechanical response under high pressure. The single cubic B1 phase structure of the high-entropy carbide is retained under extreme hydrostatic pressure. An abnormal cubic-to-cubic phase transition was observed unexpectedly under nonhydrostatic compression. This result reflects the effect of the severe lattice distortion of the initial B1 phase high-entropy carbide and the shear strain caused by deviatoric stress under high nonhydrostatic pressure. The physical mechanism about electronic/magnetic characteristics behind findings is an interesting issue for future studies.

Insights into the Bond Behavior and Mechanical Properties of Hafnium Carbide under High Pressure and High Temperature.

Hafnium carbide (HfC) is a potential candidate of ultrahigh-temperature ceramics (UHTCs) and has attracted significantly widespread interest in recent years. Here, we have synthesized high-purity HfC samples with NaCl-type structure by using a high-pressure solid-solid reaction. The structural stability, equation of state, plastic deformation, yield strength, and bonding properties under high pressure are investigated by a series of in situ high-pressure synchrotron-radiation angle-dispersive X-ray diffraction experiments combined with first-principles calculations. The yield strength of HfC (∼18 GPa) is obtained from analyzing the plastic deformation behavior under high pressure. In addition, we have successfully prepared bulk HfC ceramics with high density using a high-pressure and high-temperature method. The synthesized sample possesses a desirable Vickers hardness of 24.2 GPa and an excellent fracture toughness of 5.0 MPa·m1/2. The present results offer insights into the achievable application of HfC ceramics under extreme conditions and provide a powerful guide for the further design and synthesis of other high-performance UHTCs.