Search

In this work, we presented investigations on the microstructural, magnetic, dielectric and conductive properties of (1-x)Na0.5Bi0.5TiO3-xCoFe2O4 (NBT-CFO) composite ceramics (x = 0.1, 0.2, 0.3 and 0.5). The composites were synthesized using the in-situ sol-gel method, and the resulting CFO/NBT embedded structure was observed in the 0.9NBT–0.1CFO sample. Rietveld refinement confirmed that the embedded structure had remarkable effects on the c/a ratios of NBT-CFO composites. Increasing normalization saturation magnetizations were observed with increasing of CFO contents, thus revealing the influence of phase interfaces on magnetism. Dielectric relaxation anomalies were observed on both temperature dependent permittivity and loss spectra. Two sets of anomalies in these spectra were caused by space charges on the phase interfaces of embedded CFO grains and the NBT matrix. The conduction behavior of ceramics with and without the embedded structure obeyed the double power law and Jonscher's fraction power law, respectively, which inferred a transformation from the AC to the DC conduction mechanism with the disappearance of the embedded structure.

Layered transition-metal trichalcogenides have become one of the research frontiers as two-dimensional magnets and candidate materials used for phase-change memory devices. Herein we report the high-pressure synchrotron X-ray diffraction and resistivity measurements on Cr2Ge2Te6 (CGT) single crystal by using diamond anvil cell techniques, which reveal a mixture of crystalline-to-crystalline and crystalline-to-amorphous transitions taking place concurrently at 18.3-29.2 GPa. The polymorphic transition could be interpreted by atomic layer reconstruction and the amorphization could be understood in connection with randomly flipping atoms into van der Waals gaps. The amorphous (AM) phase is quenchable to ambient conditions. The electrical resistance of CGT shows a bouncing point at ~ 18 GPa, consistent with the polymorphism phase transition. Interestingly, the high-pressure AM phase exhibits metallic resistance with the magnitude comparable to that of high-pressure crystalline phases, whereas the resistance of the AM phase at ambient pressure fails to exceed that of the crystalline phase, indicating that the AM phase of CGT appeared under high pressure is quite unique and similar behavior has never been observed in other phase-change materials. The results definitely would have significant implications for the design of new functional materials., 14 pages,7 figures, 2 tables, 1 supporting information. JPCC in press

Orthorhombic (space group: Pnma) Nb2P5 is a high-pressure phase that is quenchable to ambient pressure, which could viewed as the zigzag infinite P chain-inserted NbP2. We report herein the high-pressure crystal growth of Nb2P5 and the discovery of its superconducting transition at Tc ~ 2.6 K. The electrical resistivity, magnetization, and specific heat capacity measurements on the high-quality crystal unveiled a conventional type-II weakly coupled s-wave nature of the superconductivity, with the upper critical field Hc2(0) ~ 0.5 T, the electron-phonon coupling strength {\lambda}ep ~ 0.5 - 0.8, and the Ginzburg-Landau parameter \k{appa} ~ 100. The ab initio calculations on the electronic band structure unveiled nodal-line structures protected by different symmetries. The one caused by band inversion, for example, on the {\Gamma}-X and U-R paths of the Brillouin zone, likely could bring nontrivial topology and hence possible nontrivial surface state on the surface. The surface states on the (100), (010) and (110) surfaces were also calculated and discussed. The discovery of the phosphorus-rich Nb2P5 superconductor would be instructive for the design of more metal phosphides superconductors which might host unconventional superconductivity or potential technical applications., Comment: 8 pages,8 figures in the main text + 4 pages, 3 figures, 1 table in the supplementary information

We present extensive investigations of the crystallographic phase diagram and electronic properties of the Fe-based superconductor FeSe under extreme conditions (high pressure (HP) and low temperature (LT)) by synchrotron X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). An isostructural phase transition (Tetragonal (T) →high-pressure Tetragonal (Tʹ)) is discovered in FeSe at ∼2.8 GPa based on the axial ratio c/a with finer pressure step as observed in Fe-As-based superconductor such as EuFe2As2. We also find a pressure-induced Tʹ → MnP-type phase transition at 7.6 GPa in FeSe, which is consistent with the documented pressure-induced high-spin → low-spin transition (∼6–7 GPa). These results reveal the pressure-induced structural phase transition sequence in FeSe at room temperature to be T → Tʹ → Tʹ+MnP-type at pressures of 0–10.6 GPa, enriching the crystallographic phase diagram. The HPLT XRD data also indicate that a sluggish structural phase transition (Cmma → Pnma) begins at 7.5 GPa, and these two phases coexist up to 26.5 GPa. The HP X-ray absorption near-edge spectroscopy (XANES) measurement shows that Eo of Se experiences a pressure-induced shift to high energy, evidencing strongly charge transfer between Fe and Se under high pressure. Our results shed lights on the correlation between crystallographic/electronic structure and superconductivity in this material.

Topological semimetal, a novel state of quantum matter hosting exotic emergent quantum phenomena dictated by the nontrivial band topology, has emerged as a new frontier in condensed-matter physics. Very recently, the coexistence of triply degenerate points of band crossing and Weyl points near the Fermi level was theoretically predicted and experimentally identified in MoP. Via high-pressure electrical transport measurements, we report here the emergence of pressure-induced superconductivity in MoP with a critical transition temperature Tc of ca. 2.5 K at ca. 30 GPa. No structural phase transition is observed up to ca. 60 GPa via synchrotron X-ray diffraction study. Accordingly, the topologically nontrivial band protected by the crystal structure symmetries and superconductivity are expected to coexist at pressures above 30 GPa, consistent with density functional theory calculations. Thus, the pressurized MoP represents a promising candidate of topological superconductor. Our finding is expected to stimulate further exploitation of exotic emergent quantum phenomena in novel unconventional fermion system. Molybdenum phosphide becomes superconducting above 30 GPa, with a critical transition temperature of 2.5 K. A team led by Youguo Shi, Xiaoping Yang and Zhaorong Yang from the Chinese Academy of Sciences perform high-pressure electrical transport measurements in molybdenum phosphide, which is a recently proved topological material hosting exotic fermions. They apply pressure over 90 GPa and observe a superconducting transition above 30 GPa with a critical transition temperature of 2.5 K, which increases to 4 K at 95 GPa. Meanwhile, the compound crystalizes into a (tungsten carbide) WC-type structure where the topological properties are predicted from ambient pressure up to 60 GPa. They expect that topological fermions and superconductivity could coexist under a pressure between 30 GPa and 60 GPa, suggesting a possible candidate for topological superconductivity.

Manganese (Mn)-based antiperovskite structures (Mn3AX, where A and X represent the 3d transition-metal elements and N or C atoms, respectively) have attracted growing attention because of their novel electronic and magnetic properties. However, the lack of an effective approach to regulate the magnetic coupling in Mn3AX crystal structure, particularly in antiferromagnetic ground states, hinders their further design and applications. Herein, robust high-temperature ferrimagnetic order with a Curie temperature (TC) in the range of ∼390–420 K was successfully achieved in Mn3GaxNx (x = 0.5, 0.6, and 0.7) via composition-deficient engineering. A systematic investigation, including synchrotron X-ray diffraction, neutron powder diffraction, pair distribution function, X-ray absorption near-edge structure, magnetic characterization, and first-principles calculations, convincingly indicated that the redistribution of partial atoms in the antiferromagnetic ground state was responsible for the observed long-range magnetic order. These results not only provide a new perspective into the design and construction of high-temperature ferrimagnets based on the Mn3AX structure, but also open up a promising avenue for the further design of Mn3AX-based spintronic or other multifunctional devices.

We report the discovery of superconductivity in high-quality single crystals of transition-metal pnictides WP grown by chemical vapor transport method. Bulk superconductivity is observed at ${T}_{\text{c}}\ensuremath{\sim}0.8\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ at ambient pressure by electrical resistivity, ac magnetic susceptibility, and specific-heat measurements. The effects of magnetic field on the superconducting transitions are studied, leading to a large anisotropy parameter around 2 with the in-plane and out-of-plane upper critical fields of ${\ensuremath{\mu}}_{0}{H}_{\text{c}2,\ensuremath{\parallel}}=17.2\phantom{\rule{0.28em}{0ex}}\mathrm{mT}$ and ${\ensuremath{\mu}}_{0}{H}_{\text{c}2,\ensuremath{\perp}}=8.5\phantom{\rule{0.28em}{0ex}}\mathrm{mT}$, respectively. The low value of electron-phonon coupling estimated from the normal-state resistivity and specific-heat measurements suggest that WP is a weak-coupling BCS superconductor. Our finding demonstrates that WP is the first superconductor among $5d$--transition metal pnictides with MnP-type structure at ambient pressure, which will help in the search for new superconductors in transition-metal pnictides.

We systematically investigate the influence of high pressure on the electronic transport properties of layered ferromagnetic materials; in particular, those of ${\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$. Its crystal sustains a hexagonal phase under high pressures up to 25.9 GPa, while the Curie temperature decreases monotonously with the increasing pressure. By applying appropriate pressures, the experimentally measured anomalous Hall conductivity, ${\ensuremath{\sigma}}_{xy}^{A}$, can be efficiently controlled. Our theoretical study reveals that this finding can be attributed to the shift of the spin--orbit-coupling-induced splitting bands of Fe atoms. With loading compression, ${\ensuremath{\sigma}}_{xy}^{A}$ reaches its maximal value when the Fermi level lies inside the splitting bands and then attenuates when the splitting bands float above the Fermi level. Further compression leads to a prominent suppression of the magnetic moment, which is another physical cause of the decrease in ${\ensuremath{\sigma}}_{xy}^{A}$ at high pressure. These results indicate that the application of pressure is an effective approach in controlling the anomalous Hall conductivity of layered magnetic materials, which elucidates the physical mechanism of the large intrinsic anomalous Hall effect.

Recently, A2B3-type tetradymites have developed into a hot topic in physical and material research fields, where the A and B atoms represent V and VI group elements, respectively. In this study, in situ angle-dispersive X-ray diffraction measurements were performed on Bi2Te2Se, BiSbTeSe2, and Sb2Te2Se tetradymites under high pressure. Bi2Te2Se transforms from a layered rhombohedral structure (phase I) into 7-fold monoclinic (phase II) and body-centered tetragonal (phase IV) structures at about 8.0 and 14.3 GPa, respectively, without an 8-fold monoclinic structure (phase III) similar to that in Bi2Te3. Thus, the compression behavior of Bi2Te2Se is the same as that of Bi2Se3, which could also be obtained from first-principles calculations and in situ high-pressure electrical resistance measurements. Under high pressure, BiSbTeSe2 and Sb2Te2Se undergo similar structural phase transitions to Bi2Te2Se, which indicates that the compression process of tellurides can be modulated by doping Se in Te sites. According to these high-pressure investigations of A2B3-type tetradymites, the decrease of the B-site atomic radius shrinks the stable pressure range of phase III and expands that of phase II, whereas the decrease of the A-site atomic radius induces a different effect, i.e. expanding the stable pressure range of phase III and shrinking that of phase II. The influence of the atomic radius on the compression process of tetradymites is closely related to the chemical composition and the atom arrangement in the quintuple layer.

The high-pressure effect on valence fluctuation of the ThCr2Si2-type intermetallic compound EuNi2P2 has been investigated using in situ synchrotron Mossbauer spectroscopy (SMS). The isomer shift of 151Eu in EuNi2P2 increases monotonically with increasing pressure up to 50 GPa, suggesting a valence transition of the Eu from mixed toward trivalent. The synchrotron angle-dispersive X-ray diffraction (AD-XRD) experiment shows that EuNi2P2 remains in the tetragonal structure up to 32.5 GPa at room temperature. We propose that the evolutions of bonding distance with pressure have an obvious effect on the valence fluctuation.

The extremely large magnetoresistance (XMR) in nonmagnetic semimetals has inspired growing interest owing to both intriguing physics and potential applications. We report results of synchrotron X-ray diffraction (SXRD) and electrical transport measurements on TaAs2 under pressure up to ~ 37 GPa, which revealed an anisotropic compression of the unit cell, formation of unusual As-As bonds above 9.5 GPa, and enhancement of metallicity. Interestingly, the MR of TaAs2 under pressure changed gently, which at 1.7 GPa is 96.6% and at 36.6 GPa is still 36.7%. The almost robust MR under pressure could be related to the nearly stable electronic structure unveiled by the ab initio calculations. The discovery would expand the potential use of XMR even under high pressure., Comment: 15 pages, 7 figures including those in supporting information

The layered Cr2Ge2Te6 (CrGT) has captured vast attentions because it is capable of retaining the long-range ferromagnetic order even in its monolayer form, thus offering potential applications in spintronic devices. We report herein the Raman spectra of CrGT under pressure up to 35.1 GPa at room temperature. We observed a structural phase transition at ∼14 GPa as was revealed by a change in pressure derivate frequencies of the E g 3 and A g 1 modes. Both of the two peaks became broad and the A g 1 mode eventually disappeared with further increasing the pressure. Raman modes above 26.4 GPa could not be experimentally observed, suggesting that the CrGT was pressed into the amorphous phase. Interestingly, a pressure-induced soft phonon mode is identified in CrGT at ∼23 GPa by dynamical property calculation. The phonon instability at the Brillouin zone boundary T point is suggested as the driving force for the experimentally observed crystalline-to-crystalline phase transition at ∼ 18 GPa. The appearance of soft phonon might indicate the possibility of superconductivity induced by much higher pressure.

CrAs was observed to possess the bulk superconductivity under high-pressure conditions. To understand the superconducting mechanism and explore the correlation between the structure and superconductivity, the high-pressure structural evolution of CrAs was investigated using the angle-dispersive X-ray diffraction (XRD) method. The structure of CrAs remains stable up to 1.8 GPa, whereas the lattice parameters exhibit anomalous compression behaviors. With increasing pressure, the lattice parameters a and c both demonstrate a nonmonotonic change, and the lattice parameter b undergoes a rapid contraction at similar to 0.18-0.35 GPa, which suggests that a pressure-induced isostructural phase transition occurs in CrAs. Above the phase transition pressure, the axial compressibilities of CrAs present remarkable anisotropy. A schematic band model was used to address the anomalous compression behavior of CrAs. The present results shed light on the structural and related electronic responses to high pressure, which play a key role toward understanding the superconductivity of CrAs.

As a follow-up of our previous work on pressure-induced metallization of the 2H_{c}-MoS_{2} [Chi et al., Phys. Rev. Lett. 113, 036802 (2014)PRLTAO0031-900710.1103/PhysRevLett.113.036802], here we extend pressure beyond the megabar range to seek after superconductivity via electrical transport measurements. We found that superconductivity emerges in the 2H_{a}-MoS_{2} with an onset critical temperature T_{c} of ca. 3 K at ca. 90 GPa. Upon further increasing the pressure, T_{c} is rapidly enhanced beyond 10 K and stabilized at ca. 12 K over a wide pressure range up to 220 GPa. Synchrotron x-ray diffraction measurements evidenced no further structural phase transition, decomposition, and amorphization up to 155 GPa, implying an intrinsic superconductivity in the 2H_{a}-MoS_{2}. DFT calculations suggest that the emergence of pressure-induced superconductivity is intimately linked to the emergence of a new flat Fermi pocket in the electronic structure. Our finding represents an alternative strategy for achieving superconductivity in 2H-MoS_{2} in addition to chemical intercalation and electrostatic gating.

The 6H and 6M Ba 1− x Sr x RuO 3 at x ⩽0.6 with the normal and distorted hexagonal BaTiO 3 structures were synthesized by using high-pressure and high-temperature method. It is found that the unit cell volume deviates from Vegard's law between 0.3 and 0.4 for the solid solutions due to the increasing distortion degree of crystal structure. With the increasing x , the electrical resistivity at the same temperature is increasing. With the substitution of Sr for Ba ion, the 6H BaRuO 3 transforms to a Fermi-liquid metal at x =0.25 from the primal non-Fermi-liquid metal, and then becomes a semiconductor at low temperature when x is larger than 0.4.

The high-pressure phase of iridium-based compound Ba3CaIr2O9 was synthesized using high-pressure sintering. Being different from the distorted hexagonal BaTiO3 structure of the ambient Ba3CaIr2O9, the high-pressure phase crystals into the 1:2 B-site-ordered perovskite structure with the space group P-3m1 (Z=1). Through fitting the X-ray powder diffraction (XRD) data with Rietveld analysis, in which the obtained Rp, Rwp, and Rexp factors are 7.49%, 11.4%, and 4.82%, respectively, the lattice parameters are a=5.8296(1) A and c=7.1659(2) A. The atomic coordinates and the main interatomic distances and bond angles were also obtained. The relationship of electrical resistivity versus temperature shows that the high-pressure phase of Ba3CaIr2O9 is a semiconductor in the temperature range of 5–300 K. The measurement of temperature dependence of magnetic susceptibility indicates that it is paramagnetic.

The 6H BaRu{sub 1-x}Mn{sub x}O{sub 3} with the hexagonal BaTiO{sub 3} structure was synthesized using high-pressure sintering method. It is found that the lattice parameter deviates from Vegard's law at x=0.3 for the solid solutions due to the charge transfer effects at B-site. The substitution of Mn for Ru cations gives rise to the short-range magnetic ordering, due to the disordered arrangement of Ru and Mn cations. The compounds are weak ferromagnetic in the x range 0.05-0.40, with the maximal Curie temperature T{sub c} 175.2 K at x=0.10. They are of spin-glass-like magnetism at lower temperature at x{>=}0.1. With Mn doping, the 6H BaRuO{sub 3} transforms to a semiconductor from the primal metal at x=0.30. The resistance as a function of temperature below about 70 K follows the two-dimensional variable-range hopping conduction mechanism in BaRu{sub 0.50}Mn{sub 0.50}O{sub 3}. - Graphical abstract: The 6H BaRu{sub 1-x}Mn{sub x}O{sub 3} was synthesized using high-pressure sintering. The substitution of Mn for Ru cations gives rise to the short-range magnetic ordering, with the maximal Curie temperature T{sub c} 175.2 K at x=0.10. The compounds are of spin-class magnetism at lower temperature at x{>=}0.05. Mn doping results in the transition from the primal metal to semiconductormore » at x=0.30.« less

The single-phase 6H BaRuO{sub 3} with the hexagonal BaTiO{sub 3} structure was synthesized at 5 GPa and 1000 deg. C. Rietveld refinement of the powder X-ray data for the 6H phase resulted in the lattice parameters to be a=5.7127(1) A and c=14.0499(2) A; the average Ru-O distance and direct Ru-Ru distance in the Ru{sub 2}O{sub 9} dioctahedron being 1.992(6) and 2.5658(14) A, respectively. The electrical resistivity of the 6H BaRuO{sub 3} follows a relationship of {rho} versus T{sup 3/2} below 60.0 K, a signature deviation from the Fermi-liquid behavior. Both magnetic susceptibility and specific heat data indicate that the 6H BaRuO{sub 3} is an exchange-enhanced Pauli paramagnet due to the electron correlation effect with large Wilson ratio R{sub W} and Stoner enhancement factor. The comprehensive evolution of transport and magnetic properties from 9R across 4H to 6H BaRuO{sub 3} was discussed. - Graphical abstract: The single-phase 6H BaRuO{sub 3} was synthesized under high pressure high temperature. The electrical resistivity of 6H BaRuO{sub 3} follows a T{sup 3/2} relation below 60.0 K. Both magnetic susceptibility and specific heat data indicate that 6H BaRuO{sub 3} is an exchange-enhanced Pauli paramagnet with the large Wilson ratio and Stoner enhancement factor.

Raman spectroscopy and angle dispersive X-ray diffraction (XRD) experiments of bismuth selenide (Bi2Se3) have been carried out to pressures of 35.6 and 81.2 GPa, respectively, to explore its pressure-induced phase transformation. The experiments indicate that a progressive structural evolution occurs from an ambient rhombohedra phase (Space group (SG): R-3m) to monoclinic phase (SG: C2/m) and eventually to a high pressure body-centered tetragonal phase (SG: I4/mmm). Evidenced by our XRD data up to 81.2 GPa, the Bi2Se3 crystallizes into body-centered tetragonal structures rather than the recently reported disordered body-centered cubic (BCC) phase. Furthermore, first principles theoretical calculations favor the viewpoint that the I4/mmm phase Bi2Se3 can be stabilized under high pressure (>30 GPa). Remarkably, the Raman spectra of Bi2Se3 from this work (two independent runs) are still Raman active up to ~35 GPa. It is worthy to note that the disordered BCC phase at 27.8 GPa is not observed here. The remarkable difference in atomic radii of Bi and Se in Bi2Se3 may explain why Bi2Se3 shows different structural behavior than isocompounds Bi2Te3 and Sb2Te3.

Phase transition of solid-state materials is a fundamental research topic in condensed matter physics, materials science and geophysics. It has been well accepted and widely proven that isostructural compounds containing different cations undergo same pressure-induced phase transitions but at progressively lower pressures as the cation radii increases. However, we discovered that this conventional law reverses in the structural transitions in 122-type iron-based superconductors. In this report, a combined low temperature and high pressure X-ray diffraction (XRD) measurement has identified the phase transition curves among the tetragonal (T), orthorhombic (O) and the collapsed-tetragonal (cT) phases in the structural phase diagram of the iron-based superconductor AFe2As2 (A = Ca, Sr, Eu, and Ba). The cation radii dependence of the phase transition pressure (T → cT) shows an opposite trend in which the compounds with larger ambient radii cations have a higher transition pressure.

Here, we report comprehensive studies on the high-pressure structural and electrical transport properties of the layered transition metal chalcogenide (Cr2S3) up to 36.3 GPa. A structural phase transition was observed in the rhombohedral Cr2S3 near 16.5 GPa by the synchrotron angle dispersive X-ray diffraction measurement using a diamond anvil cell. Through in situ resistance measurement, the electric resistance value was detected to decrease by an order of three over the pressure range of 7–15 GPa coincided with the structural phase transition. Measurements on the temperature dependence of resistivity indicate that it is a semiconductor-to-metal transition in nature. The results were also confirmed by the electronic energy band calculations. Above results may shed a light on optimizing the performance of Cr2S3 based applications under extreme conditions.