A study of high pressure solid Te was carried out at room temperature using Raman spectroscopy and Density Functional Theory (DFT) calculations. The analysis of the P-dependence of the experi- mental phonon spectrum reveals the occurrence of phase transitions at 4 GPa and 8 GPa confirming the high-pressure scenario recently proposed. The effects of the incommensurate lattice modulation on the vibrational properties of Te is discussed. DFT calculations agree with present and previous experimental data and show the metallization process at 4 GPa being due to the development of charge-bridges between atoms belonging to adjacent chains. A first-principles study of the stability of the 4 GPa phase is reported and discussed also in the light of the insurgence of lattice modulation.
Recent progress in understanding the electronic band topology and emergent topological properties encourage us to reconsider the band structure of well-known materials including elemental substances. Controlling such a band topology by external field is of particular interest from both fundamental and technological view point. Here we report the pressure-induced topological phase transition from a semiconductor to a Weyl semimetal in elemental tellurium probed by transport measurements. Pressure variation of the periods of Shubnikov-de Haas oscillations, as well as oscillations phases, shows a…
Tellurium trioxide, TeO3, is the only example of a trioxide adopting at ambient conditions the VF3-type structure (a distorted variant of the cubic ReO3 structure). Here we present a combined experimental (Raman scattering) and theoretical (DFT modelling) study on the influence of high pressure (exceeding 100 GPa) on the phase stability of this compound. In experiment the ambient-pressure VF3-type structure (R-3c symmetry) is preserved up to 110 GPa. In contrast, calculations indicate that above 66 GPa the R-3c structure should transform to a YF3-type polymorph (Pnma symmetry) with the coordin…
The phonon dispersion and the electron-phonon interaction for the $β$-Po and the bcc high pressure phases of tellurium are computed with density-functional perturbation theory. Our calculations reproduce and explain the experimentally observed pressure dependence of the superconducting critical temperature (T$_{\rm c}$) and confirm the connection between the jump in T$_{\rm c}$ and the structural phase transition. The phonon contribution to the free energy is shown to be responsible for the difference in the structural transition pressure observed in low and room temperature experiments.
Observation of high-temperature superconductivity in sulfur hydrides at megabar pressures has generated an irresistible wave on searching for new superconductors in other compressed hydrogen-rich compounds. An immediate effort is towards exploration of the relevant candidate of tellurium hydrides, where tellurium is isoelectronic to sulfur but it has a heavier atomic mass and much weaker electronegativity. The hitherto unknown phase diagram of tellurium hydrides at high pressures was investigated by a first-principles swarm structure search. Four energetically stable and metallic stoichiometri…
Based on density functional perturbation theory, we have studied the vibrational properties of three crystalline phases of tellurium dioxide: paratellurite alpha-TeO2, tellurite beta-TeO2, and the new phase gamma-TeO2, recently identified experimentally. Calculated Raman and IR spectra are in good agreement with available experimental data. The vibrational spectra of alpha- and beta-TeO2 can be interpreted in terms of vibrations of TeO2 molecular units.
A room-temperature infrared spectroscopy study of elemental tellurium at pressures up to 8.44 GPa in the energy range 0.015-2 eV is reported. Optical signatures of the high-pressure polymorphs are investigated and compared to the results of density-functional band-structure calculations. A Drude peak is first seen in the optical conductivity around 3.5 GPa indicating a semiconductor-to-metal transition within trigonal Te-I. A sharp increase in the Drude spectral weight and dc-conductivity around 4.3 GPa signals the transformation toward the triclinic Te-II polymorph. An absorption peak around …
This study investigates the transport properties of a chiral elemental semiconductor tellurium (Te) under magnetic fields and pressure. Application of hydrostatic pressure reduces the resistivity of Te, while its temperature dependence remains semiconducting up to 4 GPa, contrary to recent theoretical and experimental studies. Application of higher pressure causes structural as well as semiconductor--metal transitions. The resulting metallic phase above 4 GPa exhibits superconductivity at 2 K along with a noticeable linear magnetoresistance effect. On the other hand, at ambient pressure, we id…
Employing high-pressure infrared spectroscopy we unveil the Weyl semimetal phase of elemental Te and its topological properties. The linear frequency dependence of the optical conductivity provides clear evidence for metallization of trigonal tellurium (Te-I) and the linear band dispersion above 3.0 GPa. This semimetallic Weyl phase can be tuned by increasing pressure further: a kink separates two linear regimes in the optical conductivity (at 3.7 GPa), a signature proposed for Type-II Weyl semimetals with tilted cones; this however reveals a different origin in trigonal tellurium. Our density…
We have computed second and third nonlinear optical susceptibilities of two crystalline bulk tellurium oxide polymorphs: $α$-TeO$_{2}$ (the most stable crystalline bulk phase) and $γ$-TeO$_{2}$ (the crystalline phase that ressembles the more to the glass phase. Third order nonlinear susceptibilities of the crystalline phases are two orders of magnitude larger than $α$-SiO$_{2}$ cristoballite, thus extending the experimental observations on glasses to the case of crystalline compounds. While the electronic lone pairs of Te contribute to those large values, a full explanation of the anisotropy o…
Superconductivity of Tellurium Polyhydride with Tc above 90K
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所凝聚态物理国家实验室）；School of Physics, University of Chinese Academy of Sciences（中国科学院大学物理学院）；Shanghai Key Laboratory of Material Frontiers Research in Extreme Environments (MFree), Shanghai Advanced Research in Physical Sciences (SHARPS)（上海极端环境材料前沿研究重点实验室（MFree），上海物理研究所先进物理科学中心）；Center for High Pressure Science & Technology Advanced Research (HPSTAR)（高压科学和技术先进研究中心（HPSTAR））；Department of Chemistry, Johann Wolfgang Goethe University（约翰·沃尔夫冈·歌德大学化学系）
We consider different structures, which can be obtained by polymerization of aromatic organic molecules under high pressures. These 2D and 3D covalently bonded organic polymers and their functionalization can pave the way to production of energy storage and conversion devices. High-pressure synthesis might serve as a useful hint for production of these structures and their functionalized analogs by means of wet chemical synthesis.
Two new crystalline compounds, pentazole (N_{5}H) and ammonium pentazolate (NH_{4})(N_{5}), both featuring cyclo-{\rm N_{5}^{-}} are discovered using first principles evolutionary search of the nitrogen-rich portion of the hydro-nitrogen binary phase diagram (N_{x}H_{y}, x\geqy) at high pressures. Both crystals consist of the pentazolate N_{5}^{-} anion and ammonium NH_{4}^{+} or hydrogen H^{+} cations. These two crystals are predicted to be thermodynamically stable at pressures above 30 GPa for (NH_{4})(N_{5}) and 50 GPa for pentazole N_{5}H. The chemical transformation of ammonium azide (NH_…
An approach combining the Rietveld method and pairs distribution function analysis was developed and can help to understand the effect of temperature and/or high-pressure on crystalline materials. It was applied to orthorhombic Ta2O5 compound, and the obtained results were compared with the experimental pairs distribution function reported in the literature, and an excellent agreement was reached. The approach permitted to simulate average pairs distribution functions GTa-Ta(R), GTa-O(R), and GO-O(R) and the average radial distribution functions RDFTa-Ta(R), RDFTa-O(R), and RDFO-O(R). The coor…
We report a comprehensive magneto-transport study on single crystalline p-type topological crystalline insulator (TCI) SnTe, across the cubic-to-rhombohedral (R3m) transition which occurs as a function of temperature. The electrical resistivity of a well-characterized SnTe crystal shows evidence for the cubic-to-rhombohedral structural transition at T$_s$ $\sim$64,K and a carrier density of $\sim$1.8$\times$10$^{20}$ at 77,K. As a function of applied magnetic field perpendicular to the (100) plane, SnTe exhibits a large unsaturated linear magnetoresistance (LMR) reaching a value of 42% at 5K a…
Germanium dioxide ($GeO_2$) is a chemical analogue of $SiO_2$. Furthermore, it is also to some extent a structural analogue, as the low and high-pressure short-range order (tetrahedral and octahedral) is the same. However, a number of differences exist. For example, the $GeO_2$ phase diagram exhibits a smaller number of polymorphs, and all three $GeO_2$ phases (crystalline, glass, liquid) have an increased sensitivity to pressure, undergoing pressure induced changes at much lower pressures than their equivalent $SiO_2$ analogues. In addition, differences exist in $GeO_2$ glass in the medium ra…
Given the unique optical properties of LiF, it is often used as an observation window in high-temperature and pressure experiments; and, hence, estimates of its transmission properties are necessary to interpret observations. Since direct measurements of the thermal conductivity of LiF at the appropriate conditions are difficult, we resort to molecular simulation methods. Using the Belonoshko et al. (2000) empirical potential validated against ab initio phonon density of states, we estimate the thermal conductivity of LiF at high temperatures (1000-4000K) and pressures (100-400 GPa) with the G…
Pressure-mediated crystalline g-C$_3$N$_4$ with enhanced spatial charge transport for solar H$_2$ evolution and photocathodic protection of 304 stainless steels
An approach combining the Rietveld method and pairs distribution function analysis to study crystalline materials under high pressure or temperature was early proposed by us, and in this study, it was applied to investigate de effect of high pressure on the rhombohedral Bi2Te3 phase. The refined structural parameters obtained from the Rietveld refinement of the XRD patterns measured for pressures up to 9.1 GPa were used as input data to simulate the partial and total structure factors SBiBi, SBiTe, STeTe, and SBi2Te3. Fourier transformation of the Sij factors permitted to obtain the partial an…
The control of the crystalline phases of the nanoparticles grown in a direct-current transferred-arc plasma-assisted reactor is reported. The crystalline phases of the as synthesized nanoparticles are shown to critically depend on the operating gas pressure. The paper reports about the change in the crystalline phases of three distinct compounds namely aluminium oxide (Al2O3), aluminium nitride (AlN) and iron oxide (FexOy). The major outcome of the present work is that the phases having higher defect densities are more probable to form at the sub-atmospheric operating pressure. The variations …
Ab initio molecular dynamic method within the framework of density functional theory is applied to analyze the structural and electronic properties of crystalline molecular hydrogen at temperature 100\,K. Pressure, pair correlation function and band structure are calculated. The crossover of molecular crystalline hydrogen from the state of a semiconductor to a semimetallic and metallic state is observed upon compression in the pressure range of 302-626\,GPa. At pressures below 361\,GPa, the molecular crystal with the C2/c structure is a semiconductor with an indirect gap. In the pressure range…
We study the gap closure with pressure of crystalline molecular hydrogen. The gaps are obtained from grand-canonical Quantum Monte Carlo methods properly extended to quantum and thermal crystals, simulated by Coupled Electron Ion Monte Carlo. Nuclear zero point effects cause a large reduction in the gap ($\sim 2eV$). \CP{Depending on the structure,} the fundamental indirect gap closes \CP{between 380GPa and} 530GPa for ideal crystals and 330-380GPa for quantum crystals. Beyond this pressure the system enters into a bad metal phase where the density of states at the Fermi level increases with p…
We used ab initio random structure searching (AIRSS) to investigate polymorphism in C3N4 carbon nitride as a function of pressure. Our calculations reveal new framework structures, including a particularly stable chiral polymorph of space group P43212 containing mixed sp2 and sp3-bonding, that we have produced experimentally and recovered to ambient conditions. As pressure is increased a sequence of structures with fully sp3-bonded C atoms and three-fold coordinated N atoms is predicted, culminating in a dense Pnma phase above 250 GPa. Beyond 650 GPa we find that C3N4 becomes unstable to decom…
Single crystals of AlFe$_{2}$B$_{2}$ have been grown using the self flux growth method and then measured the structural properties, temperature and field dependent magnetization, and temperature dependent electrical resistivity at ambient as well as high pressure. The Curie temperature of AlFe$_{2}$B$_{2}$ is determined to be $274$~K. The measured saturation magnetization and the effective moment for paramagnetic Fe-ion indicate the itinerant nature of the magnetism with a Rhode-Wohlfarth ratio $ \frac{M_{C}}{M_{sat}}\approx 1.14$. Temperature dependent resistivity measurements under hydrostat…
We investigate the low temperature structural and electronic properties of the type-I clathrate Eu8Ga16Ge30 under pressure using x-ray powder diffraction (XRD), x-ray absorption near-edge structure (XANES) and x-ray magnetic circular dichroism (XMCD) techniques. The XRD measurements reveal a transition to an amorphous phase above 18 GPa. Unlike previous reports on other clathrate compounds, no volume-collapse is observed prior to the crystalline-amorphous phase transition which takes place when the unit cell volume is reduced to 81% of its ambient pressure value. Fits of the pressure-dependent…
e report the effect of pressure on the steplike magnetostriction of single crystalline bilayered manganite (LaPr)1.2Sr1.8Mn2O8, for our understandings of the ultrasharp nature of the field-induced first-order transition from a paramagnetic insulator to a ferromagnetic metal phase. The application of pressure suppresses a steplike transformation and causes a broad change in the magnetostriction. The injection of an electric current to the crystal also weakens the steplike variation in both the magnetostriction and magnetoresistance. The stabilization of ferromagnetic interaction or the delocali…
We have studied the amorphization process of SnI4 up to 26.8GPa with unprecedented experimental details by combining Sn and I K edge X-ray absorption spectroscopy and powder X-ray diffraction. Standards and reverse Monte Carlo extended X-ray absorption fine structure (EXAFS) refinements confirm that the SnI4 tetrahedron is a fundamental structural unit that is preserved through the crystalline phase-I to crystalline phase-II transition about 7 to 10GPa and then in the amorphous phase that appears above 20GPa. Up to now unexploited Iodine EXAFS reveals to be extremely informative and confirms t…
In this paper we present a Path Integral Monte Carlo (PIMC) simulation of the orthorhombic phase of crystalline polyethylene, using an explicit atom force field with unconstrained bond lengths and angles. This work represents a quantum extension of our recent classical simulation (J. Chem. Phys. 106, 8918 (1997)). It is aimed both at exploring the applicability of the PIMC method on such polymer crystal systems, as well as on a detailed assessment of the importance of quantum effects on different quantities. We used the $NpT$ ensemble and simulated the system at zero pressure in the temperatur…
We report a phonon density of states measurement of $α$-ZnCl$_{2}$ using the coherent inelastic neutron scattering technique and a lattice dynamical calculation in four crystalline phases of ZnCl$_{2}$ using a transferable interatomic potential. The model calculations agree reasonably well with the available experimental data on the structures, specific heat, Raman frequencies and their pressure variation in various crystalline phases. The calculated results have been able to provide a fair description of the vibrational as well as the thermodynamic properties of ZnCl$_{2}$ in all its four pha…
We study the low-pressure (0 to 10 GPa) phase diagram of crystalline benzene using quantum Monte Carlo (QMC) and density functional theory (DFT) methods. We consider the $Pbca$, $P4_32_12$, and $P2_1/c$ structures as the best candidates for phase I and phase II. We perform diffusion quantum Monte Carlo (DMC) calculations to obtain accurate static phase diagrams as benchmarks for modern van der Waals density functionals. We use density functional perturbation theory to compute phonon contribution in the free-energy calculations. Our DFT enthalpy-pressure phase diagram indicates that the $Pbca$ …
Study of condensed matter in certain extreme conditions allows one to better understand the mechanisms of microscopic structural transformations and to develop materials with completely new mechanical properties. In the present work, we study the structure and crystallization kinetics of bulk metallic glass (BMG) Ni$_{62}$Nb$_{38}$. We show that this BMG is rapidly crystallized under the combined effect of shear deformation and ultrahigh pressure. A threshold pressure required to initiate the formation of a stable crystalline phase is revealed. Shear deformation and pressure lead to the phase …
The effect of pressure on the Raman spectrum of PbWO4 has been investigated up to 48 GPa in a diamond-anvil cell using neon as pressure-transmitting medium. Changes are detected in the Raman spectrum at 6.8 GPa as a consequence of a structural phase transition from the tetragonal scheelite structure to the monoclinic PbWO4-III structure. Two additional phase transitions are detected at 15.5 and 21.2 GPa to the previously unknown crystalline phases IV and V. The last one remains stable up to 43.3 GPa. At 47.7 GPa all Raman modes disappear, which could be caused by a pressure-induced amorphizati…
We report experimental and theoretical evidence that solid molecular ammonia becomes unstable at room temperature and high pressures and transforms into an ionic crystalline form. This material has been characterised in both hydrogenated NH3 and deuterated ND3 ammonia samples up to about 180 and 200 GPa, respectively, by infrared absorption, Raman spectroscopy and x-ray diffraction. The presence of a new, strong IR absorption band centered at 2500 cm-1 in NH3 (1900 cm-1 in ND3) signals the ionization of ammonia molecules into NH2- and NH4+ ions, in line with previous theoretical predictions. W…
Disorder driven superconductor to insulator transition via intermediate metallic regime is reported in nano-crystalline thin films of molybdenum. The nano-structured thin films have been deposited at room temperature using DC magnetron sputtering at different argon pressures. The grain size has been tuned using deposition pressure as the sole control parameter. A variation of particle sizes, room temperature resistivity and superconducting transition has been studied as a function of deposition pressure. The nano-crystalline molybdenum thin films are found to have large carrier concentration b…
The pressure-induced phase transition of bismuth telluride, Bi2Te3, has been studied by synchrotron x-ray diffraction measurements at room temperature using a diamond-anvil cell (DAC) with loading pressures up to 29.8 GPa. We found a high-pressure body-centered cubic (bcc) phase in Bi2Te3 at 25.2 GPa, which is denoted as phase IV, and this phase apperars above 14.5 GPa. Upon releasing the pressure from 29.8 GPa, the diffraction pattern changes with pressure hysteresis. The original rhombohedral phase is recovered at 2.43 GPa. The bcc structure can explain the phase IV peaks. We assumed that th…
We study the pressure-induced structural transformation of Ge$_{136}$ clathrate by ab initio molecular dynamics and metadynamics. The system under pressure first undergoes amorphization followed by an ultrafast recrystallization to the $β$-tin structure on the time scale of 30 ps. The initial pressure-induced amorphization of clathrate is triggered by high pressure while the subsequent fast recrystallization to $β$-tin is driven by low temperature. Interestingly, the amorphous intermediate is still diffusive even at room temperature, in spite of very strong undercooling, making the ultrafast r…
Our investigations on porous Si show that on increase of pressure it undergoes crystalline phase transitions instead of pressure induced amorphization - claimed earlier, and the amorphous phase appears only on release of pressure. This amorphous phase, when subjected to higher pressures, transforms reversibly to a higher coordinated primitive hexagonal phase showing a kind of memory effect which may be the only example of its kind in the elemental solids. First principles calculations and thermodynamic arguments help understand these observations.
Understanding the conditions which favor crystallisation or vitrification of liquids has been a long-standing scientific problem. Another connected, and not yet well understood question is the relationship between the glassy and the various possible crystalline forms a system may adopt. In this context, B2O3 is a puzzling case of study since i) it is one of the best glass-forming systems despite an apparent lack of low-pressure polymorphism ii) it vitrifies in a glassy form abnormally different from the only known crystalline phase at ambient pressure iii) it never crystallises from the melt u…
Ternary C_{x}N_{y}O_{z} compounds are actively researched as novel high energy density and ultrahard materials. Although some synthesis work has been performed at ambient conditions, very little is known about the high pressure chemistry of of C_{x}N_{y}O_{z} compounds. In this work, first principles variable-composition evolutionary structure prediction calculations are performed with the goal of discovering novel mixed C_{x}N_{y}O_{z} materials at ambient and high pressure conditions. By systematically searching ternary variable composition crystalline materials, the full ternary phase diagr…
Nanocrystals can exist in multiply twinned structures like the icosahedron, or single crystalline structures like the cuboctahedron or Wulff-polyhedron. Structural transformation between these polymorphic structures can proceed through diffusion or displacive motion. Experimental studies on nanocrystal structural transformations have focused on high temperature diffusion mediated processes. Thus, there is limited experimental evidence of displacive motion mediated structural transformations. Here, we report the high-pressure structural transformation of 6 nm Au nanocrystals under nonhydrostati…
The crystalline garnet Mn3Fe2Si3O12 and an amorphous phase of the same nominal composition are synthesized at high pressure. The magnetic properties of the two forms are reported. Both phases order antiferromagnetically. The crystalline phase exhibits a Curie-Weiss theta of -47.2 K, with a sharp ordering transition at 12 K. The glassy phase exhibits a larger antiferromagnetic Curie-Weiss theta, of -83.0 K, with a broad ordering transition observed at 2.5 K. Both phases can be classified as magnetically frustrated, although the amorphous phase shows a much higher degree of frustration. The amor…
There have existed for a long time a paradigm that TiO phases at ambient conditions are stable only if structural vacancies are available. Using an evolutionary algorithm, we perform an ab initio search of possible zero-temperature polymorphs of TiO in wide pressure interval. We obtain the Gibbs energy of the competing phases taking into account entropy via quasiharmonic approximation and build the pressure-temperature diagram of the system. We reveal that two vacancy-free hexagonal phases are the most stable at relatively low temperatures in a wide range of pressures. The transition between t…
It is a long-time pursuit of computations with \emph{ab initio} precision of thermal contributions to phase behaviors of condensed matters under extreme conditions. In this work, the pressure induced structural phase transitions of crystalline aluminum up to $600$ GPa at room temperature are investigated based on the criterion of Gibbs free energy derived directly from the partition function that formulated in the ensemble theory with the interatomic interactions characterized by density functional theory computations. The transition pressures of the FCC$\rightarrow$HCP$\rightarrow$BCC phase t…
We report the effect of hydrostatic pressure on the magnetotransport properties of the Weyl semimetal NbAs. Subtle changes can be seen in the $ρ_{xx}(T)$ profiles with pressure up to 2.31 GPa. The Fermi surfaces undergo an anisotropic evolution under pressure: the extremal areas slightly increase in the $\mathbf{k_x}$-$\mathbf{k_y}$ plane, but decrease in the $\mathbf{k_z}$-$\mathbf{k_y}$($\mathbf{k_x}$) plane. The topological features of the two pockets observed at atmospheric pressure, however, remain unchanged at 2.31 GPa. No superconductivity can be seen down to 0.3 K for all the pressures…
Our earlier Monte Carlo simulations of metastable supercooled-liquid and glass phases of Lennard-Jones atoms found several distinct signatures for identifying the glass transition boundary; i.e., the density, enthalpy, and pair distribution function dependences on temperature and pressure are different for the two phases (F. F. Abraham, J. Chem. Phys., 72, 359 (1980)). In this extension of that study, we base our analysis on the Ackland-Jones (A-J) method for determining the local crystal packing about each atom (G. Ackland & A. Jones, PRB 73, 054104 (2006)). It focuses on the angular distribu…
Crystallization of pristine cubic ice from liquid at ambient pressure
University College London（伦敦大学学院）；ISIS Neutron and Muon Source, Science and Technology Facilities Council, Rutherford Appleton Laboratory（ISIS中子与μ子源，科学与技术设施委员会，拉瑟福德阿普尔顿实验室）；Uppsala Universitet（乌普萨拉大学）；University of Oxford（牛津大学）；The University of Manchester（曼彻斯特大学）
研究立方冰 Ic 能否从液态直接形成等问题，通过冷却介孔二氧化硅内水溶液在常压下获得原始立方冰 Ic，利用中子散射技术证明其可重复形成且为唯一稳定晶相，揭示该多晶型物在自然和合成过程中作用更重要。
The crystal structures of LaAlO3 films grown by pulsed laser deposition on SrTiO3 substrates at oxygen pressure of 10-3 mbar or 10-5 mbar, where kinetics of ablated species hardly depend on oxygen background pressure, are compared. Our results show that the interface between LaAlO3 and SrTiO3 is sharper when the oxygen pressure is lower. Over time, the formation of various crystalline phases is observed while the crystalline thickness of the LaAlO3 layer remains unchanged. X-ray scattering as well as atomic force microscopy measurements indicate three-dimensional growth of such phases, which a…
The first principles approaches, density functional theory (DFT) and quantum Monte Carlo, have been used to examine the balance between van der Waals (vdW) forces and hydrogen (H) bonding in ambient and high pressure phases of ice. At higher pressure, the contribution to the lattice energy from vdW increases and that from H bonding decreases, leading vdW to have a substantial effect on the transition pressures between the crystalline ice phases. An important consequence, likely to be of relevance to molecular crystals in general, is that transition pressures obtained from DFT functionals which…
