arXiv subjects

Tian Cui

Publications and source records attributed to Tian Cui.

67 records · Page 4Linked to original sources

arXiv2017

Nanotwinned diamond synthesized from multi-core onion carbon

Nano-polycrystalline diamond (NPD) and nanotwinned diamond (NtD) were successfully synthesized in multi-anvil high pressure apparatus at high pressure and high temperature (HPHT) conditions using precursors of onion carbons. We found that the choices of distinct onion carbons with hollow or multi-cored microstructures lead to the synthesis of different diamond products of NPD or NtD ones. High quality NtD with an average twin size of 6.8 nm has been synthesized whose Vickers hardness reaches as high as 180 GPa as measured by indentation hardness experiment. The existence of stacking faults other than various defects in the onion carbon is found to be crucial to form twin boundary in the product. The origin of the extraordinarily high Vickers hardness in the NtD sample is attributable to the high concentration of twin boundary. Our work gave a direct support on the argument that pursuit of nanotwinned microstructure is an effective strategy to harden materials, in good coincidence with the well-known Hall-Petch effect.

cond-mat.mtrl-sci↗

arXiv2016

Direct Meissner Effect Observation of Superconductivity in Compressed H2S

Recently, an extremely high superconducting temperature (Tc) of ~200 K has been reported in the sulfur hydride system above 100 GPa. This result is supported by theoretical predictions and verified experimentally. The crystal structure of the superconducting phase was also identified experimentally, confirming the theoretically predicted structure as well as a decomposition mechanism from H2S to H3S+S. Even though nuclear resonant scattering has been successfully used to provide magnetic evidence for a superconducting state, a direct measurement of the important Meissner effect is still lacking. Here we report in situ alternating-current magnetic susceptibility measurements on compressed H2S under high pressures. It is shown that superconductivity suddenly appears at 117 GPa and that Tc reaches 183 K at 149 GPa before decreasing monotonically with a further increase in pressure. This evolution agrees with both theoretical calculations and earlier experimental measurements. The idea of conventional high temperature superconductivity in hydrogen-dominant compounds has thus been realized in the sulfur hydride system under hydrostatic pressure, opening further exciting perspectives for possibly realizing room temperature superconductivity in hydrogen-based compounds.

cond-mat.supr-con↗

arXiv2016

Crossover from Metal to Insulator in Dense Lithium-Rich Compound $\textrm{CLi}_{4}$

Crystal structures of $\textrm{CLi}_4$ compounds are explored through \emph{ab} \emph{initio} evolutionary methodology. Phase transition from metal to semimetal and semiconductor, and eventually to insulator with increasing pressure are revealed under pressure. Pressure-induced evolution of anti-metallization has been described quantitatively by Fermi Surface Filling Ratio and electron energy band gap using \emph{ab} \emph{initio} \emph{GW} calculations. Anti-metallization is attributed to the hybrid valence electrons and their repulsion by core electrons into the lattice interstices. Very weak electron-phonon coupling interactions are found in the metallic phases, resulting in very low superconducting temperature.

cond-mat.mtrl-sci↗

arXiv2015

High-pressure structures and superconductivity of bismuth hydrides

We have systematically searched for the ground state structures of bismuth hydrides based on evolutionary algorithm method and particle swarm optimization algorithm method. Given only rich-hydrogen region, except for BiH$_{3}$, other hydrides (BiH, BiH$_{2}$, BiH$_{4}$, BiH$_{5}$, BiH$_{6}$) have been predicted to be stable with pressurization. With the increase of hydrogen content, hydrogen exists in bismuth hydrides with the different forms and presents the characteristics of ionicity. Under high pressure, the remarkable structural feature is the emergence of H$_{2}$ units in BiH$_{2}$, BiH$_{4}$ and BiH$_{6}$, and BiH$_{6}$ adopts a startling layered structure intercalated by H$_{2}$ and the linear H$_{3}$ units. Further calculations show these energetically stable hydrides are good metal and their metallic pressures are lower than that of pure solid hydrogen because of the doping impurities. The $T_{c}$ in the range of 20-119 K has been calculated by the Allen-Dynes modified McMillan equation, which indicates all these stable hydrides are potential high-temperature superconductors. Remarkably, it is the H-Bi-H and Bi atoms vibrations rather than the high-frequency H$_{2}$ or H$_{3}$ units that dominate the superconductivity. In addition, hydrogen content has a great influence on the superconducting transition temperature.

cond-mat.supr-con↗

arXiv2015

The unexpected binding and superconductivity in SbH4 at high pressure

The semimetal antimony (Sb) element doped into hydrogen has been performed theoretically to explored high-pressure crystal structure and superconductivity of antimony hydrides. The unexpected stoichiometry $\textrm{SbH}_\textrm{4}$ with $P6_3/mmc$ symmetry is found to have most negative enthalpy and embody the coexistence of covalent and ionic bonds. It is a metallic phase and stable in the pressure ranges of 127-300 GPa. Furthermore, a superconducting critical temperature ($T_c$) of 106 K is obtained at 150 GPa by employing the Allen-Dynes modified McMillan equation. In addition, an extrusive distinguishing feature is the presence of soft phonon modes, which is primary contribution to the strength of electron-phonon coupling.

cond-mat.supr-con↗

arXiv2015

Decomposition of solid hydrogen bromide at high pressure

The stability of different stoichiometric H$_n$Br ($n$=1-7) compounds under pressure are extensively studied using density functional theory calculations. Five new energetically stable stoichiometries of H$_2$Br, H$_3$Br, H$_4$Br, H$_5$Br, and H$_7$Br were uncovered at high pressure. The results show that HBr is stable below 64 GPa, then decomposes into new compound H$_2$Br and Br$_2$ molecular crystal. For H$_2$Br and H$_3$Br compounds, they were found to become stable above 30 GPa and 8 GPa, respectively. In addition, we accidentally discovered the triangular H$_3^+$ species in H$_5$Br compounds at 100 GPa. Further electron-phonon coupling calculations predicted that hydrogen-rich H$_2$Br and H$_4$Br compounds are superconductors with critical temperature of superconductivity $T_c$ of 12.1 K and 2.4 K at 240 GPa, respectively.

cond-mat.supr-con↗

arXiv2015

Phase diagram and superconductivity of polonium hydrides under high pressure

High pressure structures, phase diagram and superconductivity of polonium hydrides have been systematically investigated through the first-principles calculations based on the density functional theory. With the increasing pressure, several stoichiometries (PoH, $\textrm{PoH}_\textrm{2}$, $\textrm{PoH}_4$ and $\textrm{PoH}_6$) are predicted to stabilize in the excess hydrogen environment. All of the reported hydrides, exception of PoH, exhibit intriguing structural character with the appearing $\textrm{H}_2$ units. Moreover, our electronic band structure and the projected density of states (PDOS) demonstrate that these energetically stable phases are metallic. The application of the Allen-Dynes modified McMillan equation with the calculated electron-phonon coupling parameter reveals that $\textrm{PoH}_4$ is a superconductor with a critical temperature $T_c$ of 41.2-47.2 K at 300 GPa.

cond-mat.supr-con↗

arXiv2015

Pressure-induced decomposition of solid hydrogen sulfide

Solid hydrogen sulfide is well known as a typical molecular crystal but its stability under pressure is still under debate. Particularly, Eremets et al. found the high pressure superconductivity with $T_{c}\approx$ 190 K in a H$_{2}$S sample [arXiv: 1412.0460 (2014)] which is associates with the elemental decomposition into H$_{3}$S [Sci. Rep. 4, 6968 (2014)]. Therefore, on what pressure H$_{2}$S can decompose and which kind of the products of decomposition urgent need to be solved. In this paper, we have performed an extensive structural study on different stoichiometries H$_{n}$S with ${n> 1}$ under high pressure using $ab$ $initio$ calculations. Our results show that H$_{2}$S is stable below 50 GPa and decomposes into H$_3$S and sulfur at high pressure, while H$_{3}$S is stable at least up to 300 GPa. The other hydrogen-rich H$_{4}$S, H$_{5}$S, and H$_{6}$S are unstable in the pressure range from 20 to 300 GPa.

cond-mat.supr-con↗

arXiv2013

The study of high pressure structural stability of CeO2 nanoparticles

In situ high pressure XRD diffraction and Raman spectroscopy have been performed on 12 nm CeO2 nanoparticles. Surprisingly, under quasihydrostatic condition, 12 nm CeO2 nanoparticles maintain the fluorite-type structure in the whole pressure range (0-51 GPa) during the experiments, much more stable than the bulk counterpart (PT=31 GPa). In contrast, they experienced phase transition at pressure as low as 26 GPa under non-hydrostatic condition (adopting CsCl as pressure medium). Additionally, 32-36 nm CeO2 nanoparticles exhibit an onset pressure of phase transition at 35GPa under quasihydrostatic condition, and this onset pressure is much lower than our result. Further analysis shows both the experimental condition (i.e., quasihydrostatic or non-hydrostatic) and grain size effect have a significant impact on the high pressure behaviors of CeO2 nanomaterials.

cond-mat.mtrl-sci↗

arXiv2012

Ethylene glycol-mediated synthesis of nanoporous anatase TiO2 rods and rutile TiO2 self-assembly chrysanthemums

Nanoporous anatase TiO2 rods and rutile TiO2 chrysanthemums were successfully synthesized via a simple ethylene glycol-mediated synthesis route. Their morphologies, phase compositions and components were characterized by SEM, TEM, Raman and IR, respectively. The results show that a self-assembly growth takes place in the calcination under vacuum, which makes the titanium glycolate rods transform into rutile TiO2/C chrysanthemums rather than anatase TiO2 rods. It also indicates that the carbon plays an important role in the phase transition process which promotes the phase transition to rutile TiO2 at a lower temperature (400 oC). It provides a new approach to prepare nanoporous rutile TiO2 nanomaterials under low temperature.

cond-mat.mtrl-sci↗

arXiv2012

Pressure-induced amorphization and polyamorphism in one-dimensional single crystal TiO2 nanomaterials

The structural phase transitions of single crystal TiO2-B nanoribbons were investigated in-situ at high-pressure using the synchrotron X-ray diffraction and the Raman scattering. Our results have shown a pressure-induced amorphization (PIA) occurred in TiO2-B nanoribbons upon compression, resulting in a high density amorphous (HDA) form related to the baddeleyite structure. Upon decompression, the HDA form transforms to a low density amorphous (LDA) form while the samples still maintain their pristine nanoribbon shape. HRTEM imaging reveals that the LDA phase has an α-PbO2 structure with short range order. We propose a homogeneous nucleation mechanism to explain the pressure-induced amorphous phase transitions in the TiO2-B nanoribbons. Our study demonstrates for the first time that PIA and polyamorphism occurred in the one-dimensional (1D) TiO2 nanomaterials and provides a new method for preparing 1D amorphous nanomaterials from crystalline nanomaterials.

cond-mat.mtrl-sci↗

arXiv2007

Ab initio investigation of optical properties of high-pressure phases of ice

We report a detailed ab initio investigation on the optical properties of ice under a wide high pressure range. The ice X phase (up to 380GPa), the theoretical proposed higher pressure phase ice XV (300GPa), as well as the ambient pressure low-temperature phase ice XI are involved. Our results show that the dispersion relations of optical properties of ice under high pressure are quite different from those under ambient pressure. Under higher pressure, there is whole tendency of blue shift in all optical properties of ice, and the energy region for optical response become broader, such as the absorption band and reflection band. In addition to the augmented absorption edge, all absorption peaks are found to be rising, and the reflection peaks are also enhanced a bit; hence the transmissivity of ice is inferred to be dropping. The photoconductivity is enhanced, and we explain this behavior from the increase of earner density. The static optical properties are found to be pressure independent, and principally due to the network topology of hydrogen bonding.

cond-mat.mtrl-sci↗

arXiv2007

Ab initio investigation of hydrogen bonding and electronic structure of high-pressure phases of ice

We report a detailed ab initio investigation on hydrogen bonding, geometry, electronic structure, and lattice dynamics of ice under a large high pressure range, including the ice X phase (55-380GPa), the previous theoretically proposed higher-pressure phase ice XIIIM (Refs. 1-2) (380GPa), ice XV (a new structure we derived from ice XIIIM) (300-380GPa), as well as the ambient pressure low-temperature phase ice XI. Different from many other materials, the band gap of ice X is found to be increasing linearly with pressure from 55GPa up to 290GPa, the electronic density of states (DOS) shows that the valence bands have a tendency of red shift (move to lower energies) referring to the Fermi energy while the conduction bands have a blue shift (move to higher energies). This behavior is interpreted as the high pressure induced change of s-p charge transfers between hydrogen and oxygen. It is found that ice X exists in the pressure range from 75GPa to about 290GPa. Beyond 300GPa, a new hydrogen-bonding structure with 50% hydrogen atoms in symmetric positions in O-H-O bonds and the other half being asymmetric, ice XV, is identified. The physical mechanism for this broken symmetry in hydrogen bonding is revealed.

cond-mat.mtrl-sci↗