Showing papers by "Xiang-Feng Zhou published in 2019"

Potential high- T c superconductivity in CaYH 12 under pressure

Abstract: The high-pressure phases and superconductivity of ${\mathrm{CaYH}}_{12}$ have been explored by using a particle swarm optimization structure prediction methodology in combination with first-principles calculations. Our results show that ${\mathrm{CaYH}}_{12}$ becomes stable with a cubic $Fd\overline{3}m$ structure above 170 GPa, where metal atoms form body-centered-cubic (bcc) lattices and hydrogens occupy all the tetrahedral interstices of these bcc lattices, completing sodalitelike cages. The electron-phonon coupling calculations indicate that the $Fd\overline{3}m$ structure is a potential high-temperature superconductor, with a calculated ${T}_{c}$ of 258 K at 200 GPa. Our current study provides a possibility for searching new high-${T}_{c}$ superconductors in ternary hydrides.

Potential high- Tc superconductivity in CaYH12 under pressure

Abstract: The work was supported by National Natural Science Foundation of China (Grants No. 11604290 and No. 51732010), the Science Foundation for the Youth Topnotch Talent from Universities of Hebei Province (Grant No. BJ2017023), Funding Program for Recruited Oversea Scholars of Hebei Province (CL201729), and the Ph.D. Foundation by Yanshan University (Grant No. B970). A.B. acknowledges financial support from the Spanish Ministry of Economy and Competitiveness (FIS2016-76617-P) and the Department of Education, Universities and Research of the Basque Government and the University of the Basque Country (IT756-13).

Mechanical properties of boron arsenide single crystal

Abstract: As the only semiconductor material exhibiting ultrahigh thermal conductivity under ambient conditions, cubic boron arsenide (BAs) is currently attracting great interest. Thanks to the development of high-quality BAs single crystal growth techniques, investigation of its basic physical properties has now become possible. Here, the mechanical properties of BAs single crystals are studied by experimental measurements combined with first-principles calculations. A Vickers hardness of 22 GPa suggests that BAs is a hard material, although not among the hardest. The bulk and Young's moduli are measured to be 142 and 388 GPa, respectively. These important mechanical performance parameters, in conjunction with the unusual high thermal conductivity, show great potential for BAs to serve in next-generation semiconductor applications.

Continuous strengthening in nanotwinned diamond

Abstract: Strengths of nanograined (ng) and nanotwinned (nt) metals increase with decreasing grain size and twin thickness, respectively, until reaching a critical value, below which strength decreases. This behavior is known as the reverse Hall–Petch effect (RHPE), which has also been observed in nanograined cubic boron nitride (cBN) and diamond. Surprisingly, however, hardness of nt-cBN and nt-diamond increases continuously with decreasing twin thickness down to several nanometers, suggesting the absence of RHPE in these covalent materials. The mechanism responsible for such a behavior remains controversial. Here we investigate the strengthening mechanisms in ng- and nt-diamond using molecular dynamics and first-principles calculations. For ng-diamond, the competition between shuffle-set dislocation (SSD) and grain boundary atom motions gives rise to RHPE. For nt-diamond, SSDs remain dominant but their slips along twin boundaries energetically show no advantage over those along other slip planes. Twin domains are locked and mechanically stable, resisting SSD propagation and inhibiting RHPE. These findings provide new insights into the hardening mechanism of nanotwinned covalent materials.

First-principles study of crystal structures and superconductivity of ternary YSH 6 and LaSH 6 at high pressures

Abstract: The work was supported by the National Natural Science Foundation of China (Grants No. 11604290 and No. 51732010), National Key R & D Program of China (Grant No. 2018YFA0703400), Funding Program for Recruited Oversea Scholars of Hebei Province (Grant No. CL201729), the Ph.D. Foundation by Yanshan University (Grant No. B970), and the Natural Science Research Project of Education Department of Anhui Province (KJ2018A0342). A.B. acknowledges financial support from the Spanish Ministry of Economy and Competitiveness (FIS2016-76617-P) and the Department of Education, Universities and Research of the Basque Government and the University of the Basque Country (IT756-13).

Magnetic borophenes from an evolutionary search

Abstract: A computational methodology based on ab initio evolutionary algorithms and spin-polarized density functional theory was developed to predict two-dimensional magnetic materials. Its application to a model system borophene reveals an unexpected rich magnetism and polymorphism. A metastable borophene with nonzero thickness is an antiferromagnetic semiconductor from first-principles calculations, and can be further tuned into a half-metal by finite electron doping. In this borophene, the buckling and coupling among three atomic layers are not only responsible for magnetism, but also result in an out-of-plane negative Poisson's ratio under uniaxial tension, making it the first elemental material possessing auxetic and magnetic properties simultaneously.

Predicting three-dimensional icosahedron-based boron B 60

Abstract: The icosahedron-based bulk boron structures have extremely chemical and structural complexity, and are usually semiconductors at ambient conditions. Here we predict bulk boron phases with a 60-atom orthorhombic unit cell from an ab initio evolutionary structure search, termed as ${\mathrm{B}}_{60}$. The metastable structures can be either a conductor or a semimetal depending on their interstitial atomic positions. In particular, an orthorhombic structure with $Pnma$ symmetry ($Pnma\ensuremath{-}{\mathrm{B}}_{60}$), consisting of ${\mathrm{B}}_{12}$ icosahedra and twisted interstitial two-atom wide boron ribbons, is identified to be a topological node-line semimetal with potential superior electronic transport. The band structure and simulated electron-diffraction pattern of $Pnma\ensuremath{-}{\mathrm{B}}_{60}$ are in satisfactory agreement with the experimental data, suggesting that it may exist in the form of nanomaterials.

High-pressure phases of boron arsenide with potential high thermal conductivity

Abstract: Zinc-blende (zb) boron arsenide (BAs) has been confirmed to have impressively high thermal conductivity. However, studies on its phase transitions under pressure have been few. Here, through a recently developed structure search method, we predicted that many polytypes with the structural features of cubic and hexagonal diamond, which are known to be unstable even up to very high pressures for carbon and boron nitride, can become stable at low pressures and might be retained to ambient conditions. Moreover, some of these BAs polytypes are calculated to have impressively high thermal conductivities at ambient conditions and the thermal conductivities for zb- and $2H$-BAs will decrease with increasing pressure, which are mainly attributed to the stronger third anharmonic interaction. The current study will open up a route to the search for high thermal conductors.

Small onion-like BN leads to ultrafine-twinned cubic BN

Abstract: Nanotwinned cubic boron nitride (nt-cBN) with remarkable hardness, toughness, and stability has attracted widespread attention due to its distinct scientific and industrial importance. The key for nt-cBN synthesis is to adopt an onion-like BN (oBN) nano-precursor and induce phase transition under high pressure. Here, we found that the size change of oBN used greatly affected the mechanical performance of products. With the precursor size decreasing from ~320 to 90 nm, the Vickers hardness of nanostructured products improved from 61 to 108 GPa, due to the fact that large oBN nanoparticles possessed more flattened, orderly and graphite-like shell layers, in sharp contrast to the highly wrinkled and imperfect layers in small-diameter nanoparticles, thus resulting in the apparent reduction of ultrafine-twin substructure in the synthetic products. This study reveals that only small oBN precursor could produce complete ultrafine nt-cBN with outstanding performance. A practical route was proposed to further improve the performance of this important material.

Formation of copper boride on Cu(111)

Abstract: Boron forms compounds with nearly all metals, with notable exception of copper and other group IB and IIB elements. Here, we report an unexpected discovery of ordered copper boride grown epitaxially on Cu(111) under ultrahigh vacuum. Scanning tunneling microscopy experiments combined with ab initio evolutionary structure prediction reveal a remarkably complex structure of 2D-Cu8B14. Strong intra-layer p-d hybridization and a large amount of charge transfer between Cu and B atoms are the key factors for the emergence of copper boride. This makes the discovered material unique and opens up the possibility of synthesizing ordered low-dimensional structures in similar immiscible systems.

Predicted lithium oxide compounds and superconducting low-pressure LiO 4

Abstract: We study the stability of Li-O compounds as a function of pressure, with rich phase diagram, diverse properties, and fundamental chemical interest in mind. Using the ab initio evolutionary algorithm USPEX, we predict the stability of compounds ${\mathrm{LiO}}_{4}, {\mathrm{Li}}_{5}{\mathrm{O}}_{3}$, and ${\mathrm{Li}}_{6}\mathrm{O}$ under pressure. Unexpectedly, ${\mathrm{LiO}}_{2}$ will decompose to ${\mathrm{Li}}_{2}{\mathrm{O}}_{2}+{\mathrm{LiO}}_{4}$ in the pressure range 6--18 GPa. ${\mathrm{LiO}}_{4}$, formed at the pressure of just 6 GPa, can be seen as $\ensuremath{\varepsilon}\ensuremath{-}{\mathrm{O}}_{8}$ accepting two electrons from two Li atoms. This phase is superconducting, with ${T}_{c}$ up to 12.2 K at 10 GPa. This is remarkable, because elemental oxygen becomes superconducting at much higher pressure (96 GPa) and has much lower ${T}_{c}$ ($l0.6$ K), and suggests that chemical alloying with other elements has the potential of not only decreasing metallization pressure, but also of increasing ${T}_{c}$. Since $\ensuremath{\varepsilon}\ensuremath{-}{\mathrm{O}}_{8}$ is called red oxygen, ${\mathrm{LiO}}_{4}$ can be identified as ``lithium red-oxide'', and is distinct from superoxide. Additionally, ${\mathrm{Li}}_{5}{\mathrm{O}}_{3}$ is stable at pressures above 70 GPa and can be represented as a hybrid structure $4{\mathrm{Li}}_{2}\mathrm{O}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathrm{Li}}_{2}{\mathrm{O}}_{2}$, and electride suboxide ${\mathrm{Li}}_{6}\mathrm{O}$ is stable above 62 GPa.

Discovery of copper boride on Cu(111)

Abstract: Boron forms compounds with nearly all metals, with notable exception of copper and other group IB and IIB elements. However, since the surface atoms are more reactive, it could be possible to form borides of these metals on their surfaces. Here we report the discovery of Cu8B14, a two-dimensional (2D) copper boride grown epitaxially on Cu(111) under ultrahigh vacuum. Scanning tunneling microscopy experiments combined with evolutionary structure prediction based on the density functional theory reveal a remarkably complex structure of 2D-Cu8B14. This discovered material forms as a large single phase with metallic conductivity and opens up the possibility of synthesizing ordered low-dimensional structures in similar immiscible systems.