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A series of carbon-based superconductors XC6 with high Tc were reported recently. In this paper, based on the first-principles calculations, we studied the mechanical properties of these structures, and further explored the XC12 phases, where the X atoms are from elemental hydrogen to calcium, except noble gas atoms. The mechanically- and dynamically-stable structures include HC6, NC6, and SC6 in XC6 phases, and BC12, CC12, PC12, SC12, ClC12, and KC12 in XC12 phases. The doping leads to a weakening in mechanical properties and an increase in the elastic anisotropy. C6 has the lowest elastic anisotropy, and the anisotropy increases with the atomic number of doping atoms for both XC6 and XC12. Furthermore, the acoustic velocities, Debye temperatures, and the electronic properties are also studied.

Recently, a new high-pressure semiconductor phase of Ca2C (space group Pnma) was successfully synthesized, it has a low-pressure metallic phase (space group C2/m). In this paper, a systematic investigation of the pressure-induced phase transition of Ca2C is studied on the basis of first-principles calculations. The calculated enthalpy reveals that the phase transition which transforms from C2/m-Ca2C to Pnma-Ca2C occurs at 7.8 GPa, and it is a first-order phase transition with a volume drop of 26.7%. The calculated elastic constants show that C2/m-Ca2C is mechanically unstable above 6.4 GPa, indicating that the structural phase transition is due to mechanical instability. Both of the two phases exhibit the elastic anisotropy. The semiconductivity of Pnma-Ca2C and the metallicity of C2/m-Ca2C have been demonstrated by the electronic band structure calculations. The quasi-direct band gap of Pnma-Ca2C at 0 GPa is 0.86 eV. Furthermore, the detailed analysis of the total and partial density of states is performed to show the specific contribution to the Fermi level.

Motivated by a recently predicted structure of diamond-like BC2 with a high claimed hardness of 56 GPa (J. Phys. Chem. C 2010, 114, 22688–22690), we focus on whether this tetragonal BC2 (t-BC2) is superhard or not in spite of such an ultrahigh theoretical hardness. The mechanical properties of t-BC2 were thus further extended by using the first principles in the framework of density functional theory. Our results suggest that the Young’s and shear moduli of t-BC2 exhibit a high degree of anisotropy. For the weakest shear direction, t-BC2 undergoes an electronic instability and structural collapse upon a shear strain of about 0.11, with its theoretically ideal strength of only 36.2 GPa. Specifically, the plastic deformation under shear strain along the (110)[001] direction can be attributed to the breaking of d1 B–C bonds.

The elastic anisotropy and thermodynamic properties of the potential superhard carbon nitride phase C2N2(NH) have been investigated by using an ab initio plane-wave pseudopotential density theory method. The crystal parameters have been calculated at ambient as well as high pressure. The Young's modulus and shear modulus as a function of crystal orientations for C2N2(NH) have been systematically investigated. The Young's modulus is found to reach a maximum along the [100] direction. Using a set of total energy versus volume obtained with the first-principles calculations, the quasiharmonic Debye model is applied to the study of the thermal and vibrational effects. The dependence of Debye temperature, Gruneisen parameter, heat capacity, and expansion coefficient on the temperature and pressure are systematically explored in the whole pressure range from 0 to 60 GPa and temperature range from 0 to 2000 K.

Structural stability, elastic behavior, hardness, and chemical bonding of ideal stoichiometric rhenium dicarbide (ReC2) in the ReB2, ReSi2, Hex-I, Hex-II, and Tet-I structures have been systematically studied using first-principles calculations. The results suggest that all these structures are mechanically stable and ultra-incompressible characterized by large bulk moduli. Formation enthalpy calculations demonstrated that they are metastable under ambient conditions, and the relative stability of the examined candidates decreases in the following sequence: Hex-I4 Hex-II4 ReB24 Tet-I4ReSi2. The hardness calculations showed that these structures are all hard materials, among which the Hex-I exhibits the largest Vickers hardness of 32.2 GPa, exceeding the hardness of a-SiO2 (30.6 GPa) and b-Si3N4 (30.3 GPa). Density of states and electronic localization function analysis revealed that the strong C–C and C–Re covalent bonds are major driving forces for their high bulk and shear moduli as well as small Poisson’s ratio.

High-pressure and high-temperature phase relations of FeTiO3 were investigated up to a pressure of about 74 GPa and 2600 K by synchrotron X-ray diffraction and analytical transmission electron microscopy. We conclude that FeTiO3 ilmenite transforms into the following phase(s) with increasing pressure: FeTiO3 (perovskite) at 20–30 GPa, Fe2TiO4 (Ca2TiO4-type) + TiO2 (OI-type) at 30–44 GPa and high temperature, FeO (wustite) + TiO2 (OI) at 30–44 GPa and low temperature, and wustite + FeTi3O7 (orthorhombic phase) above 44 GPa. Among these dense high-pressure polymorphs, FeTi3O7 is a new compound and its structure analysis was tried using particle swarm optimization simulation. This method successfully found a new high-density FeTi3O7 structure, and Rietveld refinement based on this model structure gave an excellent fit with the experimentally obtained X-ray diffraction pattern. This new high-density FeTi3O7 structure consists of polyhedra for monocapped FeO7 prisms, bicapped TiO8 prisms, and tricapped TiO9 prisms, which develop on the b-c plane and stack along the a axis. The dense compound assemblage found in FeTiO3 is promising for investigating the behavior of ABX3 compounds under ultrahigh pressures.

Ab initio particle swarm optimization algorithm for crystal structural prediction was employed to uncover the high-pressure crystal structure of indium iodide (InI). We have predicted one tetragonal high-pressure phase for InI with P4/nmm symmetry, which is energetically much superior to the previously proposed CsCl-type structure. The P4/nmm-InI possesses alternative stacking of double I and In layers. The arrangement of adjacent I and In layers of P4/nmm-InI is similar to that of the CsCl-type structure. The calculated electronic density of states supports a metallic character for this tetragonal phase that is similar to the high-pressure behavior of IIA–VIB families. Furthermore, the phase transition path from the ambient pressure TlI–InI-P4/nmm-InI has been discussed.

Manganese sulfide (MnS) nanocrystals (NCs) with three different phases were synthesized by one-pot solvent thermal approach. The crystal structures and morphologies were investigated using powder X-ray diffraction, transmission electron microscopy, and high-resolution transmission electron microscopy. We found that the crystal structure and morphology of MnS NCs could be controlled by simply varying the reaction temperature. The detailed growth process of MnS nanobipods, including the zinc blende (ZB)-core formation and wurtzite (WZ)-arms growth, provides direct experimental evidence for the polymorphism model. Furthermore, we have studied the stability of metastable ZB- and WZ-MnS NCs under high pressure and found that ZB-nanoparticles and ZB/WZ-nanobipods are stable below their critical pressure, 5.3 and 2.9 GPa, respectively. When pressures exceed the critical point, all these metastable MnS NCs directly convert to the stable rock salt MnS.

Using an efficient structure search method combined with first-principles calculations, the structural behavior of YB2C2 has been extensively investigated in a wide pressure range of 0–200 GPa. A structural phase transition from the ambient tetragonal phase to a new high-pressure tetragonal phase was uncovered above 110 GPa. By total-energy calculations, this pressure-induced structural phase transition was determined to be first order with a volume drop of 9.1%. Inspections of crystal and electronic structures indicated that this high-pressure phase exhibits a 3D sp 3 B-C network built up from the sharply distortions and reconstructions of the 2D B-C layers in the ambient-pressure phase under compression, leading to an improved mechanical ductility and weakened elastic anisotropy.

On the basis of the evolutionary methodology for crystal structure prediction, we have clarified the long-standing debates on the ground state structures of technically important molybdenum borides: MoB2, Mo2B5, and MoB4. The earlier proposed rhombohedral structure for Mo2B5 and WB4-type structure for MoB4 are ruled out. Instead, two novel hexagonal P63/mmc structures are proposed. Moreover, the yet synthesized MoB3 was found to adopt the intriguing rhombohedral R3m structure and was suggested to be experimentally synthesizable by the calculation of convex hull. Density of states calculation revealed that the strong covalent bonding between Mo and B atoms is the driving force for the high bulk and shear modulus as well as small Poisson’s ratio of the studied borides. The hardness calculations suggest that these borides are all hard materials, among which MoB3 exhibits the largest Vickers hardness of 31.8 GPa, exceeding the hardness of α-SiO2 (30.6 GPa) and β-Si3N4 (30.3 GPa).

To evaluate the therapeutic effects of VSD combined with irrigation of oxygen loaded fluid on the growth of granulation tissue and macrophage polarization in chronic venous leg ulcers.Thiry-four patients with chronic venous leg ulcers hospitalized in our department from December 2010 to July 2014 were divided into VSD group ( A, n = 11) , VSD + irrigation group ( B, n = 11) , and VSD + oxygen loaded fluid irrigation group ( C, n = 12) according to the random number table. After admissian, debridement was performed, and granulation tissue in the center of the wound was harvested during the operation. After dehridement, the patients in group A were treated with VSD only (negative pressure from -30 to -25 kPa, the same below) ; the patients in group B were treated with VSD combining irrigation of normal saline; the patients in group C were treated with VSD combining normal saline loaded with oxygen irrigation (flow of 1 L/min) . On post treatment day (PTD) 7, the VSD devices were removed. Cross observation was conducted before debridement and on PTD 7. On PTD 7, the granulation tissue in the center of the wound was harvested for histopathological observation with HE staining and Masson staining, following calculation of granulation tissue coverage rate. After debridement but before the negative pressure therapy (hereinafter referred to as before treatment) and on PTD 7, partial pressure of oxygen of the skin around the wound was measured by transcutaneous tissue oxygen tension survey meter. On PTD 7, expression of vascular endothelial growth factor (VECF) was determined with immunohistochemistry. Before treatment and on PTD 7, cells with double positive expressions of induced nitric oxide synthase plus CD68 ( type I macro- phage) and arginase 1 plus CD68 ( type II macrophage) were observed with immunofluorescence staining and quantified. Data were processed with Fisher's exact test, one-way analysis of variance, covariance analysis, paired test, and LSD test.(1) The gross observation showed that before debridement there was a certain amount of necrotic tissue and little granulation tissue in the wounds of patients in all the 3 groups. On PTD 7, new granulation tissue was found in the wounds of patients in all the 3 groups, and in group C its amount was the largest. (2) On PTD 7, the granulation tissue coverage rate of wounds in pa- tients of group C was higher than that of group A or B ( P0.05 or P0.01). (3) On PTD 7, HE staining showed that there appeared more abundant new born microvessels and fibroblasts in the wounds of patients in group C than those in groups A and B; Masson staining showed that there was more abundant fresh collagen distributed orderly in the wounds of patients in group C compared with group A or B. (4) On PTD 7, it was found that partial pressure of oxygen of the skin around the wounds in patients of group C [(40.7 +/- 4.1) mmHg, 1 mmHg = 0.133 kPa] was higher than that of group A [ (35.0 +/- 3.1) mmHg] or B [(35.4 +/- 2.7) mmHg, with P values below 0.01]; the partial pressure of oxygen of the skin around the wounds of patients in all the 3 groups was increased significantly compared with that before treatment (with values from 10.38 to 22.52, P values below 0.01). (5) On PTD 7, the expression of VECF in the wounds of patients in group C was higher than that in group A or B ( P0.05 or P0.01). (6) On PTD 7, the number of type I macrophages in granulation tissue of patients was respectively 14.3 +/- 2.3, 11.5 +/- 3.0, and 10.7 +/- 2.3 per 400 times vision field in groups A , B, and C ( F = 25.14, P0.01), while the number in group C was less than that in group A or B ( P0.05 or P0.01). Compared with that before treatment, the number of type I macrophages was significantly decreased on PTD 7 in all the 3 groups (with values from 14.76 to 23. 73, P values below 0. 01). On PTD 7, the number of type II macrophages in granulation tissue of patients was respectively 32.7 +/- 3.2, 35.1 +/- 3.3 , and 41.3 +/- 3.2 per 400 times vision field in groups A, B, and C ( F = 81.10, P0.01), and the number in group C was lager than that in group A or B ( with P values below 0. 01). Compared with that before treatment, the number of type II macrophages in all the 3 groups was significantly increased (with t values from -69.34 to -47.95, P values below 0.01).VSD combined with irrigation of oxygen loaded fluid can raise the partial pressure of oxygen of the skin around the wounds effectively, promoting the transition of macrophages from type I to type II, thus it may promote the growth of granulation tissue, resulting in a better recipient for skin grafting or epithelization.

To explore the serum anti-Mullerian hormone (AMH) level in women of childbearing age with normal menstrual cycles.A total of 1 423 women with regular menstrual cycles were selected and divided into 5 groups according to their ages, i.e. ≤ 25, 26-30, 31-35, 36-40, ≥ 41 years. Their serum levels of AMH were measured, and the relationship between AMH and age was analyzed.The serum AMH levels of 5 groups according to ages (≤ 25, 26-30, 31-35, 36-40, ≥ 41 years) were 3.62, 3.10, 2.27, 1.07, 0.45 µg/L, respectively. The comparison of serum AMH levels in different age groups had significant difference (P0.01). Serum AMH level declined with increasing age, and dropped significantly after 36. The serum AMH level and age showed a negative correlation with significant difference (r = -0.374, P0.01). Quadratic regression of logAMH proximally reflected the relationship between AMH and age.AMH determination for women of childbearing age could provide reference for the evaluation of ovarian function.

Under high pressure, the graphite-like materials present complex structural behaviors, interesting mechanical properties, and fascinating electronic structures. We here report two novel high-pressure structures of oF40-CaB2C2 and HPtI20-CaB2C2 by using the newly developed particle swarm optimization algorithm based on first-principles calculations. The oF40-CaB2C2 and HPtI20-CaB2C2 exhibit a 3D sp 3 B-C bonded network under compression, which is verified by the inspections of their crystal structures and partial densities of states. The study of mechanical anisotropy reveals that the degree of anisotropy for CaB2C2 is gradually enhanced with its structural transformations under pressure. Intriguingly, the originally indirect band-gap CaB2C2 at ambient pressure transform into a direct band-gap semiconductor under extreme pressure (beyond 214.4 GPa).

Hair cells in posthatch chickens regenerate spontaneously through mitosis or the transdifferentiation of supporting cells in response to antibiotic injury. However, how embryonic chicken cochleae respond to antibiotic treatment remains unknown. This study is the first to indicate that unlike hair cells in posthatch chickens, the auditory epithelium was free from antibiotic injury (25–250 mg gentamicin/kg) in embryonic chickens, although FITC-conjugated gentamicin actually reached embryonic hair cells. Next, we examined and counted the cells and performed labeling for BrdU, Sox2, Atoh1/Math1, PV or p27 kip1 (triple or double labeling) in the injured cochlea ducts after gentamicin treatment at 2 h (h), 15 h, 24 h, 2 days (d), 3 d and 7 d after BrdU treatment in posthatch chickens. Our results indicated that following gentamicin administration, proliferating cells (BrdU+) were labeled for Atoh1/Math1 in the damaged areas 3d after gentamicin administration, whereas hair cells (PV+) renewed through mitosis (BrdU+) or direct transdifferentiation (BrdU-) were evident only after 5 d of gentamicin administration. In addition, Sox2 expression was up-regulated in triggered supporting cells at an early stage of regeneration, but stopped at the advent of mature hair cells. Our study also indicated that p27 kip1 was expressed in both hair cells and supporting cells but was down-regulated in a subgroup of the supporting cells that gave rise to hair cells. These data and the obtained dynamic changes of the cells labeled for BrdU, Sox2, Atoh1/Math1, PV or p27 kip1 are useful for understanding supporting cell behaviors and their fate specification during hair cell regeneration.

To evaluate the therapeutic effects of VSD combined with irrigation of oxygen loaded fluid on chronic wounds in diabetic patients.Twenty-six diabetic patients hospitalized in Nanfang Hospital of Southern Medical University from September 2010 to June 2013, with chronic ulcers on lower extremities conforming to the inclusive criteria, were divided into group VSD (n = 8), VSD + irrigation control group (VSD + IC, n = 9), VSD + oxygen loaded fluid irrigation group (VSD OLI, n = 9) according to the random number table. After gross observation was conducted and wound secretion was sent for bacterial culturing right after admission, debridement was performed. During the debridement, granulation tissue of wound center was harvested for determination of the activity of lactate dehydrogenase (LDH) and succinate dehydrogenase (SDH) with ELISA. After debridement, the patients in group VSD were treated with VSD (negative pressure from -30 to -25 kPa, the same below); the patients in group VSD + IC were treated with VSD combining irrigation of normal saline; the patients in group VSD + OLI were treated with VSD combining normal saline loaded with oxygen (flow of 1 L/min ) irrigation. Drainage tube blockage was recorded and its incidence rate was recorded during the treatment. On post treatment day (PTD) 7, tissue exudates were collected and analyzed with blood gas analyzer for determining the partial pressure of oxygen of the exudate. After the VSD was terminated, bacterial culture was conducted as before, and the bacterial clearance rate was calculated. After the calculation of granulation tissue coverage rate, the granulation tissue in the center of the wound was harvested for histopathological observation with HE staining; morphological characteristics and density of mitochondria were observed with transmission electron microscopy; the activity of LDH and SDH was estimated as before; microvascular density (MVD) was counted after CD31 antibody immunohistochemical staining. Then the second stage operation was performed. The method of second stage operation was recorded and survival rate of grafted skin or flap was calculated. Data were processed with one-way analysis of variance, LSD- t test, rank sum test, or Fisher's exact test.(1) The gross observation showed that before debridement there was only necrotic tissue without granulation tissue in the wounds of patients in all the 3 groups. On PTD 7, granulation tissue was found in the wounds of patients in all the 3 groups. HE staining showed that there were more abundant newborn microvessels and regularly arranged fibroblasts in the wounds of group VSD + OLI; less newborn microvessels and relatively sparsely fibroblasts were observed in the wounds of group VSD + IC. There were only sparse newborn microvessels and fibroblasts in the wounds of group VSD. (2) Rates of drainage tube blockage, granulation tissue coverage, and bacterial clearance showed significant differences among the 3 groups (with F values from 10.98 to 770.24, P values below 0.01). The drainage tube blockage rate was significantly lower in groups VSD + IC and VSD + OLI [(2.0 ± 0.4)% and (1.9 ± 0.6)%] than in group VSD [(16.0 ± 1.3)%, with t values respectively 28.77 and 29.20, P values below 0.01]. (3) On PTD 7, the partial pressure values of oxygen of the exudate in groups VSD + IC, VSD + OLI, and VSD were respectively (111 ± 4), (43 ± 4), and (40 ± 4) mmHg (1 mmHg = 0.133 kPa, F = 882.76, P0.01). (4) The density of mitochondria in group VSD + OLI was obviously higher than that of the other 2 groups, full in shape, with complete outer membrane and no vacuolization. (5) During debridement, the activity of LDH and SDH in 3 groups showed no significant differences (with F values respectively 0.08 and 1.03, P values above 0.05). On PTD 7, the activity of LDH was lower in group VSD + OLI [(103 ± 15) U/L] than in group VSD + IC [(136 ± 16) U/L, t = 4.49, P0.01], while it was higher in group VSD [(155 ± 16) U/L] than in group VSD + IC (t = 2.47, P0.05). The activity of SDH was higher in group VSD + OLI [(2.93 ± 0.27) U/L] than that in group VSD + IC [(1.77 ± 0.22) U/L] or group VSD [(1.61 ± 0.19) U/L, with t values respectively 10.21 and 11.65, P values below 0.01]. (6) On PTD 7, there was more positive expression of CD31 in group VSD + OLI than in the other 2 groups. The MVD of groups VSD, VSD + IC, and VSD + OLI were respectively (109 ± 5), (124 ± 5), (141 ± 6) per 400 times visual field (F = 68.78, P0.01). (7) The patients in 3 groups mainly received skin or flap grafting as the second stage operation. The survival rates of skin and flap in group VSD + OLI were higher than those of groups VSD + IC and VSD (with t values from 3.32 to 8.26, P0.05 or P0.01), and the rates were higher in group VSD + IC than in group VSD (with t values respectively 2.67 and 3.18, P values below 0.05).VSD + OLI is effective in reducing drainage tube blockage, removing necrotic tissue and bacteria, ameliorating ischemia and hypoxia of wound tissue, providing fresh wound bed for wound healing, and improving skin or flap graft survival rates.

Additional electrons can drastically change the bonding trend of light elements. For example, N atoms in alkali metal azides form the linear N3(-) anions instead of N2 molecules with the introduction of additional electrons. The effect of the additional electrons on the polymerization of N under pressure is important and thus far unclear. Using first principles density functional methods and the particle swarm optimization structure search algorithm, we systematically study the evolution of LiN3 structures under pressures up to 600 GPa. A stable structure featuring polymerized N under pressures higher than 375 GPa is identified for the first time. It consists of zig-zag N polymer chains that are formed by N5(-) five-member rings sharing N-N pairs. Throughout the stable pressure range, the structure is insulating and consists of N atoms in sp(3) hybridizations. Comparing with the atomic and electronic structures of previous phases, our study completes the structural evolution of LiN3 under pressure and reveals the structural changes which are accompanied and driven by the change of atomic orbital hybridization, first from sp to sp(2) and then from sp(2) to sp(3).

A high-pressure hexagonal semiconducting phase (space group P 6 3 mc , 2f.u./cell) of LiBC stable above 108 GPa was predicted through first-principles calculations combined with unbiased swarm structure searching techniques. This new phase consisted of three-dimensional B-C networks which originate from the dramatic out-of-plane distortions of the graphene-like B-C sublattice in the low-pressure phase under compression. Contrary to the metallizations of LiBC under high pressure previously proposed, the resulting three-dimensional B-C framework lacks the system of π bonds with mobile electrons and has more localized electrons, as a result of the semiconducting nature of this high-pressure LiBC phase.

Two novel high-pressure structures are predicted for the title compounds: a monoclinic structure (space group C2/m, Z = 4) for ScB2 and YB2 stable above 208 and 163 GPa, resp., and a tetragonal α-ThSi2 type structure (I41/amd, Z = 4) for TiB2 which is stable above 215 GPa.

We have extensively explored the high-pressure structures of transition-metal diborides (TMB(2), TM = Sc, Ti, Y, and Zr) stabilized with the AlB(2)-type structure at ambient pressure by using first-principles structural prediction. We find two novel high-pressure structures: (i) a monoclinic structure (C2/m, Z = 4) for ScB(2) and YB(2) stable above 208 and 163 GPa, respectively; and (ii) a tetragonal alpha-ThSi(2)-type phase (I4(1)/amd, Z = 4) for TiB(2) stable above 215 GPa. Our calculations show that the electron transfer from transition-metals TM to B under pressure might be the main cause for the structural phase transitions. Further phonon and hardness calculations suggest that alpha-ThSi(2) phase of TiB(2) is quenchable to ambient pressure and possesses excellent mechanical property with a Vickers hardness of 29.8 GPa. Interestingly, ZrB(2) is quite stable and persists on the ambient-pressure AlB(2)-type structure up to at least 300 GPa. We attribute the strong covalent hybridization between the transition-metal Zr and B to this ultrastability.

Based on first-principle calculations, we fundamentally investigate the anisotropic mechanical properties and atomistic deformation mechanism of the recent proposed ultra-incompressible tetragonal t-B4C4. The obtained results reveal that the t-B4C4 exhibits a pronounced elastic anisotropy according to the distributions of elastic moduli along the principal crystal orientations. It is found that the ideal shear strength in the direction of dominates the shear plastic deformation for t-B4C4. The lattice instability of the t-B4C4 can be revealed by the bond lengths and electron localization function as functions of shear strain. The results show that the failure modes of t-B4C4 can be attributed to the breaking of C-B bonds in the C2B8 double quadrangular pyramid.

First-principles calculations are employed to provide a fundamental understanding of the structural features, elastic anisotropy, shear-induced atomistic deformation behaviors, and its electronic origin of the recently proposed superhard t-SiCN. According to the dependences of the elastic modulus on different crystal directions, the t-SiCN exhibits a well-pronounced elastic anisotropy which may impose certain limitations and restrictions on its applications. The further mechanical calculations demonstrated that t-SiCN shows lower elastic moduli and ideal shear strength than those of typical hard substances of TiN and TiC, suggesting that it cannot be intrinsically superhard as claimed in the recent works. We find that the failure modes of t-SiCN at the atomic level during shear deformation can be attributed to the breaking of C-C bonds through the bonding evolution and electronic localization analyses.

A theoretical investigation on mechanical and electronic properties of Imma-carbon was performed by employing first-principles calculations based on the density functional theory. The stability at ambient condition is approved by phonon dispersion and elastic constants calculations. The analysis of elastic anisotropy and hardness anisotropy shows that Imma-carbon is nearly mechanical isotropy. The large elastic constants and ideal strength indicate Imma-carbon is a potential superhard material. Ideal strength studies show that (010) plane is the easy cleavage plane for Imma-carbon, and the cleavage mechanism was discussed in detail. The calculated band structure is a direct band gap semiconductor with 2.97 eV gap at Γ-point.

Recently, pressure-induced phase transition of CeN from ambient B1 phase to the B2 phase has been experimentally reported at the pressure of 65–70 GPa. Nevertheless, the full transformation of the high-pressure B2 phase has not been observed in the experiment as the authors said. Here we predict an unexpected anti-B10 high-pressure phase which is energetically more preferable than the B2 phase at the studied pressure range, disproving the experimental result. Our argument has been supported by the softening of both elastic constant C44 in B1 phase and transverse acoustic mode at the zone boundary M point of B2 phase.

Based on the particle swarm optimization algorithm on crystal structural prediction, we first predict that TaN undergoes a phase transition from the experimental θ-TaN to a hexagonal P63/mmc structure at 87.5 GPa with volume drop of 1.6%. This hexagonal P63/mmc structure is isostructural with anti-NiAs and can be quenchable to ambient pressure by further phonon dispersions calculations. The Young's modulus E and shear modulus G as a function of crystal orientation for TaN have thus been systematically investigated. The calculated mechanical properties suggest that the P63/mmc-TaN is ultra-incompressible and hard due to its high bulk modulus (336 GPa), large shear modulus (214 GPa), originating from a staking of “N-Ta-N” sandwiches layers linked by strong covalent Ta-N bonding.

The pressure-induced formation of a hexagonal phase (P6/m, Z = 2) of LiN3 is predicted by particle swarm optimization (PSO) structural search at zero temperature and pressure of > 34.7GPa. Compared to the ground-state insulator C2/m phase, the P6/m-LiN3 is found to possess a metallic feature dominated by the 2p(z) bands. Electron topological analysis revealed an intriguing phase change of nitrogen, from azide to novel pseudo-benzene "N-6" molecules at the formation of the P6/m phase. The occurrence of this hexagonal phase follows the polymerization of the typical linear N-3(-) anion molecular chains in the low-pressure molecular phases of LiN3 and is originated from the volume reduction favorable for a denser structure packing under compression. Copyright (C) EPLA, 2013

The structure and distributions of elastic moduli of c-W3N4 have been investigated by first-principles calculations. The formation of c-W3N4 can be understood from the fact that the N atoms partially occupy the 6b (0, 1/2, 1/2) interstitial sites in pure cubic W. Young's modulus of c-W3N4 is found to reach a maximum along the [100] direction and a minimum along the [111] direction, and the ideal shear strength along the weakest (100)[100] slip system is about 22.5 GPa. The evidence of the bonding nature of W-N which plays an important role to form a hard material is manifested by the PDOS and Mulliken population analysis.

We have extensively explored the ground-state structure of RuC using the particle swarm optimization algorithm for crystal structural prediction. A hexagonal R-3m structure has been proposed as the best candidate, which is energetically more favorable than the previously proposed zinc blend structure. The R-3m-RuC possesses alternative stacking of double hexagonal close-packed Ru atom layers and C atom layers, and it is dynamically stable evidenced by the calculation of phonon dispersion. The calculated large bulk modulus, shear modulus, and elastic constant C44 reveal that it is an ultra-incompressible and hard material. The evidence of strong covalent bonding of Ru—C, which plays an important role to form a hard material, is manifested by the partial densities of states analysis.

Using newly developed particle swarm optimization algorithm on crystal structural prediction, we first predicted that MoB2 undergoes a phase transition from the low-pressure rhombohedral phase to a tetragonal α-ThSi2-type phase with a volume drop of 4.01% when the applied pressure is 68 GPa. Phonon calculations suggest the α-ThSi2-type phase can be quenchable to ambient pressure. Then, Young’s modulus E and shear modulus G as a function of crystal orientation for the α-ThSi2-MoB2 have been systematically investigated. Further mechanical properties demonstrated that α-ThSi2-MoB2 possesses large bulk modulus of 322.3 GPa and high Vickers hardness of 32.1 GPa, exceeding the hardness of α-SiO2 (30.6 GPa) and β-Si3N4 (30.3 GPa). The excellent mechanical properties are attributed to the three-dimensional networks linked by strong covalent B-B bonding and Mo-B covalent bonds in MoB12 polyhedrons.

We theoretically investigate high-order harmonic generation (HHG) from a helium ion model in a two-color laser field, which is synthesized by a fundamental pulse and its second harmonic pulse. It is shown that a supercontinuum spectrum can be generated in the two-color field. However, the spectral intensity is very low, limiting the application of the generated attosecond (as) pulse. By adding a static electric field to the synthesized two-color field, not only is the ionization yield of electrons contributing to the harmonic emission remarkably increased, but also the quantum paths of the HHG can be significantly modulated. As a result, the extension and enhancement of the supercontinuum spectrum are achieved, producing an intense isolated 26-as pulse with a bandwidth of about 170.5 eV. In particular, we also analyse the influence of the laser parameters on the ultrabroad supercontinuum spectrum and isolated sub-30-as pulse generation.