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By means of first-principles calculations in combination with universal evolutionary structure search, we identified the crystalline structure of long-term argued M3Bi2 (M = Sr, Ca, and Ba), which crystallizes in a tubelike structure stacked by buckled graphenelike layers. The analyses of electronic structures revealed that this type of M3Bi2 is a native wide-gap three-dimensional topological insulator with the inverted band order induced mainly by crystal field effect. The spin-orbit coupling effect was found to open the band gap and further enhance the band inversion. Among them, Sr3Bi2 is most attractive due to its largest fundamental gap of about 0.3 eV and the directly inverted band gap of 0.81 eV at T obtained within the framework of Green functionals (GW). Moreover, the computation also evidences that their tubelike structure is suitable for further treatment via magnetic dopants, which prefer to occupy lb site. Interestingly, the ferromagnetic insulating state has been achieved for V- and Mn-doped cases. This may provide a further opportunity to observe the quantized anomalous Hall effect in its thin films. [ABSTRACT FROM AUTHOR]

First-principles determination of the vacancy formation enthalpies has been long-term believed to be highly successful for metals. However, a widely known fact is that the various conventional density functional theory (DFT) calculations with the typical semilocal approximations show apparent failures to yield accurate enthalpies of Ag and Au. Recently, the previously commonly assumed linear Arrhenius extrapolation to determine the vacancy formation enthalpies at T = 0 K from the high-temperature measured concentration of thermally created vacancies has been demonstrated to have to be replaced by the non-Arrhenius local Griineisen theory (LGT) [A. Glensk, B. Grabowski, T. Hickel, and J. Neugebauer, Phys. Rev. X 4,011018 (2014)]. The large discrepancies between the conventional DFT-PBE data and the unrevised experimental vacancy formation enthalpies disappear for Cu and Al. Even by following the same LGT revisions for Ag, the large discrepancies still remain substantial at T = 0K. Here, we show that the hybrid functional (HSE), by including nonlocal exchange interactions to extend the conventional DFT method, can further correct these substantial failures. Upon a comparison of the experimental valence-band spectra for Cu, Ag, and Au, we have determined the HSE exchange-correlated mixing parameters a of 0.1, 0.25, and 0.4, and further derived the HSE enthalpies of vacancy formation of 1.09, 0.94, and 0.72 eV, respectively; in nice agreement with available LGT-revised experimental data. Our HSE results shed light on how to improve the theoretical predictions to accurately determine the defect formation energies and related thermodynamical properties. [ABSTRACT FROM AUTHOR]

By means of first-principles calculations coupled with the kinetic Monte Carlo simulations, we have systematically investigated the effects of dilute substitutional solutes on the behaviors of carbon in α-Fe. Our results uncover the following, (i) Without the Fe vacancy the interactions between most solutes and carbon are repulsive due to the strain relief, whereas Mn has a weak attractive interaction with its nearest-neighbor carbon due to the local ferromagnetic coupling effect, (ii) The presence of the Fe vacancy results in attractive interactions of all the solutes with carbon. In particular, the Mn-vacancy pair shows an exceptionally large binding energy of -0.81 eV with carbon, (iii) The alloying addition significantly impacts the atomic-scale concentration distributions and chemical potential of carbon in the Fe matrix. Among them, Mn and Cr increase the carbon chemical potential, whereas A1 and Si reduce it. (iv) Within the dilute scale of the alloying solution, the solute concentration- and temperature-dependent carbon diffusivities demonstrate that Mn has a little impact on the carbon diffusion, whereas Cr (A1 or Si) remarkably retards the carbon diffusion. Our results provide a certain implication for better understanding the experimental observations related with the carbon solubility limit, carbon microsegregation, and carbide precipitations in the ferritic steels. [ABSTRACT FROM AUTHOR]

Unlike time-reversal topological insulators, surface metallic states with Dirac cone dispersion in the recently discovered topological crystalline insulators (TCIs) are protected by crystal symmetry. To date, TCI behaviors have been observed in SnTe and the related alloys Pbi1-xSnxSe/Te, which incorporate heavy elements with large spin-orbit coupling (SOC). Here, by combining first-principles and ab initio tight-binding calculations, we report the formation of a TCI in relatively lighter rocksalt SnS and SnSe. This TCI is characterized by an even number of Dirac cones at the high-symmetry (001), (110), and (111) surfaces, which are protected by the reflection symmetry with respect to the (110) mirror plane. We find that both SnS and SnSe have an intrinsically inverted band structure even without the SOC and the SOC is necessary only to open the bulk band gap. The bulk band gap evolution upon volume expansion reveals a topological transition from an ambient pressure TCI to a topologically trivial insulator. Our results indicate that the SOC alone is not sufficient to drive the topological transition. [ABSTRACT FROM AUTHOR]

By electron and x-ray diffraction we establish that the CrB4 compound discovered over 40 years ago crystallizes in the of 10 (Pnnm) structure, in disagreement with previous experiments but in agreement with a recent first-principles prediction. The 3D boron network in this structure is a distorted version of the rigid carbon sp3 network proposed recently for the high-pressure C4 allotrope. Our systematic density functional theory analysis of the electronic, structural, and elastic properties in ten related transition metal TMB4 tetraborides (TM = Ti, V, Cr, Mn, Fe and Y, Zr, Nb, Mo, Tc) identifies CrB4 as the prime candidate to be a superhard material. In particular, the compound's calculated weakest shear and tensile stresses exceed 50 GPa, and its Vickers hardness is estimated to be 48 GPa. We compare the reported and estimated Vickers hardness for notable (super)hard materials and find that the CrB4 calculated value is exceptionally high for a material synthesizable under standard ambient-pressure conditions. [ABSTRACT FROM AUTHOR]

We explore the topological behavior of the binary Zintl phase of the alkaline-earth-metal-based compounds Sr2Pb and Sr2Sn using both standard and hybrid density functional theory. It is found that Sr2Pb lies on the verge of a topological instability which can be suitably tuned through the application of a small uniaxial expansion strain (>3%). The resulting nontrivial topologically insulating state displays well-defined metallic states in the Sr2Pb(010) surface, whose evolution is studied as a function of the film thickness. [ABSTRACT FROM AUTHOR]

By means of first-principles calculations, we identified the structural phase transition of Na3Bi from the hexagonal ground state to the cubic cF16 phase above 0.8 GPa, in agreement with the experimental findings. Upon the releasing of pressure, the cF16 phase of Na3Bi is mechanically stable at ambient condition. The calculations revealed that the cF16 phase is topological semimetal (TS), similar to well-known HgTe, and it even exhibits an unusually low C' modulus (only about 1.9 GPa) and a huge anisotropy Au of as high as 11, which is the third-highest value among all known cubic crystals in their elastic behaviors. These facts render cF16-type Na3Bi very soft with a liquidlike elastic deformation in the (110)〈110〉 slip system. Importantly, accompanying this deformation, Na3Bi shows a topological phase transition from a TS state at its strain-free cubic phase to a topological insulator (TI) at its distorted phase. Because the C' elastic deformation has almost no energy cost in a reversible and liquidlike soft manner, cF16-type Na3Bi would potentially provide a fast on/off switching method between TS and TI, which would be beneficial to quantum electronic devices for practical applications. [ABSTRACT FROM AUTHOR]

By means of the first-principles calculations, we found that the early characterized 3D topological Dirac semimetal P63/mmc-Na3Bi is dynamically unstable at the ground state due to the presence of the large imaginary phonon frequencies around the K point. Alternatively, our calculations suggested a new ground-state phase crystalizing in a P3c1 structure with buckled graphite-like Na/Bi sheets, which is both energetically and dynamically stable. Moreover, the calculations also uncovered that the P3c1 phase is a 3D topological Dirac semimetal, with exactly the same electronic states of the metastable P63/mmc phase. [ABSTRACT FROM AUTHOR]