Predicted boron-carbide compounds: a first-principles study.

By using developed particle swarm optimization algorithm on crystal structural prediction, we have explored the possible crystal structures of B-C system. Their structures, stability, elastic properties, electronic structure, and chemical bonding have been investigated by first-principles calculations with density functional theory. The results show that all the predicted structures are mechanically and dynamically stable. An analysis of calculated enthalpy with pressure indicates that increasing of boron content will increase the stability of boron carbides under low pressure. Moreover, the boron carbides with rich carbon content become more stable under high pressure. The negative formation energy of predicted B5C indicates its high stability. The density of states of B5C show that it is p-type semiconducting. The calculated theoretical Vickers hardnesses of B-C exceed 40 GPa except B4C, BC, and BC4, indicating they are potential superhard materials. An analysis of Debye temperature and electronic localization function provides further understanding chemical and physical properties of boron carbide.

Phase stability and elastic properties of chromium borides with various stoichiometries.

Phase stability is important to the application of materials. By first-principles calculations, we establish the phase stability of chromium borides with various stoichiometries. Moreover, the phases of CrB3 and CrB4 have been predicted by using a newly developed particle swarm optimization (PSO) algorithm. Formation enthalpy-pressure diagrams reveal that the MoB-type structure is more energetically favorable than the TiI-type structure for CrB. For CrB2, the WB2-type structure is preferred at zero pressure. The predicted new phase of CrB3 belongs to the hexagonal P-6m2 space group and it transforms into an orthorhombic phase as the pressure exceeds 93 GPa. The predicted CrB4 (space group: Pnnm) phase is more energetically favorable than the previously proposed Immm structure. The mechanical and thermodynamic stabilities of predicted CrB3 and CrB4 are verified by the calculated elastic constants and formation enthalpies. The full phonon dispersion calculations confirm the dynamic stability of WB2 -type CrB2 and predicted CrB3. The large shear moduli, large Young's moduli, low Poisson ratios, and low bulk and shear modulus ratios of CrB4-PSC and CrB4-PSD indicate that they are potential hard materials. Analyses of Debye temperature, electronic localization function, and electronic structure provide further understanding of the chemical and physical properties of these borides.

The effect of structure and phase transformation on the mechanical properties of Re2N and the stability of Mn2N.

First-principles calculations were carried out on recently synthesized Re2N and Re3N as well as hypothetical Tc and Mn nitrides. It is found that structure and covalent bonds play an important role in determining mechanical properties. Under a large strain along (0001)<1010> direction, Re2N undergoes a phase transformation with a slight increase in ideal shear strength. On the other hand, it is transformed into a phase with weaker mechanical properties, if the strain is along Re2 <1210> direction. Mn2N can be synthesized under moderate conditions due to its more negative formation energy. Re2N, Re3N, and Mn2 N show structure-related mechanical property under larger strains to ReB2 but exhibit much lower ideal strengths, which is attributed to the larger ionicity of cation-anion bond. Three-dimensional framework of strong covalent bonds is thus highly recommended to design superhard materials.

Electronic structure and low temperature thermoelectric properties of In24M8O48 (M = Ge(4+), Sn(4+), Ti(4+), and Zr(4+)).

The electronic structure and transport properties of In24M8O48 (M = Ge(4+), Sn(4+), Ti(4+), and Zr(4+)) have been studied by using the full-potential linearized augmented plane-wave method and the semiclassical Boltzmann theory, respectively. It is found that the magnitude of powerfactor with respect to relation time follows the order of In24Sn8O48 > In24Zr8O48 > In24Ge8O48 > In24Ti8O48. The largest powerfactor is 2.7 × 10¹² W/K² ms for In24Sn8O48 at 60 K, which is nearly thirty times larger than those of conventional n-type thermoelectric materials. The origin of the different thermoelectric behavior for these compounds is discussed from the electronic structure level. It is found that, at low temperature, the dopant strongly affect the bands near the Fermi level, which consequently leads to their different thermoelectric properties. The electronic configuration and the difference in atomic number between the dopant and the host atom also play an important role on the thermoelectric properties of In24M8O48. Our calculations give a valuable insight on how to enhance the thermoelectric performance of In32O48.

Giant magnetic moment of the core-shell Co13@Mn20 clusters: first-principles calculations.

The core-shell clusters Co(13)@TM(20) with TM = Mn, Fe, Co, and Ni are investigated within first-principles simulations in the framework of density-functional theory. Huge magnetic moments have been found in the Co(13)@TM(20) clusters especially for the Co(13)@Mn(20) cluster with a giant magnetic moment of 113 µ(B). The large magnetic moments are mainly due to the special core-shell structure and the weak interaction between the TM and other atoms.

Density functional study of CO adsorbed on MnN (N = 2-8) clusters.

The geometry, electronic structure, magnetism, and adsorption properties of one CO molecule on the Mn(N) (N = 2-8) clusters have been investigated based on the density functional theory (DFT) with the spin polarized generalized gradient approximation. It is found that the CO molecule adsorbs on the atop site for N = 2, 4, 7, 8 and on the bridge site for N = 3, 5, 6. The results of the calculated second-order energy differences of bare Mn(N) cluster indicate that the Mn(3), Mn(6), and Mn(8) clusters have relatively low stability. However, their corresponding CO complexes possess high adsorption ability implied by the C-O bond length, vibrational frequency, adsorption energy, and the charge transfer between the CO molecule and the clusters. Compared with bare Mn clusters, the adsorbing of a CO molecule enhances the magnetic moments of the Mn(N) clusters for N = 4, 6-8.

First-principles study of the structural, elastic, and electronic properties of C(20), C(12)B(8), and C(12)N(8).

First-principles calculations were performed to study the structural, elastic, and electronic properties of the crystalline form of C(20), C(12)B(8), and C(12)N(8). These compounds exhibit very different elastic and electronic properties. The shear modulus of C(12)N(8) is much higher than those of C(20) and C(12)B(8). The strong covalent C-N interaction plays an important role in this high shear modulus. Compared with C(20), the relatively small Zener anisotropy of C(12)N(8) is mainly due to its large elastic constant (C(11) - C(12)). The calculated band structure shows that C(12)N(8) is an insulator with a direct band gap of 3 eV and the other two compounds (C(20) and C(12)B(8)) are metallic. Analysis of the band structure, density of states, and charge density show that the degree of filling in the non-bonding 2p(z) strongly affects the electronic properties. The full filling of the non-bonding orbital for C(12)N(8) results in its insulating behavior.

Density-functional study of structural, electronic, and magnetic properties of the EuSi(n) (n=1-13) clusters.

The geometries, stabilities, and electronic and magnetic properties of europium encapsulated EuSi(n) (n=1-13) clusters have been investigated systematically by using relativistic density functional theory with generalized gradient approximation. Starting from n=12, the Eu atom completely falls into the center of the Si frame, i.e., EuSi(12) is the smallest fully endohedral Eu silicon cluster. The interesting finding is in good agreement with the recent experimental results on the photoelectron spectroscopy of the europium silicon clusters [A. Grubisic, H. P. Wang, Y. J. Ko, and K. H. Bowen, J. Chem. Phys. 129, 054302 (2008)]. The magnetic moments of the EuSi(n) (n=1-13) clusters are also studied, and the results show that the total magnetic moments of the EuSi(n) clusters and the magnetic moments on Eu do not quench when the Eu is encapsulated in the Si outer frame cage. It is concluded that most of the 4f electrons of the Eu atom in the EuSi(12) cluster do not interact with the silicon cage and the total magnetic moments are overwhelming majority contributed by the 4f electrons of the Eu atom. According to the binding energy per atom, the second difference in energy (Delta(2)E), and vertical ionization potential, the EuSi(n) (n=4,9,12) clusters are very stable.

The competition of double-, four-, and three-ring tubular B(3n) (n = 8-32) nanoclusters.

The geometry and electronic properties of three-ring tubular B(3n) clusters (n = 8-32) are studied systematically with the density functional theory. It is composed of three staggered rings with the diameter of the middle ring larger than those of the two outer rings. With the increase in boron atom numbers, the three-ring tubular clusters are energetically more stable than the double-ring and four-ring tubular clusters and the buckled sheet clusters with hexagon holes. The average binding energy tends to the finite value. The stability is further analyzed through the natural bond orbital population analysis. The highest occupied and lowest unoccupied energy gaps become small, which demonstrates a favorable metallic property.

No quenching of magnetic moment for the GenCo (n=1-13) clusters: first-principles calculations.

The authors predict that for the Ge(n)Co (n=1-13) clusters the magnetic moment does not quench, which is dark contrast to the previous results with transition-metal-doped Si(n) clusters. It may be due to the unpaired electrons of the Co atom in the clusters. For the ground state structures of the Ge(n)Co (n>or=9) clusters, the Co atom completely falls into the center of the Ge outer frame, forming metal-encapsulated Ge(n) cages. The doping of the Co atom enhances the stability of the host Ge(n) clusters. The Ge(10)Co cluster with the bicapped tetragonal antiprism structure is more stable than others, which agrees very well with the results of the experiment of the Co/Ge binary clusters by the laser vaporization.

A density functional study of YnAl (n=1-14) clusters.

The geometries, stabilities, and electronic and magnetic properties of Y(n)Al (n=1-14) clusters have been systematically investigated by using density functional theory with generalized gradient approximation. The growth pattern for different sized Y(n)Al (n=1-14) clusters is Al-substituted Y(n+1) clusters and it keeps the similar frameworks of the most stable Y(n+1) clusters except for Y(9)Al cluster. The Al atom substituted the surface atom of the Y(n+1) clusters for n<9. Starting from n=9, the Al atom completely falls into the center of the Y-frame. The Al atom substituted the center atom of the Y(n+1) clusters to form the Al-encapsulated Y(n) geometries for n>9. The calculated results manifest that doping of the Al atom contributes to strengthen the stabilities of the yttrium framework. In addition, the relative stability of Y(12)Al is the strongest among all different sized Y(n)Al clusters, which might stem from its highly symmetric geometry. Mulliken population analysis shows that the charges always transfer from Y atoms to Al atom in all different sized clusters. Doping of the Al atom decreases the average magnetic moments of most Y(n) clusters. Especially, the magnetic moment is completely quenched after doping Al in the Y(13), which is ascribed to the disappearance of the ininerant 4d electron spin exchange effect. Finally, the frontier orbitals properties of Y(n)Al are also discussed.

First-principles study of the (001) surface of cubic SrHfO(3) and SrTiO(3).

We have performed first-principles calculations on the (001) surface of cubic SrHfO(3) and SrTiO(3) with SrO and BO(2) (B = Ti or Hf) terminations. Surface structure, partial density of states, band structure, and surface energy have been obtained. For the BO(2)-terminated surface, the largest relaxation appears on the second-layer atoms but not on the first-layer ones. The analysis of the structure relaxation parameters reveals that the rumpling of the (001) surface for SrHfO(3) with SrO termination is stronger than that for SrTiO(3). For the HfO(2)-terminated surface of SrHfO(3), the surface state appears near the M point of its band structure.