The stabilities, electronic structures and elastic properties of Rbben As systems

The structural, electronic and elastic properties of Rb-As systems (RbAs in NaP, LiAs and AuCu structures, RbAs₂ in the MgCu₂ structure, Rb₃As in Na₃As, Cu₃P and Li₃Bi structures, and Rb₅As₄ in the A₅B₄ structure) are investigated with the generalized gradient approximation in the frame of density functional theory. The lattice parameters, cohesive energies, formation energies, bulk moduli and the first derivatives of the bulk moduli (to fit Murnaghan's equation of state) of the considered structures are calculated and reasonable agreement is obtained. In addition, the phase transition pressures are also predicted. The electronic band structures, the partial densities of states corresponding to the band structures and the charge density distributions are presented and analysed. The second-order elastic constants based on the stress-strain method and other related quantities such as Young's modulus, the shear modulus, Poisson's ratio, sound velocities, the Debye temperature and shear anisotropy factors are also estimated.     

The stabilities, electronic structures and elastic properties of Rb-As systems

The structural, electronic and elastic properties of Rb–As systems (RbAs in NaP, LiAs and AuCu structures, RbAs2 in the MgCu2 structure, Rb3 As in Na3As, Cu3 P and Li3Bi structures, and Rb5 As4 in the A5B4 structure) are investigated with the generalized gradient approximation in the frame of density functional theory. The lattice parameters, cohesive energies, formation energies, bulk moduli and the first derivatives of the bulk moduli (to fit Murnaghan’s equation of state) of the considered structures are calculated and reasonable agreement is obtained. In addition, the phase transition pressures are also predicted. The electronic band structures, the partial densities of states corresponding to the band structures and the charge density distributions are presented and analysed. The second-order elastic constants based on the stress-strain method and other related quantities such as Young’s modulus, the shear modulus, Poisson’s ratio, sound velocities, the Debye temperature and shear anisotropy factors are also estimated.     

Simulation of plastic deformation in glassy polymers: Atomistic and mesoscale approaches

The mechanism of deformation in glasses is very different from that of crystals, even though their general behavior is very similar. In this study, we investigated the deformation of polycarbonate on the atomistic scale with molecular dynamics and on the continuum scale with a new simulation approach. The results indicated that high atomic/segmental mobility and low local density enabled the formation (nucleation) of highly deformed regions that grew to form plastic defects called plastic shear transformations. A continuum-scale simulation was performed with the concept of plastic shear transformations as the basic region of deformation. The continuum simulations were able to predict the primary and secondary creep behavior. The slope of the secondary creep depended on the interactions between the plastic shear transformations. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 994-1004, 2005     

Mechanical and dynamical stability of TiAsTe compound from ab initio calculations

The first-principles calculations are employed to provide a fundamental understanding of the structural features and relative thermodynamical, mechanical and phonon stability of TiAsTe compound. The calculated lattice parameters are in good agreement with available experimental results. We have computed elastic constants, its derived moduli and ratios that characterize mechanical properties for the first time. The calculated elastic constants indicate that these materials are mechanically stable at ambient condition. The minimum thermal conductivities of TiAsTe are calculated using both Clarke’s model and Cahill’s model. Furthermore, the elastic anisotropy has been visualized in detail by plotting the directional dependence of compressibility, Young’s modulus and shear modulus. Our results suggest strong elastic anisotropy for this compound. Additionally, the phonon spectra and phonon density of states are also obtained and discussed. The full phonon dispersion calculations confirm the dynamic stability of TiAsTe.     

Lattice vibrational properties of Al2X (X = Sc,Y) from density functional theory calculations

The phonon dispersion curves of the C15-type of Al₂Sc and Al₂Y compounds are investigated, using density functional theory within the generalized gradient approximation (GGA), based on the “direct method”. The obtained results show that both compounds are dynamically stable. The temperature dependence of the various thermodynamical quantities such as internal energy, free energy, entropy, and heat capacity are also predicted under the harmonic approximation, and the observed trends are discussed in detail.     

First-principles studies of the structural,elastic, and lattice dynamical properties of ZrMo2 and HfMo2

A comprehensive investigation of the structural, elastic, and lattice dynamical properties for ZrMo₂ and HfMo₂ with C14, C15, C36, and CeCu₂ phases are conducted using density functional total energy calculations. The results have showed that C15 phase for both materials is energetically more stable than C14, C36 and CeCu₂ phases. We have also estimated the mechanical behaviours of these compounds, including mechanical stability, bulk modulus, Young's modulus, shear modulus, Poisson's ratio, ductility, and anisotropy. Additionally, the lattice dynamical properties are analyzed and discussed exhaustively for these phases. The calculated properties agree well with available experimental and theoretical data.     

Theoretical predictions of the structural,mechanical and lattice dynamical properties of XW2 (X = Zr,Hf) Laves phases

We have investigated the structural, mechanical and lattice dynamical properties of ZrW₂ and HfW₂ compounds in cubic C15 (space group Fd-3m), hexagonal C14 (space group P6₃/mmc) and C36 (space group P6₃/mmc) phases using generalized gradient approximation within the plane-wave pseudo-potential density functional theory. We have found that ZrW₂ and HfW₂ in cubic C15 phase are the most stable among the considered phases. From calculated elastic constants, it is shown that all phases are mechanically stable according to the elastic stability criteria. The related mechanical properties, such as bulk, shear and Young moduli, Poisson’s ratio, Debye temperature and hardness have been also calculated. The results show that ZrW₂ and HfW₂ compounds are ductile in nature with respect to the B/G and Cauchy pressure analysis. The phonon dispersion curves, phonon density of states and some thermodynamic properties are computed and discussed exhaustively for considered phases.     

Influence of stereoerrors on the formation of helices during early stage crystallization of isotactic polyproyplene

Coarse-grained, on-lattice Monte Carlo simulations are performed to investigate the role of stereotacticity defects along an isotactic polypropylene chain on the formation of 3₁-helices, which form the basic crystalline order within the chain. For this reason, systems with various stereoerror configurations are studied and are compared to neat isotactic polypropylene. All systems are equilibrated above the melting temperature and are cooled to lower temperatures in a stepwise manner, making sure each system is equilibrated at every temperature. Results indicate that chain ends have the lowest probability of being found in helices. Addition of a single stereoerror (mrm) decreases the probability of five repeat units' participation in helices (the repeat unit that contains the stereoerror and two nearest repeat units on both sides). The probability profile becomes more complicated when the number of stereoerrors increases, however, the results indicate that the effects of many stereoerrors can be explained by a simple addition of the effect of each stereoerror considered individually. The results also indicate that the presence of even a single stereoerror eliminates (within the temperature range studied) the transition to longer, more stable helices observed in neat isotactic polypropylene. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3349–3360, 2007     

First principles study on the structural, electronic, and elastic properties of Na-As systems

We have performed the first principles calculation by using the plane-wave pseudopotential approach with the generalized gradient approximation for investigating the structural, electronic, and elastic properties Na-As systems (NaAs in NaP, LiAs and AuCu-type structures, NaAs₂ in MgCu₂-type structure, Na₃As in Na₃As, Cu₃P and Li₃Bi-type structures, and Na₅As₄ in A₅B₄-type structure). The lattice parameters, cohesive energy, formation energy, bulk modulus, and the first derivative of bulk modulus (to fit to Murnaghan’s equation of state) of the related structures are calculated. The second-order elastic constants and the other related quantities such as Young’s modulus, shear modulus, Poisson’s ratio, sound velocities, and Debye temperature are also estimated.     

Calculation of the stability and mechanical and phonon properties of NbRuB,TaRuB, and NbOsB compounds

The structural, mechanical, and phonon properties of NbRuB, TaRuB, and NbOsB compounds with orthorhombic space groups Pmma (No. 51) and Pbam (No. 55) are investigated by using first-principles calculations. The elastic constants and moduli, Debye temperature, Poisson’s ration, Pugh’s ratio, elastic anisotropy factors, and minimum thermal conductivities have been predicted to understand the mechanical behavior of these ternary compounds. The mechanical anisotropy is discussed via several anisotropy indices and two-dimensional (2D) surface constructions. We observe that all compounds ought to be classified as ductile materials and exhibit elastically anisotropy. Their mechanical and dynamical stability is confirmed via the calculated elastic constants and phonon dispersion curves, respectively. This work should provide a useful guide for designing ternary boride materials that have excellent thermoelectric performance.