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• The perovskite HoMnO 3 crystal structures are studied in this work. • The simulations using Quantum Espresso are applied and discussed. • The DFT method is applied to understand its properties. • The elastic properties are investigated. • The mechanical behavior is deduced. The structural and elastic properties of the oxide perovskite HoMnO 3 have been investigated by density-functional theory (DFT) implemented on the Quantum ESPRESSO code for the hexagonal, orthorhombic, rhombohedral and cubic crystal structures in the non-magnetic (NM), ferromagnetic (FM) and antiferromagnetic (AFM) configurations. The results show that all these compounds are thermodynamically stable, the hexagonal and the orthorhombic being the most stables in agreement with experimental results. The calculated bulk modulus, B, is about 168.5 ± 1.0 GPa and 178.9 ± 1.0 GPa for hexagonal structure in FM and configurations, respectively, and 188.7 ± 0.6 GPa and 175.6 ± 2.1GPa for orthorhombic structure in FM and AFM configurations, respectively. These values and those of the elastic constants C ij in the present work are comparable to values recently obtained with DFT calculations and experimentally for other perovskites RMnO 3 (R = Eu, Gd, Tb and Dy). The calculated Poisson's ratio is about 0.19; 0.27; 0.23 and 0.3 for hexagonal, orthorhombic, rhombohedral and cubic structure, respectively. The calculated Pugh's ratio is about 1.30; 1.77–1.98; 1.54–1.60 and 2.11, respectively. These results show that the perovskite HoMnO 3 tends to be brittle in hexagonal phase and ductile in cubic phase. In the orthorhombic or rhombohedral phase, it is near brittle-ductile boarder line. [ABSTRACT FROM AUTHOR]

Based on the results of ab-initio calculations, a new type of 3D carbon-based material, where graphene sheets are connected with each other by carbyne chains, is predicted. A simultaneous presence of three different types of hybridization (sp 1 -sp 2 -sp 3 ) is shown to be the reason for unusual mechanical properties of this material. Four basic types of such 3D-structures are considered, which differ both in the type of contact bonds of the edge atom of the carbyne chain with graphene sheet and in their location on graphene sheet. “Stress-strain” diagrams for these structures are obtained; their strength and elastic properties are ascertained. It is shown that the strength of contact bond in such a structure can reach 67% of maximum attainable strength, which is realized in five-atom carbyne chain. This is one of the main factors, which guarantees extremely high levels of strength of considered 3D-structures. The linear dependence between the strength of contact bond and its equilibrium length is established, which determines the regularities of atomic structure effect on the strength of such structures. It is shown that a combination of high strength with relatively low values of elasticity moduli is a characteristic feature of considered 3D-structures, which is very attractive for both nano-devices and medical applications. [ABSTRACT FROM AUTHOR]

The tendency to dissolve in the matrix for alloying elements such as transition metals and some main group elements in α-Nb 5 Si 3 phase as well as their effects on the structure stability and mechanical properties are important for the performance of niobium-silicide based alloys, which, however, are not clear yet. In this work, we unravel the above problems based on ab - initio calculations. Our results show that the alloyed Nb 5 Si 3 systems become less stable as the alloying elements change from group IVB to VIB in the periodic table. The occupation preferences of the alloying elements depend on their relative atomic radii respect to either Nb or Si. Furthermore, the dissolution of the alloying elements is easier at high temperatures by the Debye model analysis, from which the homogenization-treatment temperatures of alloyed Nb 5 Si 3 phases are also deduced. All alloyed Nb 5 Si 3 phases are mechanically stable, even though their mechanical properties like ductility are not improved. Finally, the electron localized function, Bader charge and densities of states are used to understand the structure stability of alloyed Nb 5 Si 3 , and we find that ionic bonding has quite significant effects on the stability of these intermetallic compounds. [ABSTRACT FROM AUTHOR]

Elastic and thermodynamic properties for the stable phases in the Nb–Al system were studied with the first-principles pseudo-potential plane-wave method based on density functional theory. The temperature dependence of the enthalpy of formation for the NbAl 3 and Nb 2 Al intermetallics are reported for the first time using the quasi-harmonic approximation. The elastic properties of Nb 2 Al and the linear thermal expansion coefficient and the elastic properties versus temperature of the Nb 3 Al are reported for the first time. The calculations would suggest that the NbAl 3 is the stiffest and most brittle of the Nb–Al intermetallics. [ABSTRACT FROM AUTHOR]

Abstract: The electronic, elastic and mechanical properties of eight binary B 2 – type intermetallic compounds of Ag and Au (AgX and AuX, X=Mg, Sc, Zn and Cd) have been studied systematically using first principles density functional theory within generalized gradient approximation. Ground state properties such as lattice constant (a 0), bulk modulus (B), its pressure derivative (B′) and elastic constants are calculated. The present results are in good agreement with the experimental and other available theoretical results. Poisson’s ratio (σ), Young’s moduli (E), shear moduli (GH ) and the ratio of elastic anisotropy factor (A) are also estimated. Ductility for these compounds is further analysed by calculating the ratio of B/GH and Cauchy pressure (C 12–C 44). Amongst all the intermetallics AuZn is found to be most ductile due to the presence of strong metallic bonding, and AgZn is least ductile in all the Au and Ag intermetallic compounds. [Copyright &y& Elsevier]

Abstract: To investigate the structural, elastic, and lattice dynamical properties of the germanium diiodide, we have performed the first-principles calculations by using the local density approximation method based on density-functional theory. Some basic physical parameters such as lattice constant, bulk modulus and its first derivatives, elastic constants, shear modulus, Young’s modulus, and Poisson’s ratio are calculated. The phonon dispersion curves, electronic band-structures, and total and partial density of states have also been calculated for ground state C6 phase of GeI2. Our results show that this structure has got 1.72eV direct band gap. Our secondary results on the temperature-dependent behavior of thermodynamical properties such as entropy, heat capacity, internal energy, and free energy are also presented for the same compounds. The obtained results are in good agreement with the available experimental and other theoretical data. [ABSTRACT FROM AUTHOR]