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Elastic anisotropies obtained by density functional theory (DFT) calculations of four iron carbides exhibiting trigonal prismatic coordination of C by Fe are presented and compared. The elastic anisotropy is considerable for the carbides θ-Fe 3 C, χ-Fe 5 C 2 and a hypothetical Fe 2 C with Pbcm symmetry, all with sheet-like arrangement of the trigonal prisms which results in a largely parallel arrangement of the prism axes. It is found that the easy shear which is mainly responsible for the large elastic anisotropy in these carbides occurs in all these three carbides in a plane perpendicular to the average axis of the prisms. In contrast to these three elastically anisotropic carbides, Fe 7 C 3 , in which the axes of the trigonal prisms are much more regularly distributed in space, shows a negligible elastic anisotropy. [ABSTRACT FROM AUTHOR]

Abstract: Influence of temperature on the elastic properties of Mg2X (X=Si, Ge, Sn, and Pb) compounds, has been studied using first-principles calculations, within the generalized gradient approximation, and compared with the available experimental data in the literature. Elastic stiffness coefficients calculated with respect to volume (c ij (V)) have been correlated to the equilibrium volume as a function of temperature V(T) from phonon calculations to obtain temperature dependence of elastic stiffness coefficients c ij (T). A good agreement between the thus predicted elastic constants and experimental data has been achieved. The general trend in the bulk modulus (B), shear modulus (G) and Young''s modulus (E) seen for the compounds is Mg2Ge>Mg2Si>Mg2Sn>Mg2Pb. Elastic anisotropy, fracture toughness and stiffness of the compounds have been analyzed as a function of temperature based on their anisotropic ratio (2c 44/(c 11–c 12)), product of bulk modulus and volume (B × V 1/3), and Young''s modulus. The results obtained herein provide a better understanding of the elastic behavior of antifluorite compounds as a function of temperature. The methodology used in this work acts as a benchmark for future first-principles work that involves calculating elastic constants as a function of temperature. [Copyright &y& Elsevier]

Abstract NiSn 4 grows in diffusion couples (Ni plates with electrodeposited Sn) in a temperature range from room temperature up to 353 K. Previously, the crystal structure of NiSn 4 grown at ambient temperature was identified to be an intermediate stacking variant between the orthorhombic PtSn 4 -type and the tetragonal β-IrSn 4 -type structure, with a slight tendency towards the tetragonal type (Schimpf et al., Mater. Design 109 (2016) 324–333). Now it is reported that NiSn 4 forming at 333 K and above, exhibits a 12-layer polytype (space group P 4 2 / nbc , a = b = 6.250 Å, c = 69.00 Å) due to ordered incorporation of stacking faults into the structure. This NiSn 4 structure was derived by comparison of its diffraction patterns with those of the exhaustively generated NiSn 4 polytypes. First-principles calculations on a series of selected NiSn 4 polytypes demonstrate that the experimentally observed polytype is energetically favoured as compared to others. Highlights • Ni electroplated with Sn and annealed at 293–353 K leads to formation of NiSn4 intermetallic. • Whereas at 293–313 K NiSn4 shows disordered stacking, at 333–353 K a 12-layer polytype forms. • The 12-layer polytype is identified as of P42/nbc symmetry. • DFT calculations reveal a lower energy of the 12-layer polytype as compared to others. [ABSTRACT FROM AUTHOR]

Abstract: Thermodynamic and mechanical properties of the six known phases in the La–Mg phase diagram, viz. LaMg, LaMg2, LaMg3, La5Mg41, La2Mg17, and LaMg12, and their elemental antecedents, Mg and La, are computed with density functional theory (DFT) using the PBE and PBEsol exchange-correlation functionals. Phase stability analyses show that both LaMg2 and La5Mg41 are metastable at low temperatures which is consistent with experiments and vibrational spectra. We generalize an existing approach for computing the crystallographic dependence of Young''s modulus and Poisson''s ratio, which is presently limited to cubic systems, to address any space group symmetry using 0K elasticity tensor components (C ij ) from DFT. Isothermal and isentropic C ij (T) are computed with the quasiharmonic approximation (QHA) as are the linear thermal expansion of the cubic compounds, the average linear thermal expansion for the non-cubic compounds, the bulk modulus, and the constant pressure heat capacity. A critical comparison of theoretical results from the PBE and PBEsol functionals is made with available experimental data. [Copyright &y& Elsevier]