First-principles study of elastic and thermal properties of refractory carbides and nitrides

CRYSTAL23: A Program for Computational Solid State Physics and Chemistry

The Crystal program for quantum-mechanical simulations of materials has been bridging the realm of molecular quantum chemistry to the realm of solid state physics for many years, since its first public version released back in 1988. This peculiarity stems from the use of atom-centered basis functions within a linear combination of atomic orbitals (LCAO) approach and from the corresponding efficiency in the evaluation of the exact Fock exchange series. In particular, this has led to the implementation of a rich variety of hybrid density functional approximations since 1998. Nowadays, it is acknowledged by a broad community of solid state chemists and physicists that the inclusion of a fraction of Fock exchange in the exchange-correlation potential of the density functional theory is key to a better description of many properties of materials (electronic, magnetic, mechanical, spintronic, lattice-dynamical, etc.). Here, the main developments made to the program in the last five years (i.e., since the previous release, Crystal17) are presented and some of their most noteworthy applications reviewed.

Residual Stress Evolution in Low-Alloyed Steel at Three Different Length Scales

Quantitative and qualitative residual stress evolution in low-alloyed steel during heat treatment is investigated on three different length scales for sourgas resistant seamless steel tubes: on the component level, on the level of interdendritic segregation and on precipitate scale. The macroscopic temperature, phase and stress evolution on the component scale result from a continuum model of the heat treatment process. The strain and temperature evolution is transferred to a mesoscopic submodel, which resolves the locally varying chemistry being a result of interdendritic segregation. Within the segregation area and the surrounding matrix precipitates form. They are categorized with respect to their tendency for formation of microscopic residual stresses. After rapid cooling macroscopic stresses up to 700 MPa may form dependent on the cooling procedure. Mesoscopic stresses up to Δ50 MPa form depending on the extent of segregation. Carbides and inclusions occuring in low-alloyed steel are ranked by their tendency for residual stress formation in the iron matrix. This scale bridging study gives an overview of residual stresses, their magnitude and evolution on three different length scales in low-alloyed steels and the results presented can serve as a input for steel design.

Mechanical properties of nonstoichiometric cubic titanium carbide TiCy

The changes of elastic constants c_ij in the homogeneity interval from TiC_0.50 to TiC_1.00 of nonstoichiometric disordered cubic titanium carbide are evaluated by a semiempirical method for the first time. It is established that the elastic stiffness constants c_ij of a disordered TiC_y decrease as the titanium carbide composition deviates from stoichiometry. The distributions of the Young's modulus E, the Poisson's ratio μ, shear G and bulk B moduli of monocrystalline particles of cubic TiC_y on the crystallographic direction [hkl] and on relative carbon content y are calculated. The lowest values of G_hkl for TiC_y are observed in the (111) plane. It is found that the deviation of titanium carbide from stoichiometric composition TiC_1.0 is accompanied by increase of the elastic anisotropy. It is shown that the calculated Vickers hardness H_V of titanium carbide on the relative carbon content y increases non-linearly with growing y.

The thermodynamic stability and mechanical properties of TiCxN1-x(0x⩽ 1) compounds by cluster expansion method and first-principles calculations

The thermodynamic stability and mechanical properties of titanium carbonitrides TiC_xN_1-x(0 ⩽x⩽ 1) are investigated by a combination of the universal cluster expansion method and the first-principles calculations. By considering the ordering of the N/C distributions on the anion sublattice sites of TiC_xN_1-x, a binary diagram of the heat of formation is constructed, and seven kinds of ground-state structures are predicted in the whole range of 0 ⩽x⩽ 1. These predicted ground-state TiC_xN_1-xstructures are further proved to be dynamically and mechanically stable by examining their phonon dispersion spectra and elastic constants. Further studies indicate that the mechanical and thermodynamic properties of the ternary TiC_xN_1-xstructures are generally better than those of the binary TiC or TiN, while the differences within the ternary systems are insignificant. The possible origin of the enhancement of the mechanical and thermodynamic properties of the predicted ground-state TiC_xN_1-xare discussed together with the electronic structures.

Thermoelasticity of Flexible Organic Crystals from Quasi-harmonic Lattice Dynamics: The Case of Copper(II) Acetylacetonate

A computationally affordable approach, based on quasi-harmonic lattice dynamics, is presented for the quantum-mechanical calculation of thermoelastic moduli of flexible, stimuli-responsive, organic crystals. The methodology relies on the simultaneous description of structural changes induced by thermal expansion and strain. The complete thermoelastic response of the mechanically flexible metal-organic copper(II) acetylacetonate crystal is determined and discussed in the temperature range 0-300 K. The elastic moduli do not just shrink with temperature but they do so anisotropically. The present results clearly indicate the need for an explicit account of thermal effects in the simulation of mechanical properties of elastically flexible organic materials. Indeed, predictions from standard static calculations on this flexible metal-organic crystal are off by up to 100%.