Finite-temperature elastic constants of paramagnetic materials within the disordered local moment picture fromab initiomolecular dynamics calculations

Abstract

We present a theoretical scheme to calculate the elastic constants of magnetic materials in the high-temperature paramagnetic state. Our approach is based on a combination of disordered local moments picture and ab initio molecular dynamics (DLM-MD). Moreover, we investigate a possibility to enhance the efficiency of the simulations of elastic properties using the recently introduced method: symmetry imposed force constant temperature-dependent effective potential (SIFC-TDEP). We have chosen cubic paramagnetic CrN as a model system. This is done due to its technological importance and its demonstrated strong coupling between magnetic and lattice degrees of freedom. We have studied the temperature-dependent single-crystal and polycrystalline elastic constants of paramagentic CrN up to 1200 K. The obtained results at T = 300 K agree well with the experimental values of polycrystalline elastic constants as well as the Poisson ratio at room temperature. We observe that the Young's modulus is strongly dependent on temperature, decreasing by ∼14% from T = 300 K to 1200 K. In addition we have studied the elastic anisotropy of CrN as a function of temperature and we observe that CrN becomes substantially more isotropic as the temperature increases. We demonstrate that the use of Birch law may lead to substantial errors for calculations of temperature induced changes of elastic moduli. The proposed methodology can be used for accurate predictions of mechanical properties of magnetic materials at temperatures above their magnetic order-disorder phase transition.

Efficient determination of the true magnetic structure in a high-throughput ab initio screening: the MDMC method

“…They introduced a so-called disordered local moment -molecular dynamics (DLM-MD) method, where MD runs in a maintained DLM state, so all the multisite interactions should be implicitly included up to the atomic distances within the size of the supercell. The reported results are in a rea-sonable agreement with experiment for the equation of state of CrN [23] and its elastic constants [24]. Nevertheless the method has other drawbacks.…”

Temperature-dependent elastic properties of binary and multicomponent high-entropy refractory carbides

“…In contrast, information concerning finite-temperature elastic properties is rather sparse for these classes of materials. Finite-temperature estimations of SOEC can be efficiently obtained using static ab initio calculations that employ the quasiharmonic approximation (QHA) [23,27,62] or volume-scaling methods [28,31]. These methods account for lattice thermal expansion while neglect anharmonic vibrational effects.…”

“…However, explicit inclusion of lattice vibrations in theoretical modeling is shown to result in significant differences in second-order elastic moduli and a change of elastic anisotropy of materials at moderate or high temperatures [28,29]. Ab initio molecular dynamics (AIMD) simulations, which inherently reproduce anharmonic vibrational effects, provide more reliable predictions of elastic constants at elevated temperature [29][30][31].…”

“…Recently, we have reported on an ab initio molecular dynamics (AIMD) approach for computing stress/strain curves of single-crystal B1 ceramics subject to tensile and shear deformation at any temperature of interest [32][33][34]. The simulations employ deformations well beyond the elastic limit conventionally used to calculate the elastic constants [28][29][30][31], and allow evaluating mechanical properties including, e.g., ideal strengths, moduli of resilience, toughness, and elongation at fracture. The method detailed in Refs.…”

Anomalous Phonon Lifetime Shortening in Paramagnetic CrN Caused by Spin-Lattice Coupling: A Combined Spin and Ab Initio Molecular Dynamics Study

“…In this way we approximate the adiabatic limit where individual spin states live too short to be "seen" by the atomic vibrations. This can be regarded as a generalization of previous DLM-AIMD works on CrN [16,37]. It considers non-collinear disorder in the limit of an infinitely fast change of magnetic states.…”