Melting behaviors of defective crystals under extreme conditions are theoretically investigated using the statistical moment method. In our theoretical model, heating processes cause missing atoms or vacancies in crystal structures via dislocating them from their equilibrium positions. The coordination number of some atoms is assumed to be removed by one unit and the defect depends on temperature and external pressure. We formulate analytical expressions to directly connect the equilibrium vacancy concentration, the elastic modulus, and the melting temperature. Numerical calculations are carried out for six transition metals including Cu, Ag, Au, Mo, Ta, and W up to 400 GPa. The obtained results show that vacancies strongly drive the melting transition. Ignoring the vacancy formation leads to incorrect predictions of the melting point. The good agreement between our numerical data and prior experimental works validates the accuracy of our approach.
A detailed description of the statistical distribution of thermal vacancies near the melting point is presented, using copper as an example. As the temperature is increased, the average number of thermal vacancies generated by atoms migrating to neighboring sites also increase, according to Arrhenius' law. We present for the first time a model for the distribution of thermal vacancies, which according to our results follow a Gamma distribution. All the simulations are carried out by classical molecular dynamics and the recognition of vacancies is achieved via a recently developed algorithm. Ou…
In a large variety of ideal crystals we found that when rapidly migrating atoms squash or annihilate a neighbouring vacancy and produce a disordered cluster, the heat of migration stored in the system exceeds the enthalpy increase required for the coordinating atoms of the vacancy to form a liquid phase, i.e. the liquid phase nucleates from vacancies. Furthermore volumetric analysis supports this well. This vacancy-decomposition model provides quantitative information on the melting point, the latent heat and the volume change upon melting and hence clarifies the mechanism of melting.
The features of alternative approach of non-equilibrium evolution thermodynamics are shown on the example of theory of vacancies by opposed to the classic prototype of Landau theory. On this foundation a strict theory of the melting of metals, based on development of Frenkel ideas about the vacancy mechanism of such phenomena, is considered. The phenomenon of melting is able to be described as a discontinuous phase transition, while the traditional Frenkel's solution in the region of low-concentration of vacancies can describe such transition only as continuous one. The problem of limiting tra…
Large-scale atomistic simulations using the reactive empirical bond order force field approach is implemented to investigate thermal and mechanical properties of single-layer (SL) and multi-layer (ML) molybdenum disulfide (MoS$_2$). The amplitude of the intrinsic ripples of SL-MoS$_2$ are found to be smaller than those exhibited by graphene (GE). Furthermore, because of the van der Waals interaction between layers, the out-of-plane thermal fluctuations of ML-MoS$_2$ decreases rapidly with increasing number of layers. This trend is confirmed by the buckling transition due to uniaxial stress whi…
We present molecular dynamics simulations of the influence of point defects on the shear elastic coefficients of copper. We find that vacancies do not influence these coefficients at all, while the introduction of interstitials causes a large reduction in the elastic coefficients. The simulations establish a phase diagram of the melting temperature as a function of the density of interstitials. A crystal having no free surface undergoes bulk mechanical melting as a result of the vanishing of C'=(C/sub/11-C/sub/12)/2 once the specific volume reaches a critical value, equal to the experimental v…
We report a combined experimental and theoretical study of boron carbide under stress/deformation. A special rotating anvil press, the rotating tomography Paris Edinburgh cell (RotoPEC), has been used to apply torsional deformation to boron carbide under a pressure of 5~GPa at ambient temperature. Subsequent damages and point defects have been analysed at ambient pressure by energy dispersive X-ray microdiffraction at the synchrotron and by Raman spectroscopy, combined with calculations based on the density functional theory (DFT). We show that apart from the signals due to B$_4$C, new peaks a…
A combination of systematic density functional theory (DFT) calculations and machine learning techniques has a wide range of potential applications. This study presents an application of the combination of systematic DFT calculations and regression techniques to the prediction of the melting temperature for single and binary compounds. Here we adopt the ordinary least-squares regression (OLSR), partial least-squares regression (PLSR), support vector regression (SVR) and Gaussian process regression (GPR). Among the four kinds of regression techniques, the SVR provides the best prediction. In ad…
Accurate prediction of thermodynamic properties requires an extremely accurate representation of the free energy surface. Requirements are twofold -- first, the inclusion of the relevant finite-temperature mechanisms, and second, a dense volume-temperature grid on which the calculations are performed. A systematic workflow for such calculations requires computational efficiency and reliability, and has not been available within an ab initio framework so far. Here, we elucidate such a framework involving direct upsampling, thermodynamic integration and machine-learning potentials, allowing us t…
We present an ab initio study on the structural and electronic distortions of modified graphene by creation of vacancies, inclusion of boron atoms, and the coexistence of both, by means of thermodynamics and band structure calculations. In the case of coexistence of boron atoms and vacancy, the modified graphene presents spin polarization only when B atoms locate far from vacancy. Thus, when a boron atom fills single- and di-vacancies, it suppresses the spin polarization of the charge density. In particular when B atoms fill a di-vacancy a new type of rearrangement occurs, where a stable BC4 u…
We present the results of molecular dynamics (MD) studies of hexane physisorbed onto graphite for eight coverages in the range $0.875 \le ρ\le 1.05$ (in units of monolayers). At low temperatures the adsorbate molecules form a uniaxially incommensurate herringbone (UI-HB) solid. At high coverages the solid consists of adsorbate molecules that are primarily rolled on their side perpen-dicular to the surface of the substrate. As the coverage is decreased, the amount of molecular rolling diminishes until $ρ$ = 0.933 where it disappears (molecules become primarily parallel to the surface). If the d…
With density functional theory we have performed molecular dynamics simulations of ZrC which displayed spontaneous Frenkel pair formation at a temperature of 3200 K, some 500 K below the melting point. To understand this behaviour, rarely seen in equilibrium simulations, we quenched and examined a set of lattices containing a Frenkel pair. Five metastable structures were found, and their formation energies and electronic properties were studied. Their thermal generation was found to be facilitated by a reduction of between 0.7 and 1.5 eV in formation energy due to thermal expansion of the latt…
Lattice stability and metastability, as well as melting, are important features of the physics and chemistry of dense hydrogen. Using ab initio molecular dynamics (AIMD), the classical superheating limit and melting line of metallic hydrogen are investigated up to 1.5 TPa. The computations show that the classical superheating degree is about 100 K, and the classical melting curve becomes flat at a level of 350 K when beyond 500 GPa. This information allows us to estimate the well depth and the potential barriers that must be overcome when the crystal melts. Inclusion of nuclear quantum effects…
Nonstoichiometric ceria(CeO$_{2-δ}$) is a candidate reaction medium to facilitate two step water splitting cycles and generate hydrogen. Improving upon its thermodynamic suitability through doping requires an understanding of its vacancy thermodynamics. Using density functional theory(DFT) calculations and a cluster expansion based Monte Carlo simulations, we have studied the high temperature thermodynamics of intrinsic oxygen vacancies in ceria. The DFT+$U$ approach was used to get the ground state energies of various vacancy configurations in ceria, which were subsequently fit to a cluster e…
We present the first quantum molecular dynamics calculation of zero-pressure isobar of solid and liquid uranium that account for spin-orbit coupling. We demonstrate that inclusion of spin-orbit interaction leads to higher degree of the thermal expansion of uranium, especially in the liquid phase. Full accounting of relativistic effects for valence electrons, particularly spin-orbital splitting of the 5f band, is substantial for the reproduction of the experimental density of molten uranium at the melting temperature. Influence of the spin-orbit interaction on the thermodynamic properties at hi…
The modification of the properties of CeO$_2$ through aliovalent doping are investigated within the \emph{ab-initio} density functional theory framework. Lattice parameters, dopant atomic radii, bulk moduli and thermal expansion coefficients of fluorite type Ce$_{1-x}$M$_{x}$O$_{2-y}$ (with M$ = $ Mg, V, Co, Cu, Zn, Nb, Ba, La, Sm, Gd, Yb, and Bi)are presented for dopant concentrations in the range $0.00 \leq x \leq 0.25$. The stability of the dopants is compared and discussed, and the influence of oxygen vacancies is investigated. It is shown that oxygen vacancies tend to increase the lattice…
Additive manufacturing of a single crystalline metallic column is studied using molecular dynamics simulations. In the model, a melt pool is incrementally added and cooled to a target temperature under isobaric conditions to build a metallic column from bottom up. Common neighbor analysis (CNA) is used to observe the evolution of atomic scale defects during this process. The solidification is seen to proceed along two directions for an added molten layer. The molten layer in contact with the cooler lattice has a fast solidification front that competes with the slower solidification front start…
Crystal growth of MnBi$_{2}$Te$_{4}$ has delivered the first experimental corroboration of the 3D antiferromagnetic topological insulator state. Our present results confirm that the synthesis of MnBi$_{2}$Te$_{4}$ can be scaled-up and strengthen it as a promising experimental platform for studies of a crossover between magnetic ordering and non-trivial topology. High-quality single crystals of MnBi$_{2}$Te$_{4}$ are grown by slow cooling within a narrow range between the melting points of Bi$_{2}$Te$_{3}$ (586 °C) and MnBi$_{2}$Te$_{4}$ (600 °C). Single crystal X-ray diffraction and electron m…
With single crystal X-ray diffraction studies, we compare the structures of three sample showing optimal superconductivity, K0.774(4)Fe1.613(2)Se2, K0.738(6)Fe1.631(3)Se2 and Cs0.748(2)Fe1.626(1)Se2. All have an almost identical ordered vacancy structure with a ({\sqrt}5 x {\sqrt}5 x 1) super cell. The tetragonal unit cell, space group I4/m, possesses lattice parameters at 250K of a = b = 8.729(2) Å and c = 14.120(3) Å, a = b = 8.7186(12) Å and c = 14.0853(19) Å and at 295 K, a = b = 8.8617(16) Å and c = 15.304(3) Å for the three crystals, respectively. The structure contains two iron sites; o…
