The phase diagram of the Al-Li system was determined by means of first principles calculations in combination with the cluster expansion formalism and statistical mechanics. The ground state phases were determined from first principles calculations of fcc and bcc configurations in the whole compositional range while the phase transitions as a function of temperature were ascertained from the thermodynamic grand potential and the Gibbs free energies of the phases. Overall, the calculated phase diagram was in good agreement with the currently accepted experimental phase diagram but the simulations provided new insights that are important to optimize microstructure of these alloys by means of heat treatments. In particular, the structure of the potential GP zones, made up of Al0.5Li0.5 (001) monolayers embedded in Al matrix, was identified. It was found that Al3Li is a stable phase although the energy barrier for the transformation of Al3Li into AlLi is very small (a few meV) and can be overcome by thermal vibrations. Moreover, bcc AlLi was found to be formed by martensitic transformation of fcc configurations and Al3Li precipitates stand for favorable sites for the nucleation of AlLi because they contain the basic blocks of such fcc ordering. Finally, polynomial expressions of the Gibbs free energies of the different phases as a function of temperature and composition were given, so they can be used in mesoscale simulations of precipitation in Al-Li alloys.
First Principles Phase Diagram Calculations for the Octahedral-Interstitial System $HfO_{X}$, $0 \leq X \leq 1/2$
用第一性原理计算了六方密排八面体间隙固溶体体系$αHfO_{X} ($ αHf[ ]_{1-X} O_{X} $; [ ]=Vacancy; $ 0 \leq X \leq 1/2 $). The cluster expansion method was used to do a ground state analysis, and to calculate the phase diagram. The predicted diagram has four ordered ground-states in the range $ 0 \leq X \leq 1/2 $, but one of these, at X=5/12, is predicted to disproportionate at T$ \approx 220 K $. At X$ \approx 1/3（$Hf_{3}O$）和X $\approx 1/2 ($ Hf_{2} O$），序参数与温度的关系图预测了紧密相关的有序结构的魔鬼阶梯。
The whole Al-Li phase diagram is predicted from first principles calculations and statistical mechanics including the effect of configurational and vibrational entropy. The formation enthalpy of different configurations at different temperatures was accurately predicted by means of cluster expansions that were fitted from first principles calculations. The vibrational entropic contribution of each configuration was determined from the bond length vs. bond stiffness relationships for each type of bond and the Gibbs free energy of the different phases was obtained as a function of temperature fr…
We have performed first principles study for bismuth crystal structure at pressure from 0~14 GPa based on structure search and quasi-harmonic approximation. A new stable phase with Pbcm symmetry is predicted by structure search method. We find that the most stable structures of bismuth at 0 K are R-3m(0 ~ 3.29 GPa), Pbcm (3.29 GPa ~ 4.91 GPa), Cmce (4.91 GPa ~ 10.57 GPa) and Im-3m ( > 10.57 GPa), respectively. By quasi-harmonic approximation, we predicted the phase diagram of bismuth from first principles calculations. We found that the phase transition pressure reduces with increasing tempera…
First principles phase diagram calculations, that included van der Waals interactions, were performed for the bulk transition metal dichalcogenide system $(1-X) \cdot WS_{2} - (X) \cdot WTe_{2}$. To obtain a converged phase diagram, a series of cluster expansion calculations were performed with increasing numbers of structure-energies, ($N_{Str}$) up to $N_{Str}=435$, used to fit the cluster expansion Hamiltonian. All calculated formation energies are positive and all ground-state analyses predict that formation energies for supercells with 16 or fewer anion sites are positive; but when $\appr…
Alloying is a successful strategy for tuning the phases and properties of two-dimensional (2D) transition metal dichalcogenides (TMDCs). To accelerate the synthesis of new TMDC alloys, we present a method for generating temperature-composition equilibrium phase diagrams by combining first-principles total energy calculations with thermodynamic solution models. This method is applied to three representative 2D TMDC alloys: an isostructural alloy, MoS2(1-x)Te2x, and two heterostructural alloys, Mo1-xWxTe2 and WS2(1-x)Te2x. We show that the mixing enthalpy of the entire composition range of these…
We studied for the first time the magnetic phase diagram of the rare-earth manganites series Gd$_{1-x}$Ca$_{x}$MnO$_{3}$ (GCMO) over the full concentration range based on density functional theory. GCMO has been shown to form solid solutions. We take into account this disordered character by adapting special quasi random structures at different concentration steps. The magnetic phase diagram is mainly described by means of the magnetic exchange interactions between the Mn sites and Monte Carlo simulations were performed to estimate the corresponding transition temperatures. They agree very wel…
The intercalation phase diagram of Mg in V$_2$O$_5$ from first principles
我们已经研究了镁嵌入到正交V $_2$ O $_5$中，已知仅有的三种可逆嵌入镁离子的阴极之一。通过计算Mg $_x$ V $_2$ O $_5$的基态组态，并对$δ$ -V $_2$ O $_5$的组态无序进行团簇展开，得到了完整的温度-组成相图。我们的计算表明完全脱镁的$α$ -V $_2$ O $_5$和完全镁化的$δ$ -V $_2$ O $_5$之间存在平衡相分离行为，但也激发了两相中潜在的亚稳固溶体转变路径的存在。我们发现Mg在$δ$多晶型物中具有显著更好的流动性，这表明通过使Mg在$δ$相中循环可以实现更好的性能。
Perovskite structure SrMnO$_3$ is a rare example of a multiferroic material where strain-tuning and/or cation substitution could lead to coinciding magnetic and ferroelectric ordering temperatures, which would then promise strong magnetoelectric coupling effects. Here, we establish the temperature and strain dependent ferroic phase diagram of SrMnO$_3$ using first-principles-based effective Hamiltonians. All parameters of these Hamiltonians are calculated using density functional theory, i.e., no fitting to experimental data is required. Temperature dependent properties are then obtained from …
In this work, an assessment of the CALPHAD method trained on only density functional theory (DFT) data is performed for the Li-Si binary system, as a case study. Using a parameter sampling approach based on the Bayesian Error Estimation Functional (BEEF-vdW) exchange-correlation potential. By using built-in ensemble of functionals from BEEF-vdW, the uncertainties of the Gibbs Free Energy fitting parameters are obtained and can be propagated to the resulting phase diagram. To find the best fitting form of the CALPHAD model, we implement a model selection step using the Bayesian information crit…
The phase diagram of Al-Li-Cu system in the Al-rich region was determined by means of first-principles calculations and statistical mechanics. The mixing enthalpies of many configurations for different lattices in the whole Al-Li-Cu system were determined by density functional theory simulations to find the stable phases in the convex hull. They were fitted with a cluster expansion to calculate the free energy of the configurations with different compositions as a function of temperature in the Al-rich region (Al content > 40 at. %) by means of Monte Carlo simulations. It was found that the gr…
The fundamental properties of infinite-layer rare-earth nickelates (RNiO2) are carefully revisited and compared with those of CaCuO2 and RNiO3 perovskites. Combining first-principles and finite-temperature second-principles calculations, we highlight that bulk NdNiO2 compound are far from equivalent to CaCuO2, together at the structural, electronic, and magnetic levels. Structurally, it is shown to be prone to spin-phonon coupling induced oxygen square rotation motion, which might be responsible for the intriguing upturn of the resistivity. At the electronic and magnetic levels, we point out o…
This study is inspired by the recent development of "virtual material testing laboratory" in which the main equipment is a full field crystal plasticity modeling tool. Ample examples have demonstrated its applications to sheet forming operations. In those applications, the mechanical anisotropy originated from the crystallographic texture can be adequately described, such as r-values and earing. Formability is another important property in sheet metal forming, which has yet not been equipped in these virtual laboratories. Though theoretical models for formability can be dated back to 1885 due …
In this work, global search for crystal structures of ternary Mg-Sc-H hydrides (Mg$_x$Sc$_y$H$_z$) under high pressure ($100 \le P \le 200$ GPa) were performed using the evolutionary algorithm and first-principles calculations. Based on them, we computed the thermodynamic convex hull and pressure-dependent phase diagram of Mg$_x$Sc$_y$H$_z$ for $z/(x+y) < 4$. We have identified the stable crystal structures of four thermodynamically stable compounds with the higher hydrogen content, i.e., $R\bar{3}m$-MgScH$_{6}$, $C2/m$-Mg$_{2}$ScH$_{10}$, $Immm$-MgSc$_{2}$H$_{9}$ and $Pm\bar{3}m$-Mg(ScH$_{4}$…
The thermodynamic properties of α-Al and other phases (GP zones, θ'', θ' and θ) in the Al-rich part of the Al-Cu system have been obtained by means of the cluster expansion formalism in combination with statistical mechanics. This information was used to build the Al-rich part of the Al-Cu phase-diagram taking into account vibrational entropic contributions for θ', as those of the other phases were negligible. The simulation predictions of the phase boundaries between α-Al and either θ'', θ' or θ phases as a function of temperature are in good agreement with experimental data and extend the ph…
Mixed-halide perovskite alloys $\text{CsPb}(\text{I}_{1-x}^{}\text{Br}_x^{})_3^{}$ and $\text{CsPb}(\text{Br}_{1-x}^{}\text{Cl}_x^{})_3^{}$: New insight of configuration entropy effect from first principles and phase diagrams
Density Functional Theory (DFT) calculations have been widely used to predict the activity of catalysts based on the free energies of reaction intermediates. The incorporation of the state of the catalyst surface under the electrochemical operating conditions while constructing the free energy diagram is crucial, without which even trends in activity predictions could be imprecisely captured. Surface Pourbaix diagrams indicate the surface state as a function of the pH and the potential. In this work, we utilize error-estimation capabilities within the BEEF-vdW exchange correlation functional a…
Based on first principles calculations, we predict and characterize the flexibility of two aluminium-based Metal-Organic Frameworks (MOFs), CAU-13 and NOTT-300. Both materials have a wine rack topology similar to that of MIL-53(Al), the archetypal breathing MOF, yet their flexibility has not been demonstrated so far.
We study the structural and vibrational properties of the high-temperature superconducting sulfur trihydride and trideuteride in the high-pressure $Im\bar{3}m$ and $R3m$ phases by first-principles density-functional-theory calculations. On lowering pressure, the rhombohedral transition $Im\bar{3}m \rightarrow R3m$ is expected, with hydrogen bond desymmetrization and occurrence of trigonal lattice distortion. In hydrostatic conditions we find that, contrary to what suggested in some recent experiments, if the rhombohedral distortion exists it affects mainly the hydrogen-bonds, whereas the resul…
Knowledge of phase diagrams is essential for material design as it helps in understanding microstructure evolution during processing. The determination of phase diagrams is thus one of the central tasks in materials science. When exploring new materials for which the phase diagram is unknown, experimentalists often try to determine the key experiments that should be performed by referencing known phase diagrams of similar systems. To enhance this practical strategy, we attempted to estimate unknown phase diagrams based on known phase diagrams using a machine learning-based classification appro…
First principles phase diagram calculation and theoretical investigation of electronic structure properties of $\mathrm{KCuTe_{1-x}Se_{x}}$ for photocathode applications
本研究通过第一性原理计算 KCuTe₁₋ₓSeₓ 的温度-成分相图与电子结构，证明其在 322 K 以上形成六方固溶体，并具备高迁移率、强光吸收及合适析氢能级对齐，适用于光电阴极。
The emergence of superconductivity in slightly-misaligned graphene bilayer [1] and moiré excitons in MoSe$_2$-WSe$_2$ van der Waals (vdW) heterostructures [2] is intimately related to the formation of a 2D superlattice in those systems. At variance, perfect primitive lattice matching of the constituent layers has also been reported in some vdW-heterostructures [3-5], highlighting the richness of interfaces in the 2D world. In this work, the determination of the nature of such interface, from first principles, is demonstrated. To do so, an extension of the Frenkel-Kontorova (FK) model [6] is pr…
