According to previous interpretations of experimental data, sodium-scandium double-cation borohydride NaSc(BH$_4$)$_4$ crystallizes in the crystallographic space group $Cmcm$ where each sodium (scandium) atom is surrounded by six scandium (sodium) atoms. A careful investigation of this phase based on \textit{ab initio} calculations indicates that the structure is dynamically unstable and gives rise to an energetically and dynamically more favorable phase with $C222_1$ symmetry and nearly identical x-ray diffraction pattern. By additionally performing extensive structural searches with the minima-hopping method we discover a class of new low-energy structures exhibiting a novel structural motif in which each sodium (scandium) atom is surrounded by four scandium (sodium) atoms arranged at the corners of either a rectangle with nearly equal sides or a tetrahedron. These new phases are all predicted to be insulators with band gaps of $7.9-8.2$ eV. Finally, we estimate the influence of these structures on the hydrogen-storage performance of NaSc(BH$_4$)$_4$.
We report the superconductivity at Tc ~ 0.049 K in Laves phase HfZn2. Based on the first principles method we have studied the details structural and electronic properties of HfZn2 according to which there are 4.67 states eV-1 fu-1 at Fermi level. Using this value we calculate the specific heat coefficient γ theoretically as 10.97 mJ/ K2 mol and then systematically calculate the electron-phonon coupling constant λ as 0.45. Finally applying these values in McMillan formula we get the superconducting critical temperature of HfZn2 as approximately 0.049 K.
Structural, elastic and electronic properties of Ir-based carbides-antiperovskites Ir3MC (M = Ti, Zr, Nb and Ta) as predicted from first-principles calculations
Transmission electron microscopy has been a promising candidate for mapping atomic orbitals for a long time. Here, we explore its capabilities by a first principles approach. For the example of defected graphene, exhibiting either an isolated vacancy or a substitutional nitrogen atom, we show that three different kinds of images are to be expected, depending on the orbital character. To judge the feasibility of visualizing orbitals in a real microscope, the effect of the optics aberrations is simulated. We demonstrate that, by making use of energy-filtering, it should indeed be possible to map…
We searched for stable crystal structures of YFe$_{12}$ using a crystal structure prediction technique based on a genetic algorithm and first-principles calculations. We obtained two monoclinic $C2/m$ structures as metastable phases that are different from the well-known ThMn$_{12}$ structure. These two phases have advantages in their magnetism over the ThMn$_{12}$ structure: The total magnetization $M$ is increased from 25.6 $μ_{\text{B}}$/f.u. up to 26.8 $μ_{\text{B}}$/f.u. by the transformations. We also calculated Curie temperature $T_{\text{C}}$ for these structures within the mean-field …
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…
Finding electrode materials with high capacity is a key challenge for developing Lithium-ion batteries (LIBs). Graphene was once expected to be a promising candidate, but it turns out to be too inert to interact with Li. Here, by using the first-principles calculations, we predict that Germanium doped graphene, termed as Germagraphene which has been achieved in recent experiment, is a promising LIB anode material. We find that at the optimal Ge concentration, which corresponds to the chemical formula C17Ge, the specific capacity for Germagraphene can be as high as 1734 mAh/g, over four times l…
Recently, Bafekry et al. [Appl. Phys. Lett. 119, 142102 (2021)] reported their density functional theory (DFT) results on the elastic constants of a novel C6N7 monolayer. They predicted a very soft elastic modulus of 36.29 GPa for the C6N7 monolayer, which is remarkably low for carbon-nitride 2D lattices. Using DFT calculations, we predict a remarkably higher elastic modulus of 267 GPa for this monolayer. The maximum tensile strength is also predicted to be 20.5 GPa, revealing the outstanding mechanical properties of the C6N7 monolayer.
The formation of precipitated zirconium (Zr) hydrides is closely related to the hydrogen embrittlement problem for the cladding materials of pressured water reactors (PWR). In this work, we systematically investigated the crystal structures of zirconium hydride (ZrHx) with different hydrogen concentrations (x = 0~2, atomic ratio) by combining the basin hopping algorithm with first principles calculations. We conclude that the P3m1 ζ-ZrH0.5 is dynamically unstable, while a novel dynamically stable P3m1 ZrH0.5 structure was discovered in the structure search. The stability of bistable P42/nnm Zr…
Two novel three-dimensional (3D) crystal structures of carbon (C) and germanium carbide (GeC2) were predicted using first-principles density-functional theory (DFT) calculations. These newly discovered 3D carbon allotrope and GeC2 are in the space group of Fmmm (space group number 69). Their crystal structures have unique tetragonal/hexagonal rings formed by either C or Ge/C atoms. Both structures were proven to be thermodynamically stable through the phonon spectrum calculations. The C allotrope is a semiconductor with a wide band gap of 3.65 eV predicted by the hybrid density functional HSE0…
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。
MgO-barrier magnetic tunnel junctions with half-metallic Heusler alloy electrodes attracted much attentions for spintronics applications. However, a couples of issues related to materials still remain to be resolved for practical uses. Recently, quarterly equiatomic Heusler alloys attracted attentions as advanced Heusler alloys. CoIrMnZ (Z = Al, Si, Ga, and Ge) half-metallic Heusler alloys were designed and predicted to have moderate Curie temperatures and to be a lattice-matched with the MgO barrier, being advantageous to traditional Co2 Heusler alloys [T. Roy et al., J. Magn. Magn. Mater. 49…
Two-dimensional (2D) multiferroics exhibit cross-control capacity between magnetic and electric responses in reduced spatial domain, making them well suited for next-generation nanoscale devices; however, progress has been slow in developing materials with required characteristic properties. Here we identify by first-principles calculations robust 2D multiferroic behaviors in decorated Fe2O3 monolayer, showcasing N@Fe2O3 as a prototypical case, where ferroelectricity and ferromagnetism stem from the same origin, namely Fe d-orbit splitting induced by the Jahn-Teller distortion and associated c…
The Janus structure, by combining properties of different transition metal dichalcogenide (TMD) monolayers in a single polar material, has attracted increasing research interest because of their particular structure and potential application in electronics, optoelectronics and piezoelectronics. In this work, Janus SnSSe monolayer is predicted by means of first-principles calculations, which exhibits dynamic and mechanical stability. By using generalized gradient approximation (GGA) plus spin-orbit coupling (SOC), the Janus SnSSe monolayer is found to be an indirect band-gap semiconductor, whos…
Structures and magnetic properties of Fe16-xCoxN2 are studied using adaptive genetic algorithm and first-principles calculations. We show that substituting Fe by Co in Fe16N2 with Co/Fe ratio smaller than 1 can greatly improve the magnetic anisotropy of the material. The magnetocrystalline anisotropy energy from first-principles calculations reaches 3.18 MJ/m3 (245.6 μeV per metal atom) for Fe12Co4N2, much larger than that of Fe16N2 and is one of the largest among the reported rare-earth free magnets. From our systematic crystal structure searches, we show that there is a structure transition …
From the spin polarized density functional total energy calculations, we shown that the ground state of cubic perovskite RbMnF$_3$ is an antiferromagnetic (AFM) insulator due to the super-exchange mechanism, in agreement with the other theoretical and experimental results. After tetragonal distortion along the c-axis, keeping the predicted volume, our results indicated that the strain-induced magnetic phase transition from an AFM insulator to a half metallic ferromagnetic (HM-FM) state is available by the tetragonal distortion due to the insulator -- half metallic transition. The predicted ele…
The high pressure phases of HBr are explored with an ab initio crystal structure search. By taking into account the contribution of zero-point energy (ZPE), we find that the P4/nmm phase of HBr is thermodynamically stable in the pressure range from 150 to 200 GPa. The superconducting critical temperature (Tc) of P4/nmm HBr is evaluated to be around 73 K at 170 GPa, which is the highest record so far among binary halogen hydrides. Its Tc can be further raised to around 95K under 170 GPa if half of the bromine atoms in the P4/nmm HBr are substituted by the lighter chlorine atoms. Our study shows…
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 $\appr…
We have performed first principles study for bismuth crystal structure at pressure from 0~14 GPa based on structure search and quasi-harmonic approximation. A new stable phase with Pbcm symmetry is predicted by structure search method. We find that the most stable structures of bismuth at 0 K are R-3m(0 ~ 3.29 GPa), Pbcm (3.29 GPa ~ 4.91 GPa), Cmce (4.91 GPa ~ 10.57 GPa) and Im-3m ( > 10.57 GPa), respectively. By quasi-harmonic approximation, we predicted the phase diagram of bismuth from first principles calculations. We found that the phase transition pressure reduces with increasing tempera…
Molecular simulations of polymer rely on accurate force fields to describe the inter-atomic interactions. In this work, we use first-principles density functional theory (DFT) calculations to parameterize a united-atom force field for polyacetylene (PA), a conjugated polymer potentially of high thermal conductivity. Different electron correlation functionals in DFT have been tested. Bonding interactions for the alternating single and double bonds in the conjugated polymer backbone are explicitly described and Class II anharmonic functions are separately parameterized. Bond angle and dihedral i…
Rutile germanium dioxide (r-GeO$_2$) is a recently predicted ultrawide-band-gap semiconductor with potential applications in high-power electronic devices, for which the carrier mobility is an important material parameter that controls the device efficiency. We apply first-principles calculations based on density functional and density functional perturbation theory to investigate carrier-phonon coupling in r-GeO$_2$ and predict its phonon-limited electron and hole mobilities as a function of temperature and crystallographic orientation. The calculated carrier mobilities at 300 K are $μ_{\text…
Beryllium polynitrides, BeN4 is a novel layered material, which has been most recently fabricated under high pressure (Phys. Rev. Lett. 126(2021), 175501). As a new class of 2D materials, in this work we conduct first-principles calculations to examine the stability and explore the electronic nature of MN4 (M= Be, Mg, Ir, Rh, Ni, Cu, Au, Pd, Pt) monolayers. Acquired results confirm the dynamical and thermal stability of BeN4, MgN4, IrN4, PtN4 and RhN4 monolayers. Interestingly, BeN4 and MgN4 monolayers are found to show anisotropic Dirac cones in their electronic structure. While PtN4 monolaye…
We have given a summary on our theoretical predictions of three kinds of topological semimetals (TSMs), namely, Dirac semimetal (DSM), Weyl semimetal (WSM) and Node-Line Semimetal (NLSM). TSMs are new states of quantum matters, which are different with topological insulators. They are characterized by the topological stability of Fermi surface, whether it encloses band crossing point, i.e., Dirac cone like energy node, or not. They are distinguished from each other by the degeneracy and momentum space distribution of the nodal points. To realize these intriguing topological quantum states is q…
First-principles approach is demonstrated to calculate the work function of Bi2Te3. The reference potential and the vacuum energy levels are extracted from the Bi2Te3 (0001) surface structure using the reference potential method based on the density functional theory. The one-shot GOWO many-body perturbation theory is used to place the bulk band edge energies with respect to the reference level and the vacuum energy. At last, the work function of 5.301 - 5.131 eV is predicted for Bi2Te3 (0001) surface and compared to various elements.
We have developed a method to improve the doping computation efficiency, this method is based on first principles calculations and cluster expansion. First principles codes produce highly accurate total energies and optimized geometries for any given structure. Cluster expansion method constructs a cluster expansion using partial first principles results and computes the energies for other structures derived from a parent lattice. Using this method, energies for multiple doping structures can be predicted quickly without series of first principles calculations. This method has been packaged in…
Computationally-guided material discovery is being increasingly employed using a descriptor-based screening through the calculation of a few properties of interest. A precise understanding of the uncertainty associated with first principles density functional theory calculated property values is important for the success of descriptor-based screening. Bayesian error estimation approach has been built-in to several recently developed exchange-correlation functionals, which allows an estimate of the uncertainty associated with properties related to the ground state energy, for e.g. adsorption en…
With the increasing applications of Phosphorene in nano/optoelectronics and thermoelectrics, a comprehensive study on its thermal transport properties is necessary. It has been concluded from previous studies that there exist vast differences and uncertainty in the theoretically predicted thermal conductivity of single-layer phosphorene, which is generally attributed to the selection of XC functionals. However, even though the selection of functional groups in the first-principles calculations is particularly essential for predicting the thermal conductivity of phosphorene, there is no compreh…
The colours of metals have attracted the attention of humanity since ancient times, and coloured metals, in particular gold compounds, have been employed for tools and objects symbolizing the aesthetics of power. In this work we develop a comprehensive framework to obtain the reflectivity and colour of metals, and show that the trends in optical properties and the colours can be predicted by straightforward first-principles techniques based on standard approximations. We apply this to predict reflectivity and colour of several elemental metals and of different types of metallic compounds (inte…
