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Register a new userPrediction of semi-metallic tetragonal Hf2O3 and Zr2O3 from first-principles
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A tetragonal phase is predicted for Hf2O3 and Zr2O3 using density functional theory. Starting from atomic and unit cell relaxations of substoichiometric monoclinic HfO2 and ZrO2, such tetragonal structures are only reached at zero temperature by introducing the oxygen vacancy pair with the lowest formation energy. The tetragonal Hf2O3 and Zr2O3 structures belong to space group P-4m2 and are more stable than their corundum structure counterparts. These phases are semi-metallic, as confirmed through further G0W0 calculations. The carrier concentrations are estimated to be 1.77E21 cm^{-3} for both electrons and holes in tetragonal Hf2O3, and 1.75E21 cm^{-3} for both electrons and holes in tetragonal Zr2O3. The tetragonal Hf2O3 phase is probably related to the low resistivity state of hafnia-based resistive random access memory (RRAM).
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Electronic, magnetic, and transport properties of the antiferromagnetic (AFM) CuMnAs alloy with tetragonal structure, promising for the AFM spintronics, are studied from first principles using the Vienna ab-initio simulation package. We investigate the site-occupation of sublattices and the lattice parameters of three competing phases. We analyze the factors that determine which of the three conceivable structures will prevail. We then estimate formation energies of possible defects for the experimentally prepared lattice structure. Mn$_{rm Cu}$- and Cu$_{rm Mn}$-antisites as well as Mn$leftrightarrow$Cu swaps and vacancies on Mn or Cu sublattices were identified as possible candidates for defects in CuMnAs. We find that the interactions of the growing thin film with the substrate and with vacuum as well as the electron correlations are important for the phase stability while the effect of defects is weak. In the next step, using the tight-binding linear muffin-tin orbital method for the experimental structure, we estimate transport properties for systems containing defects with low formation energies. Finally, we determine the exchange interactions and estimate the Neel temperature of the AFM-CuMnAs alloy using the Monte Carlo approach. A good agreement of the calculated resistivity and Neel temperature with experimental data makes possible to draw conclusions concerning the competing phases.
Using first principles calculations, we show the high hydrogen storage capacity of a new class of compounds, metalloboranes. Metalloboranes are transition metal (TM) and borane compounds that obey a novel-bonding scheme. We have found that the transition metal atoms can bind up to 10 H2 molecules.
Finite-temperature Kohn--Sham density-functional theory (KS-DFT) is a widely-used method in warm dense matter (WDM) simulations and diagnostics. Unfortunately, full KS-DFT-molecular dynamics models scale unfavourably with temperature and there remains uncertainty regarding the performance of existing approximate exchange-correlation (XC) functionals under WDM conditions. Of particular concern is the expected explicit dependence of the XC functional on temperature, which is absent from most approximations. Average-atom (AA) models, which significantly reduce the computational cost of KS-DFT calculations, have therefore become an integral part of WDM modelling. In this paper, we present a derivation of a first-principles AA model from the fully-interacting many-body Hamiltonian, carefully analysing the assumptions made and terms neglected in this reduction. We explore the impact of different choices within this model -- such as boundary conditions and XC functionals -- on common properties in WDM, for example equation-of-state data. Furthermore, drawing upon insights from ground-state KS-DFT, we speculate on likely sources of error in KS-AA models and possible strategies for mitigating against such errors.
Ten new ternary fluorooxoborate structures were obtained from first-principles prediction. Coplanar aligned triangle structure units [BO2F]2- and [BOF2]- like [BO3]3- in borates were found from the computational simulation. We identified new covalent coordination patterns of the F atom connected with the B atoms which are located in the bridging site, -B--F--B-. Besides, one molecular crystal with [B4O4F4] molecular unit was attached.
A novel stable crystallographic structure is discovered in a variety of ABO3, ABF3 and A2O3 compounds (including materials of geological relevance, prototypes of multiferroics, exhibiting strong spin-orbit effects, etc...), via the use of first principles. This novel structure appears under hydrostatic pressure, and is the first post-post-perovskite phase to be found. It provides a successful solution to experimental puzzles in important systems, and is characterized by one-dimensional chains linked by group of two via edge-sharing oxygen/fluorine octahedra. Such unprecedented organization automatically results in anisotropic elastic properties and new magnetic arrangements. Depending on the system of choice, this post-post-perovskite structure also possesses electronic band gaps ranging from zero to ~ 10 eV being direct or indirect in nature, which emphasizes its universality and its potential to have striking, e.g., electrical or transport phenomena.
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