Inversion,chemical complexity,and interstitial transport in spinels

Spinels with the generic chemical formula AB₂O₄ have potential applications in nuclear energy and batteries. In both cases, their functionality is related to mass transport through the crystal. Here, using long-time atomistic simulations, we examine the impact of the cation structure on interstitial transport in two spinel chemistries, inverse MgGa₂O₄ and double MgAlGaO₄. We emphasize two aspects of the transport properties: the unit mechanisms that are described by individual barriers, for which we introduce pole-figure-like plots, and the aggregate behavior of those unit mechanisms. Compared to previous work on normal spinels, we find that inversion significantly reduces the rate of interstitial transport in these structures and has an impact on the stability of defects as they move through the lattice. In particular, B cation interstitials are found to be kinetically stable only in the inverse MgGa₂O₄. These results provide new insight into relationship between structure, chemistry, and transport in spinels.     

Computational strategies for polymer dielectrics design

The present contribution provides a perspective on the degree to which modern computational methods can be harnessed to guide the design of polymeric dielectrics. A variety of methods, including quantum mechanical ab initio methods, classical force-field based molecular dynamics simulations, and data-driven paradigms, such as quantitative structure–property relationship and machine learning schemes, are discussed. Strategies to explore, search and screen chemical and configurational spaces extensively are also proposed. Some examples of computation-guided synthesis and understanding of real polymer dielectrics are also provided, highlighting the anticipated increasing role of such computational methods in the future design of polymer dielectrics.     

Dielectric permittivity of ultrathin PbTiO3 nanowires from first principles

We propose an efficient method to compute the dielectric permittivity of nanostructures by combining first principles density functional perturbation theory with effective medium theory. Specifically, ultrathin axially symmetric ferroelectric PbTiO₃ nanowires are considered. As established previously by Pilania and Ramprasad (Phys Rev B 82:155442, 2010), (4 × 4) PbO-terminated nanowire and (4 × 4) TiO₂-terminated nanowire display, respectively, a uniform axial and a vortex polarization in their ground state configurations (the latter with a non-zero axial toroidal moment). Both nanowires, regardless of the lateral surface termination, display a significantly larger dielectric constant value along the axial direction, and diminished values along the off-axis directions, as compared to the corresponding bulk values. Our results further suggest that the nanowires with unconventional vortex-type polarization states are expected to have an increased dielectric response as compared to those with conventional uniform axial polarization. The method proposed here is quite general and readily extendable to other zero-, one-, and two-dimensional nanostructures.     

Recent advances in computational materials design: methods,applications, algorithms,and informatics

Journal of Materials Science -     

Strong Zeeman splitting in orbital-hybridized valleytronic interfaces

Journal of Materials Science - Stacked heterointerfaces of two-dimensional (2D) materials are well-suited to obtain novel electronic functionality at small length scales, because the intrinsic...     

Ab initio study of antiferroelectric PbZrO<Subscript>3</Subscript> (001) surfaces

We have carried out first-principles total-energy calculations of bulk and (001) surfaces of PbZrO₃. The ground state for bulk PbZrO₃ is determined to be the antiferroelectric orthorhombic phase, with the ferroelectric rhombohedral and paraelectric cubic phases being 0.14 and 0.39 eV per formula unit higher in energy, respectively. PbO- and ZrO₂-terminated (001) surfaces, either clean or when hydroxyl species were adsorbed were considered. Surface relaxations, in-plane antiferroelectric distortions and modifications to the electronic structure due to the surfaces, and hydroxyl adsorbates on the surfaces were investigated. We find that while clean surfaces retained bulk-like behavior, hydroxyl adsorbates induce significant changes to the surface geometry as well as introduce electronic states in the band gap possibly rendering the surfaces metallic.     

Barriers to carriers: faults and recombination in non-stoichiometric perovskite scintillators

Journal of Materials Science - Tuning the efficiency and speed of charge carrier recombination in inorganic scintillators can potentially improve their performance in diverse applications. Recent...     

First principles investigations of structural, electronic, elastic, and dielectric properties of KMgF3

An ab initio study of structural, electronic, elastic, and dielectric properties of KMgF₃ in cubic perovskite structure is presented in the framework of density functional theory. The calculations presented here employ generalized gradient approximation with projector augmented wave method. The fully relaxed structural parameters are found to be in reasonable agreement with available experimental data and with previous theoretical work. The independent elastic constants of cubic KMgF₃ are derived from the derivative of total energy as a function of lattice strain in full detail. The bulk modulus and its first pressure derivative are obtained by fitting total energy versus volume data to a Murnaghan equation of state. The electronic band structure, total density of states, and projected density of states on each of the K, Mg, and F atoms are calculated and found to be in good agreement with previous theoretical results. First principles computed phonon dispersions for the cubic KMgF₃ are reported for the first time. The imaginary part of frequency-dependent dielectric function is determined by summing over all possible transitions from occupied to unoccupied states and taking the appropriate transition matrix element into account. The Born effective charges computed by linear response within density functional perturbation theory are used together with the mode eigenvectors to decompose the lattice dielectric susceptibility tensor into contributions arising from individual IR-active phonon modes. Our results for the static and optical dielectric constant are in good agreement with previously reported experimental results.