During the last century, inorganic oxide compounds laid foundations for materials synthesis, characterization, and technology translation by adding new functions into devices previously dominated by main-group element semiconductor compounds. Today, compounds with multiple anions beyond the single-oxide ion, such as oxyhalides and oxyhydrides, offer a new materials platform from which superior functionality may arise. Here we review the recent progress, status, and future prospects and challenges facing the development and deployment of mixed-anion compounds, focusing mainly on oxide-derived materials. We devote attention to the crucial roles that multiple anions play during synthesis, characterization, and in the physical properties of these materials. We discuss the opportunities enabled by recent advances in synthetic approaches for design of both local and overall structure, state-of-the-art characterization techniques to distinguish unique structural and chemical states, and chemical/physical properties emerging from the synergy of multiple anions for catalysis, energy conversion, and electronic materials.

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Expanding frontiers in materials chemistry and physics with multiple anions

Real-space grids are a powerful alternative for the simulation of electronic systems. One of the main advantages of the approach is the flexibility and simplicity of working directly in real space where the different fields are discretized on a grid, combined with competitive numerical performance and great potential for parallelization. These properties constitute a great advantage at the time of implementing and testing new physical models. Based on our experience with the Octopus code, in this article we discuss how the real-space approach has allowed for the recent development of new ideas for the simulation of electronic systems. Among these applications are approaches to calculate response properties, modeling of photoemission, optimal control of quantum systems, simulation of plasmonic systems, and the exact solution of the Schrodinger equation for low-dimensionality systems.

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Real-space grids and the Octopus code as tools for the development of new simulation approaches for electronic systems

The generalization of the Local Density Approximation (LDA) method for the systems with strong Coulomb correlations is presented which gives a correct description of the Mott insulators. The LDA+U method is based on the model hamiltonian approach and allows to take into account the non-sphericity of the Coulomb and exchange interactions. parameters. Orbital-dependent LDA+U potential gives correct orbital polarization and corresponding Jahn-Teller distortion. To calculate the spectra of the strongly correlated systems the impurity Anderson model should be solved with a many-electron trial wave function. All parameters of the many-electron hamiltonian are taken from LDA+U calculations. The method was applied to NiO and has shown good agreement with experimental photoemission spectra and with the oxygen Kα X-ray emission spectrum.

http://absimage.aps.org/image/MAR06/MWS_MAR06-2005-007233.pdf

First-principles calculations of electronic structure and spectra of strongly correlated systems: the LDA+U method

The orthosilicate phosphors SrxBa2–xSiO4:Eu2+ have now been known for over four decades and have found extensive recent use in solid-state white lighting. It is well-recognized in the literature and in practice that intermediate compositions in the solid-solutions between the orthosilicates Sr2SiO4 and Ba2SiO4 yield the best phosphor hosts when the thermal stability of luminescence is considered. We employ a combination of synchrotron X-ray diffraction, total scattering measurements, density functional theory calculations, and low-temperature heat capacity measurements, in conjunction with detailed temperature- and time-resolved studies of luminescence properties to understand the origins of the improved luminescence properties. We observe that in the intermediate compositions, the two cation sites in the crystal structure are optimally bonded as determined from bond valence sum calculations. Optimal bonding results in a more rigid lattice, as established by the intermediate compositions possessing the hi...

Consequences of Optimal Bond Valence on Structural Rigidity and Improved Luminescence Properties in SrxBa2–xSiO4:Eu2+ Orthosilicate Phosphors

The electronic, mechanical properties and theoretical hardness of chromium carbides by first-principles calculations

First principles calculations were performed to investigate the structural, elastic, and electronic properties of IrN2 for various space groups: cubic Fm-3m and Pa-3, hexagonal P3(2)21, tetragonal P4(2)/mnm, orthorhombic Pmmn, Pnnm, and Pnn2, and monoclinic P2(1)/c. Our calculation indicates that the P2(1)/c phase with arsenopyrite-type structure is energetically more stable than the other phases. It is semiconducting (the remaining phases are metallic) and contains diatomic N-N with the bond distance of 1.414 A. These characters are consistent with the experimental facts that IrN2 is in lower symmetry and nonmetallic. Our conclusion is also in agreement with the recent theoretical studies that the most stable phase of IrN2 is monoclinic P2(1)/c. The calculated bulk modulus of 373 GPa is also the highest among the considered space groups. It matches the recent theoretical values of 357 GPa within 4.3% and of 402 GPa within 7.8%, but smaller than the experimental value of 428 GPa by 14.7%. Chemical bonding and potential displacive phase transitions are discussed for IrN2. For IrN3, cubic skutterudite structure (Im-3) was assumed.

Crystal structures and elastic properties of superhard IrN2 and IrN3 from first principles

The title compound, the first transition-metal(III) carbodiimide with a 3d3 configuration is prepared from a mixture of CrCl3 and ZnNCN using LiCl/KCl as a flux (550 °C, 2 d).

A ferromagnetic carbodiimide: Cr2(NCN)3.

Publisher's Note: Crystal structures and elastic properties of superhard Ir N 2 and Ir N 3 from first principles [Phys. Rev. B 76 , 054115 (2007)]

For metallic nanoparticles less than 10 nm in diameter, localized surface plasmon resonances (LSPRs) become sensitive to the quantum nature of conduction electrons. In this regime, experimental probes of size-dependent LSPRs are particularly challenging, and contradictory results are often reported. Unfortunately, quantum mechanical simulations based on time-dependent Kohn–Sham density functional theory (TD-KSDFT) are computationally too expensive to tackle metal particles larger than 2 nm. Herein, we present a time-dependent orbital-free density functional theory (TD-OFDFT) that accurately captures the dynamic response of electrons in the presence of realistic ionic potentials. The TD-OFDFT method offers a comparable accuracy as TD-KSDFT but with a much lower computational cost. Using TD-OFDFT, we study size-dependent LSPRs on Na nanoparticles with diameters from 0.7 to 12.3 nm. The optical absorption spectra exhibit a nonmonotonic behavior from blue shift to red shift and back to blue shift as the parti...

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Size-Dependent Plasmonic Resonances from Large-Scale Quantum Simulations

Using time-dependent orbital-free density functional theory, we perform quantum mechanical simulations to understand plasmonic responses in sodium nanoparticle dimers and trimers. The electronic structure, optical absorption, electric field enhancement, and photoinduced tunneling current are examined. For dimers, the maximum field enhancement is reached at a gap distance of 6 A, below which a conductive channel is formed. The tunneling current peaks at the resonant energy of the charge-transfer plasmon (CTP). One such CTP is formed in symmetric linear trimers, and it splits into two in asymmetric linear trimers—one corresponds to a “global” and the other a “local” oscillation as a result of constructive and destructive interferences between the CTPs of the corresponding dimers. The interference leads to “nanofocusing” where the intensity of the “global” mode reaches its maximum while the “local” mode is quenched completely. Similarly, the electric field can be tuned to be localized at one of the two gaps ...

Hongping Xiang

Papers

Crystal structures and elastic properties of superhard IrN2 and IrN3 from first principles

A ferromagnetic carbodiimide: Cr2(NCN)3.

Publisher's Note: Crystal structures and elastic properties of superhard Ir N 2 and Ir N 3 from first principles [Phys. Rev. B 76 , 054115 (2007)]

Size-Dependent Plasmonic Resonances from Large-Scale Quantum Simulations

Understanding Quantum Plasmonics from Time-Dependent Orbital-Free Density Functional Theory