795 research outputs found
A Free Energy Model of Boron Carbide
The assessed phase diagram of the boron-carbon system contains a single
non-stoichiometric boron-carbide phase of rhombohedral symmetry with a broad,
thermodynamically improbable, low temperature composition range. We combine
first principles total energy calculations with phenomenological thermodynamic
modeling to propose a revised low temperature phase diagram that contains two
boron-carbide phases of differing symmetries and compositions. One structure
has composition B4C and consists of B11C icosahedra and C-B-C chains, with the
placement of carbon on the icosahedron breaking rhombohedral symmetry. This
phase is destabilized above 600K by the configurational entropy of alternate
carbon substitutions. The other structure, of ideal composition B13C2, has a
broad composition range at high temperature, with rhombohedral symmetry
throughout, as observed experimentally.Comment: 15 pages, 3 figures, submitted to J. Stat. Phys. August 9th, 201
Tunable Semiconductors: Control over Carrier States and Excitations in Layered Hybrid Organic-Inorganic Perovskites
For a class of 2D hybrid organic-inorganic perovskite semiconductors based on
-conjugated organic cations, we predict quantitatively how varying the
organic and inorganic component allows control over the nature, energy and
localization of carrier states in a quantum-well-like fashion. Our
first-principles predictions, based on large-scale hybrid density-functional
theory with spin-orbit coupling, show that the interface between the organic
and inorganic parts within a single hybrid can be modulated systematically,
enabling us to select between different type-I and type-II energy level
alignments. Energy levels, recombination properties and transport behavior of
electrons and holes thus become tunable by choosing specific organic
functionalizations and juxtaposing them with suitable inorganic components
Thermodynamic Equilibria in Carbon Nitride Photocatalyst Materials and Conditions for the Existence of Graphitic Carbon Nitride g-C3N4
We quantify the thermodynamic equilibrium conditions that govern the formation of crystalline heptazine-based carbon nitride materials, currently of enormous interest for photocatalytic applications including solar hydrogen evolution. Key phases studied include the monomeric phase melem, the 1D polymer melon, and the hypothetical hydrogen free 2D graphitic carbon nitride phase "g-C3N4". Our study is based on. density functional theory including van der Waals dispersion terms with different experimental conditions represented by the chemical potential of NH3. Graphitic carbon nitride is the subject of a vast number of studies, but its existence is still controversial. We show that typical conditions found in experiments pertain to the polymer melon (2D planes of 1D hydrogen-bonded polymer strands). In contrast, equilibrium synthesis of heptazine (h)-based g-h-C3N4 below its experimentally known decomposition temperature requires much less likely conditions, equivalent to low NH3 partial pressures around 1 Pa at 500 degrees C and around 10(3) Pa even at 700 degrees C. A recently reported synthesis of triazine (t)-based g-t-C3N4 in a salt melt is interpreted as a consequence of the altered local chemical environment of the C3N4 nanocrystallites
ELSI: A Unified Software Interface for Kohn-Sham Electronic Structure Solvers
Solving the electronic structure from a generalized or standard eigenproblem
is often the bottleneck in large scale calculations based on Kohn-Sham
density-functional theory. This problem must be addressed by essentially all
current electronic structure codes, based on similar matrix expressions, and by
high-performance computation. We here present a unified software interface,
ELSI, to access different strategies that address the Kohn-Sham eigenvalue
problem. Currently supported algorithms include the dense generalized
eigensolver library ELPA, the orbital minimization method implemented in
libOMM, and the pole expansion and selected inversion (PEXSI) approach with
lower computational complexity for semilocal density functionals. The ELSI
interface aims to simplify the implementation and optimal use of the different
strategies, by offering (a) a unified software framework designed for the
electronic structure solvers in Kohn-Sham density-functional theory; (b)
reasonable default parameters for a chosen solver; (c) automatic conversion
between input and internal working matrix formats, and in the future (d)
recommendation of the optimal solver depending on the specific problem.
Comparative benchmarks are shown for system sizes up to 11,520 atoms (172,800
basis functions) on distributed memory supercomputing architectures.Comment: 55 pages, 14 figures, 2 table
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