First-Principles Thermodynamics of Coherent Interfaces in Samarium-Doped Ceria Nanoscale Superlattices

Nanoscale superlattices of samarium-doped ceria layers with varying doping levels have been recently proposed as a novel fuel cell electrolyte. We calculate the equilibrium composition profile across the coherent {100} interfaces present in this system using lattice-gas Monte Carlo simulations with long-range interactions determined from electrostatics and short-range interactions obtained from ab initio calculations. These simulations reveal the formation of a diffuse, nonmonotonic, and surprisingly wide (11 nm at 400 K) interface composition profile, despite the absence of space charge regions

First-principles thermodynamic modeling of lanthanum chromate perovskites

Tendencies toward local atomic ordering in (A,A′)(B,B′)O_(3−δ) mixed composition perovskites are modeled to explore their influence on thermodynamic, transport, and electronic properties. In particular, dopants and defects within lanthanum chromate perovskites are studied under various simulated redox environments. (La_(1−x),Sr_x)(Cr_(1−y),Fe_y)O_(3−δ) (LSCF) and (La_(1−x),Sr_x)(Cr_(1−y),Ru_y)O_(3−δ) (LSCR) are modeled using a cluster expansion statistical thermodynamics method built upon a density functional theory database of structural energies. The cluster expansions are utilized in lattice Monte Carlo simulations to compute the ordering of Sr and Fe(Ru) dopant and oxygen vacancies (Vac). Reduction processes are modeled via the introduction of oxygen vacancies, effectively forcing excess electronic charge onto remaining atoms. LSCR shows increasingly extended Ru-Vac associates and short-range Ru-Ru and Ru-Vac interactions upon reduction; LSCF shows long-range Fe-Fe and Fe-Vac interaction ordering, inhibiting mobility. First principles density functional calculations suggest that Ru-Vac associates significantly decrease the activation energy of Ru-Cr swaps in reduced LSCR. These results are discussed in view of experimentally observed extrusion of metallic Ru from LSCR nanoparticles under reducing conditions at elevated temperature

First-principles thermodynamic modeling of atomic ordering in yttria-stabilized zirconia

Yttria-stabilized zirconia YSZ is modeled using a cluster expansion statistical thermodynamics method built upon a density-functional theory database. The reliability of cluster expansions in predicting atomic ordering is explored by comparing with the extensive experimental database. The cluster expansion of YSZ is utilized in lattice Monte Carlo simulations to compute the ordering of dopant and oxygen vacancies as a function of concentration. Cation dopants show a strong tendency to aggregate and vacate significantly sized domains below 9 mol % Y_2O_3, which is likely important for YSZ aging processes in ionic conductivity. Evolution of vibrational and underlying electronic properties as a function of Y doping is explored

Self-driven lattice-model Monte Carlo simulations of alloy thermodynamic

Monte Carlo (MC) simulations of lattice models are a widely used way to compute thermodynamic properties of substitutional alloys. A limitation to their more widespread use is the difficulty of driving a MC simulation in order to obtain the desired quantities. To address this problem, we have devised a variety of high-level algorithms that serve as an interface between the user and a traditional MC code. The user specifies the goals sought in a high-level form that our algorithms convert into elementary tasks to be performed by a standard MC code. For instance, our algorithms permit the determination of the free energy of an alloy phase over its entire region of stability within a specified accuracy, without requiring any user intervention during the calculations. Our algorithms also enable the direct determination of composition-temperature phase boundaries without requiring the calculation of the whole free energy surface of the alloy system

Nuclear structure study around Z=28

Yrast levels of Ni, Cu and Zn isotopes for $40 \leq N \leq50$ have been described by state-of-the-art shell model calculations with three recently available interactions using $^{56}$Ni as a core in the $f_{5/2}pg_{9/2}$ model space. The results are unsatisfactory viz. large $E(2^+)$ for very neutron rich nuclei, small B(E2) values in comparison to experimental values. These results indicate an importance of inclusion of $\pi f_{7/2}$ and $\nu d_{5/2}$ orbitals in the model space to reproduce collectivity in this region.Comment: 12 pages, 14 figure

Dangling Bonds in Hexagonal Boron Nitride as Single-Photon Emitters.

Hexagonal boron nitride has been found to host color centers that exhibit single-photon emission, but the microscopic origin of these emitters is unknown. We propose boron dangling bonds as the likely source of the observed single-photon emission around 2 eV. An optical transition where an electron is excited from a doubly occupied boron dangling bond to a localized B p_{z} state gives rise to a zero-phonon line of 2.06 eV and emission with a Huang-Rhys factor of 2.3. This transition is linearly polarized with the absorptive and emissive dipole aligned. Because of the energetic position of the states within the band gap, indirect excitation through the conduction band will occur for sufficiently large excitation energies, leading to the misalignment of the absorptive and emissive dipoles seen in experiment. Our calculations predict a singlet ground state and the existence of a metastable triplet state, in agreement with experiment

Beached birds on the Flemish beaches

The history of collecting beached birds along the Flemish coast goes back to 1962. The study aims to determine the oil-rate (the proportion of oiled birds) of the beach washed birds, which is a good tool to determine trends in the chronic pollution of our coastal waters with oil. Since 1992, the Research Institute for Nature and Forest (INBO) is responsible for the Beached Bird Surveys (BBS) and organises systematic counts once a month during October - March.The numbers of beached birds fluctuated a lot over the previous decades, depending on the number of birds wintering along our coast, oil spills and weather conditions. During the winter 2006/2007, 447 beached birds were counted along the Flemish coast, being 0.99 birds km-1, close to the average number of 1.06 birds km-1 since the winter 1991/1992 (numbers of beached birds due to the Tricolor oil spill during the winter 2002/2003 are ignored in this analysis). The highest densities were encountered during February, a general tendency. Auks and gulls were the most numerous last winter, and that was always the case since the winter 1991/1992. A of Uria aalge and Razorbills) last winter will be remembered as the winter with the highest proportion of auks since the winter 1992.Guillemots and Razorbills are especially vulnerable for oil pollution because they spend most of their life swimming at beach were auks. During the first half of the winter, the oil-rate was relatively low, but the second half was characterised by a relatively high score. Recently, OSPAR (the commission for the Protection of Marine Environment of the North-East Atlantic) has recognised the oil-rate among Guillemots as EcoQ, an indicator to evaluate trends in chronic oil pollution of the marine environment. The EcoQ-objective is to force back the oil rate of Guillemots to 10% or less. When examining the changes in the oil-rate of Guillemots that washed ashore our beaches since 1962, three periods are noticeable. Between 1962 and 1990, the oil rate of beached Guillemots was very high (> 95% of all beached Guillemots were fouled). Since 1990, there was a remarkable decline with an average oil-rate of about 60%. Since 2004 the decline continued with an all time lowest score of an oil-rate of ‘only’ 17% last winter. On the other hand, there is no decline in the density of Guillemots found death along the Flemish coast, so possibly a problem in food viability is responsible for the death of many auks since 1990. Diet studies on collected auks will hopefully tell us something why auks wash ashore without any visible reason