2,751 research outputs found
CO on Pt(111) puzzle; A possible solution
CO adsorption on the Pt(111) surface is studied using first-principles
methods. As found in a recent study [Feibelman, et al., J. Phys. Chem. B 105,
4018 (2001)], we find the preferred adsorption site within density functional
theory to be the hollow site, whereas experimentally it is found that the top
site is preferred. The influence of pseudopotential and exchange-correlation
functional error on the CO binding energy and site preference was carefully
investigated. We also compare the site preference energy of CO on Pt(111) with
the reaction energy of formaldehyde formation from H and CO. We show that
the discrepancies between the experimental and theoretical results are due to
the generalized gradient approximation (GGA) treating different bond orders
with varying accuracy. As a result, GGA results will contain significant error
whenever bonds of different bond order are broken and formed
Anisotropic Local Correlations and Dynamics in a Relaxor Ferroelectric
Relaxor ferroelectrics have been a focus of intense attention due to their
anomalous dielectric characteristics, diffuse phase transitions, and strong
piezoelectricity. Understanding the structure and dynamics of relaxors has been
one of the long-standing challenges in solid-state physics, with the current
model of polar nanoregions in a non-polar matrix providing only a qualitative
description of the relaxor phase transitions. In this paper, we investigate the
local structure and dynamics in 75%PbMgNbO-25%PbTiO
(PMN-PT) using molecular dynamics simulations and the dynamic pair distribution
function technique. We show for the first time that relaxor transitions can be
described by local order parameters. We find that structurally, the relaxor
phase is characterized by the presence of highly anisotropic correlations
between the local cation displacements. These correlations resemble the
hydrogen bond network in water. Our findings contradict the current polar
nanoregion model; instead, we suggest a new model of a homogeneous random
network of anisotropically coupled dipoles.Comment: We combine our manuscript and supplementary information in one file.
5 pages and 3 figures in main text. 3 pages and 3 figures in supplementary
informatio
Accurate construction of transition metal pseudopotentials
We generate a series of pseudopotentials to examine the relationship between
pseudoatomic properties and solid-state results. We find that lattice constants
and bulk moduli are quite sensitive to eigenvalue, total-energy difference and
tail norm errors, and clear correlations emerge. These trends motivate our
identification of two criteria for accurate transition metal pseudopotentials.
We find that both the preservation of all-electron derivative of tail norm with
respect to occupation and the preservation of all-electron derivative of
eigenvalue with respect to occupation {[Phys. Rev. B {\bf 48}, 5031 (1993)]}
are necessary to give accurate bulk metal lattice constants and bulk moduli. We
also show how the fairly wide range of lattice constant and bulk modulus
results found in the literature can be easily explained by pseudopotential
effects.Comment: submitted to Phys. Rev
Substantial bulk photovoltaic effect enhancement via nanolayering.
Spontaneous polarization and inversion symmetry breaking in ferroelectric materials lead to their use as photovoltaic devices. However, further advancement of their applications are hindered by the paucity of ways of reducing bandgaps and enhancing photocurrent. By unravelling the correlation between ferroelectric materials' responses to solar irradiation and their local structure and electric polarization landscapes, here we show from first principles that substantial bulk photovoltaic effect enhancement can be achieved by nanolayering PbTiO3 with nickel ions and oxygen vacancies ((PbNiO2)x(PbTiO3)(1-x)). The enhancement of the total photocurrent for different spacings between the Ni-containing layers can be as high as 43 times due to a smaller bandgap and photocurrent direction alignment for all absorption energies. This is due to the electrostatic effect that arises from nanolayering. This opens up the possibility for control of the bulk photovoltaic effect in ferroelectric materials by nanoscale engineering of their structure and composition
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