235 research outputs found
Rules and mechanisms governing octahedral tilts in perovskites under pressure
The rotation of octahedra (octahedral tilting) is common in ABO3 perovskites
and relevant to many physical phenomena, ranging from electronic and magnetic
properties, metal-insulator transitions to improper ferroelectricity.
Hydrostatic pressure is an efficient way to tune and control octahedral
tiltings. However, the pressure behavior of such tiltings can dramatically
differ from one material to another, with the origins of such differences
remaining controversial. In this work, we discover several new mechanisms and
formulate a set of simple rules that allow to understand how pressure affects
oxygen octahedral tiltings, via the use and analysis of first-principles
results for a variety of compounds. Besides the known A-O interactions, we
reveal that the interactions between specific B-ions and oxygen ions contribute
to the tilting instability. We explain the previously reported trend that the
derivative of the oxygen octahedral tilting with respect to pressure (dR/dP)
usually decreases with both the tolerance factor and the ionization state of
the A-ion, by illustrating the key role of A-O interactions and their change
under pressure. Furthermore, three new mechanisms/rules are discovered. We
further predict that the polarization associated with the so-called hybrid
improper ferroelectricity could be manipulated by hydrostatic pressure, by
indirectly controlling the amplitude of octahedral rotations.Comment: Submitted to Phys. Re
Effects of Vacancies on Properties of Relaxor Ferroelectrics: a First-Principles Study
A first-principles-based model is developed to investigate the influence of
lead vacancies on the properties of relaxor ferroelectric Pb(Sc1/2Nb1/2)O3
(PSN). Lead vacancies generate large, inhomogeneous, electric fields that
reduce barriers between energy minima for different polarization directions.
This naturally explains why relaxors with significant lead vacancy
concentrations have broadened dielectric peaks at lower temperatures, and why
lead vacancies smear properties in the neighborhood of the ferroelectric
transition in PSN. We also reconsider the conventional wisdom that lead
vacancies reduce the magnitude of dielectric response.Comment: 11 pages, 1 figur
Origin of negative electrocaloric effect in Pnma-type antiferroelectric perovskites
Anomalous electrocaloric effect (ECE) with decreasing temperature upon
application of an electric field is known to occur in antiferroelectrics
(AFEs), and previous understanding refers to the field-induced canting of
electric dipoles if there is no phase transitions. Here, we use a
first-principle-based method to study the ECE in Nd-substituted BiFeO3 (BNFO)
perovskite solid solutions, which has the Pnma-type AFE ground state. We
demonstrate another scenario to achieve and explain anomalous ECE, emphasizing
that explicit consideration of octahedral tiltings is indispensable for a
correct understanding. This mechanism may be general for AFEs for which the
antipolar mode is not the primary order parameter. We also find that the
negative ECE can reach a large magnitude in BNFO
Pintura mural romana en ámbito doméstico durante el s. i d. C. en el conventus Caesaraugustanus
Presentamos un análisis de los restos pictóricos hallados en los ambientes domésticos del conventus Caesaraugustanus, fechados en el s. i d. C. El estudio, basado en una metodología a través de la cual hemos examinado los aspectos técnicos y estilísticos de los fragmentos procedentes del ámbito doméstico de nueve yacimientos insertos en dicho territorio, nos ha permitido profundizar en el conocimiento tanto de la representación del iii y iv estilos pompeyanos en el mundo provincial como de los talleres de artesanos encargados de enlucir las estancias. Los restos decorativos también nos han permitido acercarnos a los comitentes que seleccionaron determinados motivos ornamentales para sus viviendas. A partir de ello nos planteamos si es posible documentar, a través de la pintura mural romana y en el territorio que nos ocupa, la mutación social acaecida en la Península Ibérica con la llegada de Augusto, siempre teniendo en cuenta el sustrato indígena precedente y el fuerte impulso que en este momento alcanzó el proceso de adhesión a la nueva cultura.
We present an analysis of pictorial remains found in domestic environments of conventus Caesaraugustanus, dated in the first century ad. The study, based on a methodology through which we examined the technical and stylistic aspects of the fragments from the domestic sphere of nine sites located in that territory, has allowed us to deepen the understanding of both the representation of iii and iv Pompeian styles in the provincial world, and craft workshops that made them. The decorative remains will also bring us out to those patrons who selected certain ornamental motives for their homes. So, we will consider whether it is possible to document, through the Roman wall painting and in the territory concerned, the social mutation occurred in the Iberian Peninsula with the advent of Augustus, taking into account the precedent indigenous substrate and the strong impulse that the accession process to the new culture reached at this time
Effects of atomic short-range order on the properties of perovskite alloys in their morphotropic phase boundary
The effects of atomic short-range order on the properties of
Pb(Zr_{1-x}Ti_x)O_3 alloy in its morphotropic phase boundary (MPB) are
predicted by combining first-principles-based methods and annealing techniques.
Clustering is found to lead to a compositional expansion of this boundary,
while the association of unlike atoms yields a contraction of this region.
Atomic short-range order can thus drastically affect properties of perovskite
alloys in their MPB, by inducing phase transitions. Microscopic mechanisms
responsible for these effects are revealed and discussed.Comment: 4 pages, with 2 postscript figures embedded. Uses REVTEX4 and
graphicx macro
Ab-initio design of perovskite alloys with predetermined properties: The case of Pb(Sc_{0.5} Nb_{0.5})O_{3}
A first-principles derived approach is combined with the inverse Monte Carlo
technique to determine the atomic orderings leading to prefixed properties in
Pb(Sc_{0.5}Nb_{0.5})O_{3} perovskite alloy. We find that some arrangements
between Sc and Nb atoms result in drastic changes with respect to the
disordered material, including ground states of new symmetries, large
enhancement of electromechanical responses, and considerable shift of the Curie
temperature. We discuss the microscopic mechanisms responsible for these
unusual effects.Comment: 5 pages with 2 postscript figures embedde
First-principles study of (BiScO3){1-x}-(PbTiO3){x} piezoelectric alloys
We report a first-principles study of a class of (BiScO3)_{1-x}-(PbTiO3)_x
(BS-PT) alloys recently proposed by Eitel et al. as promising materials for
piezoelectric actuator applications. We show that (i) BS-PT displays very large
structural distortions and polarizations at the morphotropic phase boundary
(MPB) (we obtain a c/a of ~1.05-1.08 and P_tet of ~1.1 C/m^2); (ii) the
ferroelectric and piezoelectric properties of BS-PT are dominated by the onset
of hybridization between Bi/Pb-6p and O-2p orbitals, a mechanism that is
enhanced upon substitution of Pb by Bi; and (iii) the piezoelectric responses
of BS-PT and Pb(Zr_{1-x}Ti_x)O3 (PZT) at the MPB are comparable, at least as
far as the computed values of the piezoelectric coefficient d_15 are concerned.
While our results are generally consistent with experiment, they also suggest
that certain intrinsic properties of BS-PT may be even better than has been
indicated by experiments to date. We also discuss results for PZT that
demonstrate the prominent role played by Pb displacements in its piezoelectric
properties.Comment: 6 pages, with 3 postscript figures embedded. Uses REVTEX and epsf
macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/ji_bi/index.htm
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