Schottky defect

Résumé

A Schottky defect is an excitation of the site occupations in a crystal lattice leading to point defects named after Walter H. Schottky. In ionic crystals, this defect forms when oppositely charged ions leave their lattice sites and become incorporated for instance at the surface, creating oppositely charged vacancies. These vacancies are formed in stoichiometric units, to maintain an overall neutral charge in the ionic solid. Definition Schottky defects consist of unoccupied anion and cation sites in a stoichiometric ratio. For a simple ionic crystal of type A−B+, a Schottky defect consists of a single anion vacancy (A) and a single cation vacancy (B), or v + v following Kröger–Vink notation. For a more general crystal with formula AxBy, a Schottky cluster is formed of x vacancies of A and y vacancies of B, thus the overall stoichiometry and charge neutrality are conserved. Conceptually, a Schottky defect is generated if the crystal is expanded by one unit cell, whose a p

Source officielle

https://en.wikipedia.org/wiki/Schottky_defect

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One-Shot Approach for Enforcing Piecewise Linearity on Hybrid Functionals: Application to Band Gap Predictions

Stefano Falletta, Alfredo Pasquarello, Jing Yang

We present an efficient procedure for constructing nonempirical hybrid functionals to accurately predict band gaps of extended systems. We determine mixing parameters by enforcing the generalized Koopmans’ condition on localized electron states, which are achieved by inserting an optimized potential probe. Application of this scheme to a large set of materials yields band gaps with a mean error of 0.30 eV with respect to experiment. Next, we consider a perturbative one-shot approach in which the single-particle eigenvalues are calculated with the wave functions obtained at the semilocal level. In this way, the computational cost is reduced by ∼85% without loss of accuracy. The scheme is found to be robust upon consideration of different defect species and functional forms.

2022

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In this work, the emergence of polarization and electro-mechanical coupling in Pb(Mg1/3Nb2/3)O3 and Pb(Mg1/3Nb2/3)O3 â PbTiO3 was investigated by means of thermally stimulated current, and nonlinear dielectric and electro-mechanical measurements. The presence of complex time-dependent and interrelated mechanisms, and their influence on macroscopic functional properties was demonstrated. Widely accepted interpretations related to the nature of relaxor ferroelectrics were questioned. Persistent macroscopic polarization in the nominally ergodic and centrosymmetric relaxor state of pristine Pb(Mg1/3Nb2/3)O3 single crystals was revealed by dynamic pyroelectric measurements. Annealing experiments below and above Burns temperature related this polarization then to the presence of nano-sized polar entities. Poling experiments of Pb(Mg1/3Nb2/3)O3 associated a maximum in the electric-field dependent nonlinear dielectric permittivity and an attendant anomaly in the second harmonic polarization response to the reorientation of nano-sized polar entities.In the following, changes in the polar structure of Pb(Mg1/3Nb2/3)O3 were characterizedin-situ with thermally stimulated depolarization current measurements. It was shown thatanomalies can be induced by electrical or mechanical poling at temperatures related to staticand dynamic nano-sized polar regions. An aging effect was revealed with dynamic dielectricand thermally stimulated current measurements. A current reversal was observed around thecoherence temperature in electrical polarized and unpolarized aged samples, suggesting a director indirect correlation between oxygen vacancies and nano-sized polar regions.Based on these observations, the effect of aging on macroscopic functional properties wasinvestigated in Pb(Mg1/3Nb2/3)O3 â PbTiO3 by means of thermally stimulated depolarization current, and dielectric and electro-mechanical measurements. For this purpose, the response ofunpoled, direct current poled and alternating current poled samples was compared for annealedand aged samples. Aging was related to reduced piezoelectric properties and depolarizationtemperatures. An apparent contradicting duality of polarization stabilization and property enhancement was observed for alternating current poling. It was proposed that the applied bipolar poling field leads to a distribution of oriented defect clusters throughout the volume, which could improve dielectric and piezoelectric properties through locally broken symmetry and mediated reversible polarization rotation. The existence of an ergodic relaxor state and the model of a single polar entity below TB are not consistent with the results presented in this work. It is proposed that functional and structural properties of undoped Pb(Mg1/3Nb2/3)O3 and undoped Pb(Mg1/3Nb2/3)O3 â PbTiO3 are considerably affected by defect structures, which depend on the thermal, electric field and mechanical field history, and on time. It was speculated that the multitude of sometimes contradicting experimental results reported in the literature are related to these findings. The outcomes of this work are expected to further the fundamental understanding of relaxor ferroelectrics and to contribute to an enhanced reproducibility of experimental results found in the literature.

EPFL2022

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Defect structures in deformed FCC metals

Yong Dai

A high density of small defect clusters, similar to those observed in irradiated or quenched metals. has been observed in the deformed f.c.c. metals Cu, Au and Ni. The preliminary results show that the defect clusters are predominantly stacking fault tetrahedral (SFT). The SFT number density, rather than the size distribution, is deformation dependent. The defect cluster density is greater in the vicinities of dislocation tangles and grain boundaries. Their size distribution is wider than that produced by irradiation with an important number of larger clusters being formed. It is argued that these deformation-produced clusters may play a role in determining the flow stress and work hardening at low deformations. (C) 1997 Acta Metallurgica Inc.

1997

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EPFL2022