摘要:

We study the ferroelectric phase transition inPbSc1/2Nb1/2O3(PSN) using a first-principles effective Hamiltonian approach. Results for PSN with NaCl-type ordering of Sc and Nb on the B sites shows that a Pb-centered effective Hamiltonian is appropriate for the ordered cell. We obtain a complete effective Hamiltonian for ordered PSN that has the form of effective Hamiltonians for simple perovskites. We modify the effective Hamiltonian to include the effects of Sc-Nb disorder by adding one additional term giving the effective force on the Pb site due to the nearest neighbor B-site cations. Monte Carlo simulations of our model shows that disordered PSN has a lower Curie temperature than ordered PSN, in agreement with a previous first-principles study based on a virtual crystal approach. No evidence for relaxor behavior is seen in our model. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1399704

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

We present and discuss results of the calculations forBaTiO3andSrTiO3surface relaxation with different terminations using a semi-empiricalshell model(SM) as well asab initiomethods based on Hartree-Fock (HF) and Density Functional Theory (DFT) formalisms. Using the SM, the positions of atoms in 16 near-surface layers placed atop a slab of rigid ions are optimized. This permits us determination of surface rumpling and surface-induced dipole moments (polarization) for different terminations of the (100) and (110) surfaces. We also compare results of theab initiocalculations based on both HF with the DFT-type electron correlation corrections, several DFT with different exchange-correlation functionals, and hybrid exchange techniques. Our SM results for the (100) surfaces are in a good agreement with both ourab initiocalculations and LEED experiments. For the (110) surfaces O-termination is predicted to be the lowest in energy. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1399705

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

We generate a series of Zr pseudopotentials and use them to calculate the properties ofPbZrO3,in order to examine the relationship between pseudo-atomic properties and solid-state oxide results. We find that lattice constants and bond lengths within the oxide unit cell are quite sensitive to pseudopotential construction errors, and clear correlations emerge. These trends motivate our identification of two criteria for accurate transition metal pseudopotentials for use in oxide calculations, which are similar to the criteria for metal use. We find that both the preservation of all-electron tail norm and the preservation of all-electron ionization energy are necessary to give good lattice constants for oxides. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1399706

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

We extend our first-principles study of novel ferroelectric perovskite systems in which compositional inversion symmetry is broken [N. Sai, B. Meyer and D. Vanderbilt, Phys. Rev. Lett. 84, 5636 (2000)] by (i) focusing on the piezoelectric response of the triple-cell heterovalent systemBa(Ti-&dgr;,Ti,&hthinsp;Ti+&dgr;)O3,and (ii) studying the strength of the symmetry breaking in the double-cell system(Ba,&hthinsp;Sr)(Ti-&dgr;,Ti+&dgr;)O3with simultaneousA-site andB-site substitutions. We observe the enhanced piezoelectric response coefficiente33when increasing the compositional parameter &dgr; in the triple-cell system. This enhancement is quite drastic for the metastable minimum, but only modest for the stable minimum, corresponding respectively to the minority and majority wells of the ferroelectric double-well structure. In the double-cell system, we find that the increase in strength of symmetry breaking with the chemical or concentration perturbation is dominated by the term linear in &dgr;, in contrast with the&dgr;3dependence found in the triple-cell system. A symmetry-based justification of the dominance of the linear term is provided. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1399707

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

The current experimental and theoretical knowledge of new polaronic-type excitons in ferroelectric oxides—charge transfer vibronic excitons (CTVE)—is discussed. It is shown that Hartree-Fock-type INDO calculations as well as photoluminescence studies in ferroelectric oxygen-octahedral perovskites confirm the CTVE-concept. Single CTVE as well as a new phase of strongly correlated CTVEs are analyzed. It is shown also that polaron- and CTVE-trapping effects including the oxygen vacancy clusters play an important role in polar cluster formation. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1399708

出版商:AIP    

年代:1901

数据来源: AIP