摘要:

PbTiO3is a prototypical ferroelectric material that exhibits a single structural phase transition (cubic to tetragonal): it is a soft mode driven, predominantly displacive, transition. In this paper, we study the behavior of PbTiO3at finite temperature by ab‐initio molecular dynamics simulations. In this approach classical mechanics is used to describe nuclear dynamics, while the interatomic potential is generated on the fly from the ground state of the electrons within density functional theory. Fluctuations of volume and shape of the simulation cell are included by means of Parrinello‐Rahman constant pressure scheme. Extensive convergence studies based on static calculations indicate that a 3×3×3 supercell containing 135 atoms, with a singlek‐point sampling, is sufficient to represent accurately the T = 0 energetics of this material. Although computationally demanding, ab‐initio molecular dynamics simulations for PbTiO3using a 3×3×3 cell are feasible with current computational methodologies. Here we report preliminary results of simulations that are both below and above the phase‐transition temperature. We discuss, in particular, how phonon softening occurs with temperature and how thermal expansion affects the results. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1609951

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

We performedab initiocalculations to study ferroelectric (FE) instabilities and isotope effects in the H‐bonded ferroelectric KH2PO4(KDP). We demonstrate that the source of the FE instability is the hydrogen off‐centering. This ordering, produces an electronic charge redistribution within the PO4tetrahedral units, which polarize alongc. Cluster distortions following the H off‐centered relaxation pattern in a mean‐field paraelectric (PE) phase, lead to instabilities which are significant only when the heavy ions P and K are also allowed to relax. Subsequent quantization in small clusters, leads to tunneling only for distortions including heavy ions relaxations. This explains the H double‐site occupancy observed experimentally in the PE phase, and is also in agreement with the P‐atom multi‐site distribution detected experimentally in deuterated KDP (DKDP). Mass changes due to deuteration at fixed structural parameters cannot account for the huge isotope effect. However, the main effect of deuteration is a depletion of the proton probability density at the O‐H‐O center, which in turn weakens the proton‐mediated covalency in the bridge. A lattice expansion follows then, which is coupled self‐consistently with the proton off‐centering. This self‐consistent mechanism is illustrated with a non‐linear model deduced from theab initiocalculations, and allows us to explain the huge isotope effect observed and the importance of geometrical effects proved by high‐pressure experiments. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1609952

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

We study from first principles the groundstate properties of the high‐symmetry phases of K2SeO4: two ideal ordered high‐temperature hexagonal ones and an orthorhombic one. We analyze in detail the dielectric properties of these phases and discuss certain aspects of the zone‐center dynamical properties. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1609953

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

The strong dependence of the properties of ferroelectric lithium niobate crystals on the concentration of intrinsic (non‐stoichiometric) and extrinsic (impurity) defects is analyzed. Spectra of optical absorption and magnetic resonance spectroscopy are compared for crystals with different compositions. The results show clearly that the possibility to vary the concentration of intrinsic defects offers extraordinary informative opportunities for the investigation of the fundamental physics of the material. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1609954

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

We present the state‐of‐the art of large scale computer modeling of electron and hole polarons in advanced ABO3perovskites performed by means of semi‐empirical quantum chemical (INDO) method. Our calculations confirm existence of the self‐trapped electron polarons in KNbO3, KTaO3, BaTiO3, and PbTiO3crystals. The self‐trapped electron is mostly localized on B‐type ion due to a combination of breathing and Jahn‐Teller modes of nearest 6 O ion displacements. The relevant lattice relaxation energies are typically 0.2–0.3 eV, whereas the optical absorption lies at 0.7–0.8 eV, respectively. The optical absorption energies for the electron polaron bound to cation impurity in KNbO3(0.88 eV) and hole polaron bound to K vacancy (≈ 1 eV) are also in a good agreement with the relevant experimental data. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1609955

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Results of calculations for the (110) polar surfaces of three ABO3perovskites — STO, BTO and LMO — are discussed. These are based onab initioHartree‐Fock method and classical Shell Model. Both methods agree well on both surface energies and on near‐surface atomic displacements. A novel model of the “zig‐zag” surface termination is suggested and analyzed. Considerable increase of the Ti&sngbnd;O chemical bond covalency nearby the surface is predicted for STO. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1609956

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

We demonstrate how a phenomenological model which reproduces first‐principles density functional theory (DFT) calculations can be constructed to study complex ferroelectric oxides. This model is derived from a well‐known crystal chemistry approach, in which Brown’s Rules of Valence[1, 2] are used to determine the configurational energy of the system. Our previous work has shown that this model can be used to explain the atomic interactions in the Pb(Zr1−xTix)O3(PZT) solid solution. We extend this model by parameterizing to a distorted DFT structure as well as a structural minimum. Finally, we shall comment on the applicability of this model for finite‐temperature and composition‐dependent studies. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1609957

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

The large‐scale modeling of the atomic and electronic structure of KNbxTa1−xO3(KTN) perovskite solid solutions is performed using the Intermediate Neglect of the Differential Overlap (INDO) method based on the Hartree‐Fock formalism. It is found that periodic Nb impurities in KTaO3reveal coherent off‐center displacements already at the smallest calculated concentration,x=0.125. The calculated magnitude of ⟨ 111 ⟩ Nb off‐center displacement is 0.27 a.u., which is close to the XAFS observation at 70 K andx=0.09. In contrast, Ta impurities in KNbO3remain on‐center, due to higher ionicity of Ta, as compared to Nb. Using the calculated energy gain caused by the off‐center displacements of Nb atoms, the non‐empirical Ginzburg‐Landau‐Devonshire functional with concentration‐dependent coefficients is constructed. Analysis of INDO results for several Nb concentrations in KTN allows calculate the lowest critical Nb concentration,xcr=0.025, corresponding to the quantum displacive limit for the paraelectric‐ferroelectric transition at 0 K. This value is only slightly higher than the experimental one. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1609958

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

We present a report on recent progress in application of quantum Monte Carlo (QMC) methods to accurate electronic structure calculations of ferroelectric transition metal oxides. There are three major aspects: i) construction of accurate many‐body trial functions using orbitals from one‐particle approaches such as Hartree‐Fock and density functional theory, in particular, the hybrid functionals such as B3LYP; ii) efficient calculations of excitation energies both in variational and diffusion QMC; iii) method for calculating the small energy differences such as subtle features on total energy surfaces related to ferroelectric distortions. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1609959

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Fermion quantum Monte Carlo (QMC) methods that work in a general basis space may be more effective for some problems than the traditional diffusion QMC method. Projection of the ground state energy is achieved by random walks in the space of Slater determinants whose one‐particle orbitals are expressed in terms of the chosen basis set. A complication is that the determinants will in general acquire complex phases. This a consequence of ground state Monte Carlo projection using the Hubbard‐Stratonovich transformation of the two‐body interaction. To control the resulting “sign” decay, we describe a method we have recently introduced for the propagation of phaseless determinants. The approximation relies on importance sampling with a trial wave function. The approximation has features in common with diffusion MC fixed node and lattice‐model constrained path methods. Using a planewave basis and non‐local pseudopotentials, we apply the method to Si atom, dimer, and 2, 16, 54 atom (216 electrons) bulk supercells. Single Slater determinant wave functions from density functional theory calculations were used as |&PSgr;T⟩ with no additional optimization. The calculated binding energy of Si2and cohesive energy of bulk Si are in excellent agreement with experiments and are comparable to the best existing theoretical results. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1609960

出版商:AIP    

年代:1903

数据来源: AIP