摘要:

Interatomic potentials forBaTiO3are determined, in the framework of a shell model, by mapping first-principles potential energy surfaces for various ferroelectric distortions. Several bulk properties, such as lattice parameters, phase transition sequence, thermal expansivity, etc., are correctly reproduced by molecular dynamics simulations. To investigate whether the model will also prove successful for describing surface properties, such as structural relaxations and surface energies, we perform static calculations on periodic slabs to compare with recentab initiostudies ofBaTiO3surfaces, which are taken as benchmark results. The agreement is quite good in spite that the model was developed for the bulk material. The good description of the static surface properties constitutes an important first step to a further finite-temperature simulation study of the thickness dependence of ferroelectric transitions inBaTiO3thin films. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324464

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

A new hypothesis regarding the role of rare earth dopants in improving the lifetime of multilayer capacitors with base metal electrodes is presented. The hypothesis is based on the experimental findings, which have shown that the site occupancy of the important dopants is a function of the overall A/B ratio in the perovskite dielectric material. The main assumption is that deviation from A/B stoichiometry significantly controls the mobility of oxygen vacancies along the grain boundaries. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324465

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

High temperature oxide melt solution calorimetry is a versatile technique for studying the energetics of formation, solid solution mixing, phase transition, and order/disorder in complex perovskites. The methodology is described and examples of both present and possible future applications given. The stability of the perovskite structure diminishes as the tolerance factor deviates from unity and increases as the size (and basicity) of the A-site cation increases. High pressure lithium niobate and perovskite structures are formed for a number of silicates, germanates, and titanates. Their energetics become less favorable as the A-site cation radius decreases. Ordered double perovskites are favored at high pressure. Such materials as well as the vacancy-ordered brownmillerite structures are only marginally energetically stable with respect to end-member perovskites. Thermochemical data suggest that “disordered” perovskite phases in fact contain pervasive clustering and/or short range order. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324466

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

The spontaneous polarization of ferroelectrics becomes temperature independent below a characteristic temperature(T≪&thgr;s)due to the freezing of fluctuational degrees of freedom. Displacive transitions are well described by a low temperature expansion of Landau theory with&Dgr;G=1/2A&thgr;s[coth&hthinsp;&thgr;s/T−coth&hthinsp;&thgr;s/Tc]p2+1/4Bp4+1/6p6+&ellip;A more general solution of the&fgr;nmodel yields mean field self-consistency equations which agree rather well with numerical solutions of 3D-computer simulations. The application of secondary fields x leads to highly non-linear phase boundariesTc(x)in(Tc,x)space. Pe´rez–Mato and Salje showed that in several cases the functionTc(x)can be found by direct inversion of the functionQ(T)with the mapping&ggr;pn−2→x(Hcoupling=&ggr;xp2).An example of multi-phonon coupling is discussed. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324467

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

A summary of the work recently carried out on the morphotropic phase boundary (MPB) of PZT is presented. By means of x-ray powder diffraction on ceramic samples of excellent quality, the MPB has been successfully characterized by changing temperature in a series of closely spaced compositions. As a result, an unexpected monoclinic phase has been found to exist in between the well-known tetragonal and rhombohedral PZT phases. A detailed structural analysis, together with the investigation of the field effect in this region of compositions, have led to an important advance in understanding the mechanisms responsible for the physical properties of PZT as well as other piezoelectric materials with similar morphotropic phase boundaries. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324468

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Room temperature neutron inelastic scattering measurements of the polar transverse optic (TO) phonon mode in the cubic relaxorPb(Mg1/3Nb2/3)O3(PMN) reveal anomalous behavior, similar to that recently observed inPb(Zn1/3Nb2/3)0.92Ti0.08O3,in which the optic branchappearsto drop precipitously into the acoustic branch at a finite value of the momentum transferq=0.23&hthinsp;Å−1,measured from the zone center. By contrast, a recent neutron study indicates that PMN exhibits a normal TO phonon branch at much higher temperature (800 K). We thus speculate that this unusual feature is common to all relaxor materials at low temperatures, and is the result of the presence of nanometer-scale polarized domains in the crystal that form below a temperatureTd,which effectively prevent the propagation of long wavelength(q=0)phonons. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324469

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

We show that mean field theory for the dipole compled Ising model for the specific lattice ofLiHoF4predicts a ferromagnetic critical temperatureTc=2.29&hthinsp;K,which is about 50&percent; above the experimental valueTc=1.53&hthinsp;K.The inclusion of antiferromagnetic exchange interactions lowers the mean fieldTc.We show however that to reach the experimental value, the antiferromagnetic interactions would need to have a strength greater than the ferromagnetic interactions. Hence it can be concluded that mean field theory is not well suited for predicting the correct ordering ofLiHoF4.Including thermal fluctuations we resolve the discrepancy and predict from Monte Carlo simulations that a pair of parallel nearest neighbor Ho-ions have an antiferromagnetic exchange energyE/kB=(0.14±0.01)&hthinsp;K.This is consistent with ferromagnetic ordering. The Monte Carlo results also clearly show that needle-shaped domains form in this system, the extended direction being along the crystalc-axis. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324470

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

An account is presented of our studies of the order-disorder phase transitions in NaCN and KCN. These are based on parameter-free inter-ionic potentials based on the Gordon-Kim modified electron gas formalism extended to molecular ions. We performed static structural relaxations and supercell molecular dynamics and reproduced two transitions known in each of these systems. We also calculated upper bounds to the barrier of rotation of a cyanide ion in a ground state of NaCN and KCN and discuss possible shortcomings of our model. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324471

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

The structural transformation in aragoniteCaCO3is simulated by molecular dynamics. The simulations are based on the potentials calculated from the Gordon-Kim modified electron gas formalism. We found two phase transitions in aragonite at high temperature. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324472

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Results on a variety of mixedABO3oxides have revealed a pressure-induced ferroelectric-to-relaxor crossover and the continuous evolution of the energetics and dynamics of the relaxation process with increasing pressure. These common features have suggested a mechanism for the crossover phenomenon in terms of a large decrease in the correlation length for dipolar interactions with pressure a unique property of soft mode or highly polarizable host lattices. The pressure effects as well as the interplay between pressure and dc biasing fields are illustrated for some recent results on PZN−9.5 PT, PMN and PLZT 6/65/35. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324473

出版商:AIP    

年代:1900

数据来源: AIP