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F. H. Dacol

Bio: F. H. Dacol is an academic researcher from IBM. The author has contributed to research in topics: Raman spectroscopy & Phonon. The author has an hindex of 28, co-authored 73 publications receiving 2932 citations.


Papers
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Journal ArticleDOI
Gerald Burns1, F. H. Dacol1
TL;DR: In this paper, the optic index of refraction as a function of temperature was measured in the crystalline ferroelectrics having the simple perovskite structure and it was shown that these crystals possess a local, randomly oriented, nonreversible polarization below a temperature several hundred degrees above the ferroelectric phase-transition temperature.
Abstract: We report measurements of the optic index of refraction as a function of temperature, $n (T)$, in the crystalline ferroelectrics having the simple perovskite ($\mathrm{AB}{\mathrm{O}}_{3}$) structure. We show that these crystals possess a local, randomly oriented, nonreversible polarization below a temperature ${T}_{d}$ several hundred degrees above the ferroelectric phase-transition temperature ${T}_{c}$. Using a simple model, we account for this behavior and understand quantitatively the values of ${T}_{d}$. This model, we believe, contains the basic physical understanding of ferroelectrics with a diffuse phase transition.

594 citations

Journal ArticleDOI
Gerald Burns1, F. H. Dacol1
TL;DR: In this article, the temperature dependence of the optic index of refraction, n(T), at several wave lengths in two ferroelectric compounds that have the simple perovskite ABO3 structure was investigated.

585 citations

Journal ArticleDOI
Gerald Burns1, F. H. Dacol1
TL;DR: In this paper, a model of these polarization effects in terms of a local, randomly oriented polarization (Pr) that may occur hundreds of degrees above Tc is presented. But the model is not suitable for the case of crystalline ferroelectrics.
Abstract: In this paper we discuss some of the measurements and properties of materials that are crystalline ferroelectrics with a glassy polarization phase. We review measurements of the temperature dependence of the optical index of refraction, n(T), which were first used to observe these properties. Then we discuss a model of these polarization effects in terms of a local, randomly oriented polarization (Pr) that may occur hundreds of degrees above Tc. This local polarization is allowed by the strong breakdown of the translational symmetry that occurs in these systems. We also show how the temperature dependence of the strain, x(T), complements the n(T) results. Lastly, we discuss some recent x-ray and EXAFS measurements and how they are in agreement with the model, and the connection between these high temperature effects and the low-temperature glass-like excitations that are found in these systems.

146 citations

Journal ArticleDOI
TL;DR: Temperature-dependent measurements of the optic index of refraction, the birefringence, and the strain in the ferroelectric, tungsten-bronze crystal suggest the crystals appear to possess a local, randomly oriented (up or down), polarization.
Abstract: We report accurate temperature-dependent measurements of the optic index of refraction, the birefringence, and the strain in the ferroelectric, tungsten-bronze crystal ${\mathrm{Ba}}_{0.4}$${\mathrm{Sr}}_{0.6}$${\mathrm{Nb}}_{2}$${\mathrm{O}}_{6}$. This composition congruently melts, so large homogeneous high-quality crystals can be grown. From the experimental data, it appears that far above the ferroelectric phase transition temperature (${T}_{c}$\ensuremath{\approxeq}88 \ifmmode^\circ\else\textdegree\fi{}C), up to a temperature ${T}_{d}$ (\ensuremath{\approxeq}300 \ifmmode^\circ\else\textdegree\fi{}C), the crystals appear to possess a local, randomly oriented (up or down), polarization ${P}_{d}$. The values of ${T}_{d}$ obtained from the index and completely independent strain measurements are in very good agreement with each other, as are the values of ${P}_{d}$. Various aspects of our understanding of the polarization behavior and other effects in this ferroelectric system are discussed.

140 citations

Journal ArticleDOI
Gerald Burns1, F. H. Dacol1
TL;DR: In this paper, the temperature dependence of the optic index of refraction, n(T), in ferroelectric BaTiO 3 for T > T c (T c ≡ the cubic to tetragonal transition temperature).

132 citations


Cited by
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TL;DR: In this article, the most characteristic properties of spin glass systems are described, and related phenomena in other glassy systems (dielectric and orientational glasses) are mentioned, and a review summarizes recent developments in the theory of spin glasses, as well as pertinent experimental data.
Abstract: This review summarizes recent developments in the theory of spin glasses, as well as pertinent experimental data. The most characteristic properties of spin glass systems are described, and related phenomena in other glassy systems (dielectric and orientational glasses) are mentioned. The Edwards-Anderson model of spin glasses and its treatment within the replica method and mean-field theory are outlined, and concepts such as "frustration," "broken replica symmetry," "broken ergodicity," etc., are discussed. The dynamic approach to describing the spin glass transition is emphasized. Monte Carlo simulations of spin glasses and the insight gained by them are described. Other topics discussed include site-disorder models, phenomenological theories for the frozen phase and its excitations, phase diagrams in which spin glass order and ferromagnetism or antiferromagnetism compete, the Ne\'el model of superparamagnetism and related approaches, and possible connections between spin glasses and other topics in the theory of disordered condensed-matter systems.

3,926 citations

Journal ArticleDOI
TL;DR: In this article, the lattice dynamics and the peculiar dielectric relaxation in relaxors are discussed, and theoretical models for the mechanisms of PNR formation and freezing into nonergodic glassy state are also presented.
Abstract: Relaxor ferroelectrics were discovered almost 50 years ago among the complex oxides with perovskite structure. In recent years this field of research has experienced a revival of interest. In this paper we review the progress achieved. We consider the crystal structure including quenched compositional disorder and polar nanoregions (PNR), the phase transitions including compositional order-disorder transition, transition to nonergodic (probably spherical cluster glass) state and to ferroelectric phase. We discuss the lattice dynamics and the peculiar (especially dielectric) relaxation in relaxors. Modern theoretical models for the mechanisms of PNR formation and freezing into nonergodic glassy state are also presented.

1,784 citations

Journal ArticleDOI
TL;DR: In this paper, highly crystalline monoclinic and tetragonal BiVO4 photocatalysts were obtained by the reaction of layered potassium vanadate powder (KV3O8 and K3V5O14) with Bi(NO3)3 for 3 days in aqueous media at room temperature.
Abstract: BiVO4 photocatalysts for O2 evolution, which work under visible light irradiation, were prepared by an aqueous process. The BiVO4 photocatalysts were obtained by the reaction of layered potassium vanadate powder (KV3O8 and K3V5O14) with Bi(NO3)3 for 3 days in aqueous media at room temperature. Highly crystalline monoclinic and tetragonal BiVO4 were selectively synthesized by changing the ratio of vanadium to bismuth in the starting materials. X-ray diffraction and scanning electron microscopy measurements showed that the monoclinic BiVO4 was formed via a tetragonal BiVO4 intermediate. Tetragonal BiVO4 with a 2.9 eV band gap mainly possessed an ultraviolet absorption band while monoclinic BiVO4 with a 2.4 eV band gap had a characteristic visible light absorption band in addition to the UV band. The UV bands observed in the tetragonal and monoclinic BiVO4 were assigned to the band transition from O2p to V3d whereas the visible light absorption was due to the transition from a valence band formed by Bi6s or ...

1,761 citations

Journal ArticleDOI
TL;DR: Crystal14 as discussed by the authors is an ab initio code that uses a Gaussian-type basis set: both pseudopotential and all-electron strategies are permitted; the latter is not much more expensive than the former up to the first second transition metal rows of the periodic table.
Abstract: The capabilities of the Crystal14 program are presented, and the improvements made with respect to the previous Crystal09 version discussed. Crystal14 is an ab initio code that uses a Gaussian-type basis set: both pseudopotential and all-electron strategies are permitted; the latter is not much more expensive than the former up to the first-second transition metal rows of the periodic table. A variety of density functionals is available, including as an extreme case Hartree–Fock; hybrids of various nature (global, range-separated, double) can be used. In particular, a very efficient implementation of global hybrids, such as popular B3LYP and PBE0 prescriptions, allows for such calculations to be performed at relatively low computational cost. The program can treat on the same grounds zero-dimensional (molecules), one-dimensional (polymers), two-dimensional (slabs), as well as three-dimensional (3D; crystals) systems. No spurious 3D periodicity is required for low-dimensional systems as happens when plane-waves are used as a basis set. Symmetry is fully exploited at all steps of the calculation; this permits, for example, to investigate nanotubes of increasing radius at a nearly constant cost (better than linear scaling!) or to perform self-consistent-field (SCF) calculations on fullerenes as large as (10,10), with 6000 atoms, 84,000 atomic orbitals, and 20 SCF cycles, on a single core in one day. Three versions of the code exist, serial, parallel, and massive-parallel. In the second one, the most relevant matrices are duplicated, whereas in the third one the matrices in reciprocal space are distributed for diagonalization. All the relevant vectors are now dynamically allocated and deallocated after use, making Crystal14 much more agile than the previous version, in which they were statically allocated. The program now fits more easily in low-memory machines (as many supercomputers nowadays are). Crystal14 can be used on parallel machines up to a high number of cores (benchmarks up to 10,240 cores are documented) with good scalability, the main limitation remaining the diagonalization step. Many tensorial properties can be evaluated in a fully automated way by using a single input keyword: elastic, piezoelectric, photoelastic, dielectric, as well as first and second hyperpolarizabilies, electric field gradients, Born tensors and so forth. Many tools permit a complete analysis of the vibrational properties of crystalline compounds. The infrared and Raman intensities are now computed analytically and related spectra can be generated. Isotopic shifts are easily evaluated, frequencies of only a fragment of a large system computed and nuclear contribution to the dielectric tensor determined. New algorithms have been devised for the investigation of solid solutions and disordered systems. The topological analysis of the electron charge density, according to the Quantum Theory of Atoms in Molecules, is now incorporated in the code via the integrated merge of the Topond package. Electron correlation can be evaluated at the Moller–Plesset second-order level (namely MP2) and a set of double-hybrids are presently available via the integrated merge with the Cryscor program. © 2014 Wiley Periodicals, Inc.

1,172 citations

Journal ArticleDOI
TL;DR: In this article, the authors trace the evolution of understanding related to the modification of sharp ferroelectric phase transition behavior that occurs in composition systems which exhibit diffuse and relaxor Ferroelectric properties.
Abstract: The paper will trace the evolution of understanding related to the modification of sharp ferroelectric phase transition behavior that occurs in composition systems which exhibit diffuse and relaxor ferroelectric properties. The focus will be primarily upon the perovskite structure families where cations of different valence occupying similar crystallographic sites in the structure appear to play an important role. Limited ordering in the Pb(B1B2)O3 systems will be discussed and possible mechanisms for self limiting to nanometer scales in some systems explored. New studies of the break up of the simple ferroelectric behavior in lanthanum modified lead zirconate titanate (PLZT) and in lead titanate (PLT) systems will be discussed and the relevance to the general problem of relaxor behavior examined. Evidence for enhanced polarization fluctuations and super paraelectric behavior at high temperatures will be discussed and random field and spin glass models for the lower temperature state considered. ...

1,031 citations