G. A. Samara

The relaxational properties of compositionally disordered ABO3 perovskites

Random lattice disorder produced by chemical substitution in ABO3 perovskites can lead to the formation of dipolar impurities and defects that have a profound influence on the static and dynamic properties of these materials that are the prototypical soft ferroelectric (FE) mode systems. In these highly polarizable host lattices, dipolar entities form polar nanodomains whose size is determined by the dipolar correlation length, rc, of the host and that exhibit dielectric relaxation in an applied ac field. In the very dilute limit (< 0.1at.%) each domain behaves as a non-interacting dipolar entity with a single relaxation time. At higher concentrations of disorder, however, the domains can interact leading to more complex relaxational behaviour. Among the manifestations of such behaviour is the formation of a glass-like relaxor (R) state, or even an ordered FE state for a sufficiently high concentration of overlapping domains. After a brief discussion of the physics of random-site electric dipoles in dielectrics, this review begins with the simplest cases, namely the relaxational properties of substitutional impurities (e.g., Mn, Fe and Ca) in the quantum paraelectrics KTaO3 and SrTiO3. This is followed by discussions of the relaxational properties of Li-and Nb-doped KTaO3 and of the strong relaxors in the PbMg1/3Nb2/3O3 and La-substituted PbZr1−xTixO3 families. Some emphasis will be on the roles of pressure and applied dc biasing electric fields in understanding the physics of these materials including the R-to-FE crossover.

Synthesis and optical properties of MoS2 and isomorphous nanoclusters in the quantum confinement regime

Highly crystalline nanoclusters of hexagonal (2H polytype) MoS2 and several of its isomorphous Mo and W chalcogenides have been synthesized with excellent control over cluster size down to ∼2 nm. These clusters exhibit highly structured, bandlike optical absorption and photoluminescence spectra which can be understood in terms of the band-structures for the bulk crystals. Key results of this work include: (1) strong quantum confinement effects with blue shifts in some of the absorption features relative to bulk crystals as large as 4 eV for clusters ∼2.5 nm in size, thereby allowing great tailorability of the optical properties; (2) the quasiparticle (or excitonic) nature of the optical response is preserved down to clusters ≲2.5 nm in size which are only two unit cells thick; (3) the demonstration of the strong influence of dimensionality on the magnitude of the quantum confinement. Specifically, three-dimensional confinement of the carriers produces energy shifts which are over an order of magnitude lar...

Tailorable, Visible Light Emission From Silicon Nanocrystals

Crystalline, size-selected Si nanocrystals in the size range 1.8–10 nm grown in inverse micellar cages exhibit highly structured optical absorption and photoluminescence (PL) across the visible range of the spectrum. The most intense PL for the smallest nanocrystals produced (∼2 nm) was in the blue (∼365 nm) with a radiative lifetime of ∼1 ns and is attributed to direct recombination at zone center.

Pressure and temperature dependence of the dielectric properties and phase transitions of the ferroelectric perovskites: PbTiO3 and BaTiO3

The effects of temperature and hydrostatic pressure on the ferroelectric (FE) properties of PbTiO3 and on certain properties of BaTiO3 which have not been treated or emphasized earlier were investigated. For PbTiO3, the FE-paraelectric (PE) transition is of first-order, and the transition temperature, Tc, decreases with pressure with an initial slope of -8.4 ± 0.3°K/kbar. It is estimated that the latent heat associated with the transition is 350 ± 45 cal/mole, with a corresponding entropy change of 0.46 ± 0.05 cal/mole°K. In the PE phase, the static dielectric constant, e, obeys the Curie-Weiss law e = C/(T- T0) over a wide temperature range. Both C and T0 decrease with pressure. At constant % e varies with pressure according to e = C*/(p-po), where p0 decreases strongly with increasing T while C* is very weakly T-dependent. The results can be explained in terms of an increase of the frequency of the soft FE mode with pressure, and the Gruneisen parameter and its Tdependence are determined for this mode. ...

Ferroelectricity revisited—Advances in materials and physics

Publisher Summary This chapter discusses field of ferroelectricity. It deals with selected salient properties and associated physics. The chapter reviews the phenomenological, soft mode, and first-principles theories of ferroelectrics. There are similarities between spin glasses and dielectric dipolar glasses. The dipolar interaction in both a normal, or ordinary, polarizable medium leads to the formation of a dipolar glass at low temperatures, and in a highly polarizable medium that leads to either long-range ferroelectric order or to a glass-like relaxor state. The properties of relaxor ferroelectrics and the contrast between properties and normal ferroelectrics have also been discussed in the chapter. Discoveries coupled with the scientific challenges of the many facets of ferroelectric phenomena and the technological importance of the field ensure health and vitality of research in ferroelectricity.

Pressure Dependence of the Elastic Constants of SrTiO3

The effects of hydrostatic pressure between 0 and 22 kbar on the elastic constants of SrTiO3 were measured at 295°K. All constants increased linearly with pressure in this range. The pressure derivatives of the adiabatic constants dCij/dP are: 10.21±0.15, 3.51±0.15, and 1.24±0.05 for C11, C12, and C44, respectively. These pressure derivatives, in conjunction with the recent second‐harmonic generation results of Mackey and Arnold, allow us to determine the complete set of third‐order elastic constants. The results, in units of 1012 dyn/cm2, are: C111=−49.6±4.3, C112=−7.7±1.6, C123=+0.2±4.3, C144=−8.1±2.4, C166=−3.0±1.2, and C456=0.9±2.7. Combined with the temperature derivatives of the (second‐order) elastic constants at constant pressure, the pressure results allow a determination of the explicit volume and temperature dependences of these constants. In addition, the present results allow us to calculate the acoustic mode Gruneisen parameters, an approximate value of the limiting Gruneisen constant, γ0=1....

Low‐temperature dielectric properties of candidate substrates for high‐temperature superconductors: LaAlO3 and ZrO2 : 9.5 mol % Y2O3

The effects of temperature (4–400 K), hydrostatic pressure and frequency on the dielectric constant, e’, and dielectric loss of single crystals of LaAlO3 and cubic yttria(9.5 mol %)‐stabilized zirconia (ZrO2) were investigated. Both crystals are relatively low‐loss, low‐dispersion dielectrics in the temperature range (T<150 K) of most interest for high‐temperature superconductors. The pressure results make it possible to evaluate the various contributions to the temperature dependence of e’. It is found that this dependence is dominated by the change of polarizability with volume. The dielectric properties of the two crystals are compared with those of other candidate substrate materials.

High pressure studies of ionic conductivity in solids

Publisher Summary The chapter presents a discussion on high-pressure studies of ionic conductivity in solids. The chapter presents a review and discusses hydrostatic pressure studies performed on a variety of ionic conductors. The chapter presents discussion on the experimental results and their interpretation in terms of relevant concepts and theory. The chapter presents examples, which illustrate special features or show systematic trends or both. Much of the available pressure work has been on relatively simple materials and crystal structures because these are amenable to theoretical treatment. Some of the materials investigated (for example, PbF2 and the thallous halides) are especially interesting because, among other properties, they possess large dielectric constants and also exhibit relatively soft, low-lying phonon modes. These factors are important in relation to ionic conduction because the larger the dielectric constant of an ionic crystal, the lower the energy of formation of lattice defects. Also, physically, ionic transport occurs by hopping motion across an energy barrier, and this barrier might be expected to become smaller the “softer” the lattice. The chapter presents analysis on the evidence for the connection between these properties and the transport properties. The chapter presents a brief theoretical background with some of the concepts and results necessary for the analysis and interpretation. For this purpose it was found most convenient to divide the substances of interest into several groupings as follows: alkali halides (both NaCl and CsCl types), silver halides, thallium halides, fluorites and related structures, and fast ion conductors.

Pressure-induced crossover from long‑to‑short‑range order in [Pb(Zn1/3Nb2/3)O3]0.905 (PbTiO3)0.095 single crystal,

A pressure-induced crossover from normal Ferroelectric-to-Relaxer behavior has been observed in single crystal [Pb(Zn{sub 1/3}Nb{sub 2/3})O{sub 3}]{sub 0.905}(PbTiO{sub 3}){sub 0.0095}, or PZN - 9.5% PT. Analogy with similar observations for other perovskites indicates that this crossover is a general feature of compositionally-disordered soft mode ferroelectrics. The Pressure-Temperature phase diagram has been also determined.

The effects of pressure on the β molecular relaxation and phase transitions of the ferroelectric copolymer P(VDF0.7TrFe0.3)

Abstract The effects of hydrostatic pressure on the dynamics and energetics of the β molecular relaxation process and on the ferroelectric (Tc ) and melting (Tm ) transition temperatures of the ferroelectric copolymer P(VDF0.7TrFE0.3) have been investigated. Pressure causes a large slowing down of the β relaxation as well as large increases in Tc and Tm , but the magnitudes of the effects are different for the different transitions. These effects can be qualitatively understood in terms of pressure-induced hinderance of the molecular motions and/or reorientations that occur at the various transitions. The slowing down of the β relaxation is interpreted in terms of the Vogel-Fulcher equation and results from increases in both the energy barrier to dipolar motion and the reference temperature (T 0) for the kinetic relaxation which represents the “static” dipolar freezing temperature for the process. In terms of both its dynamics and energetics, the β relaxation of the copolymer is nearly indistinguishable f...