Abstract The possibility of inducing new polar and/or magnetic transient states through the pumping of optical phonons towards the non-linear regime has renewed the scientific interest in ...orthoferrites. Nonetheless, to perform these studies it is fundamental to have a deep knowledge of the lattice excitations at equilibrium conditions. In this work, we present a complete characterization of the optically-active zone-center phonons in NdFeO 3 single crystals at room temperature by means of polarized Raman and infrared spectroscopies. The study is complemented with polarized infrared spectroscopy at 4 K and unpolarized Raman scattering at 10 K. The predicted polar phonons were successfully observed together with some of the crystal-field excitations. First-principles simulations further allow the eigenmode and symmetry assignments of the optical phonons. The calculated atomic motions of each mode are of significant interest, as they are common for all orthoferrites and to most of the large family of orthorhombic Pbnm perovskites.
Giant permittivity (GP) materials are of interest as possible capacitors for energy storage. Ferroelectric and related homogeneous materials suffer from strongly temperature dependent dielectric ...properties. Therefore, within the last two decades, the interest has been focused to dielectric studies of various inhomogeneous GP materials, where the essential role was shown to be due to weak but inhomogeneous conductivity. Here we study, within the known models based on effective medium approximation, the broadband dielectric spectra of conducting core - less conducting shell structures, which are expected to be the most advantageous for revealing the GP effects. Examples of core-shell structures, in which the dielectric properties of both components are independent of frequency, are studied analytically and numerically within the generalized brick model. For the simple case of coated-spheres model the dielectric dispersion is of a single Debye-relaxation type and the optimal GP properties are for highly conducting cores and thinnest possible nonconducting and high-permittivity shells. For non-spherical core-shell particles the conditions for macroscopically isotropic composite are briefly discussed.
We describe the first-principles design and subsequent synthesis of a new material with the specific functionalities required for a solid-state-based search for the permanent electric dipole moment ...of the electron. We show computationally that perovskite-structure europium barium titanate should exhibit the required large and pressure-dependent ferroelectric polarization, local magnetic moments and absence of magnetic ordering at liquid-helium temperature. Subsequent synthesis and characterization of Eu(0.5)Ba(0.5)TiO(3) ceramics confirm the predicted desirable properties.
In this paper, we discuss broadband dielectric spectroscopy from mHz up to the infrared range mainly for materials with inhomogeneous weak conductivity, including conductor-dielectric nanocomposites. ...Our discussion is based on the effective medium approximation (EMA) and experiments modeled by this approach are reviewed. We discuss core-shell composites modeled by coated-spheres (Hashin-Shtrikman model) and normal composites with a possible percolation of the conductor component resulting in sharp or smeared percolation threshold of the DC conductivity and diverging static permittivity in the former case. The sharp percolation threshold is modeled by the Bruggeman EMA or by general EMA with arbitrary percolation threshold and arbitrary critical exponents of the DC conductivity and static permittivity. For the case of smeared percolation threshold in the case of complex topologies, we use the Lichtenecker model allowing for partial percolation of both the components. Finally, numerous papers reporting negative permittivity in weakly conducting materials are criticized and concluded to be due to spurious effects.
Searching for giant dipolar defects, suggested as the explanation of the colossal permittivity (CP) in (Nb + In) co-doped rutile, we have recently published (J. Appl. Phys. 119, 154105 (2016) and ...Phys. Rev. Mat., in press) temperature-dependent dielectric spectra from sub-Hz to THz range (including 4-point DC conductivity) of several (Nb + In) co-doped rutile ceramics. Using the model of two leaky capacitor in series below ∼1 MHz, we estimated the depletion layer thickness and conductivity. The spectra below ∼10 GHz were fitted with two Cole-Cole relaxations obeying the Arrhenius law, assigned to the effect of depletion layers and grain boundaries. In this paper, using effective medium models, we have for the first time modelled the spectra as a composite of semiconducting grains and substantially less-conducting grain boundaries and near-electrode depletion layers. The CP effect has been fully explained by the combination of surface and internal barrier-layer capacitor effects, without any measurable contribution from the dipolar defects.
Interfaced conducting polymers Stejskal, J.; Bober, P.; Trchová, M. ...
Synthetic metals,
February 2017, 2017-02-00, 20170201, Volume:
224
Journal Article
Peer reviewed
•Polyaniline, polypyrrole and poly(p-phenylenediamine) have been synthesized.•They were used as templates for the coating with the same or another polymer.•FTIR and Raman spectroscopies indicate ...complete coating of the template polymer.•Modeling of dielectric and conductivity spectra of composites was performed.
The materials composed of pairs of conducting polymers, polyaniline, polypyrrole and non-conducting poly(p-phenylenediamine), were prepared by the coating of one polymer with the other. The course of polymerizations and morphology of the resulting composites have been recorded and the products were characterized by FTIR and Raman spectroscopies, DC and broad-band AC conductivity and permittivity measurements. The interfacial interaction between conducting polymers does not introduce any new effects concerning the conductivity. On the other hand, using composites where the conducting polymer is embedded in non-conducting polymer matrix allows for the control of structure at nanoscale and for the design of materials with new conductivity properties. This is illustrated by coating the conducting polymers with the poly(p-phenylenediamine) which has the conductivity by eight orders of magnitude lower, which yields composites with an intermediate conductivity, by three orders of magnitude lower than that of the conducting polymers.
We present a study of KTaO
3
:Er (≈0.05%) single crystals based on a multifaceted approach including the use of optical absorption, far-infrared reflectivity, electron paramagnetic resonance, ...photoluminescence spectra, and ab initio simulations. We describe briefly the fundamental consequences of Er doping, in particular, stiffening of TO1 soft mode of KTaO
3
. We provide information about energy level structures controlling f - f optical transitions in Er
3+
, on formation of minor cubic octahedral and major orthorhombic Er
3+
centers. It is revealed that the temperature shift of narrow zero-phonon emission lines is strikingly unusual being much larger than that typical for trivalent rare-earth impurities.
Display omitted
•IR and THz permittivity and conductivity spectra of conducting polyaniline salt and nonconducting base compared.•Vibrational spectra agree with the absorbance on powder in KBr pellet ...and thin film.•Broad absorption in the mid- and far-IR assigned to conductivity of localized carriers and Boson peak, respectively.•Spectra of salt modeled as a nanocomposite of base and metallic islands using effective medium approach.•Permittivity below the THz range increases with lowering frequency, in contrast to some published data.
Conducting polyaniline (PANI) salt and dielectric PANI base pellets were studied in the infrared (IR) and terahertz (THz) range at room temperature and compared with the IR transmission spectra of powdered samples dispersed in potassium bromide and with those of thin films. The IR reflectivity combined with THz transmission was fitted to calculate the permittivity and IR conductivity spectra. In addition to vibration modes and Drude contribution of the free carriers in the conducting PANI, strong and broad mid-IR absorption was detected and assigned to localized carriers. Additional absorption in the far IR, much stronger in case of the conducting PANI, was assigned to Boson peak. Good agreement between the IR conductivity peaks and those from the absorbance evaluated from the transmission measurements was obtained and differences between the spectra of PANI salt and base are discussed. The dielectric and conductivity spectra of PANI salt were modeled as the spectra of a composite of PANI base and metallic PANI reported by Lee et al., Nature 441, 65 (2006). Effective medium approach using three different models was applied and discussed. The best results, including the semiconductor-like temperature dependence of the low-frequency conductivity, were obtained with the generalized Lichtenecker model. The strong permittivity increase on decreasing frequency below the THz range is discussed and compared with the literature data.
Microwave dielectric spectra of the relaxor ferroelectric PbMg
1/3
Nb
2/3
O
3
single crystals, (Nb + In) co-doped rutile ceramics, inhomogeneous conducting polyaniline pellets, composites of ...poly(ethylene terephthalate) and multi-walled carbon nanotubes were used for calculation of transmission and reflection spectra of the material slabs in the free space and estimation of absorbing and shielding properties of the materials. Studied high-permittivity high-loss materials with a strong and broad dielectric dispersion in the microwave range were shown to have a high potential for microwave shielding and absorbing applications. Broadband (two decades of frequency) 40 dB shielding and high attenuation constant were demonstrated in the microwave range.
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