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ABSTRACT: Composites of multiwalled carbon nanotubes with poly(ethylene terephthalate) (PET-MWCNT) with up to 3 vol% MWCNTs were prepared and characterized by broad-band AC conductivity and dielectric spectroscopy up to the infrared range using several techniques. A very low electrical percolation threshold of 0.07 vol% MWCNTs was revealed from the low-frequency conductivity plateau as well as from DC conductivity, whose values show the same critical power dependence on MWCNT concentration with the exponent t = 4.3. Above the plateau, the AC conductivity increases with frequency up to the THz range, where it becomes overlapped with the absorption of vibrational modes. The temperature dependence down to ∼5 K has shown semiconductor behaviour with a concentration-independent but weakly temperature-dependent small activation energy of ∼3 meV. The behaviour is compatible with the previously suggested fluctuation-induced tunnelling conductivity model through a thin (∼1 nm) polymer contact layer among the adjacent MWCNTs within percolated clusters. At higher frequencies, deviations from the simple universal conductivity behaviour are observed, indicating some distribution of energy barriers for an electron hopping mechanism.
Nanotechnology 02/2013; 24(5):055707. · 3.98 Impact Factor
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ABSTRACT: We performed factor-group analysis of all phonons in possible monoclinic C2/c
and C2 structures of BiMnO3 and compared it with our experimental infrared and
Raman spectra. We conclude that the crystal structure is centrosymmetric C2/c
in the whole investigated temperature range from 10 to 550 K, therefore BiMnO3
cannot be ferroelectric. We revealed a dielectric relaxation in THz spectra
above the structural phase transition taking place at T_C1=475 K giving
evidence in strong lattice anharmonicity and a large dynamical disorder of Bi
cations above T_C1. Step-like dielectric anomaly observed at T_C1 in THz
permittivity reminds antiferroelectric phase transition. Nevertheless, the
low-temperature dielectric studies did not reveal any antiferroelectric or
ferroelectric hysteresis loop. Our experimental results support theoretical
paper of P. Baettig et al. (J. Am. Chem. Soc. 129, 9854 (2007)) claiming that
BiMnO3 is not multiferroic, but only antipolar ferromagnet.
09/2012;
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ABSTRACT: Dielectric responses of ceramics from the lead-free isovalent BaZrO3-BaTiO3 (BZT) system were investigated from Hz frequencies up to the infrared in a broad temperature range, 10–700 K. Pure BaZrO3 is a displacive weak-incipient ferroelectric with a simple cubic perovskite structure down to low temperatures, whose dielectric response is fully determined by polar phonons, the lowest-frequency one being of the Last type, unlike BaTiO3, where it is of the Slater type. BaZr0.4Ti0.6O3 is a relaxor ferroelectric whose dielectric anomaly is caused by a strong, overdamped excitation, which softens from the THz down to MHz range according to the Arrhenius law and merges into a constant-loss background at low temperatures. Such a reponse is similar to lead-containing and heterovalent relaxors, but unlike them, the lowest-frequency TO1 polar phonon does not soften appreciably. In the case of BaZr0.2Ti0.8O3 we have investigated the dynamic response connected with a diffuse ferroelectric phase transition. The main dielectric anomaly is again due to similar overdamped THz-microwave excitation, which, however, softens only to the GHz range near the transition temperature and below it merges with a near-constant-loss background. The picture of polar nanoregions in BZT differs from that in heterovalent relaxors, because they are pinned to the regions of the off-centered Ti4+ ions, which are frozen in our temperature range. Therefore we assign the soft relaxations to hopping of the off-centered Ti4+ ions. This is compared with the behavior of pure BaTiO3 ceramics, in which the hopping of the off-centered Ti4+ ions also substantially contributes to the phase transition dynamics. Unlike BaTiO3, the dynamic instability, which is responsible for the diffuse ferroelectric and relaxor behavior in BZT, is fully due to the hopping dynamics of the off-centered Ti4+ ions rather than due to soft phonons, and therefore the diffuse transition is of the order-disorder type.
Phys. Rev. B. 07/2012; 86(1).
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S Kamba,
V Goian,
M Orlita, D Nuzhnyy,
J H Lee,
D G Schlom,
K Z Rushchanskii,
M Leifmmode checkzelse vzfiaiifmmode acutecelse 'cfi,
T Birol,
C J Fennie,
P Gemeiner,
B Dkhil,
V Bovtun,
M Kempa,
J Hlinka,
J Petzelt
Physical Review B 03/2012; 85(9):094435. · 3.69 Impact Factor
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S. Kamba,
V. Goian,
M. Orlita, D. Nuzhnyy,
J. H. Lee,
D. G. Schlom,
K. Rushchanskii,
M. Lezaic,
T. Birol,
C. J. Fennie,
P. Gemeiner,
B. Dkhil,
V. Bovtun,
M. Kempa,
J. Hlinka,
J. Petzelt
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ABSTRACT: Compressively strained epitaxial (001) EuTiO3 thin films of tetragonal
symmetry have been deposited on (001) (LaAlO3)_0.29-(SrAl_{1/2}Ta_{1/2}O3)_0.71
(LSAT) substrates by reactive molecular-beam epitaxy. Enhancement of the Neel
temperature by 1 K with 0.9% compressive strain was revealed. The polar phonons
ofthe films have been investigated as a function of temperature and magnetic
field by means of infrared reflectance spectroscopy. All three infrared active
phonons show strongly stiffened frequencies compared to bulk EuTiO3 in
accordance with first principles calculations. The phonon frequencies exhibit
gradual softening on cooling leading to an increase in static permittivity. A
new polar phonon with frequency near the TO1 soft mode was detected below 150
K. The new mode coupled with the TO1 mode was assigned as the optical phonon
from the Brillouin zone edge, which is activated in infrared spectra due to an
antiferrodistortive phase transition and due to simultaneous presence of polar
and/or magnetic nanoclusters. In the antiferromagnetic phase we have observed a
remarkable softening of the lowest-frequency polar phonon under an applied
magnetic field, which qualitatively agrees with first principles calculations.
This demonstrates the strong spin-phonon coupling in EuTiO3, which is
responsible for the pronounced dependence of its static permittivity on
magnetic field in the antiferromagnetic phase.
01/2012;
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ABSTRACT: Three types of BaTiO3 core - amorphous nano-shell composite ceramics were
processed from the same core-shell powder by standard sintering, spark-plasma
sintering and two-step sintering techniques and characterized by XRD, HRSEM and
broad-band dielectric spectroscopy in the frequency range 10^3 - 10^13 Hz
including the THz and IR range. The samples differed by porosity and by the
amount of interdiffusion from the cores to shells, in correlation with their
increasing porosity. The dielectric spectra were also calculated using suitable
models based on effective medium approximation. The measurements revealed a
strong dielectric dispersion below the THz range, which cannot be explained by
the modeling, and whose strength was in correlation with the degree of
interdiffusion. We assigned it to an effect of the interdiffusion layers,
giving rise to a strong interfacial polarization. It appears that the
high-frequency dielectric spectroscopy is an extremely sensitive tool for
detection of any gradient layers and sample inhomogeneities even in dielectric
materials with negligible conductivity.
11/2011;
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ABSTRACT: Domain structures of 100–300 nm thin epitaxial films of ferroelectric PbTiO3 grown by metalorganic chemical vapor deposition technique on LaAlO3 perovskite substrate have been investigated by piezoresponse force microscopy and infrared reflectance spectroscopy techniques. Normal-incidence reflectance spectra reveal both E- and A1-symmetry modes of PbTiO3. The latter ones demonstrate the presence of a minor fraction of a-domains (with in-plane orientation of the spontaneous polarization) in the film. The piezoresponse force microscopy images allow to get local insight in the complex nanodomain architecture composed by few hundred nm large areas of primarily c/a/c/a but also a1/a2/a1/a2 domain pattern types, with participation of all six tetragonal ferroelectric domain states and both 180 ° and 90 ° ferroelectric walls.
Journal of Applied Physics 10/2011; 110(8):084115-084115-6. · 2.17 Impact Factor
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ABSTRACT: High-frequency dielectric studies of Bi_0.85Nd_0.15FeO_3 ceramics performed
betweeen 100 and 900 K reveal hardening of most polar phonons on cooling below
antiferroelectric phase transition, which occurs near 600 K. Moreover, a strong
THz dielectric relaxation is seen in paraelectric phase. Its relaxation
frequency softens on cooling towards Tc ~ 600 K, its dielectric strength
simultaneously decreases and finaly the relaxation disappears from the spectra
below 450 K. Both phonon and dielectric relaxation behavior is responsible for
a decrease in the dielectric permittivity at the antiferroelectric phase
transition. Origin of unusual strong THz dielectric relaxation in paraelectric
phase is discussed. Bi_0.85Nd_0.15FeO_3 structrure lies on the phase boundary
between polar rhombohedral and non-polar orthorhombic phase and owing to this
the polarization rotation and polarization extension can enhance the
piezoelectric response of this system. Similarities and discrepancies with
lead-based piezoelectric perovskites, exhibiting morphotrophic phase boundary
between two ferroelectric phases, are discussed.
10/2011;
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ABSTRACT: Polarized infrared reflectivity was measured between 5 and 300 K on a 17 nm thick, 1.1% compressively strained epitaxial (001) SrTiO(3) film and the orthorhombic (110) NdGaO(3) substrate upon which it was grown. A strong in-plane infrared anisotropy of the NdGaO(3) substrate was observed and polar modes with B(1u)-and a mixture of B(2u) + B(3u)-symmetry were seen. At low temperatures three new modes arose in the 90-130 cm( - 1) range, which we assigned to 4f Nd electronic transitions. The in-plane SrTiO(3) film phonons showed strong stiffening compared to the phonon frequencies of bulk unstrained SrTiO(3), particularly the soft mode, and the in-plane phonon peaks were found to split. No anomalies were detected as a function of temperature in either the infrared response or lattice parameters of the compressively strained SrTiO(3) film, providing an absence of evidence for the out-of-plane ferroelectric phase transition predicted by theory.
Journal of Physics Condensed Matter 02/2011; 23(4):045901. · 2.55 Impact Factor
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ABSTRACT: Polarized infrared reflectivity was measured between 5 and 300 K on a 17 nm thick, 1.1% compressively strained epitaxial (001) SrTiO3 film and the orthorhombic (110) NdGaO3 substrate upon which it was grown. A strong in-plane infrared anisotropy of the NdGaO3 substrate was observed and polar modes with B1u-and a mixture of B2u + B3u-symmetry were seen. At low temperatures three new modes arose in the 90–130 cm − 1 range, which we assigned to 4f Nd electronic transitions. The in-plane SrTiO3 film phonons showed strong stiffening compared to the phonon frequencies of bulk unstrained SrTiO3, particularly the soft mode, and the in-plane phonon peaks were found to split. No anomalies were detected as a function of temperature in either the infrared response or lattice parameters of the compressively strained SrTiO3 film, providing an absence of evidence for the out-of-plane ferroelectric phase transition predicted by theory.
Journal of Physics Condensed Matter 01/2011; 23(4):045901. · 2.55 Impact Factor
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V Goian,
S Kamba, D Nuzhnyy,
P Vaněk,
M Kempa,
V Bovtun,
K Knížek,
J Prokleška,
F Borodavka,
M Ledinský,
I Gregora
[show abstract]
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ABSTRACT: Dielectric properties of Eu(0.5)Ba(0.5)TiO(3) ceramics were investigated between 10 and 300 K in the frequency range of 1 MHz-100 THz. Permittivity exhibits a strong peak near the ferroelectric phase transition at 215 K. This is mainly due to softening of the lowest frequency polar phonon revealed in THz and infrared spectra. Dielectric relaxation was observed also below the ferroelectric soft mode frequency in the whole investigated temperature region, but it is probably caused by some defects such as Eu(3 + ) cations or oxygen vacancies. This implies that the ferroelectric phase transition has predominantly a displacive character. Raman scattering spectra revealed a lowering of crystal symmetry in the ferroelectric phase and XRD analysis indicated orthorhombic A2mm symmetry below 215 K. The magnetic measurements performed at various frequencies in the field cooled and field heating regime after cooling in zero magnetic fields excluded spin glass behavior and proved an antiferromagnetic order below 1.9 K in Eu(0.5)Ba(0.5)TiO(3).
Journal of Physics Condensed Matter 01/2011; 23(2):025904. · 2.55 Impact Factor
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ABSTRACT: We report an investigation of hexagonal Y1− x Eu x MnO3 ceramics with x = 0, 0.1, and 0.2 using infrared (IR) and terahertz (THz) spectroscopies in the temperature range between 5 and 900 K. The temperature dependence of the THz permittivity reveals a kink near the antiferromagnetic (AFM) phase transition temperature T N ≈ 70 K, giving evidence of a strong spin–lattice coupling. Below T N, two absorption peaks were revealed in the THz spectra close to 43 and 73 cm−1. While the first peak corresponds to a sharp AFM resonance exhibiting softening on heating towards T N, the second one may be attributed to an impurity mode or a multiphonon absorption peak. High-temperature THz spectra measured up to 900 K reveal only small gradual increase of the permittivity in agreement with a weak phonon softening observed in the IR reflectance spectra upon heating. This corresponds to an improper ferroelectric character of the phase transition proposed from first principle calculations by Fennie and Rabe (Phys. Rev. B 72: 100103(R), 2005).
Phase Transitions 10/2010; 83(Nos. 10–11):931-941. · 1.01 Impact Factor
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K Z Rushchanskii,
S Kamba,
V Goian,
P Vanek,
M Savinov,
J Prokleska, D Nuzhnyy,
K Knízek,
F Laufek,
S Eckel,
S K Lamoreaux,
A O Sushkov,
M Lezaić,
N A Spaldin
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ABSTRACT: 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.
Nature Material 08/2010; 9(8):649-54. · 32.84 Impact Factor
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S. Kamba,
V. Goian,
M. Savinov,
E. Buixaderas, D. Nuzhnyy,
M. Marysko,
M. Kempa,
V. Bovtun,
J. Hlinka,
K. Knizek,
P. Vanek,
P. Novak,
J. Bursik,
Y. Hiraoka,
T. Kimura,
K. Kouril,
H. Stepankova
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ABSTRACT: We prepared multiferroic Y-type hexaferrite Ba <sub>0.5</sub> Sr <sub>1.5</sub> Zn <sub>2</sub> Fe <sub>12</sub> O <sub>22</sub> ceramics and compared their magnetic and dielectric properties with single crystal. Magnetic susceptibility and microwave resonance measurement revealed magnetic phase transition at T<sub>C</sub>=312 K , similar as in single crystal. Ferroelectric (FE) phase can be induced by external magnetic field in all investigated samples and the phase diagram in ceramics qualitatively resembles that of the single crystal. The range of magnetic fields, where the FE phase is induced, broadens after annealing of single crystal. Ceramics quenched after sintering exhibit several orders of magnitude lower conductivity than the single crystal. Heavily damped magnetic resonance was discovered in terahertz spectra at 10 K and its frequency softens below 5 GHz near T<sub>C</sub> . Number and symmetry of observed infrared (IR) and Raman active phonons correspond to paraelectric phase with D<sub>3d</sub><sup>5</sup> hexagonal structure. No evidence for a structural phase transition was found in the IR and Raman spectra on cooling (in zero magnetic field) or in the room-temperature IR spectra with external static magnetic field up to 0.3 T.
Journal of Applied Physics 06/2010; · 2.17 Impact Factor
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K. Z. Rushchanskii,
S. Kamba,
V. Goian,
P. Vanvek,
M. Savinov,
J. Provlevska, D. Nuzhnyy,
K. Knivsek,
F. Laufek,
S. Eckel,
S. K. Lamoreaux,
A. O. Sushkov,
M. Lezaic,
N. A. Spaldin
[show abstract]
[hide abstract]
ABSTRACT: 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 even at liquid helium temperature. Subsequent synthesis and characterization of Eu$_{0.5}$Ba$_{0.5}$TiO$_3$ ceramics confirm the predicted desirable properties. Comment: Nature Materials, in press
02/2010;
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ABSTRACT: Dielectric spectra of two K <sub>0.5</sub> Na <sub>0.5</sub> NbO <sub>3</sub> ceramics with different grain sizes (10 and 0.5 μ m ) were measured from 10<sup>2</sup> to 10<sup>14</sup> Hz in a broad temperature range. The sequence of first-order phase transitions (cubic-tetragonal-orthorhombic-rhombohedral) was detected by differential scanning calorimetry, dielectric spectroscopy, and time-domain terahertz spectroscopy. The grain size affects all the phase transitions, which are more smeared in the small-grain sample. In the large-grain ceramics, two well-separated near-Debye relaxations are seen in the tetragonal phase, which suddenly merge on cooling across the tetragonal-orthorhombic transition, and on further cooling the lower-frequency relaxation strongly broadens. On reducing the grain size, the higher-frequency relaxation shifts from ∼1 to ∼20 GHz and the lower-frequency one strongly broadens. Without quantitative understanding, these effects could be assigned to domain-wall dynamics and its temperature and grain-size dependences. Similar to pure KNbO <sub>3</sub> , an overlapped central-mode–soft-mode type excitation was detected in the terahertz range related to the effective hopping and oscillations of the off-centered Nb ions in a multiwell potential.
Journal of Applied Physics 02/2010; · 2.17 Impact Factor
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ABSTRACT: SrTiO3:Mn ceramics, prepared according to the chemical formulae Sr1-xMnxTiO3 and SrTi1-yMnyO3, are studied by Fourier transform infrared and time-domain terahertz spectroscopy in the temperature range of 10-300 K to support the incorporation of Mn ions into the perovskite lattice of SrTiO3, and to ascertain their different lattice site locations. The polar soft mode of the incipient ferroelectric SrTiO3 is found to be hardened in the whole temperature range by the substitution of Mn ions on Ti sites, and only in the low-temperature range by the Sr site substitution. Activation of the mode, associated with the R point condensation of the Brillouin zone due to the doubling of the unit cell by antiphase rotations of the O-octahedra below the structural transition temperature T-a, shows that the substitution of Mn ions on the Sr sites increases T-a, whereas the Ti-site substitution suppresses T-a with respect to the undoped SrTiO3.
Acta Materialia 01/2010; 58(2):577. · 3.76 Impact Factor
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ABSTRACT: Infrared reflectance, terahertz transmittance, and microwave resonance measurements show that SrTiO3 films, strained by ∼ 1% in biaxial tension by growing them on (110) DyScO3 substrates, undergo a pronounced phonon softening near 270 K. This in-plane soft-mode drives the ferroelectric transition. The appearance of two new low-frequency modes and splitting of the high-frequency TO4 mode provide evidence of an antiferrodistortive phase below ∼ 180 K.
Applied Physics Letters 12/2009; 95(23):232902-232902-3. · 3.84 Impact Factor
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ABSTRACT: A nanocomposite of porous glass and a NaNO(2) ferroelectric (channels of approximately 7 nm diameter) was studied using infrared reflectivity, THz transmission and Raman spectroscopy as a function of temperature in the range of 300-500 K, including the ferroelectric transition. From the infrared and THz response the effective dielectric function was calculated and compared with the dielectric functions calculated from the Bruggeman and Lichtenecker models of the effective medium, using the known data on the polar phonon modes of the NaNO(2) single crystals. The results show good qualitative agreement, indicating that the stiffening of the effective modes is due to local depolarization fields on the glass-ferroelectric interfaces. The nonpolar Raman modes show no substantial modification compared to those of the bulk NaNO(2). Some signatures of the ferroelectric transition were even seen. The results indicate that the intrinsic size effect (phonon confinement) is negligible in this nanocomposite.
Nanotechnology 10/2009; 20(39):395706. · 3.98 Impact Factor
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ABSTRACT: Phonons in both ceramic samples KNbO<sub>3</sub> (KN) and K<sub>0.5</sub>Na<sub>0.5</sub>NbO<sub>3</sub> (KNN-50) were investigated from 10 to 900 K by means of Raman, infrared, and THz spectroscopy. First-order transitions from cubic to tetragonal phases were detected at about 755 K (in KN) and 710 K (in KNN-50), where the first component of the polarization appears. Transitions from the tetragonal to the orthorhombic phases take place around 510 and 475 K, respectively. The last transitions from orthorhombic to rhombohedral phases are strongly first-order type. T<sub>C</sub> is shifted from 200 K in KN to about 90 K in KNN-50. All Raman active modes below 200 cm<sup>-1</sup>. disappear in KN but not in KNN-50. The overdamped soft mode present at high temperatures in the THz range changes its dielectric strength at each phase transition (when the corresponding component jumps to higher frequencies) and abruptly disappears from THz spectra in the rhombohedral phase, because it stiffens up to 200 cm<sup>-1</sup>. This mode has lower frequency and higher dielectric strength in KNN-50.
IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control 10/2009; · 1.69 Impact Factor
