42 research outputs found
Fast polarization mechanisms in the uniaxial tungsten-bronze relaxor strontium barium niobate SBN-81
The high-frequency dielectric response of the uniaxial strontium barium niobate crystals with 81% of Sr has been studied from 1 kHz to 30 THz along the polar c axis by means of several techniques (far infrared, time domain terahertz, high-frequency and low-frequency dielectric spectroscopies) in a wide temperature interval 20–600 K. Relaxor properties were observed in the complex dielectric response and four main excitations were ascertained below the phonon frequencies. These fast polarization mechanisms take place at THz, GHz and MHz ranges and show different temperature evolution. The central mode excitation in the THz range, related to anharmonic dynamics of cations, slightly softens from high temperatures and then hardens below T ~ 400 K. Below the phase transition (at T ~ 330 K) an additional microwave excitation appears near 10 GHz related to micro domain wall oscillations. The strongest relaxation appears in the GHz range and slows down on cooling according to the Arrhenius law. Finally, another relaxation, present in the MHz range at high temperatures, also slows down on cooling at least to the kHz range. These two relaxations are due to polar fluctuations and nanodomains dynamics. Altogether, the four excitations explain the dielectric permittivity maximum in the kHz range
Polar lattice vibrations and phase transition dynamics in Pb(Zr1-xTix)O-3
Infrared (IR) reflectivity spectra of nominally pure Pb(Zr1-xTix)O-3 ceramics with different Ti/Zr concentration (x = 0.42-0.58) were measured and evaluated, along with the time-domain terahertz transmittance spectra in the temperature range 10 K-900 K. The temperature dependence of the low-frequency vibrations, related to Pb atoms, was analyzed in terms of two overdamped modes-a soft mode and an anharmonic hopping central mode-in the cubic and high-temperature ferroelectric phase and three main vibrations in the low-temperature ferroelectric phase with the doubled unit cell: two E-symmetry modes (the soft mode and a mode corresponding to antiphase vibrations of neighboring Pb atoms in the terahertz range) and the antiferrodistortive mode producing the antiphase tilts of the oxygen octahedra. The last bare mode is not IR active, but it becomes activated by coupling with the soft mode. As predicted by theory, the intrinsic permittivity of Pb(Zr1-xTix)O-3 has a maximum at the morphotropic phase boundary, although this represents just a small percentage of the total permittivity at lower frequencies. Its maximum is linked to the softening of the anharmonic vibrations of Pb ions, perpendicular to the polarization, and shifts from x = 0.48 at room temperature to x = 0.52 at 10 K
Searching for the Slater Transition in the Pyrochlore CdOsO with Infrared Spectroscopy
Infrared reflectance measurements were made on the single crystal pyrochlore
CdOsO in order to examine the transformations of the
electronic structure and crystal lattice across the boundary of the metal
insulator transition at . All predicted IR active phonons are
observed in the conductivity over all temperatures and the oscillator strength
is found to be temperature independent. These results indicate that charge
ordering plays only a minor role in the MIT and that the transition is strictly
electronic in nature. The conductivity shows the clear opening of a gap with
. The gap opens continuously, with a temperature
dependence similar to that of BCS superconductors, and the gap edge having a
distinct dependence. All of these
observables support the suggestion of a Slater transition in CdOsO.Comment: 4 pages, 4 figure
Polar phonons in some compressively stressed epitaxial and polycrystalline SrTiO3 thin films
Several SrTiO3 (STO) thin films without electrodes processed by pulsed laser
deposition, of thicknesses down to 40 nm, were studied using infrared
transmission and reflection spectroscopy. The complex dielectric responses of
polar phonon modes, particularly ferroelectric soft mode, in the films were
determined quantitatively. The compressed epitaxial STO films on (100)
La0.18Sr0.82Al0.59-Ta0.41O3 substrates (strain 0.9%) show strongly stiffened
phonon responses, whereas the soft mode in polycrystalline film on (0001)
sapphire substrate shows a strong broadening due to grain boundaries and/or
other inhomogeneities and defects. The stiffened soft mode is responsible for a
much lower static permittivity in the plane of the compressed film than in the
bulk samples.Comment: 11 page
Ferroelectric and Incipient Ferroelectric Properties of a Novel Sr_(9-x)PbxCe2Ti2O36 (x=0-9) Ceramic System
Sr_(9-x)PbxCe2Ti12O36 system is derived from the perovskite SrTiO3 and its
chemical formula can be written as (Sr_(1-y)Pby)0.75Ce0.167TiO3. We
investigated dielectric response of Sr_(9-x)PbxCe2Ti12O36 ceramics (x = 0-9)
between 100 Hz and 100 THz at temperatures from 10 to 700 K using low- and
high-frequency dielectric, microwave (MW), THz and infrared spectroscopy. We
revealed that Sr9Ce2Ti12O36 is an incipient ferroelectric with the R-3c
trigonal structure whose relative permittivity e' increases from 167 at 300 K
and saturates near 240 below 30 K. The subsequent substitution of Sr by Pb
enhances e' to several thousands and induces a ferroelectric phase transition
to monoclinic Cc phase for x>=3. Its critical temperature Tc linearly depends
on the Pb concentration and reaches 550 K for x=9. The phase transition is of
displacive type. The soft mode frequency follows the Barrett formula in samples
with x=3.
The MW dispersion is lacking and quality factor Q is high in samples with low
Pb concentration, although the permittivity is very high in some cases.
However, due to the lattice softening, the temperature coefficient of the
permittivity is rather high. The best MW quality factor was observed for x=1:
Q*f=5800 GHz and e'=250. Concluding, the dielectric properties of Sr_(9-
x)PbxCe2Ti12O36 are similar to those of Ba_(1-x)SrxTiO3 so that this system can
be presumably used as an alternative for MW devices or capacitors.Comment: subm. to Chem. Mate
Local structure of relaxor ferroelectric SrxBa1-xNb2O6 from a pair distribution function analysis
Pair distribution function analysis of neutron-scattering data and of ab initio molecular dynamics results have been employed to study short-range structural correlations and their temperature dependence in a heavily disordered dielectric material SrxBa1-xNb2O6 (x = 0.35, 0.5, and 0.61). Intrinsic disorder caused by a partial occupation of the cationic sites by differently sized Sr and Ba atoms and their vacancies introduces important local strains to the structure and directly influences the Nb-O-6 octahedra tilting. The resulting complex system of tilts is found to be both temperature and Sr-doping sensitive with the biggest tilt magnitudes reached at low temperatures and high strontium contents, where ferroelectric relaxor behavior appears. We find evidence for two Nb-O-6 subsystems with different variations of niobium-oxygen bond lengths, distinct dynamics, and disparate levels of deviation from macroscopic polarization direction. These findings establish a detailed picture of the local structure of SrxBa1-xNb2O6 and provide a deeper insight into the origins of the materials dielectric properties.This work was supported by the Czech Science Foundation (Project No. 16-09142S). The computational part of this research was undertaken on the NCI National Facility in Canberra, Australia, which is supported by the Australian Commonwealth Government.
Broad-band dielectric spectroscopy of Ba
Barium sodium niobate (BNN) single crystals are studied by IR spectroscopy, time-domain
THz transmission spectroscopy, HF coaxial wave-guide technique and LF dielectric
spectroscopy to cover the frequency range 102–1014 Hz in a wide temperature interval. The
dielectric response parallel and perpendicular to the polar c-axis is discussed. The ferroelectric
transition at  K is driven by a relaxational soft mode coupled with another central-mode type relaxation which both gradually disappear on cooling in the ferroelectric phase.
Below Ti the parameters of the expected IR active amplitudon were estimated. The low-temperature permittivity increase on cooling for the field direction has been explained by
an incipient proper ferroelectric-ferroelastic transition driven by an IR and Raman active B2-symmetry soft mode
Phase transitions sequence in pyrochlore Cd
The infrared reflectivity of Cd2Nb2O7 single crystal was studied in the temperature interval of 10-540 K, together with complementary dielectric measurements. A ferroelectric soft mode was revealed above the ferroelectric phase transition at Tc = 196 K coupled with a central-mode type dispersion in the near-millimetre range. This proves the mixed displacive and order-disorder nature of the transition. Below Tc many new modes were detected due to lowering of the symmetry, especially below the previously suggested incommensurate transition at 85 K. Discussion of the possible phase transitions based on symmetry considerations is presented with the conclusion that the ferroelectric transition is proper with the symmetry of the order parameter, whereas the intermediate ferroelastic transition is improper and triggered by the coupling with the ferroelectric order parameter