S. Veljko

Microwave dielectric relaxation in cubic bismuth based pyrochlores containing titanium

Cubic pyrochlore (Bi1.5Zn0.5)(Zn0.5−x∕3TixNb1.5−2x∕3)O7 ceramics with 0⩽x⩽1.5 were synthesized and investigated between 100Hz and 100THz by means of broadband dielectric spectroscopy, time-domain terahertz transmission spectroscopy, Fourier transform infrared reflectivity spectroscopy and Raman scattering. Bi1.5ZnNb1.5O7 exhibits a microwave dielectric relaxation which slows down and broadens remarkably on cooling. Careful structural investigations confirmed that the relaxation originates in hopping of disordered Bi and part of Zn atoms in the A sites of the pyrochlore structure. Substitution of Ti atoms to the B sites, i.e., increasing x, results in an increase of the microwave permittivity from 150 to 200 of the relaxation frequency and also of the microwave quality Q. Low temperature Raman scattering experiments did not reveal any phase transition in the samples under study.

Relaxor-like behavior of lead-free Sr2LaTi2Nb3O15 ceramics with tetragonal tungsten bronze structure

Lead-free tetragonal tungsten bronze Sr2LaTi2Nb3O15 ceramics were prepared and its crystal structure determined. Dielectric response of Sr2LaTi2Nb3O15 was investigated in a broad frequency (100 Hz–90 THz) and temperature range. The observed relaxor-like behavior can be generally described in the framework of polar nanocluster dynamics concept. However, some peculiarities in the dielectric response, relaxational, and crystal lattice dynamics (e.g., hardly resolved Vogel–Fulcher part of the relaxation, reduced dielectric strength of the cluster flipping mechanism, absence of ferroelectric phase under electric field, absence of the polar soft phonon, etc.) were revealed. Therefore, we claim that Sr2LaTi2Nb3O15 is more alike to dipolar glasses than lead-based perovskite relaxor ferroelectrics.

Lattice dynamics and dielectric response of Mg-doped SrTiO3 ceramics in a wide frequency range

Atomic substitutions in the SrTiO3 (ST) perovskite lattice are foreseen to modify the lattice vibration modes, which are apparently sensitive to the site at which the substitution occurs. In order to understand the effect of Mg substitution in the ST lattice, the dielectric properties of Sr1−xMgxTiO3 and SrTi1−yMgyO3−δ ceramics were investigated in radio frequency (rf), microwave, terahertz (THz), and infrared (IR) ranges. Micro-Raman spectroscopy and rf tunability measurements were also conducted on these samples. The micro-Raman spectra and the high-frequency dielectric properties of Sr1−xMgxTiO3, including rf tunability, do not differ considerably from the properties of undoped SrTiO3, confirming only slight (if any) incorporation of Mg into the Sr site of ST perovskite lattice. At the same time, Ti-site Mg doping results in significant stiffening of the soft lattice mode observed in IR reflectivity spectra. Dielectric permittivity and dielectric losses decrease in both rf and THz ranges accompanied by...

Complex permittivity measurements of ferroelectrics employing composite dielectric resonator technique

Composite cylindrical TE0n1 mode dielectric resonator has been used for the complex permittivity measurements of ferroelectrics at frequency about 8.8 GHz. Rigorous equations have been derived that allowed us to find a relationship between measured resonance frequency and Q-factor and the complex permittivity. It has been shown that the choice of appropriate diameter of a sample together with rigorous complex angular frequency analysis allows precise measurements of various ferroelectric. Proposed technique can be used for materials having both real and imaginary part of permittivity as large as a few thousand. Variable temperature measurements were performed on a PbMg1/3Nb2/3O3 (PMN) ceramic sample, and the measured complex permittivity have shown good agreement with the results of measurements obtained on the same sample at lower frequencies (0.1 1.8 GHz)

Ultrabroadband dielectric spectroscopy and phonons in (Pb1−x/2Lax)(Zr0.9Ti0.1)O3

The dielectric behavior of (Pb1−x/2Lax)(Zr0.9Ti0.1)O3 PLZT 100x/90/10 (0≤100x≤1) was studied in the frequency range 102–1014 Hz using dielectric, time-domain terahertz (THz), far-infrared, and Raman spectroscopy in the temperature interval 10–900 K. PZT 90/10 and PLZT 2/90/10 undergo two phase transitions, from a cubic paraelectric to a rhombohedral untilted ferroelectric (FE) phase and on further cooling to a tilted FE phase. PLZT 4/90/10 and PLZT 10/90/10 are incommensurately modulated and composed of an intergrowth of polar and antipolar cation displacements. These samples consequently demonstrate a FE soft mode which exhibits minimal frequency several tens of degrees above the nominal antiferroelectric phase transition temperature. The dielectric response of PLZT 100x/90/10 is mainly governed by a gigahertz relaxation and a soft phonon component in the THz range. As in other PbO-based perovskites, the soft mode is split into a higher-frequency component (ω∼80 cm−1) and a highly damped THz component (ω...

Far-infrared and dielectric spectroscopy of relaxor ferroelectric (Pb1−xLax)(Zr0.4Ti0.6)O3

The dielectric behavior of tetragonal (Pb1−xLax)(Zr0.4Ti0.6)O3 (PLZT x∕40∕60) ceramics doped with lanthanum (x=12% and 15%) has been investigated from 102to1014Hz in the temperature range of 20–800K. Relaxor ferroelectric behavior, together with a first-order ferroelectric phase transition at TC below the dielectric maximum, was revealed. The lowest-frequency phonon partially softens to TC and splits into two components in the ferroelectric phase. It is shown that similar splitting is typical for all relaxors even for samples without ferroelectric phase transitions, because the TO1 splitting occurs due to a local distortion in the polar clusters. A relaxational mode, assigned to the dynamics of polar clusters, was revealed in the teraherz spectra below 800K. This mode slows down to the microwave range on cooling and finally anomalously broadens below TC, which appears to be characteristic for all relaxors. In comparison to the rhombohedral (Pb1−xLa0.095)(Zr0.65Ti0.35)O3, the low-temperature permittivity a...

Infrared and THz Soft-Mode Spectroscopy of (Ba,Sr)TiO3 Ceramics

We performed a study of the temperature dependence of the dielectric response of (Ba,Sr)TiO 3 (BST) ceramics for the whole composition range in a wide frequency range by means of Fourier transform infrared, time-domain THz, microwave and low-frequency dielectric spectroscopy. Room-temperature spectra of the paraelectric dielectric response were compared with the results of first-principle simulations within the effective Hamiltonian approach with a good semi-quantitative agreement. It is shown that in the paraelectric phase away from the transition temperature, the complex THz soft mode, consisting of two overlapping heavily damped oscillators, fully accounts for the low-frequency dielectric properties. However, close to T C additional central-mode type dispersion, apparently caused by the pre-transitional polar clusters, is to be expected in the microwave dielectric spectra.

Broad-band dielectric spectroscopy of tetragonal PLZT x /40/60

The dielectric behaviour of tetragonal PLZT x/40/60 ceramics doped with lanthanum (x = 5, 12 and 15%) has been investigated at 100–1014 Hz. Dielectric, THz and infrared spectra were measured from 500 to 20 K. For x ≥ 12 relaxor ferroelectric behaviour was observed. All three samples showed an additional relaxational mode below phonons. Analysis of the phonon modes for ferroelectric PLZT 5/40/60 is presented together with the complete dielectric behaviour.

Broad-Band Dielectric Spectroscopy of PZN-8%PT Single Crystal

Abstract Dielectric response of [Pb(Zn1/3Nb2/3O3]0.92-[PbTiO3]0.08 single crystal was investigated in a broad frequency (100 Hz–600 GHz) range. Near stepwise anomaly at T c≈ 415 K, characteristic of the ferroelectric phase transition, and a diffuse relaxational maximum of permittivity above 415 K, characteristic of the relaxor ferroelectrics, were observed. Broad dielectric dispersion takes place both above and below T c, resulting in the decrease in permittivity down to values of ∼ 100 at 600 GHz. The T c value shows a temperature hysteresis of ∼ 15 K, while no remarkable temperature hysteresis of the diffuse maximum is observed. Both the stepwise dielectric anomaly at T c and the diffuse maximum above T c are considered to be caused by contributions of polar nanoclusters, similar to the model relaxor ferroelectrics. The ferroelectric phase transition can be considered as a result of a stepwise increase in the nanocluster size, i.e. nanoclusters transform into domains below T c.

Comparison of the Dielectric Response of Relaxor PbMg1/3 Nb2/3O3 Ceramics and Single Crystals

Abstract Dielectric response of relaxor PMN ceramics (standard and textured) was investigated in the high-frequency range (1 MHz–1.8 GHz) and compared with the broad-band response of PMN single crystals. Similar to PMN crystals, dielectric response of PMN ceramics was found to be mainly defined by the polar nanocluster dynamics. Mesoscopic structure and imperfections of ceramics do not result in the remarkable contribution of the additional mechanisms, but influence the dynamics of polar nanoclusters and lead to the weakening of the cluster flipping mechanism contribution. The freezing temperature seems to be the same ( T f ∼ 200 K).