[en] Polarized far-infrared reflectivity and transmission measurements of an Sr₂Nb₂O₇ single crystal were performed in the temperature interval 10-550 K for the E parallel a and E parallel c polarizations. Modes at the centre of the Brillouin zone were analysed in the subsequent paraelectric-ferroelectric-incommensurate-monoclinic phases and the factor group analysis together with the correlation of the modes is presented. The ferroelectric A₁ soft mode revealed in Raman spectra below T_c=1613 K was found in the E parallel c polarized infrared spectra. An amplitudon which softens to T_i=488 K was observed in the incommensurate phase. On cooling the amplitudon anti-crosses a hard excitation near 33 cm^-1 located in discommensurations and then, at about 100 K, also the ferroelectric soft mode. No evidence was found for the earlier suggested multiplication of the unit cell below the phase transition at T₀=117 K. Neglecting the small volume of discommensurations, we suggest the Pb11-C²_S space group (Z=4) for the monoclinic phase below T₀. (author)

[en] PbZr_0.38Ti_0.62O_3 and PbZr_0.36Ti_0.64_O3 thick films deposited by screen printing on (0 0 0 1) single crystal sapphire substrates and prepared at two different sintering temperatures, were studied by Fourier-transform infrared reflectivity, time-domain TH_z transmission spectroscopy and micro-Raman spectroscopy. The dielectric response is discussed using the Lichtenecker model to account for the porosity of the films and to obtain the dense bulk dielectric functions. Results are compared with bulk tetragonal PZT 42/58 ceramics. The dynamic response in the films is dominated by an overdamped lead-based vibration in the TH_z range, as known in PZT, but its evaluated dielectric contribution is affected by the porosity and roughness of the surface. (Author)

[en] The Raman spectra of the ceramics PLZT x/40/60 have been measured in the temperature range 20-800 K. An assignment of the Raman modes is presented using the results of the effective medium approximation for the IR data. Factor-group analysis is presented for the case of off-site position of cations or unit cell doubling, which allows a higher number of active phonons in the IR and Raman spectra. The presence of Raman-active modes in the cubic phase is associated with local unit cell doubling and symmetry breakdown due to the off-site cations

[en] Highlights: • PEG 6000-epoxy composite is a solid–liquid PCM candidate for TES in buildings. • PCM solidification is experimentally and modeling evaluated in a spherical module. • The approach proposed to solidify the semicrystalline PEG is a two-solid front model. • Analytical results agree reasonable with experimental data for PEG solidification. • Guidance for PCM solidification in multi-spherical TES systems is provided. - Abstract: A polymeric phase change composite material (70 wt% polyethylene glycol, PEG, 6000)-epoxy resin (29 wt%) with aluminum nanopowder (1 wt%) as filler, P60-E, was developed and thermally tested first in a spherical macro capsule in order to be used in thermal energy storage (TES) systems in constructions with low energy consumption. Since the thermal behavior of the phase change component, PEG 6000, is highly influenced by its crystallization behavior, structural and thermal data were correlated. Consequently a high crystallinity degree of 82.6%, found by X-ray diffraction (XRD), for the PEG 6000 component is analogous with values obtained from integrated Raman spectra and DSC data (latent heat of −113.6 J/g) collected at a cooling rate of 0.4 °C/min. Both experimental and mathematical modeling of PEG 6000 solidification in the P60-E nanocomposite was conducted using a single spherical test cell. The heat transfer during solidification assumes time evolution of both liquid and the two solid radial fronts corresponding to crystalline chains of PEG and amorphous counterpart of PEG and epoxy resin in the P60-E composite. Good agreement between experimental values and calculated theoretical curves was found by using a two-front solids model.

[en] A set of composite PbZr_0.53Ti_0.47O₃ − xPb₂Ru₂O_6.5 (PZT 53/47 − xPRO) and pure PbZr_0.53Ti_0.47O₃ (PZT 53/47), PbZr_0.38Ti_0.62O₃ (PZT 38/62) and PbZr_0.36Ti_0.64O₃ (PZT 36/64) thick films (thicknesses 20–30 µm, porosity 25–50%) prepared by screen printing on (0001) single-crystal sapphire substrates, were studied using Fourier-transform infrared (FTIR) reflectivity and time-domain terahertz transmission spectroscopy. The compositions studied with 10, 15, 20 and 25 vol% PRO were around the electrical percolation threshold in the bulk ceramic composites (∼17 vol%), known from impedance spectroscopy by Bobnar et al (2008 Appl. Phys. Lett. 92 182911). To obtain the effective dielectric functions of the films, we accounted for their porosity by using the Lichtenecker model. Using the dielectric functions of bulk pure PZT 53/47 ceramics and PRO single crystals from the literature available, effective dielectric functions were compared with those obtained using the generalized effective medium model by McLachlan with percolation threshold and critical percolation exponents, which fit the low-frequency data by Bobnar et al. The effective dynamic response in all the samples was dominated by an overdamped THz conductivity peak (central mode), as in pure PZT. The percolation threshold was not particularly apparent in the polar-phonon frequency range as it characterized the localized response. Unlike the low-frequency permittivity, which diverged at the percolation threshold, the effective microwave–terahertz permittivity increased above it. (paper)

[en] Raman-scattering spectroscopy on the microscale is proposed as a method to study the distribution of defects and existing vacancies in lead zirconate titanate (PZT) fibres during industrial processes to monitor the volumetric structural order within the fibre and to estimate the spatial phase transition degree from rhombohedral to tetragonal phase. PZT fibres developed in two different sintering atmospheres, PbO and a mixture of PbZrO₃ and ZrO₂ (PZ+Z), were studied to determine optimal conditions for the production of defect-free PZT fibres. An estimation of defect distribution along the radius of the PZT fibres is presented using Raman spectroscopy and confirmed by x-ray-diffractometry measurements. The degradation of the spatial-mechanical properties is explained by structural changes produced by electrostatic interactions between Zr and Ti ions.

[en] We prepared multiferroic Y-type hexaferrite Ba_0.5Sr_1.5Zn₂Fe₁₂O₂₂ ceramics and compared their magnetic and dielectric properties with single crystal. Magnetic susceptibility and microwave resonance measurement revealed magnetic phase transition at T_C=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_C. Number and symmetry of observed infrared (IR) and Raman active phonons correspond to paraelectric phase with D_3d⁵ 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.

[en] The dielectric response of strontium barium niobate with 61% Sr has been studied from kHz to THz frequencies by means of several techniques (IR and time domain THz spectroscopy, the coaxial technique and dielectric spectroscopy) over a wide temperature interval: 20-600 K. A strong dielectric anisotropy is present in all the results. Relaxor ferroelectric properties were detected in ε_c^*(T,ν). At very high temperatures a strong relaxation appears in the microwave and THz range, which shifts and broadens to lower frequencies on cooling and then splits into two components. The high frequency one is seen in the THz range at high temperatures and the strong low frequency one weakens below the temperature T_m of the smeared permittivity maximum and broadens extremely in the spectra leaving a constant-loss background at very low temperatures. This relaxation is responsible for the dielectric anomaly near the ferroelectric transition. No anomalies in phonon frequencies were observed, which gives evidence relating to the order-disorder mechanism of the phase transition. The response perpendicular to the polar axis shows anomalous features at low temperatures, which could be connected with the shift of the low frequency limit of a broad dispersion (much weaker than along the polar axis) from the GHz range at 200 K to the kHz range at 40 K. There is no clear evidence of a new phase transition below 100 K suggested by some authors