Přemysl Vaněk

Broadband dielectric response and grain-size effect in K0.5Na0.5NbO3 ceramics

Dielectric spectra of two K0.5Na0.5NbO3 ceramics with different grain sizes (10 and 0.5 μm) were measured from 102 to 1014 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 KNbO3, an o...

Broadband dielectric spectroscopy of phonons and polar nanoclusters in PbMg 1 / 3 Nb 2 / 3 O 3 − 35 % PbTiO 3 ceramics: Grain size effects

Dielectric response e*(f,T) and polar phonon spectra of coarse grain (grain size ~ 4 mkm) and fine grain (grain size ~ 150 nm) ceramics of PbMg_(1/3)Nb_(2/3)O3-35%PbTiO3 were investigated at temperatures 10 - 900 K. e*(f,T) in coarse-grain ceramics exhibits relaxor behavior at high temperatures and a sharp anomaly at the ferroelectric phase transition. The fine-grain ceramics exhibit mainly relaxor ferroelectric behavior with a smaller dielectric constant. The difference is explained by different relaxational dynamics of polar nanoclusters, which appear to be more stabilized at high temperatures in the fine-grain ceramics by pinning at grain boundaries. Below Tc, the growth of ferroelectric domains is suppressed in fine-grain ceramics as supported also by a second harmonic generation. On the other hand, polar phonon frequencies and their temperature dependences are almost independent of the grain size, but the selection rules for the cubic symmetry are not obeyed and all phonons are split due to a locally broken symmetry by polar nanoregions and chemical disorder. The lowest-frequency polar phonon undergoes partial softening down to ~ 0.1 THz near Tc = 440 K in both ceramics, but the dielectric anomaly is caused predominantly by flipping and breathing of polar nanoclusters. Due to contribution of both the soft phonon mode and dielectric relaxations into the dielectric constant, the ferroelectric phase transition, which corresponds to the percolation threshold of the polar nanoregions into macroscopic domains, can be considered as a special case of crossover between the displacive and order-disorder type.

New series of chiral smectic chlorinated liquid crystals

Abstract Two series of ferroelectric liquid crystalline materials, containing a lateral chlorine group on the aromatic ring of the molecular core, have been synthesised and investigated. Sequences of mesophases and phase transition temperatures have been determined for all substances. The temperature dependences of the complex permittivity and the spontaneous polarisation were determined in the whole range of the ferroelectric SmC* phase. The substitution of chlorine increases the values of spontaneous polarisation more than 5 times in comparison with non-chlorinated compounds.

Dielectric, magnetic, and lattice dynamics properties of Y-type hexaferrite Ba0.5Sr1.5Zn2Fe12O22: Comparison of ceramics and single crystals

We prepared multiferroic Y-type hexaferrite Ba0.5Sr1.5Zn2Fe12O22 ceramics and compared their magnetic and dielectric properties with single crystal. Magnetic susceptibility and microwave resonance measurement revealed magnetic phase transition at TC=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 TC. Number and symmetry of observed infrared (IR) and Raman active phonons correspond to paraelectric phase with D3d5 hexagonal structure. No evidence for a structural phase transition was found in the IR and Ram...

New ferroelectric liquid crystalline substances with lateral groups in the core

Abstract New series of ferroelectric liquid crystalline materials containing methyl and methoxy lateral groups on the aromatic ring of the core unit, as well as non-substituted materials have been synthesised. All compounds exhibit a wide temperature range of the ferroelectric chiral SmC* phase. A low temperature ferroelectric phase was detected for some of the investigated compounds probably composed of the SmC* phase and a crystalline phase. Measurements of spontaneous polarisation and tilt angle, helical pitch and dielectric permittivity has been performed.

Lattice dynamics and dielectric response of undoped, soft and hard PbZr0.42Ti0.58O3

In this study, ceramic samples of lead zirconate titanate Pb(Zr0.42Ti0.58)O3 (PZT 42/58) doped with Fe (hard) and Nb (soft) were studied by Raman, infrared (IR) and THz spectroscopy in the temperature range from 900 to 20 K. From the evaluation of the IR and Raman phonons in undoped ceramics, we conclude that tetragonal PZT ceramics undergo a low-temperature phase transition to a tilted phase, revealed by anomalies in phonon parameters and the appearance of new modes. Doped samples also presented similar behaviour. The main differences between undoped and doped ceramics lie in the dielectric behaviour below phonon frequency range, where several mechanisms appear: a soft central mode (CM) located near 1 THz and two relaxations in the GHz range. Both types of doping raise the permittivity values below the phonon frequencies, not only increasing the dielectric contribution of the CM, but also modifying the dielectric response near the GHz range. Soft ceramics show higher permittivity with logarithmic increase at low frequencies, corresponding to an almost frequency-independent value of the dielectric losses below 0.1 GHz.

The effect of a lateral methoxy group on the mesomorphic properties of ferroelectric liquid crystals

Abstract A new series of ferroelectric liquid crystal materials having the methoxy group substituted in an aromatic ring of the mesogenic core has been synthetized and characterized. This electron-donor methoxy group makes the ferroelectric S∗c phase narrower in range and monotropic when compared to the series with the unsubstituted molecules, the values of the spontaneous polarization remaining unaffected.

A new series of 3,5-diamino-1,2,4-triazolium(1+) inorganic salts and their potential in crystal engineering of novel NLO materials

Seven inorganic salts of 3,5-diamino-1,2,4-triazole (dat) with inorganic acids were prepared within our project focused on the crystal engineering of novel NLO materials and their X-ray structures were determined. Three triclinic compounds, dat(1+) selenate dihydrate, dat(1+) sulphate dihydrate and dat(1+) perchlorate, crystallize in the space group P. Three monoclinic structures of dat(1+) – i.e. nitrate, chloride hemihydrate and hydrogen phosphite crystallize in P21/c, P21/n and P21/c, respectively. Finally, the most promising structure of dat(1+) dihydrogen phosphate is orthorhombic (the space group Fdd2). The structure is formed by anionic layers parallel to the ac plane and the cationic chains parallel to the c axis. The FTIR and Raman spectra of the title compounds have been recorded and discussed. The assignment of the spectra is based on a quantum-chemical calculation and the factor group analysis. Quantitative powder measurements of the second harmonic generation of the only non-centrosymmetric dat(1+) dihydrogen phosphate at 1064 nm were performed and a relative efficiency equal to KDP (i.e. potassium dihydrogen phosphate) was observed. The consequent study of single crystal samples enabled the estimation of NLO tensor coefficients of this material.

Ferroelectric liquid crystals with a fused heterocyclic core

Abstract The synthesis of a new heterocyclic core has been developed. Its standard derivatisation afforded new types of FLCs and their physical properties were determined.

Absence of ferroelectricity in BiMnO3 ceramics

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 TC1 = 475 K giving evidence in strong lattice anharmonicity and a large dynamical disorder of Bi cations above TC1. Step-like dielectric anomaly observed at TC1 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 Baettig et al. (J. Am. Chem. Soc. 129, 9854 (2007)) claiming that BiMnO3 is not multiferroic but only antipolar ferromagnet.