Jan Kroupa

Multiple Soft-Mode Vibrations of Lead Zirconate

Polarized Raman, IR, and time-domain THz spectroscopy of orthorhombic lead zirconate single crystals have yielded a comprehensive picture of temperature-dependent quasiharmonic frequencies of its low-frequency phonon modes. It is argued that these modes primarily involve vibrations of Pb ions and librations of oxygen octahedra. Their relation to phonon modes of the parent cubic phase is proposed. Counts of the observed IR and Raman active modes belonging to distinct irreducible representations agree quite well with group-theory predictions. Analysis of the results yields insight into the phase transition mechanism, involving a soft ferroelectric branch coupled by a trilinear term to another two oxygen octahedra tilt modes.

Novel hockey‐stick mesogens with the nematic, synclinic and anticlinic smectic C phase sequence

New hockey‐stick mesogens have been synthesised and their mesomorphic properties studied. The molecular structure consists of a laterally substituted central naphthalene unit prolonged by two non‐symmetrical arms, both of which contain ester units of identical orientation. The shorter arm possesses only one and the longer arm three benzene rings. Nematic, synclinic smectic C and anticlinic smectic C phases were identified and their physical properties studied. No banana‐type ordering producing dipolar phases has been found.

Novel polymerizable bent‐shaped monomeric molecules

Novel bent‐shaped mesogens based on substituted naphthalene‐2,7‐diol, possessing a double bond at the end of terminal branches, have been synthesized and characterized. A variety of banana‐mesophases has been found, depending on the type of lateral substitution. Increasing the length of the alkyl chain leads to the appearance of the SmCAPA phase for two of the compounds synthesized. Under an electric field a transition into the SmCSPF phase takes place and a tilt angle close to 45° has been found. These materials are appropriate for the preparation of polymeric materials.

Spatiotemporal transformations of ultrashort terahertz pulses

We have experimentally studied the propagation in free space and through focusing optics of subpicosecond half- and single-cycle terahertz pulses emitted by large-aperture emitters. The spatial transformations of the beams are connected to phase changes and effective frequency filtering and lead to spectacular changes in the pulse time profile. The experimental results are in excellent agreement with the predictions of a simple analytical model based on a Gaussian-beam approximation.

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.

Electro-optic properties of KNN–STO lead-free ceramics

The electro-optic (EO) properties of a new lead-free ceramic based on a (1−x)K0.5Na0.5NbO3–xSrTiO3 solid solution with x = 0.2 have been investigated. This material has a pseudo-cubic perovskite structure and typical relaxor-like properties for x ~0.15–0.25 (see Kosec M et al 2004 J. Mater. Res. 19 1849–54 and Bobnar V et al 2004 Appl. Phys. Lett. 85 994–6).Measurements of the electric field induced changes in birefringence at room temperature showed a rather large quadratic EO Kerr effect (n3Reff ~1.9 × 10−16 m2 V−2), with strong dispersion (decrease) in the kilohertz region. Electric field induced second harmonic generation was also observed.

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.

Structural evolution and properties of solid solutions of hexagonal InMnO3 and InGaO3.

Solid solutions InMn(1-x)Ga(x)O(3) (0 ≤ x ≤ 1) have been investigated using magnetic, dielectric, specific heat, differential scanning calorimetry (DSC), and high-temperature powder synchrotron X-ray diffraction (HT-SXRD) measurements. It was found that samples with 0.5 ≤ x ≤ 1 crystallize in space group P6(3)/mmc with a ~ 3.32 Å and c ~ 11.9 Å, and samples with 0.0 ≤ x ≤ 0.4 crystallize in space group P6(3)cm with a ~ 5.8 Å and c ~ 11.6 Å at room temperature. HT-SXRD data revealed the existence of a P6(3)cm-to-P6(3)/mmc phase transition at about 480 K in InMn(0.6)Ga(0.4)O(3) and at 950 K in InMn(0.7)Ga(0.3)O(3). However, no dielectric, phonon, second-harmonic-generation, or DSC anomalies were found to be associated with these phase transitions. The phase transition should be improper ferroelectric from the symmetry point of view, but the above-mentioned experimental facts, together with the absence of ferroelectric hysteresis loops, revealed no evidence for ferroelectricity in the low-temperature P6(3)cm structure. We suggest that InMn(1-x)Ga(x)O(3) corresponds to a nonferroelectric phase of hexagonal RMnO(3) with P6(3)cm symmetry. The antiferromagnetic phase-transition temperature decreases from 118 K for x = 0 to 105 K for x = 0.1 and 73 K for x = 0.2, and no long-range magnetic ordering could be found for x ≥ 0.3. Specific heat anomalies associated with short-range magnetic ordering were observed for 0.0 ≤ x ≤ 0.5. InMn(1-x)Ga(x)O(3) with small Mn contents (0.8 ≤ x ≤ 0.98) has a bright-blue color.

Ferroelectric-like behaviour of the SmCP phase in liquid crystalline compounds with asymmetrical bent-core molecules

A new series of asymmetrical bent-shaped mesogens has been synthesised and their mesomorphic properties studied. All compounds exhibit the B2 (SmCP) phase. For some compounds we found a very low coercive field for switching to the saturated ferroelectric state and only one peak in the switching current. Due to the lack of SHG signal in the zero field state and the behaviour of the relaxation mode we cannot unambiguously assign ferroelectric or antiferroelectric character to the B2 phase observed. The observation of planar samples revealed a striped texture. Large optically active domains appear on very slow cooling, which are attributed to a non-homogeneous (twisted) in-plane structure imposed by surfaces. The twisted domains are suppressed under a critical electric field. A simple model of such a structure and its behavior in the electric field is proposed. The low temperature phase detected on further cooling is attributed to a crystalline-like smectic phase.

Semi-organic salts of aniline with inorganic acids: prospective materials for the second harmonic generation

Three novel inorganic salts of aniline with sulfuric and selenic acids were prepared and characterized by X-ray structural analysis. Anilinium(1+) selenate, (C6H5NH3+)2SeO42−, and anilinium sulfate, (C6H5NH3+)2SO42−, crystallize in the monoclinic space groupC2. The crystal structures are based on hydrogen bonded layers of alternating anilinium cations and inorganic anions. Anilinium(1+) selenate dihydrate, (C6H5NH3+)2SeO42−·2H2O, crystallizes in the monoclinic space groupC2/c. The crystal structure is formed by a network of alternating anilinium cations, selenate anions and water molecules connected by a system of intermolecular hydrogen bonds. The FTIR and Raman spectra of all the compounds have been recorded and discussed as well as their crystal structures. According to the DSC curves and temperature dependence of lattice parameters, anilinium sulfate exhibits phase transitions at 217 and 182 K. The appropriate changes of vibrational spectra were also recorded during cooling of the sample especially in the N–H stretching and sulfate antisymmetric stretching (ν3SO42−) spectral regions. The quantitative measurements of the second harmonic generation at 1064 nm were performed using powdered samples of anilinium sulfate, anilinium chloride and anilinium selenate, and the relative efficiencies deff = 0.05dKDP, deff = 2.33dKDP and deff = 0.05dKDP (KDP; i.e.KH2PO4) have been observed, respectively.