Unusual features of lattice dynamics in lawsonite near its phase transitions.

Lattice dynamics of a single crystal of lawsonite were studied over a broad range of frequencies (1 Hz to 20 THz) using impedance, THz time-domain and infrared spectroscopies. Based on polarized spectra of complex permittivity [Formula: see text] measured as a function of temperature between 10 K and 500 K, we analyzed the properties of the two known phase transitions-an antiferrodistortive one near [Formula: see text] and a ferroelectric one, occurring at [Formula: see text]. The former one is accompanied by a flat maximum in the THz-range permittivity [Formula: see text] near [Formula: see text], which is due to an overdamped polar excitation in the [Formula: see text] spectra reflecting the dynamics of water and hydroxyl groups. The strength of this mode decreases on cooling below [Formula: see text], and the mode vanishes below [Formula: see text] due to hydrogen ordering. At the pseudoproper ferroelectric phase transition, two independent anomalies in permittivity were observed. First, [Formula: see text] exhibits a peak at [Formula: see text] due to critical slowing down of a relaxation in the GHz range. Second, infrared and THz spectra revealed an optical phonon softening towards [Formula: see text] which causes a smaller but pronounced maximum in [Formula: see text]. Such anomaly, consisting in a soft mode polarized perpendicularly to the ferroelectric axis, is unusual in ferroelectrics.

Polar Phonon Behaviour in Polycrystalline Bi-Doped Strontium Titanate Thin Films.

Strontium titanate-based materials with ferroelectric or relaxor-like properties have drawn vast attention as polar dielectrics for electronics and telecommunications. Here, we study the lattice dynamics in sol-gel-derived Sr1-1.5xBixTiO3 thin films with x = 0.0053 and 0.167, deposited on Al2O3 substrates, using a variable-temperature far-infrared spectroscopy in a transmittance mode. Bi doping, known to induce a low-frequency dielectric relaxation in SrTiO3 (ST) ceramics and films, due to off-centre dopant ion displacements generating electric dipoles, is shown to affect the polar phonon behaviour of thin films. We show that in weakly Bi-doped films, the low-frequency polar TO1 mode softens on cooling but less than in undoped ST. In heavily Bi-doped ST films, this mode displays no significant frequency variation with temperature from 300 to 10 K. The polar phonon behaviour of polycrystalline Bi-doped ST thin films is comparable with that of Bi-doped ST ceramics, which exhibit dielectric relaxations and harden soft-mode behaviour instead of the ferroelectric phase transition.

Proton dynamics in superprotonic Rb3H(SeO4)2 crystal by broadband dielectric spectroscopy.

Broadband dielectric and AC conductivity spectra (1 Hz to 1 THz) of the superprotonic single crystal Rb3H(SeO4)2 (RHSe) along the c axis were studied in a wide temperature range 10 K < T < 475 K that covers the ferroelastic (T < 453 K) and superprotonic (T > 453 K) phases. A contribution of the interfacial electrode polarization layers was separated from the bulk electrical properties and the bulk DC conductivity was evaluated above room temperature. The phase transition to the superprotonic phase was shown to be connected with the steep but almost continuous increase in bulk DC conductivity, and with giant permittivity effects due to the enhanced bulk proton hopping and interfacial electrode polarization layers. The AC conductivity scaling analysis confirms validity of the first universality above room temperature. At low temperatures, although the conductivity was low, the frequency dependence of dielectric loss indicates no clear evidence of the nearly constant loss effect, so-called second universality. The bulk (intrinsic) dielectric properties, AC and DC conductivity of the RHSe crystal at frequencies up to 1 GHz are shown to be caused by the thermally activated proton hopping. The increase of the AC conductivity above 100 GHz could be assigned to the low-frequency wing of proton vibrational modes.

Broadband dielectric spectroscopy of La0.65Sr0.35MnO3@TiO2 core-shell nanocomposites.

Core-shell composites of ferromagnetic conducting nanoparticles La0.65Sr0.35MnO3 (LSMO) embedded in an insulating matrix of TiO2 (LSMO@TiO2) have been processed, structurally and magnetically characterized, and their DC magnetoresistivity and complex dielectric response measured and fitted from Hz up to the infrared (IR) range (1014 Hz). XRD indicates that the TiO2 shells are amorphous. Modelling of the IR spectra using standard models based on the effective medium approximation has it confirmed and has characterized the effective phonon modes of the LSMO nanoceramics and LSMO@TiO2 composite. Modelling of the lower-frequency spectra has shown that TiO2 shell thicknesses are rather non-uniform down to thin nm values, which leads to giant low-frequency permittivity values and non-negligible free-carrier tunnelling among the LSMO cores. Two main dielectric dispersion regions were observed and shown to be due to the inhomogeneous conductivity-the one occuring in the 1011-1012 Hz range relates to nonmagnetic less-conducting dead layers on the surface of LSMO nanocrystallites and the broad second one below the 1010 Hz range is due to the non-uniform thicknesses of the dielectric TiO2 shells. In the IR range, effective phonon modes of the LSMO nanoceramics and LSMO@TiO2 composite were characterized from the reflectivity spectra.

PLD prepared bioactive BaTiO3 films on TiNb implants.

BaTiO3 (BTO) layers were deposited by pulsed laser deposition (PLD) on TiNb, Pt/TiNb, Si (100), and fused silica substrates using various deposition conditions. Polycrystalline BTO with sizes of crystallites in the range from 90nm to 160nm was obtained at elevated substrate temperatures of (600°C-700°C). With increasing deposition temperature above 700°C the formation of unwanted rutile phase prevented the growth of perovskite ferroelectric BTO. Concurrently, with decreasing substrate temperature below 500°C, amorphous films were formed. Post-deposition annealing of the amorphous deposits allowed obtaining perovskite BTO. Using a very thin Pt interlayer between the BTO films and TiNb substrate enabled high-temperature growth of preferentially oriented BTO. Raman spectroscopy and electrical characterization indicated polar ferroelectric behaviour of the BTO films.