Structural and dielectric properties of La- and Ti-modified K0.5Na0.5NbO3 ceramics

We report the results of studies of rectifying behavior, positive magneoresistance (MR), the charge-transport mechanism and the effect of an electric field on a ZnO (n)/La_0.5Pr_0.2Sr_0.3MnO₃ (LPSMO) (p)/SrNb_0.002Ti_0.998O₃ (SNTO) (n) heterostructure comprising two p–n junctions fabricated using the pulsed laser deposition technique. The heterostructure exhibits rectifying behavior over a wide temperature and field range having hysteresis in I–V behavior (forward bias) due to the tunneling of charge carriers. It is also observed that, depending on the nature of the electric field bias to n-type ZnO and SNTO, the junction resistance becomes modified, which has been explained on the basis of spin injection in the heterostructure. The observation of unconventional positive MR at room temperature has been justified on the basis of interface effects and the reduction in barrier height obtained using fitting of the I–V data by a thermionic emission model.     

Forced Vibrations and Nonstationary Heating of a Rectangular Viscoelastic Plate with Prestresses*

International Applied Mechanics - The problem of forced resonant vibrations and stationary, as well as nonstationary dissipative heating of a prestressed viscoelastic elastomeric rectangular plate...     

Synthesis and characterization of A-site deficient rare-earth doped BaZrxTi1−xO3 perovskite-type compounds

A-site deficient rare-earth doped BaZr_xTi_1?xO₃ (BZT) ceramics were prepared from a soft-chemistry route and by solid-state reaction (SSR). Perovskite-like single-phase diagrams for the BaTiO₃–La_2/3TiO₃–BaZrO₃ system were constructed for each method of synthesis. Infrared spectroscopy on (Ba_1?yLa_2y/3)Zr_xTi_1?xO₃ solid solution revealed a dramatic stress on the M–O (M = Ti, Zr) bonds due to the combined effect of A-site vacancies and the lower ionic radius of La³⁺ than that of Ba²⁺. A relationship between the M–O stretching vibration (υ) and the tolerance factor (t) was determined. (Ba_1?yLn_2y/3)Zr_0.09Ti_0.91O₃ (Ln = La, Pr, Nd) samples synthesized by SSR were selected for detailed studies. X-ray diffraction data were refined by the Rietveld method. Scanning electron microscopy on sintered compacts detected abnormal crystal growth and grain sizes in the range of about 1 μm up to 10 μm when the dopant concentration is 6.7 at. %. Impedance measurements exhibited that ferroelectric to paraelectric phase-transition temperature shifted to lower values as increasing rare-earth content. (Ba_1?yLn_2y/3)Zr_0.09Ti_0.91O₃ system showed a diffuse phase transition with a relaxor-like ferroelectric behaviour. Furthermore, the dielectric constant was enhanced with respect to non-doped BZT system.     

The influence of A-site rare-earth for barium substitution on the chemical structure and ferroelectric properties of BZT thin films

Rare-earth (RE) doped Ba(Zr,Ti)O₃ (BZT) thin films were prepared by rf-magnetron sputtering from a Ba_0.90Ln_0.067Zr_0.09Ti_0.91O₃ (Ln=La, Nd) target. The films were deposited at a substrate temperature of 600 °C in a high oxygen pressure atmosphere. X-ray diffraction (XRD) patterns of RE-BZT films revealed a 〈001〉 epitaxial crystal growth on Nb-doped SrTiO₃, 〈001〉 and 〈011〉 growth on single-crystal Si, and a 〈111〉-preferred orientation on Pt-coated Si substrates. Scanning electron microscopy (SEM) showed uniform growth of the films deposited, along with the presence of crystals of about half-micron size on the film's surface. Transmission electron microscopy (TEM) evidenced high crystalline films with thicknesses of about 100 nm for 30 min of sputtering. Electron-probe microanalysis (EPMA) corroborated the growth rate (3.0-3.5 nm/min) of films deposited on Pt-coated Si substrates. X-ray photoelectron spectroscopy (XPS), in depth profile mode, showed variations in photoelectron Ti 2p doublet positions at lower energies with spin-orbital distances characteristic of BaTiO₃-based compounds. The XPS analysis revealed that lanthanide ions positioned onto the A-site of the BZT-perovskite structure increasing the MO₆-octahedra distortion (M=Ti, Zr) and, thereby, modifying the Ti-O binding length. Polarization-electric field hysteresis loops on Ag/RE-doped BZT/Pt capacitor showed good ferroelectric behavior and higher remanent polarization values than corresponding non-doped system.