Structural, magnetic, dielectric and mechanical properties of (Ba,Sr)MnO3_3 ceramics

Ceramic samples, produced by conventional sintering method in ambient air, 6H-SrMnO$_3$(6H-SMO), 15R-BaMnO$_3$(15R-BMO), 4H-Ba$_{0.5}$Sr$_{0.5}$MnO$_3$(4H-BSMO) were studied. In the XRD measurements for SMO the new anomalies of the lattice parameters at 600-800 K range and the increasing of thermal expansion coefficients with a clear maximum in a vicinity at 670 K were detected. The N$\acute{e}$el phase transition for BSMO was observed at $T_N$=250 K in magnetic measurements and its trace was detected in dielectric, FTIR, DSC, and DMA experiments. The enthalpy and entropy changes of the phase transition for BSMO at $T_N$ were determined as 17.5 J/mol and 70 mJ/K mol, respectively. The activation energy values and relaxation times characteristic for relaxation processes were determined from the Arrhenius law. Results of ab initio simulations showed that the contribution of the exchange correlation energy to the total energy is about 30%.Comment: 12 pages, 12 figure

Dielectric and Ferroelectric Properties of Lead-Free NKN and NKN-Based Ceramics

Lead-free ceramics of Na 0.5K 0.5NbO 3 (NKN), Na 0.5K 0.5(Nb 0.94Sb 0.06)O 3 (NKNS6) and Na 0.5K 0.5(Nb 0.94Sb 0.06)O 3 + 0.5%MnO 2 (NKNS6 + 0.5%MnO 2) have been prepared by a solid phase hot pressing sintering process. X-ray diffraction results show that the obtained samples possess the perovskite structure. The micrograph of the fractured surface showed a dense structure in a good agreement with that of 91-94% relative density determined by the Archimedes method. An average grain size decreases with Sb and Mn doping (from about 20 μm for NKN to about 5 and 2 μm for NKNS6 and NKNS6 + 0.5%MnO 2, respectively). Low frequency (100 Hz-200 kHz) investigations revealed the diffuse phase transitions. It was found that Mn or Sb doping influence dielectric and ferroelectric properties. The pyroelectric and hysteresis loops measurements show that the samples are ferroelectric with a relatively large remanent polarization (18- 55 μC/cm 2) and a low coercive field (7-10 kV/cm). The obtained results are discussed in the framework of foreign ions/lattice imperfections, which create local electric and elastic fields. The obtained materials are considered to be promising candidates for lead-free electronic ceramic