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Register a new userStructural and dielectric properties of Pb(1-x)(Na0.5Sm0.5)xTiO3 ceramics
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A correlation between structure and vibrational properties related to a ferroelectric to paraelectric phase transition in perovskite Pb(1-x)(Na0.5Sm0.5)xTiO3 (PNST - x) polycrystalline powders is discussed. Substitution leads to reduction of tetragonality which is associated with a shift of the phase transition to lower temperatures. The nature of the phase transition gets diffused with increasing substitution.
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Dielectric and magnetic properties of Eu0.5Ba0.25Sr0.25TiO3 are investigated between 10 K and 300 K in the frequency range from 10 Hz to 100 THz. A peak in permittivity revealed near 130 K and observed ferroelectric hysteresis loops prove the ferroelectric order below thistemperature. The peak in permittivity is given mainly by softening of the lowest frequency polar phonon (soft mode revealed in THz and IR spectra) that demonstrates displacive character of the phase transition. Room-temperature X-ray diffraction analysis reveals cubic structure, but the IR reflectivity spectra give evidence of a lower crystal structure, presumably tetragonal I4/mcm with tilted oxygen octahedra as it has been observed in EuTiO3. The magnetic measurements show that the antiferromagnetic order occurs below 1.8 K. Eu0.5Ba0.25Sr0.25TiO3 has three times lower coercive field than Eu0.5Ba0.5TiO3, therefore we propose this system for measurements of electric dipole moment of electron.
We have measured dielectric properties of Ca$_{1+x}$Cu$_{3-x}$Ti$_4$O$_{12}$ ($x$ = 0, 0.1, 0.5, 1, 1.5, 2, 2.9 and 3), and have found that Ca$_2$Cu$_2$Ti$_4$O$_{12}$ (a composite of CaCu$_3$Ti$_4$O$_{12}$ and CaTiO$_3$) exhibits a high dielectric constant of 1800 with a low dissipation factor of 0.02 below 100 kHz from 220 to 300 K. These are comparable to (or even better than) those of the Pb/Ba-based ceramics, which could be attributed to a barrier layer of CaTiO$_3$ on the surface of the CaCu$_3$Ti$_4$O$_{12}$ grains. The composite dielectric ceramics reported here are environmentally benign as they do not contain Ba/Pb.
We combine the results of magnetic and transport measurements with neutron diffraction data to construct the structural and magnetic phase diagram of the entire family of SrMn$_{1-x}$Ru$_{x}$O$_3$ ($0 leqslant x leqslant 1$) perovskites. We have found antiferromagnetic ordering of the C type for lightly Ru-substituted materials ($0.06 leqslant x leqslant 0.5$) in a similar manner to $R_{y}$Sr$_{1-y}$MnO$_3$ ($R$=La, Pr), due to the generation of Mn$^{3+}$ in both families of manganite perovskites by either $B$-site substitution of Ru$^{5+}$ for Mn$^{4+}$ or $A$-site substitution of $R^{3+}$ for Sr$^{2+}$. This similarity is driven by the same ratio of $d^4$ / $d^3$ ions in both classes of materials for equivalent substitution level. In both cases, a tetragonal lattice distortion is observed, which for some compositions ($0.06 leqslant x leqslant 0.2$) is coupled to a C-type AF transition and results in a first order magnetic and resistive transition. Heavily substituted SrMn$_{1-x}$Ru$_{x}$O$_3$ materials are ferromagnetic due to dominating exchange interactions between the Ru$^{4+}$ ions. Intermediate substitution ($0.6 leqslant x leqslant 0.7$) leads to a spin-glass behavior instead of a quantum critical point reported previously in single crystals, due to enhanced disorder.
Polycrystalline bulk Mn:ZnO ceramics with Mn nominal concentrations of 6, 11, 17 and 22 at.% were prepared trough solid-state reaction method and subjected to a heat treatment in reducing atmosphere (Ar (95%) and H2 (5%)). The samples were studied with particular emphasis on their compositions, structural, and magnetic properties. A detailed microstructural and chemical analysis confirms the Mn doping of the wurtzite ZnO structure mainly at the surface of the ZnO grains. For the samples with higher Mn content, the secondary phases ZnMn2O4 and Mn1-xZnxO (Zn-doped MnO) were detected for the as prepared and the heat treated samples, respectively. The structural change of the secondary phases under heat treatment, from ZnMn2O4 to Mn1-xZnxO, confirms the effectiveness of the heat treatment in to reduce the valence of the metallic ions and in the formation of oxygen vacancies into the system. In spite of the induced defects, the magnetic analysis present only a paramagnetic behavior with an antiferromagnetic coupling between the Mn ions. In the context of the bound magnetic polaron theory, it is concluded that oxygen vacancies are not the necessary defect to promote the desired ferromagnetic order at room temperature.
Multiferroic (Bi1-xLaxFeO3)0.5(PbTiO3)0.5 ceramics was prepared from mechanical synthesized nanopowders. The XRD studies revealed the tetragonal structure and the tetragonality decreased with La content. Dielectric response of the compounds was found to contain three anomalies: 1) relaxor-like behavior due to lattice disorder (below 300 K); 2) dielectric permittivity maxima at~400 K attributed to the presence of oxygen vacancies; 3) grain boundary effect above 475 K. The Curie point at ~500 K was observed for the compound with x=0.5. The composition near the morphotropic boundary: (Bi0.8La0.2FeO3)0.5(PbTiO3)0.5 shoved the highest remnant magnetization. The irreversible magnetic properties of the (Bi1-xLaxFeO3)0.5(PbTiO3)0.5 compounds can be explained in terms of disorder induced spin-glass behavior due to random substitution of La or Pb ions for Bi sites. A sharp step in magnetization about 250 K is caused by the A-site distortion associated with tilts of FeO6 octahedra leading to modification of Fe-O-Fe angles and of antiferromagnetic coupling between magnetic Fe3+ moments.
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