A nonlinear thermodynamic formalism is developed to calculate the pyroeletric property of epitaxial single domain SrTiO_(3)/Si heterojunctions by taking into account the thermal expansion misfit strain at diferent temperatures.It has been demonstrated that the crucial role was played by the contribution associated with the structure order parameter arising from the rotations of oxygen octahedral on pyroelectricity.A dramatic decrease in the pyroelectric cofficient due to the strong coupling between the polarization and the structure order parameter is found at ferroelectric T_(p1)-T_(p2) phase transition.At the same time,the thermal expansion mismatch between film and substrate is also found to provide an additional weak decrease of pyroelectricity.The analytic relationship of the out-of-plane pyroeletric coefficient and dielectric constant of ferroeletric phases by considering the thermal expansion of thin films and substrates has been determined for the first time.Our research provides another avenue for the investigation of the pyroelectric effects of ferroic thin films,especially,such as antiferroelectric and muliferroic materials having two or more order parameters.
Gang BaiDongmei WuQiyun XieYanyan GuoWei LiLicheng DengZhiguo Liu
The multiferroic properties of Bi Fe O3-based ceramics were improved through optimizing their sintering method and doping with certain rare earth elements in pure Bi Fe O3. Some methods, especially liquid-phase sintering method has largely decreased the densities of oxygen vacancies and Fe2in Bi Fe O3-based ceramics, and thus their resistivity became high enough to measure the saturated polarization and the large piezoelectric d33 coefficient under the high electric field of [150 k V/cm. Besides,multiferroic properties were improved through the rare earth elements' doping in pure Bi Fe O3. Magnetization commonly increases with the proportional increase of Nd,La, Sm and Dy contents up to *30 %, while ferroelectric phase can transform to paraelectric phase at a certain proportion. An improved magnetoelectric coupling was often observed at ferroelectric phase with a relatively large proportion. Besides, an enhanced piezoelectric coefficient is expected in Bi Fe O3-based ceramics with morphotropic phase boundaries as they are already observed in thin epitaxial Bi Fe O3 films.
Ahmad HussainXijun XuGuoliang YuanYiping WangYing YangJiang YinJunming LiuZhiguo Liu
0.67BiFeO_(3)-0.33BaTiO_(3)multiferroic ceramics doped with x mol%MnO_(2)(x=2-10)were synthesized by solid-state reaction.The formation of a perovskite phase with rhombohedral symmetry was confirmed by X-ray diffraction(XRD).The average grain sizes were reduced from 0.80mm to 0.50mm as increasing the Mn-doped levels.Single crystalline nature of the grains was revealed by high-resolution transmission electron microscopy(HRTEM)images and electron diffraction patterns.Polar nano-sized ferroelectric domains with an average size of 9 nm randomly distributed in the ceramic samples were revealed by TEM images.Ferroelectric domain lamellae(71°ferroelectric domains)with an average width of 5 nm were also observed.Vibrational modes were examined by Raman spectra,where only four Raman peaks at 272 cm^(-1)(E-4 mode),496 cm^(-1)(A_(1)-4 mode),639 cm^(-1),and 1338 cm^(-1)were observed.The blue shifts in the E-4 and A_(1)-4 Raman mode frequencies were interpreted by a spring oscillator model.The dieletric constants of the present ceramics as a function of the Mn-doped levels exhibited a V-typed curve.They were in the range of 350-700 measured at 10^(3)Hz,and the corresponding dielectric losses were in range of 0.43-0.96,approaching to 0.09 at 10^(6)Hz.
