Chang Fanggao

Effect of Gadolinium Substitution on Dielectric Properties of Bismuth Ferrite

Multiferroic Bi1-xGdxFeO3 (x =0, 0.05, 0.1, 0.15, 0.2) ceramics were prepared by conventional solid state reaction method. X-ray diffraction was carried out to characterize the crystal structure and to detect any possible impurities existing in these ceramics. Frequency dependence of dielectric properties of Bi1-xGdxFeO3 samples at room temperature was measured in a frequency range from 100 Hz to 1 MHz using an HP4294A precision impedance analyzer. For all the samples studied, the dielectric constant and dielectric loss decreases with increasing frequency in the range between 100 Hz and 1 MHz, as can be expected from a typical orientational dielectric relaxation process. There is no indication of any dips over the whole frequency range studied, which is in direct contrast with that reported previously. It is found that both dielectric constant and dielectric loss are strongly dependent on the Gd3+ content. The effect of introducing Gd3+ is to increase the dielectric constant and to decrease the dielectric loss for slightly doped sample Bi0.95Gd0.05FeO3: the dielectric constant of the sample at 1 kHz reaches 600, six times bigger than that for pure BiFeO3. Complicated dielectric behaviors are observed at higher doping levels. Furthermore, the substitution of rare earth Gd for Bi helps to eliminate the impurity phase in BiFeO3 ceramics. There is strong evidence that both lattice constants a and c of the unit cell become smaller as the Gd3+ content is increased. The dielectric constant and loss and their frequency responses can be varied dramatically by substitution of Gd.

Investigation on dependence of BiFeO3 dielectric property on oxygen content

The influence of oxygen content on the dielectric property of BiFeO3 ceramics is studied by experiment and first-principles calculation. The experimental result demonstrates that the dielectric constant of BiFeO3 is strongly dependent on introduced oxygen and oxygen vacancies. By comparison with BiFeO3, the introduced oxygen and oxygen vacancies can lead to a reduction in dielectric constant of BiFeOδ at a lower frequency. The first-principles calculation also shows a similar result when photon energy is in a range of 2.0–4.1 eV. A likely explanation is that this oxygen content dependence may be ascribed to the distortion of Fe–O octahedron structure due to oxygen vacancies or excess oxygen ions in the crystal structure of BiFeO3.

Ultrasound anomalies in single phase (Bi1-xPbx)2Sr2CanCun+1O2n+6+y superconducting ceramics

Ultrasonic anomalies near 200 K in single-phase (Bi1-xPbx)2Sr2CanCun+1O2n+6+y (x=0-0.2, n=1, 2) superconducting samples were observed by velocity and attenuation measurements of both longitudinal and transverse sound waves. The remarkable isothermal-like anomalous changes near 200 K indicate that there are possible structural changes or phase transitions in the samples. On comparison with YBa2Cu3O7-y, the different behaviours in ultrasonic anomalies are attributed to the differences in structure. It is suggested, however, that the lattice instability appearing near T approximately=2Tc may be a common feature of the existing superconducting perovskites and may have some relation to their high Tc.

Experimental evidence for the lattice instability of Bi-based superconducting systems

Abstract Ultrasonic measurements, specific heat and thermal analysis experiments, X-ray diffraction study and infrared investigation revealed that there are anomalous structural changes or lattice instabilities near 200 K in single 2212 or 2223 phase samples of Bi(Pb)-Sr-Ca-Cu-O system. Detailed study showed that anomalous changes or lattice instabilities are isothermal-like processes and have the characteristics of a structural phase transition, accompanying with increases in lattice constants. Possible mechanism for this lattice instability is discussed.

POSSIBLE STRUCTURAL PHASE TRANSITION OF SINGLE PHASE (Bi, Pb)2Sr2Ca2Cu3O10+y SUPERCONDUCTING CERAMIC: AN INFRARED STUDY

The infrared spectra of single phase (Bi, Pb)2Sr2Ca2Cu3O10+y polycrystalline samp were studied over a temperature range of 300–120 K. Three vibrational modes were found at ambient temperature. A new band appears at 900 cm−1 near 200 K and experi evidence proved that the appearance of the new band is actually through an isothermal-like process, indicating a structural phase transition taking place. Unstable fluctuations were also observed between 200 and 140 K. This phenomenon is consistent with our previously reported ultrasonic, specific heat and lattice constants anomalies of the same sample. A possible mechanism of this phase transition is also discussed.