Volume 87, Issue 2, 15 January 2000
Index of content:
- DIELECTRICS AND FERROELECTRICITY (PACS 77)
87(2000); http://dx.doi.org/10.1063/1.371951View Description Hide Description
Superstructure reflections of lead magnesiumniobate (PMN), were measured with synchrotron radiationx-ray diffraction. Our data conclusively demonstrate that the -type reflections are the result of chemical ordering between the Mg and Nb ions in domains of 50 Å. Structure factor analysis of these peaks reveals an oxygen displacement within the ordered regions of 0.044(3) Å along a 〈100〉 direction towards the Nb ion. Single crystals of La-doped PMN (La–PMN) exhibit much larger chemically ordered regions (up to 900 Å for 10% La–PMN). Structural analysis of the La–PMN suggests that as the ordered region increases in size, the ordering changes from complete Nb:Mg ordering and approaches ordering. Although anticipated from electrostatic energy arguments, our conclusions are based on a clear trend in the bond length of the mixed site as a function of La doping.
87(2000); http://dx.doi.org/10.1063/1.371952View Description Hide Description
The dielectric response of pulsed laserablatedbariumstrontiumtitanatethin films were studied as a function of frequency and ambient temperature (from room temperature to 320 °C) by employing impedance spectroscopy. Combined modulus and impedance spectroscopic plots were used to study the response of the film, which in general may contain the grain, grain boundary, and the electrode/film interface as capacitive elements. The spectroscopic plots revealed that the major response was due to the grains, while contributions from the grain boundary or the electrode/film interface was negligible. Further observation from the complex impedance plot showed data points lying on a single semicircle, implying the response originated from a single capacitive element corresponding to the bulk grains. Conductivity plots against frequency at different temperatures suggested a response obeying the ‘universal power law’. The value of the activation energies computed from the Arrhenius plots of both ac and dc conductivities with were 0.97 and 1.04 eV, respectively. This was found to be in excellent agreement with published literature, and was attributed to the motion of oxygen vacancies within the bulk.
87(2000); http://dx.doi.org/10.1063/1.371953View Description Hide Description
when forming a solid solution with shows structural transformations over the entire compositional range. Above 70 mole % of the structure is rhombohedral and below 4 mole %, it is tetragonal. In between the structure is cubic. The ferroelectric decreases with increasing composition of and a relatively small relaxation is observed. Impedance measurements showed a structural dependence and analysis of which has clearly shown that the capacitance observed is from the bulk of the sample. Relaxation time (τ) of was estimated using the relaxation times of the intermediate compositions. Magnetization measurements showed field induced ferromagnetism. As the structure becomes cubic with increasing concentration of paramagnetism sets in, as evidenced by the electron spin resonance spectra.
87(2000); http://dx.doi.org/10.1063/1.371954View Description Hide Description
Electrical control of elasticity was performed in piezoelectric polymer films by connecting an electric circuit parallel to the sample electrodes. A polyvinylidene fluoride film and a vinylidene fluoride tetrafluoroethylene copolymer (73/27) film were used. Electrical circuits acting as a variable negative capacitor or a variable inductor were constructed using operational amplifiers. Theoretically, when the value of the external negative capacitance is increased, the observed elastic constant increases from that at an open circuit to positive infinity at around the value of the sample, turns to negative infinity, and then increases through zero to that at a short circuit. If the inductance of the external circuit is changed around the electrical resonant frequency, the elastic constant exhibits resonance and antiresonance against frequency. Experimental observations of the characteristics of the dynamic elastic constant agreed well with these theoretical predictions. By coupling the negative capacitance, the elastic constant changed between 0.5 and 2 times the original value, and the elastic loss increased to tan δ=0.7.
87(2000); http://dx.doi.org/10.1063/1.371968View Description Hide Description
The Rayleigh law and a propagating switching region of a ferroelectricpolycrystal are studied with a finite element model. Each crystallite, represented by a cubic element in a finite element mesh, is a single domain that switches completely without a simulated domain wall motion. The model involves only electric field induced (i.e., ferroelectric) switching. Under a low applied field, the model reproduces the Rayleigh law. The model predicts that a switching crystallite and its follow-up switching crystallite are likely to be the nearest neighbors and that the switching sequence of the crystallites forms the shape of an ellipsoid the major axis of which is parallel with the applied field direction. A spontaneous polarization reverses by direct 180° reorientation if only one layer of the finite elements between two electrodes is simulated but reverses by two consecutive 90° domain reorientations if the number of stacked layers between the electrodes is equal to two or greater.