Multicomponent ceramic materials on the basis of PZT for the production of piezoelectric resonators

Investigations on the influence of the PbTiO3 content on the crystalline and domain structure of PZT-based four-component solid solutions have been performed. It has been shown that there is a close relation between the structure and temperature stability of the resonance frequency (fr) of the fabricated piezoceramic transducers. Better and more stable results have been obtained by doping and modifying the basic chemical composition obtaining Pb(Zr1¡xTix)O3. The compositions with minimal ¢fr=fr, within the temperature range T = 213–358 K (in relation to room temperature Tr = 293 K), exhibited (i) the smallest internal friction (Q¡1 m ), (ii) the highest mechanical quality (Qm), (iii) high values of spontaneous deformation of the elementary cell (±T and ±Re), (iv) the complex a-c-domain structure in the tetragonal phase (T) and the simple domain structure in the rhombohedral phase (Re). The practical result of the present work was the fabrication of piezoceramic electric band filters exhibiting ¢fr=fr < 0:2% and a high mechanical quality factor (Qm = 3225)

First principles study of the multiferroics BiFeO3_{3}, Bi2_{2}FeCrO6_{6}, and BiCrO3_{3}: Structure, polarization, and magnetic ordering temperature

We present results of an {\it ab initio} density functional theory study of three bismuth-based multiferroics, BiFeO$_{3}$, Bi$_{2}$FeCrO$_{6}$, and BiCrO$_{3}$. We disuss differences in the crystal and electronic structure of the three systems, and we show that the application of the LDA+$U$ method is essential to obtain realistic structural parameters for Bi$_{2}$FeCrO$_{6}$. We calculate the magnetic nearest neighbor coupling constants for all three systems and show how Anderson's theory of superexchange can be applied to explain the signs and relative magnitudes of these coupling constants. From the coupling constants we then obtain a mean-field approximation for the magnetic ordering temperatures. Guided by our comparison of these three systems, we discuss the possibilities for designing a multiferroic material with large magnetization above room temperature.Comment: 8 Pages, 4 Figure

The nature of iron-oxygen vacancy defect centers in PbTiO3

The iron(III) center in ferroelectric PbTiO3 together with an oxygen vacancy forms a charged defect associate, oriented along the crystallographic c-axis. Its microscopic structure has been analyzed in detail comparing results from a semi-empirical Newman superposition model analysis based on finestructure data and from calculations using density functional theory. Both methods give evidence for a substitution of Fe3+ for Ti4+ as an acceptor center. The position of the iron ion in the ferroelectric phase is found to be similar to the B-site in the paraelectric phase. Partial charge compensation is locally provided by a directly coordinated oxygen vacancy. Using high-resolution synchrotron powder diffraction, it was verified that lead titanate remains tetragonal down to 12 K, exhibiting a c/a-ratio of 1.0721.Comment: 11 pages, 5 figures, accepted in Phys. Rev.

Theory of structural response to macroscopic electric fields in ferroelectric systems

We have developed and implemented a formalism for computing the structural response of a periodic insulating system to a homogeneous static electric field within density-functional perturbation theory (DFPT). We consider the thermodynamic potentials E(R,eta,e) and F(R,eta,e) whose minimization with respect to the internal structural parameters R and unit cell strain eta yields the equilibrium structure at fixed electric field e and polarization P, respectively. First-order expansion of E(R,eta,e) in e leads to a useful approximation in which R(P) and eta(P) can be obtained by simply minimizing the zero-field internal energy with respect to structural coordinates subject to the constraint of a fixed spontaneous polarization P. To facilitate this minimization, we formulate a modified DFPT scheme such that the computed derivatives of the polarization are consistent with the discretized form of the Berry-phase expression. We then describe the application of this approach to several problems associated with bulk and short-period superlattice structures of ferroelectric materials such as BaTiO3 and PbTiO3. These include the effects of compositionally broken inversion symmetry, the equilibrium structure for high values of polarization, field-induced structural phase transitions, and the lattice contributions to the linear and the non-linear dielectric constants.Comment: 19 pages, with 15 postscript figures embedded. Uses REVTEX4 and epsf macros. Also available at http://www.physics.rutgers.edu/~dhv/preprints/sai_pol/index.htm