Presence Of Free Charge Carriers Research Articles

Effect of Lanthanum-doping on the Dielectric Properties of Pb(Fe1/2Ta1/2)O3 Relaxor Ferroelectrics

This paper reports the influence of lanthanum-doping on the dielectric behavior of Pb(Fe1/2Ta1/2)O3 relaxor ferroelectrics, where the La ions are incorporated into the B-site of the perovskite constructing unit and the resultant charge imbalance is compensated by the presence of free charge carriers (holes). The formulated compositions were fabricated using one-step preparation method, and a nearly pure perovskite phase was attained, as verified by XRD analysis. Both the dielectric permittivity maximum (ε′max) and the temperature at which ε′max occurs (Tm) are progressively reduced by La-doping. The degree of frequency dispersion, however, is promoted, indicating the weakened ferroelectric couplings among the oxygen octahedra as a result of lanthanum incorporation. Moreover, the breadth of paraelectric to ferroelectric phase transition is broadened with an increase in lanthanum content, due to enhanced inhomogeneity and increase in the number of B-site cations. Frequency spectroscopy at a certain temperature below Tm reveals a widening of the size distribution of ferroelectric nanodomains, which is characteristic of relaxor microstructure, by La-doping.

Lattice instability and low-temperature phase transition in Pr2NiO4.

Some layered perovskites with general formulas {ital R}{sub 2}{ital M}O{sub 4} ({ital R}=rare earth; {ital M}=Cu, Ni or Co) undergo a sequence of structural phase transitions. The high-temperature tetragonal phase (HTT) is transformed to the low-temperature orthorhombic phase (LTO) at {ital T}{sub {ital c}2}, after further cooling, there is a new low-temperature tetragonal (LTT) phase at {ital T}{sub {ital c}1}. The soft mode behavior at {ital T}{sub {ital c}1} is observed with inelastic neutron scattering in this family of compounds for Pr{sub 2}NiO{sub 4}. One of the two branches, of the doubly degenerate {ital X}-point phonon of the HTT phase, becomes softer at {ital T}{sub {ital c}1}. In the LTT phase, this phonon branch is stiffer and appears at higher energy. The orthorhombic strain and the soft-mode frequency undergo a discontinuous jump at the transition temperature, indicating a clear first-order character of this phase transformation. From these data, the dynamics of the LTO{r arrow}LTT phase transformation in the insulator Pr{sub 2}NiO{sub 4+{delta}} ({delta}=0) can be well characterized. For La{sub 2{minus}{ital x}}Ba{sub {ital x}}CuO{sub 4} ({ital x}=1/8) the same kind of transition is observed, which coincides with the suppression of the superconducting state, but the presence of free charge carriers maymore » complicate the interpretation of the results in this case.« less