Abnormal behavior of the dielectric parameters of Ba6-xLn8+2x/3Ti18O54 (Ln=La-Gd) solid solutions

The temperature dependence of the dielectric parameters epsilon and tan delta for the entire x range of barium lanthanide titanate solid solutions (BLT ss) using the general formula Ba(6-x)Ln(8+2x/3)Ti(18)O(54) (Ln=La, Nd, Sm, Gd) was measured over wide frequency and temperature ranges. The results show the presence of diffuse regions of epsilon(T) and tan delta(T) anomalies, which are located below (in the La- and Nd-containing solid solutions) or above (in the Sm- and Gd-containing solid solutions) room temperature. However, no evidence of a structural transition responsible for these anomalies could be determined by differential scanning calorimetry and x-ray diffraction analysis. To examine the nature of the abnormal temperature behavior the dielectric parameters in the systems studied were considered in terms of a simple oscillator model. Within the framework of this model correlations among the average size of the A-site ions, tilting of the oxygen octahedra, and the lattice strain are discussed, and the effect of crystallographic peculiarities on the temperature behavior of the dielectric parameters is shown. As a consequence, the changes in epsilon(T) and tan delta(T) behavior in the BLT ss family were shown to originate from a varying relationship between the harmonic and anharmonic contributions to the phonons of the BLT ss crystal lattice. (C) 2002 American Institute of Physics.