Dielectric properties and lattice dynamics of Ca-doped K0.95Li0.05TaO3

Relaxor behavior and lattice dynamics have been studied by employing dielectric measurements and neutron-scattering methods for a single crystal of K1-xLixTaO3 (x=0.05), where a small amount of a Ca impurity (similar to 15 ppm) was incorporated during the single-crystal growth procedure. The dielectric constant epsilon(')(omega,T) shows qualitatively similar behavior to that of Ca-free KLT with x=0.043 with both compositions exhibiting relaxational properties with no evidence for a ferroelectric transition. The absolute value of epsilon(')(omega,T=0) for the present crystal is larger by an order of magnitude than that of the Ca-free sample due to charge carriers induced by the Ca doping. This large value is shown to be due to a Maxwell-Wagner relaxation process associated with the low temperature (< 8 K) activation of frozen electronic carriers. The dielectric loss tangent tan delta reveals three Debye-type relaxations with Arrhenius activation energies of 80, 135, and 240 meV that are assigned to Li+ dipoles, Ca2+-related relaxation, and the Li+-Li+ dipolar pairs, respectively. In the neutron scattering results, diffuse scattering ridges appear around the nuclear Bragg peaks along the [100] direction below similar to 150 K and phonon line broadening features start to appear at even higher temperatures suggesting that polar nanoregions (PNRs) start to form at these temperatures. These results are supported by the dielectric data that reveal relaxor behavior starting at similar to 200 K on cooling. From analyses of the diffuse intensities at different zones, we have derived atomic displacements in the PNRs. The results suggest that the displacements include a uniform phase shift of all of the atoms in addition to the atomic displacements corresponding to a polarization vector of the transverse-optic soft-ferroelectric-mode, a finding that is analogous to that in the prototypical relaxor material Pb(Mg1/3Nb2/3)O-3.