Anomalous broad dielectric relaxation inBi1.5Zn1.0Nb1.5O7pyrochlore

Abstract

The complex dielectric response of ${\mathrm{Bi}}_{1.5}{\mathrm{Zn}}_{1.0}{\mathrm{Nb}}_{1.5}{\mathrm{O}}_{7}$ cubic pyrochlore ceramics was investigated between 100 Hz and 100 THz by a combination of low-frequency capacitance bridges, a high-frequency coaxial technique, time domain transmission THz spectroscopy, and infrared spectroscopy. The data obtained between 10 K and 400 K revealed glasslike dielectric behavior: dielectric relaxation is observed over a wide frequency and temperature range, and the dielectric permittivity and loss maxima shift to higher temperature values by almost 200 K with increasing measuring frequency. The distribution of relaxation frequencies broadens on cooling and can be described by a uniform distribution. The high-frequency end of the distribution at $\ensuremath{\sim}{10}^{11} \mathrm{Hz}$ is almost temperature independent and its low-frequency end obeys the Arrhenius Law with an activation energy of $\ensuremath{\sim}0.2 \mathrm{eV}.$ The relaxation is assigned to the local hopping of atoms in the A and O' positions of the pyrochlore structure among several local potential minima. The barrier height for hopping is distributed between 0 and 0.2 eV. Such an anomalously broad distribution may have its origin in the inhomogeneous distribution of ${\mathrm{Zn}}^{2+}$ atoms and vacancies on ${\mathrm{Bi}}^{3+}$ sites, which gives rise to random fields and nonperiodic interatomic potential. Frequency independent dielectric losses $(1/f$ noise) are observed at low temperatures, which seems to be a universal behavior of disordered systems at low temperatures.