Anomalous broad dielectric relaxation in Bi_1.5Zn_1.0Nb_1.5O₇ pyrochlore

The complex dielectric response of Bi_1.5Zn_1.0Nb_1.5O₇ 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 ∼10¹¹ Hz is almost temperature independent and its low-frequency end obeys the Arrhenius Law with an activation energy of ∼0.2 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 Zn²⁺ atoms and vacancies on Bi³⁺ 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.