Relaxor-like Behavior and Structure Features of Bi2Ti2O7 Pyrochlore Single Crystals

By slow cooling of the melt, large Bi 2 Ti 2 O 7 single crystals with a composition close to stoichiometric were grown. No traces of impurity phases were found on the X-ray diffraction patterns. A structural model with the space group Fd 3 m and displacive disorder in Bi-sublattice was proposed based on X-ray single crystal diffraction data. The dielectric properties of Bi 2 Ti 2 O 7 single crystals as a function of temperature at different frequencies were studied for the first time. As in the case of ceramic samples, a step-like frequency-dependent anomaly was detected at a temperature of about 220 K at a frequency of 1 kHz. It was found that attempts to describe the dielectric relaxation using the Arrhenius equation do not lead to physically significant values of the fitting parameters. However, the relaxation behavior is well described by the empirical Vogel–Fulcher relation, which is typical for many dipole and spin glasses, as well as relaxor ferroelectrics. Based on the value of fitting parameters, Bi 2 Ti 2 O 7 occupies an intermediate position between the canonical relaxor PbMg 1/3 Nb 2/3 O 3 on one hand and lead-free weakly coupled relaxors based on BaTiO 3 on the other one. Possible mechanisms of the observed relaxor-like behavior of the Bi 2 Ti 2 O 7 single crystal are discussed in terms of correlated hopping of bismuth cations and geometric frustration of the pyrochlore lattice.