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

Abstract

By slow cooling of the melt, large Bi2Ti2O7 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 m and displacive disorder in Bi-sublattice was proposed based on X-ray single crystal diffraction data. The dielectric properties of Bi2Ti2O7 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, Bi2Ti2O7 occupies an intermediate position between the canonical relaxor PbMg1/3Nb2/3O3 on one hand and lead-free weakly coupled relaxors based on BaTiO3 on the other one. Possible mechanisms of the observed relaxor-like behavior of the Bi2Ti2O7 single crystal are discussed in terms of correlated hopping of bismuth cations and geometric frustration of the pyrochlore lattice.

Low‐temperature dielectric behavior of Sc‐ and In‐doped bismuth titanate pyrochlores

“…The parameters of the relaxation process for the BScTO and BInTO compounds are in good agreement with those for pure BTO, 15 other rare‐earth doped BTO, 21,32,33,35,56 , and bismuth zinc niobate pyrochlores 57–59 . The structural defects are supposed to play a significant role in dielectric relaxation processes 56,60 .…”

“…As shown in Figure 4, a frequency‐dependent maximum of the dielectric constant and dielectric losses is observed for all compositions. This feature of a relaxor‐type dielectric has also been revealed in other bismuth‐based pyrochlores 15,17,20,21,32,33 . For a more detailed analysis of the relaxation process, we applied the formalism of the electrical module 54,55 : where ( Z ′, M ′) and ( Z ″, M ″) are the real and imaginary parts of the impedance and electrical module, ω = 2π f is the angular frequency, C o is the vacuum capacitance.…”

“…It is known that the ideal pyrochlore structure A 2 B 2 O 6 O′ includes four different sites: two cation sites, 16 c ( A ) and 16 d ( B ), and two oxygen‐ion sites, 48 f (O) and 8 a (O′). 15,16 The location of the dopant atoms in one of the cationic positions can affect the properties of the pyrochlore solid solutions. Moreover, atomic displacements from the ideal pyrochlore positions ( A from 16 c to 96 h or 96 g ; O′ from 8 a to 32 e or 96 g ) are known to be characteristic of the Bi‐based pyrochlores 15,16,28,43 .…”

High‐temperature dielectric and energy storage properties of Bi0.5Na0.5TiO3‐based ceramics modified by Sr0.8Na0.4Nb2O6

“…Whether Bi 2 Ti 2 O 7 is ferroelectrics is still a controversial issue 43,44 . The dielectric permittivity of Bi 2 Ti 2 O 7 is found to be evidently lower than that of BNT‐BT‐xSNN ceramics 45 . And the ferroelectricity of the non‐stoichiometric pyrochlore Bi 1.74 Ti 2 O 6.624 has never been reported until now.…”

“…43,44 The dielectric permittivity of Bi 2 Ti 2 O 7 is found to be evidently lower than that of BNT-BT-xSNN ceramics. 45 And the ferroelectricity of the non-stoichiometric pyrochlore Bi 1.74 Ti 2 O 6.624 has never been reported until now. Therefore, it can be concluded that the secondary phase of Bi 1.74 Ti 2 O 6.624 may lead to the decrease of polarization.…”