Evidence for another low-temperature phase transition in tetragonal (formula presented) (formula presented)

Abstract

Results of dielectric and resonance frequency (formula presented) measurements below room temperature are presented for (formula presented) (formula presented) and 0.520. It is shown that the temperature coefficient of (formula presented) changes sign from negative to positive around 210 and 265 K for (formula presented) and 200 and 260 K for (formula presented) Anomalies in the real part of the dielectric constant (formula presented) are observed around the same temperatures at which the temperature coefficient of (formula presented) changes sign because of the electrostrictive coupling between the elastic and dielectric responses. Low-temperature powder x-ray-diffraction (XRD) data, however, reveal only one transition from the tetragonal to monoclinic phase similar to that reported by Noheda et al. [Phys. Rev. B, 61, 8687 (2000)]. Electron-diffraction data, on the other hand, reveal yet another structural transition at lower temperatures corresponding to the second anomaly in the (formula presented) vs T and (formula presented) vs T curves. This second transition is shown to be a cell-doubling transition not observed by Noheda et al. in their XRD studies. The observation of superlattice reflections raises doubts about the correctness of the Cm space group proposed by Noheda et al. for the monoclinic phase of (formula presented) below the second transition temperature.