Pulsed Photoacoustic: A Reliable Technique to Investigate Diffuse Phase Transitions and Associated Phenomena in Ferroelectrics

Abstract

Often permittivity against T curves do not provide a conclusive characterisation of dielectric features of ferroelectric materials. Particularly, when permittivity is obtained from impedance measurements, where to experimental data is gradually lost at high frequencies when the temperature rises. Here we show how a combination of permittivity measurements and photoacoustic experiments can give a much more complete characterization of the ferro-paraelectric phase transitions and associated phenomena than those obtained from permittivity data alone. Peaked correlation curves, obtained from photoacoustic experiments were interpreted to be a manifestation of large enough microscopic variations of the compressibility as the temperature changed. On one hand, using a classical ferroelectric (BaTiO ₃ ) and a well-recognized relaxor (Ba(Ti _0.65 Zr _0.35 )O ₃ ) the main differences between their photoacoustic response are established. On the other hand, dielectric and photoacoustic results from the compounds Bi ₂ WO ₆ and Bi ₂ [Ta _0.1 W _0.9 ]O _{6 − δ} , were combined to characterize the temperature dependence of their dielectric behavior. Results provide evidence of the presence of a diffuse phase transition in Bi ₂ WO ₆ at 660°C similar to that corresponding to relaxor behavior. Additionally the occurrence of a classical ferro-paraelectric phase transition above 900°C was found. This last phenomenon is detectable only by photoacoustic experiments. In Bi ₂ [Ta _0.1 W _0.9 ]O _{6 − δ} the presence of a well defined transition was not detected. Dielectric determinations throughout ac measurements were carried out in the frequency interval 5 Hz-13 MHz. A pulsed Nd:YAG laser (10 Hz, 5 ns pulse width) was used to perform the photoacoustic experiments, where the out-coming signal was detected by a piezoelectric transducer. Dielectric and photoacoustic experiments were performed in a temperature interval from 250°C up to 1000°C.

Keywords:

• Pulsed photoacoustic
• ferroelectrics
• relaxors

Acknowledgment

Financial support from DGAPA-UNAM (IN117208) and Conacyt (44318-F) is gratefully acknowledged. The authors thank Dr. Neil Bruce for the revision of the manuscript and Fis. Raúl Reyes for technical support.