Optical study on a phase transition of fresnoite Ba₂Si₂TiO₈

Fresnoite Ba₂Si₂TiO₈ is a ferroelectric with promising piezoelectric uses that exhibits unusual pyroelectric, elastic, and other property anomalies close to 433 K. X-ray diffraction analyses at 297 and 573 K by Markgraf et al. determined the space group to be P4bm at both temperatures and appeared to exclude the possibility of these anomalies being caused by a structural phase transition; a convincing alternative interpretation of the origin of these anomalies has not yet been reported. The high accuracy universal polarimeter (HAUP) method offered an alternative investigative approach. A primary difficulty in the present study was ensuring that the illuminated volume of the sample crystal was single and without domain texture. After having solved this problem, the method has established the point group of fresnoite as 4mm above 433 K, and as optically active below it. The single gyration tensor component g12 appears abruptly but continuously on cooling below this temperature and increases with decreasing temperature, reaching 5.8 × 10^-6 at 360 K. Thus fresnoite is found to undergo a structural phase transition of second order from a 4mm phase to an optically active nonenantiomorphic point group. Combining the change of optical activity with that of the birefringence leads us to the conclusion that the point group below 433 K is mm 2. The present study illustrates the unique and deep insight the HAUP method can provide into condensed-matter phase transitions.