Optical study on a phase transition of fresnoite Ba2Si2TiO8 - art. no. 094104

Fresnoite Ba2Si2TiO8 is a ferroelectric with promising piezoelectric uses t hat exhibits unusual pyroelectric, elastic, and other property anomalies cl ose 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 t o exclude the possibility of these anomalies bring caused by a structural p hase transition; a convincing alternative interpretation of the origin of t hese anomalies has not yet been reported. The high accuracy universal polar irneter (HAUP) method offered an alternative investigative approach. A prim ary difficulty in the present study was ensuring that the illuminated volum e of the sample crystal was single and without domain texture. After having solved this problem, the method has established the point group of fresnoi te as 4mm above 433 K, and as optically active below it. The single gyratio n tensor component g(12) appears abruptly but continuously on cooling below this temperature and increases with decreasing temperature, reaching 5.8x1 0(-6) at 360 K. Thus fresnoite is found to undergo a structural phase trans ition of second order from a 4mm phase to an optically active nonenantiomor phic 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 mm2. The present study illustrates the unique and deep insight the HAUP method can provide into condensed-matter phase transitions.