Brillouin study of phase transitions in KNbW2O9 bronze

Abstract

High-resolution Brillouin spectra of KNbW2O9 crystallizing in the hexagonal bronze structure were obtained for the first time in the temperature range 293-723 K. Weak anomalies in sound velocity were observed for the sound waves corresponding to the c11 elastic constant at 535, 597 and 671 K. Additionally, a strong increase in the intensity of Brillouin lines was observed near 530 K. These anomalies are attributed to phase transitions occurring in the KNbW2O9 crystal between two phases of unknown symmetry (at 535 K), two orthorhombic phases (at 597 K) and between orthorhombic and hexagonal phases (at 671 K). The character of the observed anomalies suggests that the coupling is squared in order parameter and linear in strain. Copyright © 2003 John Wiley & Sons, Ltd.