Optical phonon behaviors and unstable polar mode in transparent conducting Ba1−xLaxSnO3films from temperature dependent far-infrared reflectance spectra

Abstract

We report lattice vibration behaviors of transparent conducting Ba1−xLaxSnO3 films derived from the far-infrared reflectance spectra in the temperature range of 105–300 K. Three main phonon modes can be observed at room temperature and the frequencies approximately decrease with increasing La content. A polar mode TO1 appears at about 137 cm−1 and becomes unstable with decreasing temperature. On cooling, the TO1 mode vanishes at 270, 240, 180 and 210 K for the films with x = 0.00, 0.02, 0.04 and 0.06 and appears again with further decreasing of the temperature for the La content of 0.00 and 0.02. It can be explained by the reversal of the TO1 mode effective charge, which originates from a drastic change of the O2/3 eigendisplacement. The content dependent phonon behaviors are mainly caused by octahedral tilting with substituting La for Ba.