Optical phonon behaviors and unstable polar mode in transparent conducting Ba_1-xLa_xSnO₃ films from temperature dependent far-infrared reflectance spectra

We report lattice vibration behaviors of transparent conducting Ba _1-xLa_xSnO₃ 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 O_2/3 eigendisplacement. The content dependent phonon behaviors are mainly caused by octahedral tilting with substituting La for Ba.