A High-Temperature Optical Spectroscopy Study of the Fundamental Absorption Edge in the LiNbO₃-LiTaO₃ Solid Solution

An optical spectroscopy study is reported of single crystals of the LiNbO3-LiTaO3 solid solution. The spectra are dominated by the fundamental absorption edges. The direct band-to-band transitions in congruent LiNbO3 (LN) and LiTaO3 (LT) as well as in selected compositions of solid solutions grown from mixtures of the congruently melting end members are studied at temperatures up to 1200 degree celsius. At room temperature, the direct optical bandgaps of congruent LN and LT assume values of about 4.0 and 4.6 eV, respectively. With rising temperatures, the absorption edges are found to shift to lower energies. The isothermal composition dependence of bandgaps is a far from linear by going through a minimum. This behavior is maintained up to the highest temperatures of our study. At about 600 degree celsius, congruent LT undergoes a structural transition from the ferroelectric to the paraelectric phase. This is reflected by a change in the temperature dependence of the gap energy indicating structure-related changes in electron-phonon coupling.