Ground- and excited-state properties of LiNb₁-ₗTaₓO₃ solid solutions

LiNb₁-ₗTaₓO₃ solid solutions are investigated from first principles and by optical spectroscopy. The ground- and excited-state properties of the solid solutions are modelled within density functional theory as a function of the Ta concentration using special quasirandom structures spanning the entire composition range between LiNbO₃ and LiTaO₃. Deviations from a Vegard behavior are predicted for the lattice parameters, the heat capacity, the electronic bandgap, and consequently the absorption edge. The latter is measured for crystals of different composition by low temperature optical spectroscopy, qualitatively confirming the theoretical predictions. The LiNb₀. ₁₁Ta₀. ₈₉O₃ composition is found to be a highly unusual crystal with a permanent macroscopic electric polarization and nonetheless zero birefringence.