${\mathrm{LiNb}}_{1-x}{\mathrm{Ta}}_x{\mathrm{O}}_3$ solid solutions are investigated from first principles and by optical spectroscopy. The ground- and excited-state properties of the solid solutions are modeled within density functional theory as a function of the Ta concentration using special quasirandom structures spanning the entire composition range between ${\mathrm{LiNbO}}_3$ and ${\mathrm{LiTaO}}_3$. Deviations from a Vegard behavior are predicted for the lattice parameters, the heat capacity, the electronic band gap, 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 ${\mathrm{LiNb}}_{0.11}{\mathrm{Ta}}_{0.89}{\mathrm{O}}_3$ composition is found to be a highly unusual crystal with a permanent macroscopic electric polarization and nonetheless zero birefringence.

• Received 21 February 2024
• Revised 8 April 2024
• Accepted 11 April 2024

DOI:https://doi.org/10.1103/PhysRevMaterials.8.054403