First Principles Investigation of the LiNbO $$_3$$ 3 and LiTaO $$_3$$ 3 Lattice Dynamics Under Uniaxial Stress

Structural strain severely impacts material properties such as the linear and non-linear optical response. Although $$\upmu $$ μ -Raman spectroscopy is a well-suited technique for the investigation of such effects, it requires the knowledge on the strain dependence of the phonon frequencies. In this work, we model the phonon frequencies in the widely used ferroelectrics lithium niobate and lithium tantalate as a function of uniaxial strain via density functional theory, and compare it with existing $$\upmu $$ μ -Raman spectroscopy results. The majority of phonons shows an increase in frequency under compressive strain, while the opposite is observed for tensile strain. Moreover, for E-type phonons, we observe the lifting of degeneracy already at moderate strain fields (i.e. at $${\pm }{0.2}{\%}$$ ± 0.2 % ) along the x and y directions. This work hence allows for the systematic analysis of 3D strains in modern-type bulk and thin-film devices assembled from lithium niobate and tantalate.