Phase Transformation in Lithium Niobate-Lithium Tantalate Solid Solutions (LiNb₁-ₗTaₓO₃)

The investigation of the structural phase transition in the vicinity of the Curie temperature Tc of LiNb₁-ₗTaₓO₃ crystals is motivated by the expected combination of advantageous high-temperature properties of LiNbO₃ and LiTaO₃, including high piezoelectric modules and remarkable high-temperature stability, respectively. Tc marks the ultimate limit for exploiting the piezoelectric properties, however transition related structural modifications might impact this and other properties even below Tc. Remarkably, the phase transition from the ferroelectric to the paraelectric phase, whose temperature strongly depends on the composition x, shows a significant drop in the activation energy of the electrical conductivity. The magnitude, temperature dependence and underlying mechanisms of this drop are discussed from a microscopic perspective. Molecular dynamics calculations in the framework of the density functional theory show that substantial displacements of the cations occur below Tc for both the end compounds LiNbO₃ and LiTaO₃, and might thus affect the electrical conductivity. Above Tc, the migration of lithium ions is presumably facilitated by a shortened diffusion path for the most favorable jump of the lithium ions. Electronic contributions to the conductivity, which become important above 900 K, are explained within the polaronic picture by the formation and migration of free small polarons.