Migration from Lithium Niobate to Lithium Tantalate in LiNb(1-x)Ta(x)O3 solid solutions

Lithium Niobate (LN) and Lithium Tantalate (LT) are ferroelectric synthetic crystals with outstanding acousto-optical, electro-optical and nonlinear optical properties, currently employed for acoustic transducers and filters, electro-optic modulators, and frequency converters [1]. LN and LT exhibit the same crystal structure (space group R3c) with slightly different lattice constants [2]. Unlike other doping ions, when Tantalum is introduced in the lattice of LN, it replaces Niobium, leading to the crystalline solid solution LiNb1-xTaxO3 (LNT) with x = [Ta]/([Nb]+[Ta]). Tantalum is isomorphous to Niobium and, at least in principle, it can fully substitute Niobium in the lattice, thus envisaging the possibility of finely tuning the crystal properties by controlling x. We investigted a set of LNT sample with x increasing up to 0.25. The electro-optic unclamped coefficients rc, r33 and r13, measured through polarimetric and interferometric techniques, appeared to decrease remarkably (by at least 50%) when Tantalum concentration reached x = 0.09 and larger values. Nevertheless, the dielectric constants of the studied crystals were found to migrate from the LN values (e11=85 and e33= 29) towards the LT ones (e11= 54 and e33= 43) with increasing x. It demonstrates for the first time, to the best of our knowledge, the foreseen tuning of the dielectric properties of LNT crystals by varying the Ta/Nb ratio