Lithium niobate-tantalate, Li(Nb,Ta)O₃ (LNT) solid solutions have attracted increasing attention due to their potential to combine the advantageous properties of the end components of the LNT system, such as the high Curie temperature of LiNbO₃ and improved thermal stability of sensing relevant parameters (conductivity, acoustic loss) of LiTaO₃. These materials, grown by Czochralski technique similarly to LiNbO₃ or LiTaO₃, hold promise for applications in high-temperature sensing and actuating devices.

The intrinsic properties of LiNbO₃, LiTaO₃, and, consequently, their solid solutions, Li(Nb,Ta)O₃, are known to be strongly influenced by the lithium stoichiometry. In the current work, its impact on the electrical and acoustic properties of single crystalline Li_1-δ(Nb,Ta)_1+δ/4O₃ is investigated. The samples with nominal composition of Li_1-δ(Nb_0.55Ta_0.45)_+δ/4O₃ and Li_1-δ(Nb_0.30Ta_0.70)_+δ/4O₃ are studied and their properties are compared to those of Li_1-δNb_1+δ/4O₃ and Li_1-δTa_1+δ/4O₃. In the case of the as-grown, congruent crystals δ≈0.03.

Near-stoichiometric (ns, δ≈0.01) and sub-congruent (δ≈0.05) samples were prepared using the vapor transport equilibration (VTE) technique. Subsequently, the Li₂O content of the samples was determined by the position of the absorption edge via UV-Vis optical spectroscopy. Electrical and acoustic characterization of LNT was conducted as a function of temperature using impedance spectroscopy and resonant piezoelectric spectroscopy, respectively. The measurements reveal that at temperatures up to about 600-650 °C the electrical conductivity of Li_1-δ(Nb,Ta)_1+δ/4O₃ is governed by lithium ion migration via lithium vacancies. Above that temperature range, the increased contribution of different conduction mechanism is evident. The conductivity above 650 °C is attributed to the superposition of electronic and ionic conduction. Near-stoichiometric samples generally show lower conductivity and reduced acoustic loss, which implies better thermal stability.

Furthermore, the stability of LNT specimens was examined through the measurements of conductivity and resonance frequency at elevated temperatures in air over long periods of time. Complementary UV optical spectroscopy was employed to observe potential Li₂O evaporation resulting from thermal treatment. It is found that during annealing at 700 °C for 350 hours, the resonance frequency of LiNbO₃ remains in a ± 100 ppm range of the initial value of 3.5 MHz.