Influence of Li-stoichiometry on electrical and acoustic properties and temperature stability of Li(Nb,Ta)O_3 solid solutions up to 900 C

The current work is focused on the impact of the lithium stoichiometry on electrical conductivity, acoustic properties and high-temperature stability of single crystalline Li(Nb,Ta)O_3 at high temperatures. The crystals grown from Li-deficient melts were treated by the vapor transport equilibration (VTE) method, achieving near stoichiometric Li-content. It is shown, that the VTE-treated specimens generally exhibit lower conductivity at temperatures below 800 C, which is attributed to the reduced number of Li-vacancies in near stoichiometric Li(Nb,Ta)O_3, provided that the Li-ion migration dominates the conductivity in this temperature range. Further, it is shown, that above 600-650 C different mechanism increasingly contributes to the conductivity, which is consequently attributed to the electronic conduction. Further, it is shown that losses in LNT strongly increase above about 500 C, which is interpreted to originate from conductivity-related relaxation mechanism. Finally, the thermal stability of Li(Nb,Ta)O_3 is evaluated by the measurement of the conductivity and resonance frequency as a function of time. It is found that during annealing at 700 C for 350 hours, the resonance frequency of LiNbO_3 remains in a ± 100 ppm range of the initial value of 3.5 MHz.