Influence of Li-stoichiometry on electrical and acoustic properties and temperature stability of Li(Nb,Ta)O₃ 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)O₃ 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)O₃, 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)O₃ 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 LiNbO₃ remains in a {} 100 ppm range of the initial value of 3.5 MHz.