  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 {\deg}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 {\deg}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
{\deg}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 {\deg}C for 350 hours, the
resonance frequency of LiNbO$_{3}$ remains in a {\textpm} 100 ppm range of the
initial value of 3.5 MHz.
