  Lithium niobate and lithium tantalate are among the most widespread materials
for nonlinear, integrated photonics. Mixed crystals with arbitrary Nb-Ta ratios
provide a new degree of freedom to tune materials properties, such as the
birefringence, but also leverage the advantages of the singular compounds, for
example, by combining the thermal stability of lithium tantalate with the
larger nonlinear or piezoelectric constants of lithium niobate. Periodic poling
is the prerequisite for any nonlinear optical application. For mixed crystals
this has been challenging so far due to the lack of homogeneous, mono-domain
crystals, which severely inhibit domain growth and nucleation. In this work we
demonstrate that surface-near ($< 1$~$\mu$m depth) periodic poling on x-cut
lithium niobate tantalate mixed crystals can be achieved via electric field
poling and lithographically structured electrodes. We find that naturally
occurring head-to-head or tail-to-tail domain walls in the as-grown crystal
inhibit domain inversion at a larger scale. However, periodic poling is
possible, if the gap size between the poling electrodes is of the same order of
magnitude or smaller than the average size of naturally occurring domains. This
work provides the basis for the nonlinear optical application of lithium
niobate tantalate mixed crystals.
