Opposite domain lithium niobate structures have been grown using erbium as a dopant. The "head-to-head" and "tail-to-tail" domain walls show a variation in the Li/Nb ratio, while the Er concentration is constant along the periodic domain structure. The influence of the etching time in the domain pattern was studied. After a vapor transport equilibration process the domain structure is destroyed, and the Li/Nb ratio is constant along the whole sample due to the Li in-diffusion process.