We report a dramatic influence of lithium nonstoichiometry inLiTaO3crystals on the 180° coercive fields, internal fields, domain wall strain, optical birefringence, and domain shape. Congruent composition ofLiTaO3([Li]/([Li]+[Ta])∼0.485)show large coercive fields of ∼21 kV/mm for domain motion and built-in internal fields of ∼4.5 kV/mm, domain wall strains of|&Vegr;zz|∼(1–10)×10−6,optical birefringence in the z-plane, and triangular domains with domain walls parallel to the crystallographic x-axes. On the other hand, near-stoichiometric crystals ofLiTaO3([Li]/([Li]+[Ta])∼0.499)studied here show low coercive fields of ∼1.7 kV/mm, internal fields of <0.1 kV/mm, negligible domain wall strain or optical birefringence, and six-sided domains with domain walls parallel to crystallographicy-axes. ©2000 American Institute of Physics.