The role of A-site non-stoichiometry was investigated in lead-free piezoelectric ceramics based on compositions in the 1 - x(Bi0.5Na0.5TiO3)-xBaTiO(3) system near the morphotropic phase boundary, where x = 0.055, 0.06, and 0.07. Donor doping was introduced through the addition of excess Bi, however, there were no changes in the crystal structure. In contrast, acceptor doping was introduced through the addition of excess Na and was found to promote rhombohedral distortions. A significant improvement of dielectric properties was observed in donor-doped compositions and, in contrast, a degradation in properties was observed in acceptor-doped compositions. Compared to the stoichiometric composition, the acceptor-doped compositions displayed a significant increase in coercive field (E (c)) which is an indication of domain wall pinning as found in hard Pb(Zr (x) Ti1-x )O-3. This result was further confirmed via remanent polarization hysteresis analyses. Moreover, all A-site acceptor-doped compositions also exhibited an increase in mechanical quality factor (Q (m)) as well as a decrease in piezoelectric coefficient (d (33)), dielectric loss (tan delta), remanent polarization (P (r)), and dielectric permittivity, which are all the typical characteristics of the effects of "hardening." The mechanism for the observed hardening in A-site acceptor-doped BNT-based systems is linked to changes in the long-range domain structure and defect chemistry.