The local structure and atomic dynamics of (1-x)Na0.5Bi0.5TiO3-xBaTiO3 with x ranging from 0 to 0.074 were analyzed at room temperature by in situ single-crystal Raman spectroscopy under an external dc electric field E applied along the pseudocubic [100]c or [110]c directions, to gain further insights into the atomistic mechanism of polar coupling near the morphotropic phase boundary (MPB). The results reveal a striking compositional dependence of the E[100]c-induced changes in the Raman scattering and suggest that the origin of the enhanced longitudinal piezoelectric coefficient in alkali-Bi based solid solutions with a pseudocubic structure near MPB is due to the composition-driven reduction of the local strains and the consequent enhancement of the structural flexibility under external stimuli.