The new lead-free ternary system (0.8-x)NBT-0.2ST-xBT prepared by solid state reaction with 100%-perovskite phase, undergoes a structural change from pseudo-cubic (x <= 0.04)to tetragonal (x >= 0.06)along with a possible phase coexistence between 0.04 <= x <= 0.06. At x=0.04, the solid solution exhibits an optimum dielectric constant epsilon(max) of 2670, a strong relaxor behavior with a high diffuseness coefficient close to 2evidenced by slim P-E loops, excellent piezoelectric properties such as optimum strain value Smax of 0.424%and a normalized piezoelectric constant d(33)*of 688 pm/V. The above encouraging results may be helpful for further investigations of NBT-ST based ternary systems in search of a potential Pb-free piezoelectric material. (C) 2016 Elsevier B.V. All rights reserved