Detailed studies of the phase transitions in Pb_0.99Nb_0.02[(Zr_0.57Sn_0.43)_1-yTi_y]_0.98O₃ (abbreviated as PNZST 43/100y/2, y=0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.10, and 0.12) ceramics as a function of composition, temperature, electric field, and time were conducted using x-ray diffraction, dielectric measurement, Raman spectroscopy, and transmission electron microscopy (TEM).� It was found that for ceramics PNZST 43/100y/2 with 0.03£y£0.08, antiferroelectric order is stable at room temperature.� While for y=0.10 and 0.12, a ferroelectric order was observed at room temperature.� In the antiferroelectric phase, a characteristic nanoscale feature in the form of incommensurate modulations was directly observed by TEM.� In the ferroelectric phase, the nanoscale feature disappears.� Associated with the evolution of the nanoscale features is the ceramics' fracture resistance.� We found that antiferroelectric ceramics displays fewer microcracks under indentation than ferroelectric ceramics with similar compositions.� For the first time, we proposed a phase transition toughening mechanism is activated in antiferroelectric ceramics. �This may lead to the development of electroceramics with excellent electrical as well as mechanical properties.�