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.