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The comprehensive understanding of (Na 0.5 Bi 0.5 )TiO 3 -BaTiO 3 (NBT-BT) lattice structure is highly desired to develop lead-free ferroelectric materials. However, most of the previous studies focused on the improvement of piezoelectric properties at room temperature, and many structural puzzles are left unclear. In this work, the lattice structure of a ferroelastic phase and the ferroelectric-ferroelastic transitions in both rhombohedral NBT and tetragonal NBT-8%BT single crystals are investigated in detail. Our results illustrate the complex process of the ferroelectric-ferroelastic transition of NBT. The variation of Ti-O modes and oxygen octahedra modes clearly indicates the gradual change of lattice symmetry from R 3 c to P 4 bm during a wide temperature range between 170 and 350 °C. A ferroelectric-ferroelastic transition is also confirmed in tetragonal NBT-8BT for the first time, and the lattice symmetry of P 4 bm is found to be maintained during the ferroelastic stage. This work reveals the lattice evolutions of the ferroelectric-ferroelastic transition of NBT-BT crystals and provides new insights for understanding the ferroelasticity and the evolution of phonon modes in a lead-free relaxor.