High‐Performance Small‐Amount Fe ₂ O ₃ ‐Doped (K,Na)NbO ₃ ‐Based Lead‐Free Piezoceramics with Irregular Phase Evolution

[(K 0.43 Na 0.57 ) 0.94 Li 0.06 ][(Nb 0.94 Sb 0.06 ) 0.95 Ta 0.05 ]O 3 + x mol% Fe 2 O 3 ( KNLNST + x Fe, x = 0~0.60) lead‐free piezoelectric ceramics were prepared by conventional solid‐state reaction processing. The effects of small‐amount Fe 2 O 3 doping on the microstructure and electrical properties of the KNLNST ceramics were systematically investigated. With increasing Fe 3+ content, the orthorhombic‐tetragonal polymorphic phase transition temperature ( T O‐T ) of KNLNST + x Fe ceramics presented an obvious “V” type variation trend, and T O‐T was successfully shifted to near room temperature without changing T C ( T C = 315°C) via doping Fe 2 O 3 around 0.25 mol%. Electrical properties were significantly enhanced due to the coexistence of both orthorhombic and tetragonal ferroelectric phases at room temperature. The ceramics doped with 0.20 mol% Fe 2 O 3 possessed optimal piezoelectric and dielectric properties of d 33 = 306 pC/N, k p = 47.0%, = 1483 and tan δ = 0.023. It was revealed that the strong internal stress in the KNLNST + x Fe ceramics with higher Fe 3+ contents ( x = 0.40, 0.60) stabilized the orthorhombic phase, leading to the irregular “V” type rather than the usually observed monotonic phase transition with composition change in the ceramics.