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Second phase and defect formation mechanism of (Bi0.5(Na1-xKx)0.5)TiO3 (BNKT100x) ceramics were investigated using electron microscopy, x-ray photoelectron spectroscope (XPS) and electrical properties measurements. Experimental results indicated that second phase formation induces Bi-rich regions and compositional inhomogeneity within matrix due to thermodynamic stability of potassium titanate. Ti valence transition for BNKT ceramics sintered in air might be ascribed to formation of the secondary phase, rather than simply attributed to volatilization of bismuth. Li substitution at A-site in BNKT ceramics suppresses formation of the second phase and Ti valence transition. Appropriate Li doped BNKT ceramics suppress oxygen vacancies and titanium valence transition, and therefore decrease the leakage current.

This work demonstrates the critical role of orbital hybridizations in the FeO6 octahedral distortion, composition‐driven phase transition, and bonding covalency in multiferroic (Bi1−xSmx)FeO3 (x = 0.10‐0.20) ceramics in the vicinity of the morphotropic phase boundary (MPB). Sequential composition‐driven transitions from the polar rhombohedral R3c to antipolar orthorhombic Pbam and then Pnma phases were revealed as the system crosses the MPB with increasing Sm. A coexistence of ferroelectric (FE) rhombohedral R3c and antiferroelectric (AFE) PbZrO3‐like orthorhombic Pbam symmetries was identified by the 1/2{000}, 1/4{100}, 1/4{110}, 1/4{111}, and 1/4{121} superlattice diffractions at x = 0.12‐0.16. In addition to R3c and Pbam space groups, the nonpolar SmFeO3‐like orthorhombic Pnma space group becomes the predominant phase at x = 0.20 confirmed by the 1/2{100} superlattice diffractions. The Fe L3‐edge and oxygen K‐edge synchrotron X‐ray absorptions indicate that the O 2p–Fe 3d and O 2p–Bi 6s/6p orbital hybridizations were decreased as the system approaches the MPB.