First-principles theoretical study of pure and Ti, Zr, and Hf doped K0.5Na0.5NbO3: Structural, electronic, mechanical, and piezoelectric properties

Lead-free piezoelectric materials are gaining attention as environmentally sustainable alternatives to lead zirconate titanate (PZT)-based ceramics. Among these, potassium-sodium niobate (K 0.5 Na 0.5 NbO 3 , KNN) shows promising piezoelectric properties but faces challenges in densification and functional stability. This study uses first-principles density functional theory (DFT) calculations with the Wien2k code to explore the structural, electronic, mechanical, and piezoelectric properties of sodium-doped KNbO 3 . The optimized K 0.5 Na 0.5 NbO 3 model was doped with 12.5% transition metals (Ti, Zr, and Hf) at the B-site to investigate how ionic radius and electronic configuration affect the piezoelectric response, particularly the piezoelectric coefficient e 33 (C/m 2 ) and the piezoelectric constant d 33 (pC/N). The results indicate significant enhancements in polarization behavior and electronic band characteristics with zirconium (Zr) doping in comparison to titanium (Ti) and hafnium (Hf); notably, the piezoelectric constant d 33 reaches 118 pC/N, compared to 75 pC/N for KNN, aligning with previous experimental results. This theoretical study offers valuable insights for designing high-performance lead-free piezoelectric and lays a strong foundation for future experimental synthesis and applications in piezoelectric and ferroelectric technologies.