A defect-chemical framework for transparent piezoelectric perovskites through donor-induced phase transitions

Transparent piezoelectrics are often conceived as fundamentally distinct from conventional ferroelectric ceramics. However, this study reveals that transparency can be realized without altering the primary chemical framework but rather through a strategically controlled donor-doping protocol. From an inorganic chemical perspective, this phenomenon is rooted in the defect chemistry of ABO₃ perovskites, where donor incorporation synergistically regulates charge compensation, lattice distortion, and interfacial sintering kinetics. Aliovalent donor doping induces cation-vacancy formation and attenuates local structural distortions, guiding ferroelectric lattices toward a pseudocubic state to suppress birefringence. Simultaneously, defect-mediated mass transport facilitates liquid-phase-mediated densification and regulates grain growth, eliminating scattering from residual porosity and grain boundaries. Consequently, optical transparency emerges as an intrinsic consequence of coupled defect equilibria, phase stability, and sintering dynamics. To validate the generalizability of this design concept, we systematically investigated a representative donor-doped perovskite series, including Bi³⁺-doped (K,Na)NbO₃ (KNN) and La³⁺-doped Pb(Zr,Ti)O₃ (PZT). Crucially, this study introduces a new paradigm that allows researchers to transform their established piezoelectric compositions into transparent multifunctional systems by incorporating donors, rather than abandoning proven materials for entirely new systems. Despite their distinct parent structures, both systems exhibit a unified donor-induced transformation into transparent piezoelectric states. Our findings demonstrate that transparent functionality can be integrated into existing piezoelectric systems through strategic modulation of defect equilibria and phase stability, providing a general defect-chemical framework for understanding and designing multifunctional transparent perovskite piezoelectrics. While this study focuses on establishing the defect-chemical origin of transparency using total transmittance, detailed evaluations of haze and in-line transmittance for display and imaging applications are deferred to future work.