The effect of selective monodoping and co-doping at A/B-site on the ferroelectricity and piezoelectricity in KNbO3

Ion doping is one of the most effective strategies to tailor the piezoelectric properties of alkali niobate ceramics. However, its underlying mechanisms remain insufficiently understood. In this work, the structural, ferroelectric, and piezoelectric properties of the selected A- or B-site monodoping and codoping in orthorhombic KNbO3(KN) are studied by density-functional calculations. A-site substitutions include alkali (Li, Na, Rb, Cs), alkaline-earth (Mg, Ca, Sr, Ba), and Bi ions, while B-site doping involves Nb replacement with isovalent (V, Ta, P, As), group IVB (Ti, Zr, Hf), and Bi atoms. Two co-doping combinations, (Na, Sb) and (Ba, Zr), are also studied. The orientational averaged shear, transverse, and longitudinal piezoelectric coefficients d̄15*, d̄31*, and d̄33* of the A/B-site monodoping and codoping in KN piezoceramics are calculated from the results of single crystals. The calculated values clearly indicate that the substitution of Na, Cs, and Ca at the A-site can result in higher piezoelectricity, while the incorporation of V, Ta, Ti, Bi, and Sb to substitute Nb atoms induces better piezoelectric performance. Moreover, the codoping technique of (Na, Sb)- and (Ba, Zr)-doped KN crystals significantly enhances the piezoelectricity compared with the pure KN and those monodoping cases. These findings demonstrate that ion doping plays a critical role in flattening the energy landscape and enhancing the piezoelectric performance of perovskite ferroelectrics.