A- and B-site ratios in (Na0.5+y+zK0.5−y)(Nb1−xTax)O3 ceramics with ABO3 structure were investigated to differentiate the effects of polymorphic phase transition (PPT), morphotropic phase boundary (MPB), and Na nonstoichiometry on piezoelectric coefficient (d33). MPB was intrinsically determined by the ratio between Na and K, whereas PPT was extrinsically controlled by Ta substitution. PPT temperature between orthorhombic and tetragonal phases (TO−T) was reduced to room temperature at x = 0.45. d33 was increased to 310 pC/N at y = 0.03 and z = 0 by the combined effects of PPT and MPB and was further enhanced to 333 pC/N by the Na nonstoichiometry at z = 0.015 with improved density.

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A first-principles informed grand canonical defect chemistry model capable of accounting for non-stoichiometry and partial equilibration of different sub-lattices is developed and used to study Mg and Mn doped, and (Mg+Y) and (Mn+Y) co-doped BaTiO 3 to elucidate the role of Mn and Y in improving the resistivity and resistance degradation of BaTiO 3 as observed by Ryu et al. in Part I of this series of papers. The model qualitatively captures the behavior of the samples in all conditions, reproducing the observed carrier plateau and increased resistivity of (Mn+Y) co-doped BaTiO 3 , and expected trends in the concentrations of free oxygen vacancies with doping. These trends reflect the observed differences in degradation characteristics, and help explain the substantially improved degradation resistance of the (Mn+Y) co-doped samples. Our model adds to the mechanism proposed by Yeoh et al. that the Fermi level is pinned by the multivalent character of Mn Ti in (Mn+Y) co-doped BaTiO 3 by giving insight into the role of barium vacancies, the site preferences of the dopants, and defect complexes in this mechanism. These insights provide a set of criteria in the search for sets of co-dopants with similar behaviors.