Effects of Sn ⁴⁺ solid solubility mechanisms on the electromechanical and energy storage performance of (Ba _0.85 Ca _0.15 )(Ti _0.92 Zr _0.08 )O ₃

The electromechanical properties of piezoelectric materials are influenced significantly by the defect chemistry, which is determined by the solid-solubility and charge-compensation mechanisms. In the present work, the effects of Sn⁴⁺ doping of lead-free (Ba_0.85Ca_0.15)([Ti_0.92-xSn_x]Zr_0.08)O₃ (x = 0.00-0.10) ceramics on these parameters and the resultant electromechanical properties and energy-storage efficiencies are reported. The complex nature of the solid solubility mechanisms as a function of dopant content is elucidated through comprehensive analyses of the structures, microstructures, and surface chemistry. The corresponding charge compensation mechanisms are determined by correlating these characterization data with corresponding defect equilibria, which then provide the basis for the interpretation of the mechanisms governing the electromechanical properties and energy-storage efficiencies. The combined data for the surface Ti oxidation state (XPS) and bulk unit cell volumes (XRD) for the three observed polymorphs (orthorhombic Pmm2, tetragonal P4mm, and cubic Pm3̄m) reveal interstitial solid solubility at low (0.00 ≤ x ≤ 0.04) and high (0.08 ≤ x ≤ 0.10) Sn⁴⁺ doping levels, with intermediate (0.04 < x < 0.08) Sn⁴⁺ doping levels exhibiting mixed interstitial-substitutional solid solubility. The trends in the electromechanical properties correlate directly with the solid solubility mechanisms, with two resultant inflections at x = 0.04 (maximal defect concentration) and x = 0.08 (minimal defect concentration). These mechanisms significantly influence the electromechanical properties, where maxima occur for polarization at x = 0.04, bipolar strain at x = 0.08, and energy storage efficiency at x = 0.10. The latter is notable because this parameter reaches >95% across the wide temperature range of 25°-130 °C.