Separation of K⁺ and Bi³⁺ displacements in a Pb-free, monoclinic piezoelectric at the morphotropic phase boundary

The best piezoelectric properties of any perovskite oxide known are found in the solid solution of the relaxor Pb(Mg_1/3Nb_2/3)O₃ and ferroelectric PbTiO₃. Despite its impressive properties, this system has limited analogy. We present the compositional exploration of the Pb-free analogue (1-x)(K_1/2Bi_1/2)(Mg_1/3Nb_2/3)O₃-x(K_1/2Bi_1/2)TiO₃ (KBMN-KBT). We locate the morphotropic phase boundary between x = 0.86 and 0.88 changing from Cm to Pm symmetry and the optimally performing composition at x = 0.88. We report a piezoelectric figure of merit (d₃₃*) of 192 pm V⁻¹ from strain measurements. Diffraction methods reveal disordered displacements of K⁺ and Bi³⁺ which persist from the KBMN endmember through multiple changes in symmetry. Rearrangement of the Bi³⁺ displacements along the uncommon [011]_c direction drives the physical response. Ferroelectric, dielectric, and piezoresponse force microscopy are used to study the progression of physical properties through the MPB and attribute the mechanism to a polarization rotation. Taking account for local, short-range, and average structural features yield a balanced perspective on the structure and properties of this system, isolating the driving force within this system to the Bi³⁺ bonding configuration. This work yields a strong analogy to the Pb-based analogue, and provides strategies for further optimization.