Separation of K + and Bi 3+ 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 1/3 Nb 2/3 )O 3 and ferroelectric PbTiO 3 . Despite its impressive properties, this system has limited analogy. We present the compositional exploration of the Pb-free analogue (1-x)(K 1/2 Bi 1/2 )(Mg 1/3 Nb 2/3 )O 3 -x(K 1/2 Bi 1/2 )TiO 3 (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 (d 33 *) of 192 pm V —1 from strain measurements. Diffraction methods reveal disordered displacements of K + and Bi 3+ which persist from the KBMN endmember through multiple changes in symmetry. Rearrangement of the Bi 3+ 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 Bi 3+ bonding configuration. This work yields a strong analogy to the Pb-based analogue, and provides strategies for further optimization.