Search For New Piezoelectrics
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1110-C01-07
Search For New Piezoelectrics P. Ganesh and R. E. Cohen Geophysical Laboratory, Carnegie Institution of Washington, Washington D.C. ABSTRACT Recent first principles simulations using density functional theory and novel low temperature xray diffraction experiments show the existence of a high pressure morphtotropic phase boundary (MPB) in pure PbTiO3. In this paper we apply chemical pressure by substituting smaller atoms in the ABO3 ‘A’ and ‘B’ sites. We find that the ground state of layered PbSnTiO3 (PSnT) is Pmm2, and for rocksalt SnGeTiO3 and PbGeTiO3 is R3m. The polarization of PbSnTiO3 is large (1.13,0,0)C/m2 and is due to the large Born effective charge of the small ‘Sn’ atom. We estimate the d33 for PSnT to be about 2400 pC/N, which is as large as that of currently used relaxor ferroelectrics. INTRODUCTION Piezoelectric single crystals such as PMN-PT (PbMg2/3Nb1/3O3-PbTiO3) [1] with huge electromechanical coupling have great potential as a new generation of transducer materials. Being complex solid solutions they do not melt congruently, so that crystal boule generally have a compositional range due to fractional crystallization and therefore increases the cost of producing single phase materials. There is a big effort to understand these systems at the fundamental level which is not only an interesting problem in materials and solid state physics, but can lead to the development of new materials with improved properties. Optimally, we would like to have pure compounds that melt congruently, or piezoelectric materials that work well as thin films. The newer families of piezoelectric single crystals have strains and electromechanical coupling up to ten times that of the most commonly used piezoelectric ceramic PZT (PbZrO3PbTiO3). The best piezoelectric materials are ferroelectric solid solutions, characterized by a morphotropic phase boundary (MPB) separating tetragonal (T) and rhombohedral (R) regions. In PZT the electromechanical properties peak at the MPB, but in single crystals a wide range of compositions on the rhombohedral side of the boundary have strong coupling. The boundary region is actually not a single phase transition, but contains one or more monoclinic (M) and possibly orthorhombic (O) phases, varying among different materials. [2-5] Perovskites have a high symmetry simple cubic high temperature parent structure, so that Slater considered the perovskite ferroelectrics to be materials that could be well understood even in 1950[6]. However, the very simplicity and flexibility of the perovskite structure leads to a wide range of sensitive properties that have been intensely studied for five decades. At a fundamental level, the ferroelectric perovskites are known to have competing interactions. Long-range Ewald forces drive off-centering of the atoms from their high symmetry, centrosymmetric sites in cubic perovskite, whereas short-range forces stabilize the high symmetry structure. Hybridization between O 2p – and B-cation d-states such as Ti 3d, and the lone-pair interactions and
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