Phonon density of states of model ferroelectrics
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1262-W01-02
Phonon density of states of model ferroelectrics
Narayani Choudhury1, 5, Alexander I. Kolesnikov2, Helmut Schober3,6, Eric J. Walter4, Mark Johnson3, Douglas L. Abernathy2, Matthew S. Lucas2 1
Dept. of Physics, University of Arkansas, Fayetteville, AR 72701, USA. Neutron Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA. 3 Institut Laue-Langevin, 38042 Grenoble, Cedex 9, France. 4 Dept. of Physics, College of William and Mary, Williamsburg, VA 23185, USA. 5 Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India. 6 Université Joseph Fourier, UFR de Physique, 38041 Grenoble, Cedex 9, France 2
ABSTRACT First principles density functional calculations and inelastic neutron scattering measurements have been used to study the variations of the phonon density of states of PbTiO3 and SrTiO3 as a function of temperature. The phonon spectra of the quantum paraelectric SrTiO3 is found to be fundamentally distinct from those of ferroelectric PbTiO3 and BaTiO3. SrTiO3 has a large 70-90 meV phonon band-gap in both the low temperature antiferrodistortive tetragonal phase and in the high temperature cubic phase. Key bonding changes in these perovskites lead to spectacular differences in their observed phonon density of states.
INTRODUCTION Ferroelectric materials interconvert electrical and mechanical energies and find important applications as piezoelectric transducers and actuators, pyroelectric arrays, non-volatile memories, dielectrics for microelectronics and wireless communication, non-linear optical applications, medical imaging, etc. Oxide perovskites like PbTiO3, BaTiO3 and SrTiO3 are well studied [1-10] model ferroelectrics, due to their fundamental interest as well as due to their technological relevance. High voltage photovoltaic effects in ABO3 perovskites have also been reported [4,5] which suggest possible applications of these oxides as solar energy devices. SrTiO3 is an incipient ferroelectric with a very large static dielectric response which exhibits unusual phonon anomalies and anomalous electrostrictive response. The phonon instabilities in these perovskites have an important bearing on their piezoelectric and dielectric properties. While the ferrodistortive modes involve zone center phonon instabilities, the antiferrodistortive phases engage zone boundary modes involving rotations of the TiO6 octahedra [10]. Accurate characterization of the phonon modes in the entire Brillouin zone of these perovskites is therefore highly desirable. These perovskites have a rich phase diagram and their material properties are found to be strongly influenced by their crystal structure and bonding characteristics [1,2]. Both PbTiO3 and BaTiO3 have a simple cubic high temperature paraelectric phase which transforms to a ferroelectric tetragonal phase around 763 K and 403 K, respectively. Tetragonal PbTiO3 is a large strain material (c/a=1.06) exhibiting ferroelectricity up to high temperatures, and it has a single cubic to tetragonal transition. The spontaneous
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