Phonon density of states of model ferroelectrics Narayani Choudhury 1, 5 , Alexander I. Kolesnikov 2 , Helmut Schober 3,6 , Eric J. Walter 4 , Mark Johnson 3 , Douglas L. Abernathy 2 , Matthew S. Lucas 2 1 Dept. of Physics, University of Arkansas, Fayetteville, AR 72701, USA. 2 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 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 PbTiO 3 and SrTiO 3 as a function of temperature. The phonon spectra of the quantum paraelectric SrTiO 3 is found to be fundamentally distinct from those of ferroelectric PbTiO 3 and BaTiO 3 . SrTiO 3 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 PbTiO 3 , BaTiO 3 and SrTiO 3 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 ABO 3 perovskites have also been reported [4,5] which suggest possible applications of these oxides as solar energy devices. SrTiO 3 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 TiO 6 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 PbTiO 3 and BaTiO 3 have a simple cubic high temperature paraelectric phase which transforms to a ferroelectric tetragonal phase around 763 K and 403 K, respectively. Tetragonal PbTiO 3 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 polarization of PbTiO 3 at room temperature is Mater. Res. Soc. Symp. Proc. Vol. 1262 © 2010 Materials Research Society 1262-W01-02