An Inelastic X-ray Study of the Ferroelectric Phase Transition in SnTe

SnTe was originally studied in the context of lattice vibrations in diatomic lattices [1]. There has been a recent renewal of interest due to its identification as a crystalline topological insulator, which is related to its room temperature {\em fcc} rocksalt structure [2]. However, the material undergoes a phase transition to a rhombohedral structure upon cooling, strongly affecting its topological states. While the transition is predicted to be a displacive ferroelectric transition, no ferroelectric response has previously been seen due to free carrier screening. We report inelastic x-ray measurements showing the low energy transverse-optic phonon soften to near zero energy at the structural transition [3]. Importantly, the energy of this mode increases again at temperatures below the transition temperature, proving SnTe undergoes a ferroelectric displacement. Density functional calculations that account for the ferroelectric transition and phonon linewidth changes consistent with anharmonic coupling will also be discussed. [1] G. S. Pawley, W. Cochran, R. A. Cowley, and G. Dolling, Phys. Rev. Lett. 17, 753 (1966). [2] T. Hsieh, H. Lin, et al. Nat Comm 3, 982 (2012). [3] C.D. O'Neill, D. Sokolov, A. Hermann, A. Bossak, C. Stock and A.D. Huxley. (submitted)