Temperature dependence of the optical phonons in SrMnO(3) manganite: Evidence of a low-temperature structural transition in the hexagonal compound

Infrared and Raman spectroscopies were employed to study the temperature dependence of the optical phonons of the SrMnO3 manganite in both the hexagonal and the cubic phase. Changes in the phonon spectrum of the hexagonal SrMnO3 four-layer structure observed on decreasing temperature indicate that a subtle structural transition occurs in the system between 200 and 100 K. We propose that the transition consists in a change from the four-layer to a six-layer structure, in which the high-temperature crystal symmetry is retained.