Structural instability of the CoO4 tetrahedral chain in SrCoO3-delta thin films

Abstract

Raman scattering experiments together with detailed lattice dynamic calculations are performed to elucidate crystallographic and electronic peculiarities of SrCoO3-delta films. We observe that the 85 cm(-1) phonon mode involving the rotation of a CoO4 tetrahedron undergoes a huge hardening by 21 cm(-1) with decreasing temperature. In addition, new phonon modes appear at 651.5 and 697.6 cm(-1). The latter modes are attributed to the Jahn-Teller activated modes. Upon cooling from room temperature, all phonons exhibit an exponential-like increase of intensity with a characteristic energy of about 103-107 K. We attribute this phenomenon to an instability of the CoO4 tetrahedral chain structure, which constitutes a key ingredient to understand the electronic and structural properties of the brownmillerite SrCoO2.5. (C) 2015 AIP Publishing LLC.