Structural instability of the CoO₄ tetrahedral chain in SrCoO_3-δ thin films

Raman scattering experiments together with detailed lattice dynamic calculations are performed to elucidate crystallographic and electronic peculiarities of SrCoO_3-δ films. We observe that the 85 cm^-1 phonon mode involving the rotation of a CoO₄ tetrahedron undergoes a hardening by 21 cm^-1 when the temperature is decreased. 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 SrCoO_2.5