Structural instability of the CoO4 tetrahedral chain in SrCoO3−δ thin films

Abstract

Raman scattering experiments together with detailed lattice dynamic calculations are performed to elucidate crystallographic and electronic peculiarities of SrCoO3−δ 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.