Low-energy excitations in brownmillerites and related oxides

It is generally believed that the glasses and crystals differ very much from each other by their vibrational spectra. However, the crystalline systems with frozen disorder may exhibit the vibrational density of states (DOS) peaked in low-energy region quite resembling to the Bose peak typically observed in glasses. Common origin of these features in disordered crystalline and amorphous systems is discussed in the frameworks of two theoretical concepts: the soft potential and the broken ladder models. The experimental results are presented for large series of standard, oxygen deficient, layered perovskite-based systems and systems containing the metal-oxygen chains. They were studied with using the nuclear (Fe-57) inelastic scattering of synchrotron radiation. The largest excess of vibrational density of states with respect to the Debye-like one was found in the brownmillerite CaSrFcCoO(5) with the peak energy 7.5 meV. The peak in E-2 -weighted DOS g(E)/E-2 exceeds the Debye behavior (g(E)infinityE(2)) by similar or equal to 4 times at room temperature and grows at cooling even further. (C) 2004 Elsevier B.V. All rights reserved.