Google Search
A3: High temperature materials
Solid-state materials with high oxygen ion conductivity at moderate temperatures are of great importance and of much current scientific interest. At the same time studies to apply brownmillerites to thermoelectric systems are recently published [1]. Brownmillerite – type structure consists of alternating layers of FeO4 tetrahedra and FeO6. Due to the layered nature, the phonon scattering at the octahedral–tetrahedral interfaces could reduce thermal conductivity.
According to the recent study [2] the inelastic neutron scattering pattern of brownmillerites consists of the acoustic phonons emerging from the Bragg peaks and dominating low-frequency band of optical vibrations merging into acoustic modes. The DFT simulations show that this is a result of large displacements of apical oxygen atoms, which even able to migrate into the vacancy channels of the tetrahedral layer. The oxygen migration in Sr2Fe2O5 is based on structural instabilities and corresponding dynamical fluctuations of oxygen in the tetrahedral chains.
We recently grew the large single crystals of Sr2Fe2O5 and Ca2Fe2O5 by the floating-zone method and perform INS measurements of phonon dispersion curves. We found that acoustic transverse mode demonstrates linear behaviour at small phonon wave vectors and flattens up at energy of ~ 6 meV at q > 0.5. The optic mode has unusually low frequency at Brillouin zone centre. At phonon energies below ~ 4 meV some optical modes are overdamped and merge into acoustic intensity. In this regime phonons are dynamical fluctuations with large amplitude coupled with oxygen migration motions. This results in low thermal conductivity and gives some indications that fast oxygen transport at ambient conditions is phonon-assisted.
References
[1] E. Asenath-Smith, et al., J. of Sol. St. Chem., 183 (2010) 1670.
[2] W. Paulus, et al., J. Am. Chem. Soc., 130 (2008) 16080.