Raman spectroscopy studies of apatite-type germanate oxide ion conductors: correlation with interstitial oxide ion location and conduction

Abstract

A Raman spectroscopy study of the apatite series La₈₊xBa_2−x(GeO₄)₆O_2+x/2 is presented. The results show the presence of a new Raman band appearing at ∼645 cm⁻¹, whose intensity increases with increasing interstitial oxide ion content. This new band is also observed in samples containing cation vacancies, consistent with previous suggestions that the presence of cation vacancies enhances Frenkel-type defect formation. The fact that the new band is in the stretching region of the spectra, rather than the bending region as observed for the silicate analogues, is consistent with the interstitial oxide ions being more closely associated with the Ge. This band is attributed to the presence of interstitial oxide ions leading to the formation of five coordinate Ge, in agreement with recent neutron diffraction and modelling studies. From the observation of a reduction in the intensity of this band with increasing temperature, it is suggested that the activation energy for conduction in these apatite germanates is a combination of the energy to “free” the interstitial oxide ions from the five coordinate Ge, and the energy for their subsequent migration. The former process is ascribed to the observed reduction in Raman intensity with an activation energy of 0.32 ± 0.06 eV. Thus the higher activation energy for the germanate apatites over the related silicates can be ascribed to the defect trapping associated with the closer association of the interstitial oxide ion with the tetrahedra in the former.