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

Apatite germanates doped with tungsten: synthesis, structure, and conductivity

“…The similar high temperature conductivities (σ ≈ 0.02 S cm -1 at 820°C) of the x=0.1, and 0.5 samples is interesting, and suggests that either there is an upper limit for the magnitude of the oxide ion conductivity with increasing oxygen content, such that there is an optimum oxygen interstitial concentration, or there could be partial trapping of the interstitial oxide ion defects by W, which counterbalances the effect of the increase in oxygen content. These spectra are very similar to those of La 8+x Ba 2-x Ge 6 O 26+x/2 [40]. In particular, they present the band at 645 cm -1 whose intensity was correlated with interstitial oxygen content in Ref [40].…”

Novel Aspects of the Conduction Mechanisms of Electrolytes Containing Tetrahedral Moieties

“…These calculations supported the presence of the above conduction pathway, along with the mechanism of Frenkel defect formation. However, this MD work also highlighted that all the oxide ions were mobile, with an activation energy of 0.98 eV, decreasing to 0.61 eV at high temperatures, consistent with a trapping energy at low temperatures of ≈0.37 eV, in agreement with subsequent Raman studies, where a similar value was proposed [28]. In addition, to the fan-like pathway along the c-direction, the MD work showed that there was significant conduction perpendicular to the c-direction.…”

“…Interestingly, the modelling suggested two equally favourable interstitial sites, at the periphery of the La 6 O 2 channel or between two GeO 4 tetrahedra. However, a close inspection of the results showed that both give identical relaxed configurations leading to a pseudo 'Ga 2 O 9 ' unit ( Figure 1), the presence of this unit supported by Raman studies [9,28]. Similarly structural refinements of ger-…”

“…These materials however also contain extra oxygen (3x/2), and the location of this extra oxygen has proved non-trivial due to the substantial local distortions around the interstitial oxide ion defect. Nevertheless, modelling work and a range of experimental studies (neutron diffraction, Raman spectroscopy) now indicate that the interstitial oxide ions are associated with the framework giving rise to five coordinate Ge [9,[23][24][25][26][27][28]. Computer modelling work was performed on the nominally oxygen stoichiometric La 9.33 (GeO 4 ) 6 O 2 system, with the results indicating that the most favourable intrinsic defect was the oxygen Frenkel defect, where an oxygen in the channel moves to the periphery to create an interstitial defect.…”

Strategies for the Optimisation of the Oxide Ion Conductivities of Apatite‐Type Germanates

“…In addition to the enhanced low temperature conductivity, the conductivity plot shows a decrease in activation energy at high temperatures (>600°C). The higher activation energy at low temperatures is most likely related to an additional contribution from the trapping energy associated with the close associated of the interstitial oxide ions and the GeO 4 / NbO 4 units leading to the formation of GeO 5 (and possibly NbO 5 ) units [14,36]. Raman studies on the series La 10-x Ba xGe 6 O 27-x/2 had suggested a trapping energy of 0.32 ± 0.06 eV [36], which is close to the difference between high temperature and low temperature activation energies observed here (0.23 eV).…”