Effect of oxygen content on the 29Si NMR, Raman spectra and oxide ion conductivity of the apatite series, La8+xSr2−x(SiO4)6O2+x/2

Abstract

29Si NMR data have been recorded for the apatite series La8+xSr2-x(SiO4)6O2+x/2 (0 < or = x < or = 1.0). For x = 0, a single NMR peak is observed at a chemical shift of approximately -77 ppm, while as the La : Sr ratio and hence interstitial oxygen content is increased, a second peak at a chemical shift of approximately -80 ppm is observed, which has been attributed to silicate groups neighbouring interstitial oxide ions. An increase in the intensity of this second peak is observed with increasing x, consistent with an increase in interstitial oxide ion content, and the data are used to estimate the level of interstitial oxide ions, and hence Frenkel-type disorder in these materials. The increase in second 29Si NMR peak intensity/interstitial oxide ion content is also shown to correlate with an increase in conductivity. The effect of interstitial oxygen content can also be studied by means of Raman spectroscopy, with a new mode at 360 cm(-1) appearing for samples with x > 0 in the symmetric bending mode energy region of the SiO4 group. The intensity of this mode increases with increasing oxygen content, yielding results comparable to those from the NMR studies, showing the complementarities of the two techniques.

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

“…For the silicates it has been shown that oxygen excess in the range, 0 ≤ x ≤ 0.5 is possible, while higher oxygen content, 0 ≤ x ≤ 1.0 has been achieved for the germanates [13,18,20,21]. For the silicate series, the exact location of extrastoichiometric oxygen, x, has attracted some controversy, with reports of interstitial sites close to the SiO4 tetrahedra, while in other studies, mobile oxygen at the centre of the channels has been claimed [4,8,9,10,12,13,23,27,35,36]. For the germanates, the situation appears more clearcut, with computer modeling, neutron diffraction and Raman studies indicating occupancy by oxygen sites close to the GeO4 tetrahedra leading to the formation of five coordinate Ge [10, 20-22, 37, 40].…”

Neutron diffraction structural study of the apatite-type oxide ion conductor, La8Y2Ge6O27: location of the interstitial oxide ion site

“…In contrast to the traditional fluorite and perovskite-type oxide ion conductors, where a conduction mechanism via the presence of oxide ion vacancies has long been universally accepted, the conduction mechanism of the apatite systems has been more controversial. Initial it was assumed that these too were oxide ion vacancy conductors, although there is now an established body of evidence in support of a mechanism involving interstitial oxide ions: these interstitial oxide ions can be either present as oxygen hyperstoichiometry (x>0) or Frenkel defects [4,9,10,12,13,[20][21][22][23]28]. Despite the current acceptance of an interstitial oxide ion conduction mechanism, the location of these interstitial oxide ions and their migration pathway is still somewhat controversial [see review articles [13] and references therein], which can be related to the complex structural features of these apatite systems.…”

“…This difficulty in conclusively locating the interstitial site can be attributed to the low interstitial oxide ion contents and the considerable local distortions that arise on the introduction of an interstitial oxide ion defect. Nevertheless, there is growing support for the second of these models, initially proposed by atomistic modelling studies, and with a number of supporting experimental reports, including diffraction [10,16,20,21], Mössbauer [11], NMR and Raman studies [12,23].…”

Formation of apatite oxynitrides by the reaction between apatite-type oxide ion conductors, La8+xSr2−x(Si/Ge)6O26+x/2, and ammonia

“…In particular, previous work has shown that for the series, La 8+x Sr 2-x Si 6 O 26+x/2 , the 29 Si NMR spectrum for x=0 has a single peak at δ≈-77 ppm, while for x>0, a second peak is observed at δ≈-80 ppm, attributed to a silicate unit neighbouring an interstitial site [17,27]. In contrast the nitrided samples showed no evidence for this second peak in the sample with x=0.5, while for the x=1 sample, the peak intensity was greatly reduced [8].…”

“…Their structure may be described in terms of an (Ln/A) 4-x (Si/GeO 4 ) 6 framework, with the remaining (La/A) 6 O 2 units accommodated in the channels within the 4 framework. In these systems, the oxide ion conduction process has been shown to proceed via a mechanism involving interstitial oxide ions, which are present as a result of either oxygen hyperstoichiometry (x>0) in La 8+x Sr 2-x (Si/Ge) 6 With regard to the location of the interstitial oxide ions, computer modelling studies have predicted that for the silicates the most favourable interstitial oxide ion site is at the periphery of the oxide ion channels neighbouring the SiO 4 groups, which has been supported by neutron diffraction, 29 Si NMR, and Raman studies [14,15,17,27,36]. For the germanates, both modelling and neutron diffraction studies also indicate an interstitial position neighbouring the GeO 4 tetrahedra, although in this case the interstitial oxide ion is more closely associated, leading to five coordinate Ge [21,26].…”