Enhancement of Ionic Conductivity of Apatite‐Type Lanthanum Silicates Doped With Cations

Abstract

The oxide ion conductivity of apatite‐type lanthanum silicates has been enhanced by cation doping into both La and Si sites. The Sr and Nd fill vacancies on the La site by creating excess oxide ions and increasing the conductivity. Al and Mg, which substitute for Si at a fixed oxygen content, result in an enhancement of conductivity, accompanied by lattice enlargement. The Mg‐doped lanthanum silicate, La9.533(Si5.7Mg0.3)O26, has been shown to have the highest conductivity of 4.4 × 10−2 S/cm at 800°C. The conduction channel consists of La triangles and O triangles, and the channel sizes have been calculated from the Rietveld structure analysis of the powder X‐ray diffraction patterns. Expansions of the O triangles have been observed when introducing excess oxide ions by filling vacancies on the La site or by the substitution of Si using dopants.

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].…”

“…In order to gain information about the location of the N in the structure, 29 Si NMR data were measured for the La 8+x Sr 2-x Si 6 O 26+x/2 samples after heating at 800 ○ C in ammonia.…”

“…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].…”

“…This leads in an increase in mass, since 3O 2-will replace 2N 3-, and from the mass change observed the N content could then be determined. 29 Si NMR data for La 8+x Sr 2-x Si 6 O 26+x/2 samples after heating in NH 3 at 800 ○ C were recorded using a Varian Unity Inova spectrometer operating at 59.56 MHz. A directpolarisation experiment was used with recycle delays of 5s (x=1), 10s (x=0.5), 60s (x=0).…”

“…Structural (neutron diffraction, 29 Si NMR) data for the sample heated at 950 ○ C in NH 3 showed no evidence for the presence of interstitial oxide ions, but rather the partial replacement of O by N (3 O 2-replaced by 2N 3-) leads to a reduction in the total anion content, and hence anion vacancies. Considering the importance of interstitial oxide ions to the conductivity of these materials, the work hence suggested that nitridation would be detrimental to the conductivity.…”