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

Abstract

Recently, apatite‐type germanates La9.33+xGe6O26+3x/2 have attracted considerable interest due to their high oxide ion conductivities. Research has shown that the key defects are oxide ion interstitials which lead to the conversion of some of the GeO4 units to GeO5. Consequently there has been a large interest in the preparation of high oxygen excess samples with high defect concentration. This strategy, however, leads to a reduction in symmetry from hexagonal to triclinic for x > 0.4, and consequently to reduced oxide ion conductivity at low temperatures. We present doping strategies to stabilise the hexagonal lattice, while maintaining high oxygen content. In particular, partial substitution of La by smaller rare earths (Y, Yb) is shown to be successful in preparing x = 0.67 samples with hexagonal symmetry and hence high conductivities. In addition, doping on the Ge site with Ti, Nb or W, has been shown to be similarly successful, leading to very high oxygen contents for W doping, e.g. La10Ge5.5W0.5O27.5. In the case of Ti doping, however, there was some evidence for trapping of the interstitial oxide ions around the Ti. Preliminary results on the effect of similar doping strategies on Pr, Nd germanates (Pr/Nd)9.33+xGe6O26+3x/2, are also discussed.

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

“…This anomaly clearly needs more study, although it is suggested that it may be related to interstitial defect trapping by the dopant leading to the formation of MO 5 units and consequent local relief of lattice strain by the dopant. The Raman studies, reported here, show clear evidence for this defect trapping in the case of W, while such defect trapping has also been inferred from conductivity studies for the series La 10 Ge 6-x Ti x O 27 [44].…”

Gallium-doped lanthanum germanates as electrolyte material of solid oxide fuel cells

“…16) The electrical conductivity of the La 9.5 Ge 5.7 Ga 0.3 O 26.1 ceramic sintered at 1450°C, with respect to temperature, appeared to be lower than that of an 8YSZ ceramic 16) at a temperature below 700°C. As the temperature rose above 700°C, the former displayed higher electrical conductivity values than the latter, which reached 4.02 © 10 ¹2 Scm ¹1 at 800°C.…”

“…16 2) Different from the major ion conduction mechanism in fluorite-or perovskite-type materials closely relating to oxygen vacancy, it is thought that the conduction of the apatite materials occurs via interstitial oxide ions. The hexagonal structure is composed of isolated MO 4 tetrahedra-forming channels running parallel to the c-axis, with A cations located in two cavity sites, one 7 coordinate and one 9 coordinate, where the loosely bounded oxideconducting anions are located.…”

Ontologies and Databases – Knowledge Engineering for Materials Informatics

“…For the selection of the compounds and the crystal symmetry (space group) of the compounds the literature references are Shannon and Prewitt (1976), Rabe et al (1992), Leon-Reina et al (2003, Sansom et al (2005), Kendrick and Slater (2008aÀc), Pramana et al (2008), and Orera et al (2011). The input data for each compound was the following: • At each of three sites (Lanthanum, Germanium, and Oxygen), a quantitative measurement of each of four properties (Zunger's Pseudopotential Core Radii Sum, MartynovÀBatsanov Electronegativity, Valence Electron Number, and Shannon's Ionic Radius) for a total of 11 measurements per compound (one property, Shannon's Ionic Radius, did not apply at the Oxygen site).…”