Framework 'interstitial' oxygen in La(10)(GeO(4))(5-)(GeO(5))O(2) apatite electrolyte

Area of Science:

• Materials Science
• Solid-State Chemistry
• Electrochemistry

Background:

• Apatite materials exhibit oxygen conduction at low temperatures, making them promising solid electrolytes for solid-oxide fuel cells.
• The precise defect structures governing ion migration in these apatites, particularly inter-tunnel mobility, remain incompletely understood.

Purpose of the Study:

• To elucidate the defect structure responsible for oxygen ion migration in apatite electrolytes.
• To investigate the role of interstitial oxygen and its diffusion mechanism in La(10)(GeO(4))(6)O(3) apatite.

Main Methods:

• Neutron powder diffraction was employed to analyze the crystal structure of stoichiometric La(10)(GeO(4))(6)O(3).
• Structural analysis focused on identifying the location and concentration of oxygen defects and their impact on ion mobility.

Main Results:

• The stoichiometric compound is better described as La(10)(GeO(4))(5-)(GeO(5))O(2) apatite, indicating a significant concentration of interstitial oxygen within channel walls.
• Interstitial oxygen atoms are proposed to act as a reservoir for migrating ions, facilitating inter-tunnel oxygen diffusion.

Conclusions:

• The migration mechanism involves transient conversion of GeO(4) tetrahedra to GeO(5) trigonal bipyramids, driven by interstitial oxygen.
• This structural flexibility may be a general characteristic of oxide apatites, impacting their performance as solid electrolytes.