Defects and disorder in apatite-type silicate oxide ion conductors: implications for conductivity

• Published in: Journal of materials chemistry. A, Materials for energy and sustainability Vol. 1; no. 27; pp. 14576 - 14584
• Main Authors: Fuller, Chloe A, Gutmann, Matthias J, Ling, Chris D, Wang, Chun-Hai, Zhang, Weiguo, Halasyamani, P. Shiv, Evans, Ivana Radosavljevic, Evans, John S. O
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 12.07.2022
• Subjects: Apatite; Composition; Conductors; Crystal defects; Crystal growth; Crystal structure; Crystals; Diffraction patterns; Neutron diffraction; Second harmonic generation; Single crystals; Structural models
• ISSN: 2050-7488; 2050-7496
• DOI: 10.1039/d2ta02328h

The structure and composition of La-silicate apatite oxide ion conductor, La 10 Si 6 O 27 , in which the types of defects it contains are controversial, has been thoroughly investigated. Large crystals were grown using the floating zone method, and their structure was characterised by X-ray and neutron single crystal diffraction and second harmonic generation measurements. Structure refinements reveal extensive positional disorder on the oxygen substructure, the presence of La vacancies and no evidence of sites occupied by excess oxide ions. The actual composition of the crystal was determined through SEM-EDX and density measurements and was found to be La 9.64 Si 5.77 O 26 , differing significantly from the nominal composition but in agreement with the structure refinements. Structural models considering the actual composition, the disorder on the O sublattice, and local relaxations around these defects were able to reproduce the key features of the diffuse scattering observed in single crystal neutron diffraction patterns. Extensive diffuse scatter observed by single crystal neutron diffraction was modelled to gain insight into structural distortions in apatite-type lanthanum silicate.