A Mixed α/β Superstructure in NASICON Ionic Conductors: Neutron Diffraction Study of Li2FeTi(PO4)3 and Li2FeZr(PO4)3

✍ Scribed by Michele Catti

• Publisher: Elsevier Science
• Year: 2001
• Tongue: English
• Weight: 298 KB
• Volume: 156
• Category: Article
• ISSN: 0022-4596
• DOI: 10.1006/jssc.2000.8999

✦ Synopsis

The lithium conductors Li 2 FeTi(PO 4 ) 3 and Li 2 FeZr(PO 4 ) 3 , synthesized by solid-state reaction and characterized by X-ray powder di4ractometry, were studied structurally at room temperature by neutron powder di4raction at high resolution (HRPD, ISIS Facility, U.K.). By trial-and-error and Rietveld re5nements (R p ‫؍‬ 0.111, R(F 2 ) ‫؍‬ 0.112), the 5rst compound (orthorhombic Pbca, Z ‫؍‬ 8; a ‫؍‬ 8.5515(1), b ‫؍‬ 8.6229(1), c ‫؍‬ 23.9116(3) A > ) was shown to have a complex superstructure sharing features of both the and NASICON-type phases of LiZr 2 (PO 4 ) 3 . Four (001) layers of PO 4 and (Fe, Ti)O 6 polyhedra are present per unit-cell, and they are related both by 1 inversion centers ( structure) and by a glide planes ( structure). Ti 4؉ and Fe 3؉ order in the two interlayer regions, respectively. Owing to the structure complexity, only half of the lithium atoms could be re5ned in tetrahedral coordination with 1Li+O2 ‫؍‬ 1.99 A > . Li 2 FeZr(PO 4 ) 3 (orthorhombic Pbna, Z ‫؍‬ 4; a ‫؍‬ 8.70559(8), b ‫؍‬ 8.78572(9), c ‫؍‬ 12.2202(1) A > ) proved to be similar to -LiZr 2 (PO 4 ) 3 ; however, by Fourier synthesis and Rietveld re5nement (R p ‫؍‬ 0.0618, R(F 2 ) ‫؍‬ 0.0574) Li was located in a fully ordered tetrahedral con5guration with 1Li+O2 ‫؍‬ 2.01 A > , instead of being disordered as in the phase of LiZr 2 (PO 4 ) 3 .

Academic Press