Influence of Cationic Substitutions in Na3Fe2(AsO4)3: Transition from the Garnet to the Alluaudite Structure

✍ Scribed by S. Khorari; A. Rulmont; P. Tarte

• Publisher: Elsevier Science
• Year: 1998
• Tongue: English
• Weight: 264 KB
• Volume: 137
• Category: Article
• ISSN: 0022-4596
• DOI: 10.1006/jssc.1997.7727

✦ Synopsis

The substitution in Na 3 Fe 2 (AsO 4 ) 3 of Na ؉ by either Li ؉ or Ca 2؉ and a vacancy, is able to stabilize an alluaudite structure at the expense of either the garnet or the rhombohedral structure of the parent compound. In the system (Na 1؊x Li x ) 3 Fe 2 (AsO 4 ) 3 , pure garnet phases are obtained at 700°C in the 04x40.333 composition range, whereas pure (or nearly so) alluaudite phases are obtained at 870°C in the 0.3334x40.666 composition range (namely, from Na 2 LiFe 2 (AsO 4 ) 3 to NaLi 2 Fe 2 (AsO 4 ) 3 ). In contrast, vacancies (as in (Na 1؊x ᮀ x/2 Ca x/2 ) 3 Fe 2 (AsO 4 ) 3 ) are hardly accepted in the garnet structure, whereas the range of formation of a pure alluaudite (at 870°C) is more extended (0.1664x40.666), the last composition corresponding to NaᮀCaFe 2 (AsO 4 ) 3 . The stabilization of an alluaudite structure at the expense of a garnet structure has already been observed in previous work and is discussed in relation with the structural role of lithium and vacancies.