title: The influence of P/As substitution in the melilite-like Na2Co(P2-xAsx)O7(x = 0.40 and 0.93) solid solutions.
abstract: To investigate the influence of P/As substitution on structures and electrical properties, e.g. the effect on material densities, two new solid P/As-doped solutions, Na2CoP1.60As0.40O7(disodium cobalt diphosphorus arsenic heptaoxide) and Na2CoP1.07As0.93O7(disodium cobalt phosphorus arsenic heptaoxide), with melilite-like structures have been synthesized by solid-state reactions. Their unit-cell parameters are in agreement with Vegard's law. The obtained structural models were investigated by the bond valence sum (BVS) and charge distribution (CHARDI) validation tools and, for the latter, the structures are described as being built on anion-centred polyhedra. The frameworks can be described as layered and formed by {[Co(P,As)2O7]2-}∞slabs, with alkali cations sandwiched between the layers and with the interlayer spaces increased due to P/As substitution. The BVS model was extended to a preliminary simulation of the sodium conduction properties in the studied structural type and suggests that the most probable sodium conduction pathways are bidimensional, at the (002) planes.
authors.affiliation: Université de Tunis El Manar, Faculté des Sciences, Laboratoire de Matériaux, Cristallochimie et Thermodynamique Appliquée, El Manar II, 2092 Tunis, Tunisia.
authors.forename: Chokri
authors.initials: C
authors.lastname: Issaoui
authors.affiliation: Université de Tunis El Manar, Faculté des Sciences, Laboratoire de Matériaux, Cristallochimie et Thermodynamique Appliquée, El Manar II, 2092 Tunis, Tunisia.
authors.forename: Hammouda
authors.initials: H
authors.lastname: Chebbi
authors.affiliation: Université de Tunis El Manar, Faculté des Sciences, Laboratoire de Matériaux, Cristallochimie et Thermodynamique Appliquée, El Manar II, 2092 Tunis, Tunisia.
authors.forename: Abderrahmen
authors.initials: A
authors.lastname: Guesmi
keywords: Na2Co(P,As)2O7; P/As substitution; bidimensional conduction pathways; bond valence sum (BVS); charge distribution (CHARDI); crystal structure; sodium; sodium conduction properties; solid solutions