title: Le nitrate double NaRb2(NO3)3, composé intermédiaire du système binaire isobare NaNO3 + RbNO3: études thermiques et cristallographiques.
abstract: Crystallographic and thermodynamic investigations of the binary (NaNO3 + RbNO3) phase diagram at atmospheric pressure reveal the existence of an inter-mediate compound NaRb2(NO3)3 (sodium dirubidium trinitrate) previously predicted and now reported experimentally for the first time. According to a DSC analysis, the compound exhibits three allotropic forms. In its low-temperature allotropic form (α form, ortho-rhom-bic) there are two Rb (m.. site symmetry) and one Na (m..) independent crystallographic positions and three planar nitrate groups. The bond-valence-sum calculations for all atoms agree well with their oxidation states. The Rb cations are located in the (100) plane at x = ½ with 11 oxygen coordination. The Na ones are in the same plane at x = 0 and are coordinated to eight O atoms from six nitrate groups. The charge-distribution method has been used to evaluate the degree of distortion of the alkali polyhedra.
authors.affiliation: Université de Tunis El Manar, Faculté des Sciences, Laboratoire de Thermodynamique appliquée, El Manar II, 2092 Tunis, Tunisie.
authors.forename: Nesrine
authors.initials: N
authors.lastname: Ksiksi
authors.affiliation: Université de Tunis El Manar, Faculté des Sciences, Laboratoire de Matériaux et Cristallochimie, El Manar II, 2092 Tunis, Tunisie ; Université de Tunis El Manar, Institut Préparatoire aux Etudes d'Ingénieurs d'El Manar, El Manar II, 2092 Tunis, Tunisie.
authors.forename: Mohamed
authors.initials: M
authors.lastname: Driss
authors.affiliation: Université de Tunis El Manar, Faculté des Sciences, Laboratoire de Thermodynamique appliquée, El Manar II, 2092 Tunis, Tunisie.
authors.forename: Dalila
authors.initials: D
authors.lastname: Hellali
authors.affiliation: Université de Tunis El Manar, Faculté des Sciences, Laboratoire de Matériaux et Cristallochimie, El Manar II, 2092 Tunis, Tunisie ; Université de Tunis El Manar, Institut Préparatoire aux Etudes d'Ingénieurs d'El Manar, El Manar II, 2092 Tunis, Tunisie.
authors.forename: Abderrahmen
authors.initials: A
authors.lastname: Guesmi
authors.affiliation: Université de Tunis El Manar, Faculté des Sciences, Laboratoire de Thermodynamique appliquée, El Manar II, 2092 Tunis, Tunisie.
authors.forename: Hmida
authors.initials: H
authors.lastname: Zamali
keywords: NaNO3 + RbNO3 phase diagram; bond-valence sum; charge-distribution method; crystal structure