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 (NaNO 3 + RbNO 3) phase diagram at atmospheric pressure reveal the existence of an intermediate compound NaRb 2 (NO 3) 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 (a form, orthorhombic) 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 = 1 2 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.