Le nitrate double NaRb(2)(NO(3))(3), composé intermédiaire du système binaire isobare NaNO(3) + RbNO(3): études thermiques et cristallographiques

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 (α 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 = 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.