A study of possible extra-framework cation ordering in Pbca leucite structures with stoichiometry RbCsX2+Si5O12 (X = Mg, Ni, Cd). (September 2019)

• Record Type: Journal Article
• Authors: Bell, Anthony M. T.; Henderson, C. Michael B.
• Is Part Of: Powder diffraction. Volume 34(2019)Supplement 1
• Page Start: S2
• Page End: S7
• Publication Date: 2019-09
• Subjects: X-ray powder diffraction, -- Rietveld refinement, -- silicate framework minerals; Powder metallurgy -- Periodicals; Materials -- Analysis -- Periodicals; 620.1127
• DOI: 10.1017/S0885715619000071
• Languages: English
• ISSNs: 0885-7156

Abstract

Abstract : Leucites are silicate framework structures with some of the silicon framework cations partially replaced by divalent or trivalent cations. A monovalent extraframework alkali metal cation is also incorporated to balance the charges. We have previously reported Pbca leucite structures with the stoichiometries Cs2 X 2+ Si5 O12 ( X = Mg, Mn, Co, Ni, Cu, Zn, Cd) and Rb2 X 2+ Si5 O12 ( X = Mg, Mn, Ni, Cd). These orthorhombic leucite structures have all the silicon and non-silicon framework cations completely ordered onto separate crystallographic sites. This structure has five distinct Si sites and 1 X site; there are also two distinct sites for the extra-framework Cs or Rb. We have recently synthesised leucite analogues with two different extra-framework cations, these have the stoichiometry RbCs X 2+ Si5 O12 ( X = Mg, Ni, Cd). The initial Rietveld refinements assumed 50% Cs and 50% Rb on each of the two extra-framework cation sites. The refined structures for X = Ni and Cd have (within error limits) complete extra-framework cation site disorder. However, for X = Mg there is partial ordering of the extra-framework cation sites, the site occupancies are:- Cs1 0.37(3), Rb1 0.63(3), Cs2 0.63(3), Rb2 0.37(3).