Crystal structures and cation ordering in Cs₂MgSi₅O₁₂, Rb₂MgSi₅O₁₂ and Cs₂ZnSi₅O₁₂ leucites

Abstract

The crystal structures of the leucite analogues Cs(2)MgSi(5)O(12), Cs(2)ZnSi(5)O(12) and Rb(2)MgSi(5)O(12) have been determined by synchrotron X-ray powder diffraction using Rietveld refinement in conjunction with (29)Si MAS NMR spectroscopy. These leucites are framework structures with distinct tetrahedral sites (T sites) occupied by Si and a divalent cation (either Mg or Zn in these samples); there is also a monovalent extra-framework cation (either Cs or Rb in these samples). The refined crystal structures were based on the Pbca leucite structure of Cs(2)CdSi(5)O(12), thus a framework with five ordered Si T sites and one ordered Cd T site was used as the starting model for refinement. (29)Si MAS NMR shows five distinct Si T sites for Cs(2)MgSi(5)O(12) and Rb(2)MgSi(5)O(12), but six Si T sites for Cs(2)ZnSi(5)O(12). The refined structures for Cs(2)MgSi(5)O(12) and Rb(2)MgSi(5)O(12) were determined with complete T-site ordering, but the refined structure for Cs(2)ZnSi(5)O(12) was determined with partial disorder of Mg and Si over two of the T sites.

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

“…All are isostructural with their Pbca Rb 2 X Si 5 O 12 and Cs 2 X Si 5 O 12 analogues, with divalent X cations ordered onto separate T-sites than those occupied by Si. Table I shows the comparison of the refined lattice parameters and A cation site occupancies for RbCs X 2+ Si 5 O 12 compared to the published values for Rb 2 X Si 5 O 12 (Bell and Henderson, 1996, 2009, 2016) and Cs 2 X Si 5 O 12 (Bell and Henderson, 1996, 2009; Bell et al ., 1994b). Table II similarly shows refined interatomic distance and angles; the mean X -O and Si-O distances are close to the constraint distances, the mean O- X -O and O-Si-O angles are close to the ideal tetrahedral angle of 109.47°.…”

“…Rb 2 X 2+ Si 5 O 12 and Cs 2 X 2+ Si 5 O 12 were prepared from appropriate stoichiometric mixtures of Rb 2 CO 3 , Cs 2 CO 3 , SiO 2 , and X O ( X = Mg, Ni, Cd; Bell et al ., 1994b; Bell and Henderson, 1996, 2009, 2016) and then 50:50 mixtures of the Rb and Cs samples were made for each of the three pairs of samples with the same X 2+ cation. For X = Ni the starting materials were both glasses, the mixture was sealed in a gold capsule and heated in a furnace at 1173 K for 7 days.…”

“…Analyses of the summed powder diffraction data for each sample showed that all samples were single-phase and isostructural with the Pbca structure of Cs 2 CdSi 5 O 12 (Bell et al ., 1994b). Rietveld refinements were done using FULLPROF (Rodríguez-Carvajal, 1993), using the structures of Rb 2 X Si 5 O 12 (Bell and Henderson, 1996, 2009, 2016) as starting models but with half the Rb replaced by Cs. Backgrounds were fitted by linear interpolation between a set of background points with refinable heights.…”

“…III). Ni-O and Mg-O distances were restrained at 1.89 ± 0.01 Å; these distances are similar to those determined for X = Ni (Bell and Henderson, 1996, 2016), K 2 MgSi 5 O 12 (Bell et al ., 1994a) and Cs 2 MgSi 5 O 12 (Bell and Henderson, 2009) leucites. Cd-O distances were restrained at 2.23 ± 0.01 Å; these distances are similar to those determined for X = Cd (Bell et al ., 1994b; Bell and Henderson, 2009, 2012) leucites.…”

“…These structures all have disordered T-site cations and also have A cation sites fully occupied with either K, Rb or Cs. Crystal structures have also been refined at ambient temperature for P2 1 /c monoclinic crystal structures of leucite analogues with the general formulae A 2 B Si 5 O 12 ( A = K, B = Mg, Fe 2+ , Co, Zn; Bell et al ., 1994a; Bell and Henderson, 2018) and also for Pbca orthorhombic ( A = Rb; B = Mg, Mn, Ni, Cd; Bell and Henderson, 1996, 2009, 2016) and ( A = Cs; B = Mg, Mn, Co, Ni, Cu, Zn, Cd; Bell et al ., 1994b, 2010; Bell and Henderson, 1996, 2009) These structures all have ordered T-site cations and also have A cation sites fully occupied with either K, Rb or Cs. Cs 2 ZnSi 5 O 12 undergoes a reversible phase transition from Pbca to Pa-3 on heating to 566 K (Bell and Henderson, 2012).…”