Disordered but primitive gallosilicate hydro-sodalite: Structure and thermal behaviour of a framework with novel cation distribution

Abstract

The structure of a new gallosilicate sodalite |Na6.16(1)(H2O)8|[Ga1.04(1)Si0.96(1)O4]6 described in space group P4¯3n with lattice parameter a = 885.208(7) pm is reported. For such a sodalite with a deviation from a 1:1 ratio in the framework cations a body-centred structure in space group I4¯3m could be expected. The structure shows structural stress resulting from this unusual substitution by a high strain visible in reflections which are forbidden by symmetry in I4¯3m. Distinct amounts of Ga and Si are redistributed on the opposite crystallographic position. The second coordination sphere of Si was examined by 29Si MAS NMR, the absence of OH− in the sodalites cages was checked by FTIR- and Raman spectroscopy. The intensity distribution of MAS NMR signal is modelled using a new technique to calculate the framework metal second neighbour coordination. The confirmed distribution leads to a low thermal stability of the cubic sodalite indicated by a new intermediated phase which could be regarded as a triclinic distorted cancrinite with three-time increased c lattice parameter. This intermediate phase decomposes at around 1000 K to a beryllonite-type sodium gallosilicate.