Structural Collapse or Expansion of the Hydro-Sodalite Series Na₈[AlSiO₄]₆(OH)₂·nH₂O and Na₆[AlSiO₄]₆·nH₂O Upon Dehydratation

The two end-member series of hydro-sodalites show different structural expansion characteristics during dehydratation at elevated temperatures (300–900 K). The anomalous expansion in cell volume which is observed for series Na₆[AlSiO₄]₆ · nH₂O upon loss of 0 < n ≤ 8 H₂O is correlated to the destruction of the significant hydrogen bonding system between non-framework and framework oxygen atoms of the corresponding sodalite structures. The contraction of the unit cell of the basic sodalites Na₈[AlSiO₄]₆(OH)₂ · nH₂O upon loss of 0 < n ≤ 4 H₂O, however, is well understood in terms of space filling of the non-framework constituents and some special stereochemical features amongst sodium, oxygen and hydrogen atoms.