Zur Polymorphie des Na 2 S 2 O 3 / On Polymorphism of Na 2 S 2 O 3

Abstract

Na 2 S 2 CO 3 crystallizes in 4 modifications. The familiar monoclinic a-form stable at ambient temperature transforms into the trigonal β-form at 330 °C which at 388 °C converts to b ccy-Na 2 S 2 O 3 . These transformations are reversible. Unstable orthorhombic α'-Na 2 S 2 CO 3 (Pna2 1 ) is obtained by dehydration of the pentahydrate or by cooling the β-phase. The γ-phase decomposes in sealed ampoule to Na 2 SCO 4 and NaS 5 ; decomposition is slow below and rapid above 440 °C. Equilibrium displacement by distillation of sulphur accelerates decomposition even for β-Na 2 S 2 O 3 , and maintaining low sulphur pressure by pumping gives Na 2 SO 3 . β- and γ-Na 2 S 2 CO 3 are superionic conductors; a dynamic model is presented for the β-structure with Na + conduction pathways originating from oscillation of the S 2 O 3 2- tetrahedra. Structural relationships can be derived with respect to the arrangement of the thiosulphate groups.