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Abstract

Although it is known that unlimited solid solution exists for the high temperature polymorphs Na2S04 (1)and α-Na2C03 , the phase relationships and the variation of crystallographic parameters for different compositions are not studied. Na2S04 has four well established modifications, but only the high temperature modification Na2S04 (1) (Pб3/mmc), stable above 237°C, forms extended solid solution series with Na2C03. At the other extreme, Na2C03 shows three polymorphs and a rather complicated transition behaviour, being the hexagonal phase α-Na2C03 (Pб3/mmc)stable at T > 489°C. In this paper, the crystallographic parameters were determined at 550°C for different solid solution compositions. The study was carried out by X ray diffraction in a high temperature powder camera. The diffraction patterns show a continuous evolution through the entire compositional range. Crystalline spacings and cell parameters decrease as the sulphate content decreases. This fact and the unusual unlimited S04=-C03= substitution can be explained by the orientational disorder of tetrahedral and triangular groups. Theoretical powder diffraction diagrams, calculated assuming this orientational and substitutional disorder, show a good fit with the experimentalresults.