UNSPECIFIED. (1994) NMR-STUDIES OF THE LEUCITE ANALOGS X2YSI5O12, WHERE X=K, RB, CS - Y=MG, ZN, CD. PHYSICS AND CHEMISTRY OF MINERALS, 21 (3). pp. 176-190. ISSN 0342-1791

Abstract

A number of leucite group materials with the formula X2YSi5O12 where X=K, Rb, Cs and Y=Mg, Zn, Cd have been synthesized by dry and hydrothermal crystallization of glass starting materials. Si-29 MASNMR has been used to obtain structural information, such as the number of distinct tetrahedral sites, degree of cation ordering, and estimates of the mean T - O - T bond angles of the tetrahedra. X-ray powder diffraction gave information on cell volumes and degree of distortion from cubic symmetry for all the samples and space group and structural information for some samples. Integration of the different length-scale data obtained using these two complementary techniques allows greater reliance to be placed on the structures deduced for these leucite samples, which are only available as fine-grained powders. Hydrothermally synthesized K2MgSi5O12, K2ZnSi5O12 and Rb2ZnSi5O12 have structures with 12 distinct tetrahedral sites (T-sites) and are monoclinic P2(1)/c while the dry-synthesized equivalents are disordered with single T-sites and are cubic, Ia3d. Most of the other members of the group have structures with 6 tetrahedral sites with CS2CdSi5O12 being orthorhombic, Pbca. CS2ZnSi5O12 has an intermediate ''5+2'' structure. Decreasing the size of the X+ cation for a given Y2+ cation gives more collapsed and distorted frameworks. Cs-133 NMR was used to show that samples with 6 T-sites have 2 alkali sites. It is deduced that samples with 12 T-sites will undergo a displacive phase transition to a 6 T-site structure (possibly via a 5 + 2 intermediate in some cases) with no change in the framework topology or degree of T-site ordering.

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