NMR-STUDIES OF THE LEUCITE ANALOGS X2YSI5O12, WHERE X=K, RB, CS - Y=MG, ZN, CD

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 hydrother mal 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 me an T - O - T bond angles of the tetrahedra. X-ray powder diffraction g ave information on cell volumes and degree of distortion from cubic sy mmetry for all the samples and space group and structural information for some samples. Integration of the different length-scale data obtai ned using these two complementary techniques allows greater reliance t o be placed on the structures deduced for these leucite samples, which are only available as fine-grained powders. Hydrothermally synthesize d K2MgSi5O12, K2ZnSi5O12 and Rb2ZnSi5O12 have structures with 12 disti nct tetrahedral sites (T-sites) and are monoclinic P2(1)/c while the d ry-synthesized equivalents are disordered with single T-sites and are cubic, Ia3d. Most of the other members of the group have structures wi th 6 tetrahedral sites with CS2CdSi5O12 being orthorhombic, Pbca. CS2Z nSi5O12 has an intermediate ''5+2'' structure. Decreasing the size of the X+ cation for a given Y2+ cation gives more collapsed and distorte d 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 u ndergo a displacive phase transition to a 6 T-site structure (possibly via a 5 + 2 intermediate in some cases) with no change in the framewo rk topology or degree of T-site ordering.