EMS-6, a novel microporous gadoliniumsilicate with monteregianite structure: Synthesis, crystal structure and thermal behavior

The hydrothermal synthesis and crystal structures of the hydrated and dehydrated forms of EMS-6. a novel gadolinium mixed tetrahedral-octahedral microporous silicate with chemical composition Na8 0K4 0Gd4 0Si32 0O76 17.0 H2O. isotypic with the mineral monteregianite, are reported EMS-6 crystallizes under well controlled conditions. In particular, it requires a relatively low gadolinium content (Si/Gd = 20), appropriate alkalinity (Na/Si = 1.05 and K/Si= 0 3) and crystallization temperature (200 degrees C) The EMS-6 crystal structure is monoclinic with unit cell parameters a = 9.65954(7). b = 23 9862(2) and c = 9.52750(8) angstrom, beta = 93 757(1)degrees and space group P2(1)/n. Its structure was refined using high resolution X-ray powder diffraction data The refinement showed that one of the three crystallographically independent sodium atoms composing the EMS-6 octahedral layer is in a better defined 7-fold coordination than that previously suggested for the same site in natural monteregianite Also, the extra-framework potassium atom is 9-fold coordinated (rather than 10-fold) since only four independent water molecule positions were found in the asymmetric unit. EMS-6 is thermally stable up to 700 degrees C and rapidly rehydrates after thermal treatment, as demonstrated by coupling X-ray diffraction and thermogravimetric analyses Above 800 degrees C, EMS-6 undergoes a phase transformation into EMS-6(b). In situ X-ray diffraction, using synchrotron radiation, showed this new phase had lower space group symmetry (from P2(1)/n to P2(1)) and significant distortion compared to the original structure The phase transformation is reversed by exposing EMS-6(b) to ambient atmosphere. Structural breakdown occurs above 900 degrees C. (C) 2010 Elsevier Inc All rights reserved