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 °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) Å, β = 93.757(1)° and space group P21/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 °C and rapidly rehydrates after thermal treatment, as demonstrated by coupling X-ray diffraction and thermogravimetric analyses. Above 800 °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 P21/n to P21) 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 °C.