NMR study of phase transitions in guest-free silica clathrate melanophlogite

Reversible phase transitions in the silica clathrate melanophlogite have been studied by powder X-ray diffraction and Si-29 magic-angle-spinning (MAS) NMR on a sample previously heated to 950 degrees C in order to remove volatile guest molecules. At 200 degrees C the Si-29 MAS NMR spectrum contains three peaks in the 12:8:3 intensity ratio corresponding the cubic Pm3n polymorph. On gradual cooling, systematic peak splitting is observed, indicating the presence of cubic-cubic and cubic-orthorhombic phase transitions at ca. 140 degrees C and ca. 60 degrees C, respectively. A powder X-ray diffraction study of the thermal evolution of the cubic unit cell parameter suggests that the phase transitions are second-order. The possible symmetry breaking involved and the nature of the room temperature polymorph are examined by considering supergroup-subgroup relations for space group Pm3n in relation to Si-29 MAS NMR spectra. The analysis suggests Pm3n <----> Pm3 <----> Pmmm symmetry breaking for the 140 degrees C and 60 degrees C transitions, respectively. The spectra suggest the splitting of the 5(12) cages in the Pm3 melanophlogite structure into two crystallographically distinct types located at 000 and 1/2 1/2 1/2.