Degassing and phase transitions with temperature in melanophlogite

Melanophlogite (type I clathrate) has a microporous framework of corner sharing SiO4 tetrahedra, which can host guest gases. Here, a multi–analytical approach has been employed to understand the phase transitions with temperature and the degassing behaviour of a melanophlogite sample, which contains CO2 together with minor CH4 amount as enclathrated molecules.Synchrotron powder diffraction data collected between 110 and 380 K (ESRF, ID22 beamline) show a clear splitting of the major peaks up to about 360 K. Le Bail analysis shows that the low temperature phase is monoclinic, changing to cubic at ∼370 K. Landau theory analysis indicates that the transition is second order, with Tc = 369(1) K. Moreover, significant volume strain related to the phase transitions accounts for the lower thermal expansion at higher temperature. Differential scanning calorimetry shows evidence of a further phase transition between 220 and 250 K. The transition is confirmed by powder diffraction, with a change in slope in the b unit cell parameter and in volume expansion in the same temperature range. High temperature X-Ray powder diffraction between 313 and 673 K shows that thermal expansion decreases with temperature, possibly in relation with a structural rearrangement within the cubic structure during degassing. Thermogravimetric analysis shows that degassing of the guest gases begins at 450 K, and occurs at a higher rate above 750 K, but it is not completed even at T = 1273 K. Melanophlogite confirms to be a good host carrier for the gases, with gas loss only at higher temperature. Moreover, as degassing occurs with almost no thermal expansion, it could open promising application for its mechanical stability during degassing.

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