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The crystal structure of CO2 filled pure SiO2 LTA zeolite has been studied athigh pressures and temperatures using synchrotron based x ray powderdiffraction. Its structure consists of 13 CO2 guest molecules, 12 of themaccommodated in the large alpha cages and 1 in the beta cages, giving aSiO2:CO2 stoichiometric ratio smaller than 2. The structure remains stableunder pressure up to 20 GPa with a slight pressure dependent rhombohedraldistortion, indicating that pressure induced amorphization is prevented by theinsertion of guest species in this open framework. The ambient-temperaturelattice compressibility has been determined. In situ high pressure resistiveheating experiments up to 750 K allow us to estimate the thermal expansivity at5 GPa. Our data confirm that the insertion of CO2 reverses the negative thermalexpansion of the empty zeolite structure. No evidence of any chemical reactionwas observed. The possibility of synthesizing a silicon carbonate at hightemperatures and higher pressures is discussed in terms of the evolution of C-Oand Si-O distances between molecular and framework atoms.

Structural evolution of CO2 filled pure silica LTA zeolite under high-pressure high-temperature conditions