  The crystal structure of CO2 filled pure SiO2 LTA zeolite has been studied at
high pressures and temperatures using synchrotron based x ray powder
diffraction. Its structure consists of 13 CO2 guest molecules, 12 of them
accommodated in the large alpha cages and 1 in the beta cages, giving a
SiO2:CO2 stoichiometric ratio smaller than 2. The structure remains stable
under pressure up to 20 GPa with a slight pressure dependent rhombohedral
distortion, indicating that pressure induced amorphization is prevented by the
insertion of guest species in this open framework. The ambient-temperature
lattice compressibility has been determined. In situ high pressure resistive
heating experiments up to 750 K allow us to estimate the thermal expansivity at
5 GPa. Our data confirm that the insertion of CO2 reverses the negative thermal
expansion of the empty zeolite structure. No evidence of any chemical reaction
was observed. The possibility of synthesizing a silicon carbonate at high
temperatures and higher pressures is discussed in terms of the evolution of C-O
and Si-O distances between molecular and framework atoms.
