ADSORPTION OF CARBON-DIOXIDE BY FERROELECTRIC LITHIUM-NIOBATE

Desorption curves of CO2 are presented for powder of lithium niobate with main particle sizes of 1000, 500, 200, 100 and 50 mu m in diameter. The percentage of surface area occupied by CO2 molecules is calculated after measuring the total surface area of the samples using N-2 single point BET. Adsorption of CO2 and N-2 were measured with a Quantasorb instrument using flowing helium. The ratio of the surface area covered by CO2 with respect to N-2 changes from 11.5% for the 50 mu m particles down to 6.7% for the 100 mu m particles. Adsorption of CO2 for particles equal to or larger than 200 mu m could not be measured with our technique. XPS results on a single piece of lithium niobate indicated that CO2 is adsorbed on the surface when only Nb3+ ions are present. Induced crystallographic transformation of the crystal by an applied 2.5 kV field changes its surface composition from an initial mixture of Nb5+ and Nb3+ ions to pure Nb5+ ions and as a consequence of this change, adsorbed CO2 was released. A tentative explanation of these results is that the adsorption of the CO2 molecules by the ferroelectric substrate is due to a particular local electrostatic potential created by surface ions, in agreement with Pacchioni's work on other oxide surfaces.