INFLUENCE OF THE SIZE OF THE ALKALI ION ON THE THERMAL-EXPANSION OF ALKALI-DOPED CORDIERITES - A POWDER NEUTRON-DIFFRACTION STUDY

The crystal structure of hexagonal cordierite with the formula Cs0.5Mg 2Al4.5Si4.5O18 has been determined at 20, 275, 550 and 825 degrees C b y powder neutron diffraction. The mechanism of thermal expansion has b een analysed and compared to that of its potassium homologue. The subs titution of Cs for K results logically in an increase of the unit cell volume but in a decrease of the distortion and of the volume of the T (I)O-4 tetrahedra, and therefore in an opposite response of the tetrah edral network to the thermal expansion of the MgO6 octahedra. As a con sequence, the compensation of the increase in the a-parameter by a dec rease in the c-parameter, observed for pure and K-doped cordierites, i s no longer observed with the Cs-0.5-cordierite.