Sodalite, Na_4Si_3Al_3O_(12)Cl: Structure and Ionic Mobility at High Temperatures by Neutron Diffraction

摘要

Crystal data: Na4Si3Al3012Cl, cubic, space group P43n, Z = 2, F(000)= 233.06 fm, xn = 0.06 cm-1, lattice parameter ao(k) [T] (K) at eight temperatures: 8.882(1) [295]; 8.902(2) [500]; 8.912(1) [600]; 8.923(1) [700]; 8.951(2) [800]; 8.971(1) [900]; 8.988(1) [1000]; 9.037(1) [1200]. The crystal structure has been determined at six" temperatures (295 700 K), together with the 1:1 coupled site occupancy factors of sodium and chlorine (T = 1200K). The indices-of-fit, wR(F2), are in the range 0.015-0.028 with observation-to-parameter ratios from 7.0 to 8.6. Bond lengths and angles in the aluminosilicate framework have average e.s.d.'s less than 0.002 A and 0.08°. Between 295 and 1200K, the observed Si-O (Al-O) bond lengths differ by -0.015 A (-0.012A); corrections for librating rigid SiO4 (A1O4) groups change the difference to +0.004A (+0.006A), compared with the 295K value of 1.620A (1.741 A). The unique Si-O-Al angle increases from 138.24° (295K) to 146.87° (1200K), while the Si and Al valence angles are virtually unchanged. Between 295 and 1200 K the [Na4Cl] clusters expand with increases in the Na-Cl bond lengths of 0.200 A, with simultaneous increases in Na-O bond lengths of 0.145 A and decreases in the shortest Na- o -O contact distances of 0.126 A. The thermal expansion of sodalite is attributed to the increasing amplitudes of coupled translational motion of the Na+ ions and the librational motion of the [Al/ SiO4] tetrahedra, leading to the untwisting of the aluminosilicate framework. Maps of the probability density functions for Na+ and cr indicate ionic diffusion paths along (111) directions. There is a finite probability of finding the Na+ ion within the plane of the next-nearest O atoms, suggesting that Na+ jumps from an occupied to an unoccupied site in the next-nearest cage through the six-membered ring of [Al/SiO4] tetrahedra.