Sodalite, Na₄Si₃Al₃O₁₂Cl: structure and ionic mobility at high temperatures by neutron diffraction

Crystal data: Na₄Si₃Al₃O₁₂Cl, cubic, space group P43n, Z = 2, F(000) = 233.06 fm, μ_n = 0.06 cm⁻¹, lattice parameter a_o (Å) [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 ≤ T ≤ 1200 K) based on neutron diffraction data with (sin θ/λ) < 0.80 Å⁻¹. Besides conventional parameters, the least-squares refinement model included thermal tensor parameters up to fourth-order for sodium and chlorine (295 ≤ T ≤ 1200 K) and up to third-order for oxygen (T ≥ 700 K), together with the 1:1 coupled site occupancy factors of sodium and chlorine (T = 1200 K). 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 Å and 0.08°. Between 295 and 1200 K, the observed Si—O (Al—O) bond lengths differ by −0.015 Å (−0.012 Å); corrections for librating rigid SiO₄ (AlO₄) groups change the difference to +0.004 Å (+0.006 Å), compared with the 295 K value of 1.620 Å (1.741 Å). The unique Si—O—Al angle increases from 138.24° (295 K) to 146.87° (1200 K), 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 Å, with simultaneous increases in Na—O bond lengths of 0.145 Å and decreases in the shortest Na...O contact distances of 0.126 Å. 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 Cl⁻ 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.

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