Crystal Structure of Lithium-Sodium (Li0.7Na0.3)-Analcime

Abstract

When Na is substituted by Li in the narrow-pore zeolite analcime Na2[Al2Si4O12]⋅2H2O, the average parameter of the pseudocubic unit cell decreases from 13.67 A in the starting Na-analcime to 13.50 A in Li-analcime. Our structural data for Li-analcime [1] showed that the Li cations lie near the Na-sites in an octahedral arrangement O4(H2O)2 distorted to 4+2 or 3+3. Due to their smaller size, the Li cations are displaced toward the edge of one of the two Si,Al-tetrahedra coordination-linked to the given site, and the symmetry is reduced from Ibca to Pbca. Investigation of the Na–Li isomorphic series in analcimes showed that variation of the ratio of exchange cations leads to discontinuity in the thermal properties of the cation-substituted forms [2]. Thermal dehydration of samples with more than 30% sodium replaced by lithium led to decomposition of the solid solution into two phases, one of which (phase II) demonstrated more significant compression. According to high-temperature powder X-ray diffraction data, during dehydration, phase II undergoes a structural transformation accompanied by a change in unit cell dimensions showing that the cell has transformed from pseudocubic to pseudotrigonal and its volume has significantly decreased. This transformation is attributed to migration of the Na cations from the standard position on the twofold axis based on 4 oxygen atoms to a new position on the pseudothreefold axis based on 3 (+3) oxygen atoms. Samples with the degree of substitution 70% and higher contain phase II alone. At the first stage of our study of this structural transformation, we seek to reveal possible differences in the environment of Na and Li in the starting hydrated sample of (Li0.7Na0.3)-analcime. (Li,Na)-Analcime was obtained by ion exchange from natural analcime (Nidym River, East Siberia), whose crystal structure was refined earlier [3]. Crystals ≤0.20 mm in size were stored in a melt of LiNO3 and NaNO3 in a ratio of 0.95/0.05 at 260°C for 7 days. The ratio of the number of cations in the melt to that in the solid was 50/1. Then the sample was washed in hot distilled water to remove the salt and autoclaved in distilled water at 150°C. The Na content in the sample was determined by flame photometry. The H2O content was estimated from weight loss data obtained by sample calcination to 750°C by the thermogravimetric technique (TG-50, TA 3000, Mettler). The Li content was calculated based on the charge balance for the composition of the Li-analcime framework [1]. The formula of the resulting (Li,Na)-analcime sample is recorded as Li1.30Na0.53[Al1.83Si4.17O12]⋅2.05H2O, Z = 8; percent of Li exchange for Na is 71%. The X-ray diffraction experiment was carried out on a an Enraf-Nonius CAD-4 single crystal automatic diffractometer (MoKα radiation, 2θ = 70°, θ/2θ scan mode). The intensity data were collected twice. Collection in the full sphere of reciprocal space showed that the symmetry of the structure is not lower than orthorhombic. However, because of the wrong choice of cell dimensions in passing to the orthorhombic (pseudocubic) cell we were unable to check the deviation from I-centering of the unit cell. Therefore the intensity data were also collected in 1/8 of the sphere of reciprocal space, and the structure was solved with these data (direct methods, SHELXS-86 [4]). The full-matrix least-squares refinement was carried out anisotropically with SHELXL-93 [5] in space groups Pbca and Ibca using the complete set of F 2. Although there were 88 reflections with F0 > 3σ(F0) (five of these were “observable” reflections with F0 > 4σ), which are forbidden in a body-centered cell, structure refinement in Pbca gave unsatisfactory results. In particular, the T–O distances in the (Si,Al)O4-tetrahedra had wide variations of values in one tetrahedron (1.525-1.690 A). The correlation matrix elements were as high as 0.86. All these effects might be a consequence of understated symmetry [6]. While the Na and Li cations occupy the general positions in the given space group, their expected displacement from the center of the coordination octahedron was not observed.