Journal of Structural Chemistry, Vol. 41, No. 6, 2000 CRYSTAL STRUCTURE OF Li-ANALCIME Yu. V. Seryotkin, V. V. Bakakin, UDC 548.736:549.67 I. A. Belitskii, B. A. Fursenko, and I. S. Bazhan Li-analcime Li 1.69 Na 0.14 [Al 1.83 Si 4.17 O 12 ]⋅2.05H 2 O has been prepared and studied by single-crystal X-ray structural analysis: a = 13.510(2), b = 13.520(1), c = 13.503(2) Å, V = 2466.4(9) Å 3 , Z = 8, space group Pbca. The structure is compared with that of the starting Na-analcime. Lithium atoms lie near Na sites with a distorted octahedral coordination O 4 (H 2 O) 2 . Due to the smaller size of Li atoms, their real environment is differentiated into 3+3 or 4+2. The off-network cations and the H 2 O molecules are shifted from the sites which they normally occupy in analcime in a cooperative manner. The changes in the unit cell dimensions and symmetry are explained from crystal-chemical viewpoint. INTRODUCTION Li-analcime Li 2 [Al 2 Si 4 O 12 ]⋅2H 2 O was obtained in 1953 [1] by sequential ion exchange from Na-analcime via Ag-analcime. It had a smaller cubic cell parameter (13.50 Å) compared to the starting Na-analcime (13.67 Å). Our X-ray diffraction study on Li-substituted analcimes [2] gave similar parameters. The results of the first direct synthesis of Li-analcime from gels were published in 1971 [3]. The analcimes obtained from samples with different SiO 2 /Al 2 O 3 ratios had different refractive indices and unit cell parameters. The latter were found to be 13.64-13.69 Å; these values are much larger than those for Li-exchanged analcimes. While chemical analysis data are missing, it may be conjectured that the compositions of the analcimes synthesized in this way differ significantly from ideal ones. The Al/Si ratio in them is likely to be much greater than 2/4. The number of Li cations increases accordingly, leading to a different arrangement of these cations and H 2 O molecules affecting the unit cell parameters. Possible differences in the behavior of Li as a structure-forming template (along with H 2 O) and substituent of Na in the matrix deserve special investigations. The crystal-chemical schematic formula of analcime and its analogs is (H 2 O) 2 (Na) 3 [(Si, Al) 6 O 12 ] → W 2 [12] S 3 [4+ 2W] [T 6 O 12 ]. Here W (originating from ‘‘water’’) is the site of rather bulky species such as H 2 O, F – , K + , NH 4 + , Rb + , Cs + ; S (from ‘‘sodium’’) is the site with 4-coordination for the network O atoms plus 2 H 2 O molecules occupied by medium-sized cations Na + , Ca + , Ag + . While in the tetrahedral ANA-network the cavities have limited configuration and volume, the ‘‘classical’’ off-network W and S sites may have offset variants with slightly different environments [4, 5]. In oxides, lithium generally has coordination numbers 4, 6, 5, and (very rarely) 3 (enumerated in order of occurrence). In network aluminosilicates and their analogs, lithium generally has a tetrahedral environment. Thus in natural narrow-pore hsianghualite (LiF) 2 Ca 3 [Be 3 Si 3 O 12 ] with an ANA type network [5] and in bikitaite Li 2 [Al 2 Si 4 O 12 ]⋅2H 2 O (formula analog of analcime) [6], lithium has a tetrahedral coordination O 3 F or O 3 (H 2 O). In wide-pore zeolites, it has coordination numbers from 3 to 5 [7-11]. In Li-exchanged natrolite [12], the cation lies close to the Na site in natural natrolite in an irregular prism with coordination 5+1 and with shortened distances compared to those of sodium. In Li-exchanged albite [13], lithium is also nearly completely substituted for sodium with coordination number decreased from 7 to 5. The Li-substituted form of analcime is of special interest in view of the potential peculiarities of Li coordination and possible variations of off-network sites in ANA type structures. 0022-4766/00/4106-1021$25.00 © 2001 Plenum Publishing Corporation 1021 Institute of Mineralogy and Petrography, Siberian Branch, Russian Academy of Sciences. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated from Zhurnal Strukturnoi Khimii, Vol. 41, No. 6, pp. 1233-1241, November-December, 2000. Original article submitted November 12, 1999.