Synthesis and Structure of Large Optically Clear Crystals of Gallogermanate Sodalite D. E. W.
Vaughan,*,†
H. P.
Yennawar,#
and A. J.
Perrotta†
Materials Research Laboratory, Materials Research Institute, Department Biochemistry and Molecular Biology, Althouse Laboratory, PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 9 2072-2075
ReceiVed March 23, 2006; ReVised Manuscript ReceiVed June 23, 2006
ABSTRACT: Large transparent crystals (up to 800 µm) of a hydroxy-sodium gallogermanate sodalite have been prepared at between 100 and 150 °C in aqueous sodium tetramethylammonium hydroxide solutions. Single-crystal structure analysis gave the expected space group (P4h3n) but an unexpectedly small unit cell (8.951 Å) in which the longer (Ga,Ge)-O bond lengths are compensated by T-O-T bond angles of ∼110°. The crystals provide a readily made framework for structure studies of encapsulated chemical species. Introduction Sodalite (Na6Al6Si6O24(NaCl)2), structure code SOD,1 is a well-known mineral showing high variability in compositional polymorphs.2 Probably the best known variety is the blue semiprecious stone “lapis lazuli” in which the chloride ion is replaced by trapped anionic sulfur dimers and trimers.3 Structurally, it is similar to the clathrates in that it comprises a matrix of stacked molecular boxes 6 Å in diameter connected through 3 Å windows4,5 (Figure 1) which facilitate diffusion of anions and cations at elevated temperatures. The facility to trap removable salt species in excess of stoichiometry adds to its complexity. These manipulatable features of the structure have generated much technological interest in the material. They include its properties for storing hydrogen6,7 and the nuclear waste product 85Kr8,9 : photochromism10 and cathodochromism11,12 and trapping semiconducor nanoclusters.13-15 Much of the earlier work targeted applications in information storage and in military radar display phosphors,16 the latter commercialized by RCA. Much zeolite synthesis work on gallosilicates and gallophosphates has been reported, and it is clear that Ga substitution for Al occurs readily in many zeolite and related structures. The early geochemical work on Ga and Ge isomorphous substitution was focused on minerals made at high temperatures and pressures, such as feldspars17 and micas.18,19 Initial experiments on synthesizing Ga/Ge zeolites were done in glass reactors, and the products were highly contaminated with silica.20 Although many zeolite patents speculate on gallosilicate analogues of their primary aluminosilicate claims, rarely are they validated with specific examples. In the past decade much more interest has focused on gallogermanate reactions, resulting in the synthesis of new materials and analogues of known structures.21 Our experiments with Fe, Ga, and Ge substitution into aluminosilicate zeolites show that substitutional predictions are difficult. The longer T-O bond lengths require some compensation by changes in T-O-T bond angles, and more flexible, less rigid structures seem to tolerate greater degrees of substitution. Sodalite synthesis has been extensively studied since the late nineteenth century.22 The earlier work focused on aluminosilicate SOD, but more recent studies have turned to framework isomorphic substitution and the influence of trapped anions on * To whom correspondence should be addressed. E-mail:
[email protected]. † Materials Research Laboratory, Materials Research Institute. # Department Biochemistry and Molecular Biology.
Figure 1. Clathrate-like stacked SOD cages. Nodes are Ga3+ or Ge4+ cations covalently bonded through O2-.
aluminosilicate23 and gallogermanate compositions.24,25 The targeted synthesis of large sodalite crystals (up to 1 cm) was originally done by Bye and White26 as part of a General Electric (UK) program to make computer coding materials. Their crystals larger than about 200 µm were sometimes cloudy, and the largest ones were often opaque, particularly when made in unlined autoclaves. In their early experiments, the sodalite cocrystallized with clusters of large clear hexagonal columnar crystals of cancrinite,27 and both species seemed preferentially to grow on the oxidized wall of the steel autoclave. Others have attempted similar experiments with some success,28 making millimetersized SOD at between 600 and 700 °C, but again the larger crystals were opaque. Crystals of 120 µm were made at 200 °C by Shimizu and Hamada.29 Bibby and Dale30 made 30 µm pure silica SOD at 150 °C, and Bu et al. made gallogermanate SOD at 180 °C having maximum dimensions of 133 µm. The experiments in this report were done at either 100 or 150 °C and autogenous pressure. Our main interest was in evaluating the influence of changing SOD tetrahedral atoms on the blue-green color development in calcined tetramethylammonium (TMA)-sodium sulfate sodalites.31 Keeping the base formulation constant (0.9TMA2O/ 1.45Na2O/Al2O3/3SiO2/120H2O) and replacing Al with Ga and Si with Ge, we crystallized unusually large (up to 800 µm) transparent crystals having the SOD structure. However, calcining them at 600 and 800 °C caused the large crystals to become polycrystalline, with no development of the coloration observed
10.1021/cg060164c CCC: $33.50 © 2006 American Chemical Society Published on Web 08/18/2006
Synthesis of Gallogermanate Sodalite Crystals
Crystal Growth & Design, Vol. 6, No. 9, 2006 2073
Figure 2. L to R: 400 µm cube used for the single-crystal analysis; typical crystals from the Ostwald ripening (third) experiment; product of the seeded mother liquor (fourth) experiment. Scale ) 1 mm.
in the aluminosilicate experimental products in which the sodalite crystals were invariably
