CA5IN9SN6 : INTERPLAY OF STRUCTURAL AND ELECTRONIC FACTORS BETWEEN INTERMETALLIC AND ZINTL PHASES

Intermetallic phases formed between elements of groups 13 and 14 with one or more of the electropositive alkali or alkaline earth metals show a rich variety of very often complex structures.1 More importantly, the smooth transition of electronic properties from semiconducting Zintl phases to normal intermetallic compounds along the Zintl border provides a fertile area to search for materials with novel electronic properties and offers unique opportunities in investigating relationships between crystal structure, chemical bonding, and physical properties. In this context, the Zintl concept provides an effective and useful way to rationalize chemical bonding and electronic properties of materials in the border between metals and nonmetals.2 Our most recent results using the concept in rationalizing the synthesis and discovery of ternary and quaternary compounds successfully led to the synthesis and description of SrCa2In2Ge, which contains [IndIn-Ge]-6 chains, analogous and isoelectronic with the allyl anion chain [CHdCH-CH2]∞. Electron deficiency in compounds of group 13 (trelide) posttransition elements is usually demonstrated by the formation of 3-center 2-electron bonds that leads to the formation of cluster units. The tendency of polar intermetallic trelides to form boranelike clusters is illustrated by the extensive studies on a wide variety of novel trelide cluster compounds which can be rationalized in terms of Wade’s rules as in the boranes.4,5 Other reports on polar intermetallic trelides have resulted in intriguing questions concerning the ability of indium to accommodate high negative charges.6,7 One challenge in inorganic chemistry is the synthesis of main group compounds, other than carbon, that exhibit unsaturation and/or aromaticity.8-11 Recently, it has been reported that Sn3 cyclopropenium-like trimers, were formulated to exist in the superconducting Zintl phase BaSn3 (Ni3Sn structure type).12 Our exploratory syntheses on “electron-deficient” Zintl phases among trelides and tetrelides have led to the discovery of a novel phase, Ca5In9Sn6, with an unprecedented intergrowth structure featuring anionic indium trimer fragments, [In3], analogous and isoelectronic with cyclopropenium, C3H3. This compound exemplifies the combination of a normal intermetallic compound and a Zintl phase and addresses important issues concerning the bonding and properties of compounds containing post-transition metals along the Zintl border. The title compound was synthesized through high-temperature reactions of stoichiometric amounts of the pure elements (distilled Ca metal from APL Engineering Labs; In shots, 99.9999%; Sn chunks, 99.9999%) in welded Ta tubing within an evacuated quartz jacket. To obtain single crystals suitable for X-ray structure analysis, a temperature of 1050 °C was held for 5 days followed by slow cooling to about 300 °C. These shiny silver crystals with gem-like morphologies were found to be air sensitive. Hence, all syntheses manipulations were done only in argon atmosphere, and single crystals were sealed within thin-walled glass capillaries. Accurate lattice constants were refined from the indexed X-ray diffraction pattern measurements on a single phase sample by the least-squares method with NBS Si as an internal standard. Chemical compositions of a number of single crystals were analyzed by WDS (wavelength dispersive spectrometer). Results showed uniform composition corresponding to Ca5In9Sn6, and this stoichiometry was used in the starting model of the single-crystal structure refinement.13 Ca5In9Sn6 crystallizes in a hexagonal structure in space group P63/mmc with a ) 6.7091(5) A and c ) 26.9485(9) A, as shown in Figure 1. The structure can be derived from the intermetallic cubic AuCu3 and hexagonal Ni3Sn structure types. The relationships between these structure types are based on their differences in the stacking order of closed packed layers of (MX3) metal atoms.14 The AuCu3-type is cubic closed packed (ccp) with “abc”stacking and Ni3Sn is hexagonal closed packed (hcp) with “ab”stacking. The structure of Ca5In9Sn6 is made up of two hcp and eight ccp layers of Ca(In/Sn)3 stacked along the c-axis with “abcbacbabc” as the repeat stacking unit. The two hcp layers in the unit cell contain only In and Ca atoms while the ccp (AuCu3type) layers consist of Ca, In, and Sn atoms. Within the hcp layers, indium atoms In(1) are displaced from ideal closed packed positions resulting in the formation of In trimers with In-In distances of 3.045(3) A, a normal In-In bonding distance. The distances between trimers within this closed packed layer are much longer at 3.665(5) A. In the ccp Ca(In,Sn)3 layers, the distortions leading to In/Sn trimer formation are not observed. The distances among metalloid in the ccp layers (3.30-3.36 A) are relatively longer than the bonding distances observed in the cluster compounds and Zintl phases containing In and Sn atoms,4-6,18 and are consistent with those found in (non-Zintl)