Na7Sn12:: A binary zintl phase with a two-dimensional covalently bonded tin framework

Na7Sn12 was synthesized by quenching of stoichiometric amounts of the elements (700 degreesC) in a sealed niobium ampule and further thermal treatment at 270 degreesC for 40 days. Single crystals of Na7Sn12 were obtained from a mixture with the composition Na6SrSn16. The structure of Na7Sn12 consists of two-dimensional polyanions (2)(infinity)[Sn-12(7-)], which are separated by Na atoms. Bonding Sn-Sn contacts in the polyanion vary between 2.827(2) and 3.088(2) Angstrom. Crystal data: monoclinic, P2/n, Z = 4, a = 13.375(3) Angstrom, b = 9.239(2) Angstrom, c = 17.976(4) Angstrom, gamma = 90.15(3)degrees, V = 2243.0(8) Angstrom(3), mu = 13.22 mm(-1), d(calc) = 4.694 g cm(-3), R1(F) = 6.1% (for all reflections). Extended-Huckel tight-binding calculations with the implementation the electron localization function (ELF) reveal that Na7Sn12 can be viewed as an intermetallic compound with exclusively localized bonding and nonbonding regions as expected from the 8 - N rule. Thus Na7Sn12 is a Zintl phase with the formula (Na+)(7)[(2b)Sn2-](1)[(3b)Sn-](5)[(4b)Sn-0](6).