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The synthesis and properties of LiFeAs, a high-T c Fe-based superconducting stoichiometric compound, are reported. Single crystal x-ray studies reveal that it crystallizes in the tetragonal PbFCl type ͑P4/nmm͒ with a = 3.7914͑7͒ Å and c = 6.364͑2͒ Å. Unlike the known isoelectronic undoped intrinsic FeAs compounds, LiFeAs does not show any spin-density wave behavior but exhibits superconductivity at ambient pressures without chemical doping. It exhibits a respectable transition temperature of T c = 18 K with electronlike carriers and a very high critical field, H c2 ͑0͒ Ͼ 80 T. LiFeAs appears to be the chemical equivalent of the infinite layered compound of the high-T c cuprates. DOI: 10.1103/PhysRevB.78.060505 PACS number͑s͒: 74.70.Dd, 61.66.Fn, 74.25.Fy Until recently the chemical realm of high-T c superconductivity had been limited mainly to copper oxide-based layered perovskites. The latest search for noncuprate superconductors in strongly correlated electron layered systems has led to the discovery of high-T c superconductivity in doped quaternary rare-earth iron oxypnictides, ROFePn ͑R = rare-earth metal and Pn= pnicogen͒. 1-3 These superconductors generated enormous interest in the materials community due to the high T c 's involved ͑up to 41-55 K͒ as well as the critical presence of a magnetic component, Fe, considered antithetical to conventional s-wave superconductivity. 3,4 High-pressure studies suggest maximum T c in R͑O,F͒FeAs may be about 50 K but higher T c 's ͑Ͼ50 K͒ may yet be discovered in structurally different compounds that are electronically related to R͑O,F͒FeAs. 5 Analogous alkaline-earth iron arsenides, AeFe 2 As 2 ͑Ae= Sr and Ba͒, reportedly having formal ͑Fe 2 As 2 ͒ 2− layers as in ROFFeAs but separated by simple Ae layers as in the cuprates, were found to behave similarly. 6,7 The AeFe 2 As 2 phases become superconducting ͑maximum T c ϳ 37 K͒ with appropriate substitution of Ae atoms with alkali metals. 8,9 It was also found that isostructural compounds KFe 2 As 2 and CsFe 2 As 2 with formal ͑Fe 2 As 2 ͒ 1− layers were superconducting, having much lower T c 's of 3.8 and 2.6 K, respectively. 9 Moreover, the evolution from a superconducting state to a spin-density wave ͑SDW͒ state by chemical substitution was observed in K 1−x Sr x Fe 2 As 2 . 9 Critical to the high-T c FeAs superconductors is the need to introduce sufficient amounts of charge carriers: with electrons ͑n type͒ by F doping ͑15-20 atm %͒ or holes ͑p type͒ by Sr doping ͑4-13 atm %͒ in ROFeAs, and ͑K/Sr͒ substitution ͑40: 60 atm %͒ in AeFe 2 As 2 . These results established the unique role of ͑Fe 2 As 2 ͒ layers in high-T c superconductivity. Since simple elemental K, Cs, ͑K/ Sr͒, or ͑Cs/Sr͒ layers separate the ͑Fe 2 As 2 ͒ layers in the AFe 2 As 2 superconductors, a Li-based analog, LiFeAs, was investigated. Its crystal structure was previously reported to be of the Cu 2 Sb type that features a Fe 2 As 2 substructure similar to the known FeAs superconductors. 10 However, the locations of the Li atoms were problematic....

Single crystals of the layered organic-inorganic perovskites, [NH(2)C(I=NH(2)](2)(CH(3)NH(3))m SnmI3m+2, were prepared by an aqueous solution growth technique. In contrast to the recently discovered family, (C(4)H(9)NH(3))(2)(CH(3)NH(3))n-1SnnI3n+1, which consists of (100)-terminated perovskite layers, structure determination reveals an unusual structural class with sets of m <110>-oriented CH(3)NH(3)SnI(3) perovskite sheets separated by iodoformamidinium cations. Whereas the m = 2 compound is semiconducting with a band gap of 0.33 +/- 0.05 electron volt, increasing m leads to more metallic character. The ability to control perovskite sheet orientation through the choice of organic cation demonstrates the flexibility provided by organic-inorganic perovskites and adds an important handle for tailoring and understanding lower dimensional transport in layered perovskites.

The challenges associated with synthesizing expanded semiconductor frameworks with cage-like crystal structures continue to be of interest. Filled low-density germanium and silicon framework structures have distinct properties that address important issues in thermoelectric phonon glass-electron crystals, superconductivity and the possibility of Kondo insulators. Interest in empty framework structures of silicon and germanium is motivated by their predicted wide optical bandgaps of the same magnitude as quantum dots and porous silicon, making them and their alloys promising materials for silicon-based optoelectronic devices. Although almost-empty Na(1-x)Si136 has already been reported, the synthesis of guest-free germanium clathrate has so far been unsuccessful. Here we report the high-yield synthesis and characteristics of germanium with the empty clathrate-II structure through the oxidation of Zintl anions in ionic liquids under ambient conditions. The approach demonstrates the potential of ionic liquids as media for the reactions of polar intermetallic phases.