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....

Superconductivity and phase relationships were explored in the Na-Fe-As system. The PbFCl-type 111 phase is stable only within a Na stoichiometry range of 1.00 to ϳ0.85, and exhibits bulk superconductivity within an even narrower range around 0.90 in Na 0.9 FeAs. In particular, stoichiometric NaFeAs is not a bulk superconductor. The onset of the superconducting transition varies in a totally different way and the highest T c occurs in multiphase samples with a nominal composition of Na: Fe: As= 0.5:1:1, where the superconductive volume-fraction is almost zero. Such doping dependency is rather surprising and in disagreement with most expectations. DOI: 10.1103/PhysRevB.79.184516 PACS number͑s͒: 74.70.Dd, 74.62.Dh, 74.62.Bf The recent discovery of superconductivity in layered transition-metal oxypnictides, La͑O , F͒FeAs, 1 has attracted intense interest in the FeAs-based compounds. Superconductivity up to 55 K has been observed in three classes of FeAsbased compounds, i.e., ͑R ,Ae͒͑O , F͒FeAs, ͑Ae, A͒Fe 2 As 2 and AFeAs, where R, Ae, and A are rare earth, alkaline earth, and alkali elements, respectively.2-7 The FeAs-based superconducting compounds have often been compared with the well-investigated cuprate superconductors. The doping dependency of the superconductivity, however, appears to be rather different in the FeAs-family as it varies significantly from one member to another. 7,8 The main doping effects reported so far in the FeAs family, however, appear still to be a smooth, bell-like T c vs. carrier filling x 0 , where T c is the transition temperature. Competitions with magnetic ordering are often suggested in interpreting the data.9,10 Significant change in the superconducting volume-fraction V S , on the other hand, occurs only near the normal conductorsuperconductor boundary. The V S , it should be pointed out, is actually a convolution of the T c ͑x 0 ͒ and the local x 0 -distribution ͑composition inhomogeneity͒ if x 0 is a sole parameter. A constant V S , therefore, is expected if the superconductive range, ⌬, is much broader than the x 0 -spread, e.g., the full width at half height ͑FWHH͒ of a normal distribution. The effect on T c , in such cases, will be the main focus. At the opposite extreme of ⌬Ӷ, however, the spread would lead to the same T c distribution but a drastic V S change with x 0 , though this is rarely observed or discussed. Herein we report our observations in the superconducting system, Na y FeAs, which possesses a PbFCl-type structure isotypic to that of LiFeAs. This PbFCl-type structure as well as ͑trace͒ superconductivity exist over the whole nominalcomposition range investigated, i.e., with the nominal composition of Na y FeAs, with 0.5Յ y Յ 1.0. The samples are single phase, however, only for y Ն 0.9, and the impurity phase FeAs appears at lower y. A rather unusual doping effect is also observed. On one hand, the samples become bulk superconducting, e.g., with V S Ͼ 10%, only around y = 0.9 with an estimated spread Ӷ0.1. The apparent T c , on the other hand, monotonically...

The major skeletal elements in the (Porifera) sponges, are spicules formed from inorganic material. The spicules in the Demospongiae class are composed of hydrated, amorphous silica. Recently an enzyme, silicatein, which polymerizes alkoxide substrates to silica was described from the sponge Tethya aurantia. In the present study the cDNA encoding silicatein was isolated from the sponge Suberites domuncula. The deduced polypeptide comprises 331 amino acids and has a calculated size of M r 36 306. This cDNA was used as a probe to study the potential role of silicate on the expression of the silicatein gene. For these studies, primmorphs, a special form of aggregates composed of proliferating cells, have been used. It was found that after increasing the concentration of soluble silicate in the seawater medium from around 1 mm to approximately 60 mm, this gene is strongly upregulated. Without additional silicate only a very weak expression could be measured. Because silica as well as collagen are required for the formation of spicules, the expression of the gene encoding collagen was measured in parallel. It was also found that the level of transcripts for collagen strongly increases in the presence of 60 mm soluble silicate. In addition, it is demonstrated that the expression of collagen is also upregulated in those primmorphs which were treated with recombinant myotrophin obtained from the same sponge. Myotrophin, however, had no effect on the expression of silicatein. From these data we conclude that silicate influences the expression of the enzyme silicatein and also the expression of collagen, (via the mediator myotrophin).