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....

A new layered Ti-based pnictide oxide superconductor, Ba(1-x)Na(x)Ti(2)Sb(2)O (0.0 ≤ x ≤ 0.33), is reported. X-ray studies revealed that it crystallizes in the tetragonal CeCr(2)Si(2)C structure. The undoped parent compound, BaTi(2)Sb(2)O [P4/mmm; a = 4.1196(1) Å; c = 8.0951(2) Å], exhibits a charge density wave (CDW)/spin density wave (SDW) transition at 54 K. Upon chemical doping with Na, the CDW/SDW transition is systematically suppressed, and superconductivity arises with the critical temperature (T(c)) increasing to 5.5 K. Bulk superconductivity was confirmed by resistivity, magnetic, and heat capacity measurements. Like the high-T(c) cuprates and the iron pnictides, the superconductivity in BaTi(2)Sb(2)O arises from an ordered state. Similarities and differences between BaTi(2)Sb(2)O and the cuprate and iron pnictide superconductors are discussed.

a b s t r a c tThe newest homologous series of superconducting Fe-pnictides, LiFeAs (Li111) and NaFeAs (Na111) have been synthesized and investigated. Both crystallize with the layered tetragonal anti-PbFCl-type structure in P4/nmm space group. Polycrystalline samples and single crystals of Li111 and Na111 display superconducting transitions at $18 K and 12-25 K, respectively. No magnetic order has been found in either compound, although a weak magnetic background is clearly in evidence. The origin of the carriers and the stoichiometric compositions of Li111 and Na111 were explored.

A family of rare U(IV)-containing quaternary fluorides, Na4MU6F30 (M = Mn(2+), Co(2+), Ni(2+), Cu(2+), and Zn(2+)), was synthesized in single crystal form via a mild hydrothermal technique utilizing an in situ U(VI) to U(IV) reduction step. The modified hydrothermal route is described, and the conditions to obtain single crystals in high yield are detailed. The crystal structures were determined by single crystal X-ray diffraction. The isostructural fluorides crystallize in a new structure type in the trigonal space group P3̅c1. They exhibit a complex three-dimensional crystal structure consisting of corner- and edge-shared UF9 and MF6 polyhedra. The main building block, a U6F30(6-) group, is arranged to create two distinct hexagonal channels, inside which MF6 octahedra and Na(+) cations are located. The copper-containing member of the series, Na4CuU6F30, is unusual in that the Cu(2+) cation exhibits a rare symmetrical coordination environment consisting of six identical Cu-F bond distances, indicating the lack of the expected Jahn-Teller distortion. Magnetic susceptibility measurements of Na4ZnU6F30 yielded an effective magnetic moment of 3.42 μB for the U(4+) (f(2)) cation in the structure. Measurements of the other members containing magnetic transition-metal cations in addition to U(4+), Na4MU6F30 (M = Mn(2+), Co(2+), Ni(2+), and Cu(2+)) yielded total effective magnetic moments of 10.2, 9.84, 8.87, and 8.52 μB for the Mn-, Co-, Ni-, and Cu-containing materials, respectively. No evidence for long-range magnetic ordering was found down to 2 K. Measurements of the magnetization as a function of applied magnetic field at 2 K for Na4MnU6F30 confirmed that the U(4+) magnetic cation exhibits a nonmagnetic singlet ground state at low temperature. Thermal stability measurements and UV-vis diffuse reflectance spectroscopy are also reported.

Single crystals of several ternary alkali uranium fluorides, LiUF5, KU2F9, K7U6F31, RbUF5, RbU2F9, and RbU3F13, have been obtained in a mild hydrothermal process using UO2(CH3CO2)2(H2O)2 as the uranium source. Their crystal structures were determined by single crystal X-ray diffraction. The uranium in the starting reagent was successfully reduced from U(6+) to U(4+) in a dilute hydrofluoric acid environment, aided by the presence of a copper salt. All materials exhibit highly complex crystal structures that range from two-dimensional to three-dimensional. The U(4+) cations are found in high (UF8 and UF9) coordination environments. The magnetic susceptibility measurements yielded effective magnetic moments of 3.01-3.83 μB for the U(4+) cations. The temperature dependent magnetic susceptibility measurements confirmed that the U(4+) cation exhibits a nonmagnetic singlet ground state at low temperatures. No long-range magnetic order was observed for any of the above compositions down to 2 K. Optical and thermal behaviors of the fluorides were also investigated.