A New ZrCuSiAs-Type Superconductor: ThFeAsN

Abstract

ABSTRACT:We report the first nitrogen-containing iron-pnictide superconductor ThFeAsN, which is synthesized by a solid-state reaction in an evacuated container. The compound crystallizes in a ZrCuSiAs-type structure with the space group P4/nmm and lattice parameters a=4.0367(1) Å and c=8.5262(2) Å at 300 K. The electrical resistivity and dc magnetic susceptibility measurements indicate superconductivity at 30 K for the nominally undoped ThFeAsN.The discovery of high-temperature superconductivity in iron based compounds 1,2 triggered enormous researches, and a lot of progresses in material and mechanism aspects have been achieved. [3][4][5] Remarkably, dozens of Fe-based superconductors (FeSCs) were discovered, which can be classified into several crystallographic types. 5 All these FeSCs necessarily contain anti-fluorite-like Fe 2 X 2 (X denotes a pnictogen or a chalcogen element) layers. Another important feature for FeSCs is that the high-temperature superconductivity is mostly induced by suppressing the spin-density wave (SDW) order in a parent compound, typically through chemical dopings or applying pressures.The (Ref. 13 ). Note that the valence of the A-site cations is either 3+ or 2+. We previously succeeded in doping a tetravalent Th 4+ ion into the A site, 9,14 which produces superconductivity with a T c up to 56 K. 9 To expand the FeSC spectrum and, possibly to further increase the T c , it is of great interest to explore a possible A 4+ Z 3 combination for the fluorite-like spacer layer in the 1111 system. and AeFX (X=Cl, Br, I). 18 This structural relation suggests that iron pnictide containing [Th 2 N 2 ] 2+ layers could be thermodynamically stable. In this Communication, we report our successful synthesis of the target material, ThFeAsN, using solid-state reactions at high temperatures. The as-prepared sample shows bulk superconductivity at 30 K without any deliberate oxygen doping.In general, nitrides are synthetically challenging, presumably because of the potential oxygen incorporation. Therefore, the precursors of Th 3 N 4 , Th metal and FeAs are carefully synthesized avoiding oxygen contamination as far as possible. First, thorium powders were produced by the reduction of thorium oxide with metal calcium. 19 The yielded thorium metal powder was consolidated into a button with silver luster using an arc furnace. After that, the button was scraped in a glove box filled with argon (99.9999%) to get clean thorium powder. Then, nearly monophasic Th 3 N 4 (possibly with trace ThN) was synthesized by the reaction with 20% excess high-purity nitrogen gas (99.999%) at 1000 C for 24 hours in an evacuated quartz ampoule. FeAs was prepared by heating the mixture of iron (99.99%) and arsenic (99.999%) powders in an evacuated quartz ampoule at 700 C for 48 hours. All the precursors had been checked by X-ray diffractions (XRD) before using [figures S1 and S2 in the Supporting Information (SI)]. Similarly, the solid-state reaction of the stoichiometric mixture of Th 3 N 4 , Th and FeAs was also carried o...

Unconventional superconductivity

“…According to band structure calculations, the five Fe 3-d orbits dominate the electronic density of states at the Fermi energy of this class of superconductor, thus the name 'iron based' is appropriate. unconfirmed report of a new sub-family of the 1111's involving nitrogen, ThFeAsN, Tc=30 K (C. Wang et al [460]). The recent theoretical review by Hirschfeld [461] talks about "many .…”

On the structure of the superconducting order parameter in high-temperature Fe-based superconductors

“…Being antiferromagnetic metals with spin-density wave (SDW) ground state in stoichiometric composition, the majority of iron pnictides and selenides turn into superconductivity under electron or hole doping. However, the two exceptions, a wellknown LiFeAs [7] and recently discovered ThNOFeAs similar to an oxypnictide (1111 family) and TC ≈ 30 K [8], are nonmagnetic in the stoichiometric state. Note the Fermi energy is rather low for iron-based superconductors.…”

BaTh2Fe4As4(N0.7O0.3)2: An iron-based superconductor stabilized by inter-block-layer charge transfer

“…Because the Fe2As2 layers in BaTh2Fe4As4(N0.7O0.3)2 are electron doped, the emergence of superconductivity seems not surprising. 2 21 Second, stoichiometric mixtures of Fe, ThO2, Th3As4, Th3N4, and FeAs, and BaFe2As2 were pressed into pellets, then placed in an alumina tube. The sample-loaded alumina tube was sealed in an evacuated quartz ampule, which was heated to 950 C holding for 60 hours.…”