MgF2 Doping of SmFeAsO Superconductors Prepared by Mechanical

ARTICLE pubs.acs.org/cm

MgF2 Doping of SmFeAsO Superconductors Prepared by Mechanical Alloying and Rapid Annealing Sui-Lin Shi,† Ai-Hua Fang,† Xiao-Ming Xie,‡ Fu-Qiang Huang,*,† and Mian-Heng Jiang‡ †

CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China ‡ State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China

bS Supporting Information ABSTRACT: Magnesium and fluorine codoped Sm1
x/2Mgx/2FeAsO1
xFx (x = 0
0.3) superconductors were prepared by the process of mechanical alloying and subsequent rapid sintering. The transition temperature (Tc) increases with x and reaches 53.3 K at x = 0.3, accompanied by decreased lattice parameters. Electron-type conduction in the MgF2-doped samples was found by Hall-effect measurements. The higher Tc value of Sm1
x/2Mgx/2FeAsO1
xFx was attributed to the larger disorder and a higher internal pressure from the Sm and O sites, similar to the case of (Ln1
xMx)2CuO4 superconductors (where Ln is a rare-earth element and M = Ca, Sr, Ba). KEYWORDS: superconductivity, Sm1
x/2Mgx/2FeAsO1
xFx, mechanical alloying

’ INTRODUCTION By now, it is well-known that the superconductivity of Febased oxyarsenides (LnFeAsO, where Ln is a rare-earth element) can be enhanced by either fluorine doping or oxygen deficiency.1,2 The transition temperature (Tc) reaches 55 K in SmFeAsO0.85 and SmFeAsO1
xFx (x = 0.15
0.2) prepared by the hightemperature high-pressure method.3,4 In LnFeAsO, the FeAs layers are responsible for the charge conductivity and the LnO blocks act as a charge reservoir, tuning the electronic structure of the material and thus determining superconducting properties. As a multiband superconductor, F substitution can modulate the electronic structure by direct charge transfer (decrease the valence of Fe) and by generating lattice distortions that affect the FeAs (and LnO) layers.5 The Tc value of LnFeAsO1
xFx monotonically increases with x in the range of 12.5%
20%,6
9 and it is conceivable that further F doping is very desirable. It may further lower the Fe valence to weaken the Fe
As bonds, enlarge the Fe
As bond lengths, result in more-distorted FeAs4 tetrahedra, and lower the symmetries. However, higher F substitution had failed, because of the inability to synthesize pure-phase samples. This may indicate that the structural tension or internal stress at higher F content has exceeded the tolerance limit of the rigid edge-sharing FeAs layer, so the structure collapses and the compound decomposes. Although F replacement of O is effective to induce superconductivity, modifying the Ln
O layer offers another possibility to influence superconductivity without chemically disturbing the all important FeAs layers. Until now, size optimization of Ln (using La, Ce, Pr, Nd, Sm, and Gd) and the larger alkali-earth-ion substitution (e.g., Sr) have been studied.10 In this study, we r 2011 American Chemical Society

propose using MgF2 to simultaneously achieve electron doping (O2
substitution by F
) and hole doping (Ln3þ by Mg2þ), which allows more dopants to enter the structure, because of the charge balance. The smaller Mg cation at the Ln sites and the higher F content at the O sites can lead to a more-distorted charge-reservoir LnO layer with a less-affected chemical valence of Fe, thus affording an opportunity to probe the effect of lattice distortion or internal pressure of the FeAs layer on iron-based superconductors. In this work, the method for preparing these Sm1
x/2Mgx/2FeAsO1
xFx materials, their lattice distortions, and their improved Tc values are reported.

’ EXPERIMENTAL PROCEDURE SmAs powders were prepared by reacting Sm chips and As pieces at 900
C for 8 h. High-purity powders of SmAs, Fe, Fe2O3, As, and MgF2 were mixed according to the nominal composition of Sm1
x/2Mgx/2FeAsO1
xFx and loaded in an argon-filled stainless steel jar. Mechanical alloying11
13 was next used to increase the reactivity of the powders and improve the homogeneity of the mixtures. Mechanical alloying was done by ball milling for 4 h at a rotation speed of 480 rpm on a shake mill. The milled mixtures were pressed into small pellets and then sealed in evacuated quartz tubes. The above sample preparation procedures were all conducted in a high-purity argon atmosphere where the oxygen concentration was