Synthesis and structural characterization of LaOFeP superconductors

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2007 Supercond. Sci. Technol. 20 687

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IOP PUBLISHING SUPERCONDUCTOR SCIENCE AND TECHNOLOGY

Supercond. Sci. Technol. 20 (2007) 687–690 doi:10.1088/0953-2048/20/7/017

Synthesis and structural characterizationof LaOFeP superconductorsC Y Liang, R C Che, H X Yang, H F Tian, R J Xiao, J B Lu, R Liand J Q Li

Beijing National Laboratory for Condensed Matter Physics, Institute of Physics,Chinese Academy of Sciences, Beijing 100080, People’s Republic of China

E-mail: LJQ@aphy.iphy.ac.cn

Received 10 April 2007, in final form 15 May 2007Published 4 June 2007Online at stacks.iop.org/SUST/20/687

AbstractHigh quality LaOFeP superconducting materials have been successfullysynthesized by a two-step solid reaction method. We first prepared aLa–Fe–P ternary alloy following a pre-alloying treatment; the resultantLa–Fe–P ternary alloy was found to be the key precursor for producing thefinal LaOFeP superconductors in our experiments. The LaOFeP samples ingeneral have a superconducting transition temperature of about Tc ≈ 4.1 K asdemonstrated by the measurements of resistivity and magnetic susceptibility.The volume ratio of the superconducting phase in the high quality sample isestimated to be as high as 95% at the temperature of 2 K. Structural analysisby means of x-ray diffraction, electron diffraction and high resolutiontransmission electron microscopy shows that the LaOFeP crystal has alayered tetragonal structure with a P4/nmm space group. Structural defectssuch as stacking faults in the LaOFeP sample have also been brieflydiscussed.

1. Introduction

High Tc cuprate superconductors have been extensively stud-ied for several decades because of their essential importancefrom both academic and technological points of view [1–4];then many efforts were made to explore new superconductorsin other related compounds, such as Nax CoO2·1.3H2O [5, 6],SrRuO4 [7], UGe2 [8], Nax TaS2 [9] and URhGe [10]. In par-ticular, experimental investigations on a few Fe-based systemsclearly demonstrated the presence of superconductivity and no-table magnetic properties [11, 12]. Actually the coexistence offerromagnetism and superconductivity as a critical issue hasbeen intensively discussed in studies of the spin-triplet pair-ing superconductors [8, 10]. Recently, Kamihara et al [13] re-ported a new iron-based layered superconductor LaOFeP withTc ∼ 3.2 K. This layered material has a tetragonal ZrCuSiAs-type structure with a space group of P4/nmm [14]. Theoreti-cal analysis of the electronic bands and Fermi surface suggeststhat the inter-layer bonding could evidently affect the super-conductivity in LaOFeP [15]. It is also noted that, from eitherthe crystal structure or the electronic structure point of view,the LaOFeP superconductor shows evident differences fromthe high Tc cuprate superconductors. The Fe ion in LaOFeP oc-cupies a tetrahedral site coordinated with four P ions; theoret-

ical calculation revealed that the quasi-two-dimensional elec-tronic states in the vicinity of Fermi level are governed by theFe dx y and dx2−z2 orbitals hybridized with the P p orbitals [15].On the other hand, the Cu ions in the high Tc superconductorsoccupied a planar fourfold square site coordinated with six Oions. The charge carriers at the Fermi level are driven by thedx2−z2 orbitals. Hence, an extensive study of physical proper-ties in the LaOFeP materials would provide a new opportunityfor studying the mechanism of superconductivity in transitionmetal oxides. In the present paper, we will report on the de-velopments of our research on the synthesis, structure char-acterization and superconductivity measurements of LaOFePmaterials. In order to obtain well-crystallized LaOFeP super-conducting materials, we performed a novel pre-alloying treat-ment step in the sample preparation process. Microstructuralfeatures and crystal defects in this new type of superconductingmaterial have been carefully examined by a series of electronmicroscopy techniques.

2. Experimental details

The polycrystalline samples of LaOFeP were prepared by aconventional solid-state reaction method described as follows.First, to avoid oxidation of the pure lanthanum (La is extremely

0953-2048/07/070687+04$30.00 © 2007 IOP Publishing Ltd Printed in the UK 687

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Figure 1. (a) Powder XRD patterns of the as-synthesized LaOFeP. Inset: the atomic model of LaOFeP crystal (space group P4/nmm,a = 3.96 A and c = 8.51 A). (b) A calculated x-ray diffraction profile for LaOFeP crystal, using Cerius2 software. (c) Low magnificationSEM image of the as-synthesized LaOFeP; inset: energy dispersive x-ray spectroscopy (EDX) data recorded from the LaOFeP specimen.(d) High magnification SEM image of a LaOFeP crystal.

active in air), the lanthanum and iron were pre-alloyed by arcmelting under an argon gas flow on a water-cooled copperstage. These mixed materials were repeatedly alloyed six timesto get a chemically homogeneous ingot; then the La–Fe–Palloy was prepared by heating a mixture of La–Fe alloy andred P at 650–800 ◦C for 12 h in an evacuated quartz tube.The amounts of La–Fe and P were calculated according to thestoichiometric ratio of the LaFeP formula. The commercialLa2O3 powder was heated at 550 ◦C for 12 h for the purposeof dehydrating. Then, the stoichiometric mixtures of La–Fe–P alloy and La2O3 powder were sealed in an evacuatedquartz tube. The tube was heated to 1240 ◦C and sintered for48 h. Finally, the products were slowly cooled down to roomtemperature with a cooling rate of 1 ◦C min−1.

X-ray diffraction for the structure determination ofthe superconducting materials was performed at roomtemperature on a Rigaku RINT x-ray diffractometer withCu Kα radiation. Measurements of the temperaturedependence of magnetization were carried out using an Oxfordmultiparameter measurement system (Maglab-Exa-12). Theresistivity of the superconducting samples was measured bya standard four-point probe technique. Specimens usedfor transmission electron microscopy (TEM) analysis werepolished mechanically with a Gatan polisher to a thicknessof about 40 μm and then ion milled with a Gatan-691PIPS ion miller. TEM observations and electron energy lossspectroscopy (EELS) analysis were performed on a Tecnai F20transmission electron microscope operating at 200 kV.

3. Results and discussion

Figure 1(a) shows an experimental XRD pattern obtainedfrom a typical LaOFeP sample with a sharp superconductingtransition. All diffraction peaks in this pattern can be indexedquite well by a tetragonal cell with lattice parameters of

a = 3.962 A and c = 8.511 A and a space groupof P4/nmm. The inset shows a brief structural modelclearly illustrating the atomic layers of LaOFeP crystal. Eachprimitive cell includes eight atoms in two sub-units, i.e.,the FeP tetrahedron and the LaO tetrahedron, and each cellcorner is occupied by an Fe atom. It is also noted thatthe LaOFeP crystal, comparable with the high Tc copper-based superconductor, is also composed of two functionalsheets alternately stacked along its c-axis direction. Theiron phosphide (Fe2+P3−) layers as the electronically activeplanes are sandwiched by charge reservoir layers (lanthanumoxide La3+O2− layers). Theoretical XRD simulation withthis atomic structural model has been performed using thereflection computer code implemented in Cerius2 software.The calculated XRD profile as illustrated in figure 1(b) showsup the main structural features in good agreement with theexperimental data.

It is also noted that the experimental conditions, synthesisparameters and amount of La2O3 used in the solid reactionof the La–Fe–P ternary alloy and La2O3 play critical rolesin the formation of the superconducting LaOFeP phase andthe crystalline quality. In our experiments, we divided theLa–Fe–P ternary alloy into three batches and added threedifferent amounts of La2O3 powder. The purpose is to changethe mole ratio of the La–Fe–P component within the totalreactant. When the La2O3 amount exceeds or equals thenominal stoichiometric amount, the impurity phase of La2O3

will form in addition to the LaOFeP phase, which will degradethe superconductivity property of the final product. Whenthe La2O3 amount is slightly (10%) less than the nominalstoichiometric amount, the LaOFeP phase can be formed. Thedetailed reasons will be studied in a further study.

LaOFeP materials often show a rich variety of microstruc-tural features depending notably on the synthesis process.

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Synthesis and structural characterization of LaOFeP superconductors

Figure 1(c) is a low magnification SEM image of an as-synthesized LaOFeP sample, clearly illustrating the crystallinegrain morphology in a superconducting material. It is evidentthat the layered LaOFeP phase is quite pure in the sample andthe crystalline quality is high. The average grain size in suchsamples generally ranges from 1 to 6 μm. Figure 1(d) is a highmagnification SEM image taken from a LaOFeP crystal grain,illustrating the layered structural feature of the crystals. Thincrystal pieces with a thickness of less than 10 nm can be fre-quently observed. Energy dispersive x-ray (EDX) microanaly-sis is also employed to determine the chemical composition ofthe crystal in our products. The inset of figure 1(c) is a typicalEDX spectrum recorded from a LaOFeP specimen. This EDXspectrum clearly demonstrates that the product is composed ofLa, Fe, P and O elements. No other impurity element is found.Quantitative analysis confirms that the mole ratio among thefour elements is around 1:1:1:1.

Figure 2(a) displays the magnetic susceptibility curvesfor a LaOFeP sample under an applied field of HAC = 1Oe. The strong diamagnetic signal demonstrates the bulksuperconducting character in the LaOFeP product. Thesuperconducting critical temperature (Tc) as defined by theonset point of the real part (χ ′) of the AC susceptibility isabout 4.1 K in the susceptibility measurement. A dissipationpeak can be seen in the imaginary part of the susceptibilitycurve (χ ′′). The transition width in general is less than 1 K.The superconducting (SC) volume fraction is also estimated,based on the magnetization, the geometric volume of thespecimen and the applied magnetic field. Figure 2(b) showsthe SC volume fraction in the measured sample as a function oftemperature; it is recognizable that the value of −4πχ ′ reachesas high as 95% at 2 K, confirming the high quality of oursuperconducting sample.

Figure 2(c) shows the resistivity data (normalized to14 K) of the same sample as a function of temperature.The room temperature resistivity of our sample is about3.42 × 10−4 � cm, in agreement with the previous data.It is recognizable that this superconducting sample exhibitswell-defined metallic behaviour at low temperatures, i.e., alinear relationship between resistivity and temperature. Anabrupt decrease of resistivity is visible below 6.7 K, and zeroresistivity is reached at around 4.1 K, consistently with theresult of the above susceptibility measurements.

In order to understand the crystal structural feature ofthe layered material, we have performed a series of structuralanalysis by means of electron diffraction and high resolutionTEM observation. Figure 3(a) shows a HRTEM imagerecorded from a thin area in a LaOFeP crystal, clearly showingthe atomic layers with a lattice spacing of 8.4 A along the c-axis direction. The two sets of lattice fringes seen in figure 3(a)correspond to 0.82 and 0.28 nm, making a 90◦ intersectingangle, which could be attributed to the (001) and (110) crystalplanes of LaOFeP. The inset of figure 3(a) is an electrondiffraction pattern taken along the [110] zone axis direction.The two diffraction spots are well indexed using a tetragonalLaOFeP unit cell with lattice parameters: a = 3.962 A andc = 8.511 A (JCPDS card No 50-0971) to be (001) and(110). The LaOFeP is composed of the FeP4 tetrahedral layerssandwiched with lanthanum oxide layers. These two typesof layers stack alternately along c axis direction. Figure 3(b)

(a)

(b)

(c)

Figure 2. (a) AC magnetic susceptibility as a function of temperaturemeasured under an applied field of HAC = 1 Oe, demonstrating thesuperconducting transition at about 4.1 K. (b) Superconducting (SC)volume fraction in the LaOFeP sample. (c) Temperature dependenceof resistance for a LaOFeP sample; the resistance reaches zero at∼4 K.

shows an enlarged TEM high resolution image of an area infigure 3(a). It shows the atomic position in good agreementwith the 〈110〉 projection of the LaOFeP crystal.

Finally, we will briefly discuss the common crystal defectsin the superconducting LaOFeP materials. Figure 3(c) isa bright-field TEM image of a LaOFeP crystal, showingthe presence of stacking faults within such superconductingmaterials. Actually, several types of planar defects also existin this material due to the essential weak bonding of atomiclayers stacking along the c-axis. Figure 3(d) shows a HRTEMimage illustrating a region with a clear stacking fault in aLaOFeP crystal grain. This stacking fault is located withinthe (001) crystalline planes of LaOFeP. It is noted that such

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Figure 3. (a) High magnification TEM image taken from a thin area in a LaOFeP crystal; the inset shows the corresponding electrondiffraction pattern. (b) Enlarged image of a part in figure 3(a) together with an atomic structural model of LaOFeP. (c) Low magnification and(d) high magnification TEM images showing the stacking faults in the LaOFeP crystals.

defects are often accompanied by the notable lattice distortionand dislocation as indicated by arrows.

4. Conclusions

In summary, we have reported a new method for synthesizinghigh quality LaOFeP superconducting materials. Thismethod includes a pre-alloying treatment and two-stepsolid reactions. Temperature dependence of magneticsusceptibility and electric resistivity demonstrate the presenceof bulk superconductivity in the LaOFeP materials with asuperconducting transition temperature Tc of about 4.1 K. Thevolume fraction of the superconducting phase in the typicalsuperconducting samples is estimated to be as high as about95% at the temperature of 2 K. Structure analysis by means ofSEM and TEM techniques shows that LaOFeP has a typicallayered structure with space group of P4/nmm. The commonstructural defects, e.g. stacking faults, have been examined bymeans of high resolution TEM observations. Recently, we alsoused this new method to synthesize other related samples, suchas PrOFeP; the experimental results demonstrated that samplesprepared by this method in general have a perfect ZrCuSiAs-type layered structure without visible impurities.

Acknowledgments

The authors are grateful to Dr P Ding, G Wang, H Yang,Dr R I Walton and Professor H H Wen for their technicalassistance and helpful discussions about some measurements.

The work reported here is supported by the National NaturalScience Foundation of China and by the Ministry of Scienceand Technology of China (973 project No 2006CB601001).

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