Physica C 391 (2003) 217–222

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Narrow range of iridium-substitutionon Mg1�xIrxB2 superconductor

Morsy M.A. Sekkina, Khaled M. Elsabawy *

HTc-Ceramic Superconductors Unit, Department of Chemistry, Faculty of Science, Tanta University, Tanta, Egypt

Received 6 March 2003; accepted 30 April 2003

Abstract

The undoped and iridium-doped samples of the general formula Mg1�xIrxB2 (where x ¼ 0:0, 0.02, 0.06 and 0.12

mol%) were prepared via high temperature solid state reaction technique depending upon diffusion mechanism of Mg-

vapor through boron-matrix. The maximum iridium solubility limit m was found to be �0.06 mol%, which emphasizes

that substitution on MgB2 system is too limited and needs many precautions to be successful. The crystalline lattice

constants were evaluated as a function of Ir-doping ratio x and found to confirm that c-axis exhibits slight length

compression as x increases while a; b-axes were nearly constant. Furthermore, effect of Ir-doping was investigated

carefully on microstructure and superconducting properties of MgB2 system.

� 2003 Elsevier B.V. All rights reserved.

Keywords: Ir-doping; X-ray; Crystal structure; SEM; MgB2 superconductor

1. Introduction

Since the discovery of superconductivity in

magnesium diboride �39 K superconductor [1,2],a large progress has been achieved in the material

synthesis and processing as well as in under-

standing of its physical properties [3–8]. Since

many diborides crystallize in the same hexagonal-

type of structure as MgB2, these compounds have

* Corresponding author. Address: C/o Jansen Gp., Max

Plank Institute for Solid State Research, Heisenbergstrasse 1,

70569 Stuttgart, Germany. Fax: +49-711-689-1010.

E-mail addresses: k.elsabawy@fkf.mpg.de, ksabawy@

yahoo.com (K.M. Elsabawy).

0921-4534/$ - see front matter � 2003 Elsevier B.V. All rights reserv

doi:10.1016/S0921-4534(03)01269-3

been known and investigated carefully [9]. For

magnesium site substitutions apparently only Al

and Mn were showing good substitutions to enter

the structure unambiguously [10–14] although itwas successful only for a limited concentration

range. For boron site substitutions a number of

attempts with different elements were made includ-

ing (C, Zn, Na, K, Ag and Si). Carbon substitu-

tion was reported in several publications [15–20].

Sekkina and Elsabawy [21] have investigated effect

of fluoride substitution partially on B-site on the

superconducting, structural and microstructuralproperties of MgB2. Cava et al. [22] have proposed

the minimal criteria for judging a successful chemi-

cal doping/or substitution in MgB2 superconduct-

ing system.

ed.

Fig. 1. X-ray diffractogram for Ir-doped and undoped MgB2: # unreacted B-phase and � MgO-phase. (a) The variation of a-axislattice parameter versus Ir-dopant ratio. (b) The variation of c-axis lattice parameter versus Ir-dopant ratio. (c) Graphite-like structure

MgB2 system: (black sphere) Mg atoms and (gray sphere) B atom.

218 M.M.A. Sekkina, K.M. Elsabawy / Physica C 391 (2003) 217–222

Table 1

The calculated lattice parameter for doped and undoped MgB2

samples

Dopant

ratio

(mol%)

a (�AA) c (�AA) c=a Tc-offset(K)

0.00 3.0827(4) 3.5612(1) 1.155 37.3

0.02 3.0835(6) 3.4817(4) 1.129 33.2

0.06 3.0841(3) 3.4142(8) 1.107 28.1

0.12 3.0844(8) 3.3686(7) 1.092 18.0

M.M.A. Sekkina, K.M. Elsabawy / Physica C 391 (2003) 217–222 219

2. Experimental

2.1. Sample preparation

Mg1�xIrxB2 (where x ¼ 0:0, 0.02, 0.06 and 0.12mol%) was prepared through high temperature

solid state reaction technique for stoichiometric

amounts of the nominal compositions of starting

materials (Mg, B and Ir). The average particle size

of boron and Ir metal powder used was <50 lm.

All powders are with purity grade �99.9%. Stoi-

chiometric amounts were mixed and ground to-

gether and were pressed into pellets under apressure of 4 ton/cm2 and were placed into Ta-

ampule under argon pressure; the Ta-tube was

carefully sealed and forwarded to tubular quartz

furnace at 950 �C sintering temperature. The ther-

mal cycle of preparation of pure MgB2 and iridi-

um-doped MgB2 included intermediate fixation

step at 650 �C for 2 h, then gradual increasing till

1050 �C at the rate of 100 �C/h and finally sampleswere sintered at 1000 �C for 10 h, then the tem-

perature was brought down to RT at the rate of

)50 �C/h. It is known that Mg melts and converts

to vapor at temperatures above 648.6 �C, while

boron has a very high melting point of 2180 �C.Thus MgB2 is produced through diffusion reaction

mechanism of Mg-ion vapor into boron-matrix.

2.2. Structural measurements

2.2.1. X-ray diffraction

X-ray diffraction (XRD) measurements were

carried out at room temperature on the ground

samples using Cu-Ka radiation source and a

computerized Shimadzu (Japan) diffractometer

with 2h scan technique.

2.2.2. Scanning electron microscopy

Scanning electron microscopy (SEM) measure-

ments were carried out along ab-plane using smallpieces (cut into very thin layer by diamond cutter)

of the prepared samples by using a computerized

SEM camera with elemental analyzer unit Shima-

dzu (Japan).

2.3. Superconducting measurements

The DC-electrical resistivity of the prepared

materials were measured as a function of temper-

ature using the modified four-probe technique andthe temperature was recorded in the cryogenic

temperature zone down to 30 K using liquid he-

lium refrigerator.

3. Results and discussion

3.1. Structural measurements

Fig. 1 displays the X-ray diffraction patterns for

the investigated prepared samples; Mg1�xIrxB2

(where x ¼ 0:0, 0.02, 0.06 and 0.12 mol%) which

were found mainly belong to single hexagonal

phase with P6/mmm symmetry corresponding to

Mg1�xIrxB2 superconductors in major beside some

of secondary phases such as unreacted B and MgOwhich were assigned in the background in minor.

The evaluated lattice parameters were found to be

a ¼ 3:0827 �AA and c ¼ 3:5612 �AA for undoped MgB2

sample, while a and c axes exhibit decrease in

length with increasing iridium dopant from

x ¼ 0:02 to 0.12 mol% respectively (see Table 1). It

is clear that decrease in the c-axis is much notice-

able than increase of a-axis (see Fig. 1(a) and (b)).The decrease in c=a ratio can be explained on

the basis of (1) ionic size effect that iridium ionic

radius is smaller than magnesium ionic radius

(Mg2þ ¼ 72 pm while Ir3þ ¼ 68 pm) and (2) in-

crease of in- and inter-plane coupling depending

220 M.M.A. Sekkina, K.M. Elsabawy / Physica C 391 (2003) 217–222

upon electron-dopant effect (charge on Ir-ion

higher than that on Mg-ion). It is known that

MgB2 has AB2 structure type (see Fig. 1(c)), which

is commonly found for many metal diborides. This

structure consists of interleaved graphite-like lay-

ers of boron and triangular layers of metal atoms.

Fig. 2. SEM-images for Mg1�xIrxB2: (a) x ¼ 0:00 mol%, (b) x ¼ 0:02

sectional SEM-image with two different amplification factors (2 and

Cava et al. [22] have reported the minimal cri-

teria for judging a successful chemical substitution

in magnesium diboride system which are: (1) the

second phases should not grow systematically with

propagation of dopant ratio concentration in the

solid state solution and special care must be ap-

mol%, (c) x ¼ 0:06 mol% and (d) x ¼ 0:12 mol%. (e, f) Cross

5 lm) for polished undoped MgB2.

0.00 0.02 0.04 0.06 0.08 0.10 0.1215

20

25

30

35

40

Iridium dopant ratio mole %

T c-offs

et

(a)

(d)

(b)

(c)

(e)

Fig. 3. DC-electrical resistivity curves for Mg1�xIrxB2: (a)

x ¼ 0:00 mol%, (b) x ¼ 0:02 mol%, (c) x ¼ 0:06 mol% and (d)

x ¼ 0:12 mol%. (e) Variation of Tc-offsets as a function of

iridium dopant ratio.

M.M.A. Sekkina, K.M. Elsabawy / Physica C 391 (2003) 217–222 221

plied to peak indexing of impurity phases, (2) the

shift of lattice parameter by more than three

standard deviations in least squares lattice pa-

rameter refinements in the series of doped samples

should be seen and (3) the properties of super-

conductor should change on doping.In our opinion we are in full agreement with

Cava et al. [22], and we can add that a fourth

feature must be present in the dopant atom as

a precondition factor to substitute successfully,

which is the atomic size of the dopant element that

must be harmonic and thermodynamically com-

patible with the solid solution of system on doping

whatever the type of system.

3.2. Microstructure properties

Fig. 2(a)–(d) explains SEM-images through ab-plane for Ir-doped Mg1�xIrxB2 and pure MgB2

with boron particle size A6 50 lm. From the

analysis of SEM-images and EDX elemental

analysis for many spots on each sample we de-duced the following facts: MgO/or B impurity

phases are not noticeable at the intergrain layers

especially in samples with lower Ir-concentration,

x ¼ 0:0, 0.2 and 0.06 mol% while sample (d) with

maximum dopant ratio (x ¼ 0:12 mol%) begins to

appear in between layers in very small aggregation.

The average estimated grain size is in between 0.3

and 1.8 lm. These observations and facts are alsovery clear in our XRD patterns impurity phases

which begin to appear clearly with considerable

intensities in samples (c) and (d) (see Fig. 1).

Fig. 2(e) and (f) shows SE-micrograph of cross

sector in the polished undoped MgB2 pellet with

two different amplification factors 2 and 5 lm.

From this micrograph besides EDX analysis, one

can conclude that MgB2 is the major phase withhigh degree of homogeneity which appears in de-

gree of gray coloration.

These results obtainable from SEM explain why

Tc-offsets in DC-resistivity measurement decreases

as Ir-dopant concentration increases.

3.3. Superconducting measurements

Fig. 3(a)–(d) shows the DC-electrical resistivity

measured as a function of absolute temperature

for undoped MgB2 and iridium-doped family of

Mg1�xIrxB2 (x ¼ 0:02, 0.06 and 0.12 mol%). It isclear that the Tc-offsets decrease regularly (37.3,

33.2, 28.1 and 18 K) as Ir-dopant concentration x

222 M.M.A. Sekkina, K.M. Elsabawy / Physica C 391 (2003) 217–222

increases from x ¼ 0 to 0.12 mol% respectively (see

Fig. 3(e)). From these results, the decrease of Tc-offsets is mainly due to two factors: first is the hole

band filling caused by trivalent Ir-dopant [23,24]

and the second factor is increase in the impurity

phases concentration as x increases as shown inFig. 1. It is clear that the sample with lower Tc-offset (Tc ¼ 18 K) is that for higher iridium con-

centration (x ¼ 0:12 mol%) and higher impurity

phases (see Fig. 1). Furthermore, existence of

MgO-impurity phase which is an insulator oxide

[25] inhibits the supercurrents especially if it is

present in between the grains of MgB2. In our

work MgO was detected as a secondary phase inboth XRD and SEM pictures.

4. Conclusions

The maximum iridium solubility limit m was

found to be �0.06 mol% such that in our results

especially from XRD it is clear that no substitu-tion was over 6%, which emphasizes that substi-

tutions on MgB2 system is too limited and needs

many precautions which must be present together

to be successful especially atomic size harmoniza-

tion and thermodynamic compatibility of the do-

pant element in the solid state solution.

References

[1] J. Akimitsu, in: Symposium on Transition Metal Oxides,

Sendai, January 10, 2001.

[2] J. Nagamatsu, N. Nakagawa, T. Murakana, Y. Zenitani,

J. Akimitsu, Nature 410 (2001) 3.

[3] S.L. Bud�ko, G. Lapertot, C. Petrovic, C.E. Cunningham,

N.Anderson, P.C. Canfield, Phys. Rev. Lett. 86 (2001) 1877.

[4] D.K. Finnemore, J.E. Ostenson, S.L. Bud�ko, G. Lapertot,

P.C. Canfield, Phys. Rev. Lett. 86 (2001) 2420.

[5] P.C. Canfield, D.K. Finnemore, S.L. Bud�ko, J.E. Osten-

son, G. Lapertot, C.E. Cunningham, C. Petrovic, Phys.

Rev. Lett. 86 (2001) 2423.

[6] S.L. Bud�ko, C. Petrovic, G. Lapertot, C.E. Cunningham,

P.C. Canfield, M.-H. Jung, A.H. Lacerda, Phys. Rev. B 63

(2001) 220503.

[7] S.L. Bud�ko, V.G. Kogan, P.C. Canfield, Phys. Rev. B 64

(2001) 180506.

[8] S.L. Bud�ko, P.C. Canfield, Phys. Rev. B 65 (2002) 212501.

[9] See for example, J. Castaing, P. Costa, in: V.I. Matkovich

(Ed.), Boron and Refractory Borides, Springer-Verlag,

Berlin, 1977, p. 390.

[10] J.S. Slusky, N. Rogado, K.A. Regan, M.A. Hayward, P.

Khalifah, T. He, K. Inumaru, S.M. Loureiro, M.K. Haas,

H.W. Zandbergen, R.J. Cava, Nature 410 (2001) 343.

[11] B. Lorenz, R.L. Meng, Y.Y. Xue, C.W. Chu, Phys. Rev. B

64 (2001) 52513.

[12] J.Q. Li, L. Li, F.M. Liu, C. Dong, J.Y. Xiang, Z.X. Zhao,

Phys. Rev. B 65 (2002) 132505.

[13] H.W. Zandbergen, M.Y. Wu, H. Jiang, M.A. Hayward,

M.K. Haas, R.J. Cava, Physica C 366 (2002) 221.

[14] H. Luo, C.M. Li, H.M. Luo, S.Y. Ding, J. Appl. Phys. 91

(2002) 7122.

[15] M. Paranthaman, J.R. Thompson, D.K. Christen, Physica

C 355 (2001) 1.

[16] J.S. Ahn, E.J. Choi, 2002. Available from <cond-mat/

0103169>.

[17] T. Takenobu, T. Ito, D.H. Chi, K. Prassides, Y. Iwasa,

Phys. Rev. B 64 (2001) 134513.

[18] W. Mickelson, J. Cumings, W.Q. Han, A. Zettl, Phys. Rev.

B 65 (2002) 052505.

[19] Z.-h. Cheng, B.-g. Shen, J. Zhang, S.-y. Zhang, T.-y. Zhao,

H.w. Zhao, J. Appl. Phys. 91 (2002) 7125.

[20] A. Bharathi, S.J. Balaselvi, S. Kalavathi, G.L.N. Reddy,

V.S. Sastry, Y. Hariharan, T. Radhakrishnan, Physica C

370 (2002) 211.

[21] M.M.A. Sekkina, K.M. Elsabawy, Solid State Commun.

123 (2002) 1.

[22] R.J. Cava, H.W. Zandbergen, K. Inumaru, Physica C 385

(2003) 8.

[23] S. Xu, Y. Moritomo, K. Kato, A. Nakimura, J. Phys. Soc.

Jpn. 70 (2001) 1889.

[24] T. Takenobu, T. Ito, D.H. Chi, K. Prassides, Y. Iwasa,

Phys. Rev. B 64 (2002) 52505.

[25] R.F. Klie, J.C. Idrobo, N.D. Browning, K.A. Regan, N.S.

Rogado, R.J. Cava, Nature 410 (2001) 343.