Synthesis of high-purity RuSr2GdCu2O8 with semiconductivity

Mingde Li a, Min Yu b, Zhongbing Wang a, Hongshun Yang b, Yuan Hu c,Zuyao Chen a,*, Zhiquan Li b, Liezhao Cao b

a Structure Research Laboratory and Department of Chemistry, University of Science and Technology of China,

230026 Hefei, Anhui, Chinab Department of Physics, University of Science and Technology of China, 230026 Hefei, Anhui, China

c State Key Fire Laboratory of China, University of Science and Technology of China, 230026 Hefei, Anhui, China

Received 22 January 2001; received in revised form 25 March 2001

Abstract

The synthesis of superconductor RuSr2GdCu2O8 by common solid-state reaction is always accompanied by the

formation of small amounts of ferromagnetic SrRuO3 impurities. In this paper, we present a precursor method of pure

RuSr2GdCu2O8 preparation. Pure precursor Sr2GdRuO6 is synthesized by solid-state reaction in flowing O2 and water

vapor. The precursor crystallizes in an orthorhombic structure, a ¼ 5:814 �AA, b ¼ 5:802 �AA, c ¼ 8:206 �AA. RuSr2GdCu2O8

prepared by this method is a high-purity RuSr2GdCu2O8 phase. Stoichiometric pure RuSr2GdCu2O8 is semiconductive.

� 2002 Elsevier Science B.V. All rights reserved.

PACS: 74.25.F; 74.25.H; 74.62.B

Keywords: RuSr2GdCu2O8; Sr2GdRuO6; SrRuO3; Water vapor

1. Introduction

Two types of ruthenate–cuprate superconduc-

tors, RuSr2LnCu2O8 (Ru-1212) and RuSr2(Ln,

Ce)2Cu2O10 (Ru-1222) (Ln ¼ Sm, Eu, Gd) [1–4]had been synthesized. The synthesis of these phases

by common solid-state reaction is always accom-

panied by the formation of ferromagnetic SrRuO3

impurities, which have a devastating influence on

the superconductivity.

Recently, the material of Ru-1212 was found to

display not only superconductivity, but coexisting

ferromagnetism as well [5], and attracted a great

deal of interest. RuSr2GdCu2O8 phase can be

transformed into Sr2GdRuO6 (Ru-211) and Cu2Oby sintering in flowing nitrogen. The conven-

tional synthesis method [2] is calcining stoichio-

metric CuO, RuO2, Gd2O3 and SrCO3 to produce

initial 1212 compound, then sintering it in flow-

ing nitrogen and oxygen alternatively to reduce

SrRuO3 by the transition between Ru-1212 and

Ru-211 phases. But our effort to prepare pure

1212 samples by this method was not successful.In this paper, we present preparation of high-

purity RuSr2GdCu2O8 compound from a precur-

sor Sr2GdRuO6 (Ru-211) that was synthesized by

Physica C 382 (2002) 233–236

www.elsevier.com/locate/physc

*Corresponding author. Fax: +86-551-3631760.

E-mail address: czy@ustc.edu.cn (Z. Chen).

0921-4534/02/$ - see front matter � 2002 Elsevier Science B.V. All rights reserved.

PII: S0921-4534 (02 )01137-1

solid-state reaction in O2 and water vapor atmo-

spheres. Some results about 211 and 1212 phases

synthesized by this method are given.

2. Experimental

The starting materials were high-purity oxides

CuO (99.99%), RuO2 (99.99%), Gd2O3 (Specpure,

Johnson Matthey Chemicals) and strontium–car-

bonate SrCO3 (99.99%). The starting material of

RuO2 contained crystal water, so it was baked at

400–500 �C for 5–6 h before weighing. Then it was

taken out of furnace and weighed immediately.First, the powders of a stoichiometric RuO2,

Gd2O3, and SrCO3 mixture were calcined at 950

�C in flowing O2 and water vapor to synthesize the

pure precursor Sr2GdRuO6. The flowing O2 was

maintained during heating from room temperature

to 600 �C, then, water vapor was added. The

powders were calcined at 950 �C for 30 h in O2 and

water vapor atmospheres. To prepare Ru-1212samples, a stoichiometric mixture of pure Sr2-

GdRuO6 and CuO was milled, pressed into pellets

and sintered.

X-ray diffraction (XRD) was carried out on

a MXP18AHF diffractometer (CuKa1 radiation).

Resistivity measurements were performed by a

standard four-probe method.

3. Results and discussion

As Ru-1212, the synthesis of Ru-211 in air or

O2 is always accompanied by the formation of

SrRuO3 impurities that could not be reduced by

varying the synthesis temperature. But in flowing

O2 and water vapor, a high-purity 211 compoundwas prepared from the starting materials RuO2 þ1=2 ðGd2O3Þ þ 2ðSrCO3). The SrRuO3 phase could

not be detected by XRD diffraction as shown in

Fig. 1.

The lattice parameters of the compound Sr2-

GdRuO6 were refined to be a ¼ 5:814 �AA, b ¼5:802 �AA, c ¼ 8:206 �AA of orthorhombic symmetry.

Similar compounds having the general formulaA2BRuO6 had been prepared [6–9]. All the mate-

rials adopt a perovskite-related structure with an

alternative ordering of the B cations and Ru5þ on

the octahedral site.

Fig. 2 shows the zero-field-cooled magnetiza-

tion measurement of 211 phase. The disappear-

ance of peaks characteristic for the ferromagnetic

transition of SrRuO3 at 160 K indicates that the211 phase is pure without SrRuO3. Its ferromag-

netic transition at temperature about 50 K is in

contrast with previous A2BRuO6 compounds [6–9]

that exhibit antiferromagnetism.

Three samples were synthesized via the precur-

sor route. Sample 1 was sintered at 930 �C in air

for 24 h by one step, samples 2 and 3 were calcined

Fig. 1. The XRD spectrum for a Sr2GdRuO6 (2 1 1) sample.

Fig. 2. Temperature dependence of the zero-field-cooled mag-

netization of Sr2GdRuO6.

234 M. Li et al. / Physica C 382 (2002) 233–236

in powders in O2 at 950 �C for 24 h first, then

sintered in pellets at 1050 �C in 1 atm O2 for 24 h.

Sample 1 was high-purity 1212 compound, and in

samples 2 and 3 there were some Sr2GdRuO6 and

SrRuO3 impurities.

Fig. 3 shows the powder diffraction diagrams ofsamples 1 and 2. The lattice parameters were re-

fined to be a ¼ 3:839 �AA, c ¼ 11:51 �AA for sample 1

and a ¼ 3:832 �AA, c ¼ 11:51 �AA for sample 2. The

crystal structures are similar to that of MBa2-

LaCu2O8 (M ¼ Nb, Ta) and MA2RECu2O8 with

M ¼ Nb or Ta, A ¼ Ba or Sr, and RE ¼ Pr or Sm

[10–12].

Sample 1 is not superconductive although it is ahigh-purity phase as shown in Fig. 4. Annealing in

O2 for long time or in high-oxygen pressure could

not induce its superconductivity. Resistivity curve

shows a typical semiconductive behavior.

Fig. 5 shows resistivity measurements of sam-

ples 2 and 3. Resistivity curves exhibit ferromag-

netic transition at 133 K as that reported [5] and

superconductive transition from 24 to 30 K. Butsuperconducting transition was not detected by

magnetization measurement as shown in Fig. 6.

In summary, pure Ru-1212 phase was synthe-

sized successfully from a precursor of Sr2GdRuO6

that was prepared by calcining in O2 and water

vapor atmospheres. Stoichiometric RuSr2Gd-

Cu2O8 prepared by this method is semiconductive.

Fig. 3. The XRD diagrams for RuSr2GdCu2O8 phase: (a) high-

purity sample 1, (b) sample 2 with Sr2GdRuO6 impurities.

Fig. 4. Temperature dependence of the resistivity of RuSr2-

GdCu2O8 (sample 1).

Fig. 5. Temperature dependence of the resistivity of RuSr2-

GdCu2O8 (samples 2 and 3).

M. Li et al. / Physica C 382 (2002) 233–236 235

Acknowledgement

This work was supported by the Ministry ofScience and Technology of China (NKBRSF-

G19990646).

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Fig. 6. Temperature dependence of the zero-field-cooled mag-

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236 M. Li et al. / Physica C 382 (2002) 233–236