New superconducting sulfide - `EuNb2S5'

Article abstract of DOI:10.1016/S0921-4534(98)00065-3

A new low-temperature superconducting (T_c≈3 K) niobium sulfide `EuNb₂S₅' with a hexagonal structure (a = 3.3556(8) angstroms, c = 23.440(9) angstroms) was synthesized from a mixture of EuS, Nb(m) and S in evacuated and sealed silica tubes, heated at 1100 °C for 48 h. This superconductor was characterized by EDX analysis, X-ray powder and electron diffraction studies. Electron diffraction revealed the presence of a supercell: a_super = b_super = √7×a.

Full text of DOI:10.1016/S0921-4534(98)00065-3

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Physica C 300 1998 67–70
A new superconducting sulfide—‘EuNb2S5’
a
a
a
S.Ya. Istomin a,), A.Yu. Grippa , O.G. D’yachenko , E.V. Antipov ,
b
c
G. Svensson , J.L. Tholence
Chemical Department, Moscow State UniÕersity, 119899 Moscow, Russian Federation
a
b
Department of Structural Chemistry, Arrhenius Laboratory, Stockholm UniÕersity, S-106 91 Stockholm, Sweden
c
LEPES, CNRS, BP 166, 38042 Grenoble Cedex 9, France
Received 8 December 1997; accepted 14 January 1998
Abstract
Ž
.
Ž
Ž .
A new low-temperature superconducting Tcf3 K niobium sulfide ‘EuNb2S5’ with a hexagonal structure as3.3556 8
˚
˚
.
Ž .
Ž .
A, cs23.440 9 A was synthesized from a mixture of EuS, Nb m and S in evacuated and sealed silica tubes, heated at
11008C for 48 h. This superconductor was characterized by EDX analysis, X-ray powder and electron diffraction studies.
Electron diffraction revealed the presence of a supercell: asuper sbsuper s67=a. q 1998 Elsevier Science B.V.
Keywords: Superconductivity; Synthesis; Eu- and Nb-based sulfide
1. Introduction
probably due to this complexity and to the lack of
X-ray diffraction data from single crystals.
Considering the similar ionic radii of Eu2q and
Ž
.
The AM2 S5 AsSr, Ba and MsNb, Ta, V
compounds are known to exhibit highly diverse
transport properties. Thus, SrM2 S5, MsTa, Nb and
BaTa2 S5 are superconductors with Tc ;3 K 1–3 ,
whereas BaNb2 S5 is metallic down to 1.9 K 4 . The
recently synthesized SrV2 S5 is reported to have a
semiconductor-like type of conductivity 5 . All the
AM2 S5 compounds have hexagonal structures, ex-
cept SrV2 S5, which is rhombohedral. A specific
feature of the AM2 S5 compounds treated here is the
occurrence of complex superstructures, found both
2q
2q
Sr2q rSr s1.16 A, rEu s1.17 A, CNs6 7 , one
may logically predict the existence of an Eu-contain-
ing analogue of SrNb2 S5, but the only compound
previously reported in Eu–Nb–S system is
˚
˚
Ž
w x.
w
w x
x
w x
Eu1r6 NbS2 8 , which is suggested to have an inter-
w x
Ž . Ž
calated A x NbS2 Asalkali metal structure as
˚
˚
.
3.352 A, cs14.75 A . In this work, we present the
synthesis and characterization of the new EuNb2 S5
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phase, which is a low-Tc superconductor Tc f3 K .
w
x
by electron and X-ray diffraction studies 1,5,6 . The
structures of these compounds are still unknown
2. Experimental
EuNb2 S5 was prepared by heating powders of
)
Ž . Ž
.
intimately mixed EuS, Nb m Roth, 99.999% and S
Corresponding author. Fax: q7-095-939-4788; e-mail:
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99.99% in evacuated and sealed silica tubes at
istomin@inorg.chem.msu.su.
0921-4534r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.
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PII S0921-4534 98 00065-3

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68
S.a. Istomin et al.rPhysica C 300 1998 67–70
1050–11508C for 48 h. EuS was obtained by heating
The AC susceptibility between Ts1.5–80 K was
measured with a locally constructed mutual-induc-
tance bridge, operating at 110 Hz with an applied
field of 0.07 Oe.
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.
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.
Eu2 SO4 3 =8H2O 99.9% in a H2 S flow at 9008C
for 48 h, with one intermediate regrinding.
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X-ray powder diffraction patterns XRD for the
phase characterization were recorded in a focusing
camera of Guinier type, using CuKa radiation and
1
Ge as internal standard. X-ray powder diffraction
3. Results and discussion
data were collected on a STOE STADIrP powder
Ž
diffractometer CuKa radiation, Ge monochro-
The novel compound EuNb2 S5 was synthesized
by annealing a stoichiometric mixture of the starting
1
.
mator equipped with a scintillation detector, using a
Ž
rotating sample in symmetric transmission mode.
Electron diffraction studies were performed in a
transmission electron microscope, JEOL JEM-2000
FX II. Microanalysis was performed in a scanning
electron microscope JEOL JSM-820 equipped with
materials at 11008C for 48 h. The XRD pattern see
.
Fig. 1 was indexed with an hexagonal unit cell:
˚
˚
Ž .
Ž .
as3.3556 8 A and cs23.440 9 A. However,
some reflections in the XRD pattern remained unin-
dexed. They are shown with asterisks in Fig. 1. The
electron diffraction studies discussed below revealed
a superstructure causing these extra reflections.
The composition of ‘EuNb2 S5’ obtained by EDX
had a higher europium content than nominal. The
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an EDX energy-dispersive X-ray microanalysis
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.
system LINK AN10000 . The chemical composi-
tions of the samples were obtained by averaging
8–12-point analyses.
Fig. 1. X-ray powder diffraction pattern of EuNb2 S5. Superstructure reflections are shown by asterisks.

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S.a. Istomin et al.rPhysica C 300 1998 67–70
69
average composition of 10 grains was Eu:Nb:Ss
Ž . Ž . Ž .
17.2 6 :22 1 :61 1 , to be compared with the nomi-
nal composition 12.5:25:62.5. Occasionally, crystal-
lites with a lower europium content were found
Ž
Ž .
Ž . Ž .
Eu:Nb:Ss5.4 1 :29.8 8 :64 2 , implying them to
.
be Eu1r6 NbS2 . The results thus indicate the compo-
sition of this new compound to differ from the
stoichiometric EuNb2 S5.
A sample with a nominal composition correspond-
ing to the EDX analysis of the new phase was heated
at 11008C for 45 h. The EDX analysis showed the
product to have a composition very close to that
obtained for the first prepared sample, discussed
Ž
Ž .
Ž .
Ž ..
above Eu:Nb:Ss16 1 :23.4 5 :60.6 7 . No crystal-
lites of Eu1r6 NbS2 were found, but about 10% of
unidentified secondary phases were present in the
XRD powder pattern. The unit cell dimensions of
‘EuNb2 S5’ in this sample were significantly different
from those of ‘EuNb2 S5’ prepared from a stoichio-
˚
˚
Ž .
Ž
Ž .
metric mixture as3.3495 3 A, cs23.274 6 A
Fig. 3. Temperature dependence of the magnetic susceptibility for
the EuNb2 S5 sample.
˚
˚
Ž .
Ž .
.
and as3.3556 8 A, cs23.440 9 A, respectively .
Moreover, ‘EuNb2 S5’ prepared from a stoichiomet-
Ž
.
ric mixture at a lower annealing temperature 10508C
Ž .
also had different unit cell dimensions: as3.3544 8
A, cs23.35 1 A. Considering the information given
above, we can suggest EuNb2 S5 to be a non-
stoichiometric compound with a composition sensi-
tive to the synthetic conditions. This explains the
variation in unit cell dimensions between differently
prepared ‘EuNb2 S5’ samples.
˚
˚
Ž .
The electron diffraction pattern of EuNb2 S5
w
x
viewed along 001 is shown in Fig. 2. All strong
reflections were indexed with the hexagonal lattice
parameters mentioned above. However, the appear-
ance of weak spots in the hk0 zone indicates the
presence of a superstructure similar to that of
w x
SrNb2 S5 1 : asuper sbsuper s67=a. Reflections
corresponding to this supercell are easily observed
Ž
also in the XRD pattern shown in Fig. 1 by aster-
.
isks . It should be mentioned that occasionally, some
crystallites without the 67=a=67=a superstruc-
ture reflections were found, but EDX analysis data
showed them to be Eu1r6 NbS2.
The temperature dependence of the magnetic sus-
ceptibility for the EuNb2 S5 sample synthesized at
11008C for 48 h is shown in Fig. 3. The onset of the
superconducting transition is 3 K. The estimated
fraction of the superconducting phase is about 80%,
which indicates bulk superconductivity. The ob-
served Tc is very close to that reported for SrNb2 S5
Fig. 2. Electron diffraction pattern of EuNb2 S5 viewed along
w
x
Ž
.
Ž
'
.
001 . Subcell reflection 100 and 010 is marked by arrow. A
'
superstructure corresponding to a 7 =a= 7 =a supercell is
clearly observed.

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S.a. Istomin et al.rPhysica C 300 1998 67–70
Ž
w x.
;3 K 1 , indicating that complete replacement of
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