Superconductivity in the hexagonal-layered nanolaminates Ti2InC compound

Article abstract of DOI:10.1016/j.ssc.2007.09.028

The hexagonal-layered Ti₂InC compound is a member of the large family of lamellar materials that crystallize in the hexagonal structure with space group P 6₃ / mmc, which are isomorphs with the Cr₂AlC compound, and known as H-phase. In this work the Ti₂InC phase is investigated by X-ray diffraction, magnetization and resistivity measurements. Susceptibility and resistivity measurements display superconducting critical temperature close to 3.1 K. In spite of the great number of compounds which belong to this family, superconductivity has been reported only for four cases: Mo₂GaC, Nb₂SC, Nb₂AsC and Nb₂SnC. This work supports the existence of a new class of superconducting materials that crystallize in an H-phase.

Full text of DOI:10.1016/j.ssc.2007.09.028

Solid State Communications 144 (2007) 419–421
www.elsevier.com/locate/ssc
Superconductivity in the hexagonal-layered nanolaminates
Ti InC compound
2
∗
A.D. Bortolozo, O.H. Sant’Anna, C.A.M. dos Santos, A.J.S. Machado
Departamento de Engenharia de Materiais, Escola de Engenharia de Lorena - USP, P.O. Box 116, Lorena – SP, 12600-970, Brazil
Received 27 July 2007; received in revised form 25 September 2007; accepted 26 September 2007 by F. De la Cruz
Available online 1 October 2007
Abstract
The hexagonal-layered Ti InC compound is a member of the large family of lamellar materials that crystallize in the hexagonal structure with
2
space group P6 /mmc, which are isomorphs with the Cr AlC compound, and known as H-phase. In this work the Ti InC phase is investigated
3
2
2
by X-ray diffraction, magnetization and resistivity measurements. Susceptibility and resistivity measurements display superconducting critical
temperature close to 3.1 K. In spite of the great number of compounds which belong to this family, superconductivity has been reported only for
four cases: Mo GaC, Nb SC, Nb AsC and Nb SnC. This work supports the existence of a new class of superconducting materials that crystallize
2
2
2
2
in an H-phase.
c 2007 Elsevier Ltd. All rights reserved.
ꢀ
PACS: 74.70.Dd
Keywords: A. Superconductivity; C. H-phase; C. Nanolaminates; E. Carbides
1
. Introduction
some other interesting thermal, electrophysical, and tribological
properties [1–5]. Among those phases is Ti2InC which was first
reported by Jeitschko et al. [6] and recently synthesized in bulk
form [7]. The presence of covalent-bonded TiC octahedral, built
in 2D oriented packets connected by In metallic layers which
promote an untypical set of properties, has not been reported
for any metallic or ceramic materials.
Although Nowotny synthesized several phases with M2AX
stoichiometry, only four articles have reported superconduc-
tivity in this type of structure, which are Mo2GaC, Nb2SC,
Nb2SnC, and Nb2AsC [8–11]. However, the Nb2SnC has only
been obtained in single phase when synthesized under high
pressure. Theoretical arguments suggest that the superconduc-
tivity in the Nb2SC compound arises from pairing of elec-
trons of the sulfur atoms [12]. This suggests that a decisive
role in the superconductivity of nanolaminates is played by
electron–phonon coupling involving niobium carbide layers.
Differences in the superconducting behavior of the nanolami-
nates Nb2SnC and Nb2SC have shown that their critical tran-
sition temperatures are correlated with the orbital contributions
of the carbon atoms [12]. These results indicate that the super-
conducting behavior should not be a particularity of these four
compositions.
Recently a new family of layered ternary carbides, referred
to as “MAX” or “H-phase”, has attracted the attention of
material scientists as well as physicists and chemists [1–3].
The H-phase materials exhibit a number of interesting
properties such as high strength and modulus, damage tolerance
at room temperature, high thermal and electrical conductivities,
and being readily machinable by high speed steel tools [4,5].
The unique combination of these properties makes H-phase
materials candidates for various applications. The MAX-
phase consists of one transition metal M with A being
an A-group element (mostly III A and IV A) and X
being either carbon or nitrogen. These compounds have the
general formula Mn+1AXn, with n = 1, 2 or 3, which
are composed of molecular layers of the d-metal carbide
separated by atomic networks of p-elements. Due to this
structure, the nanolaminate compounds combine the elastic and
thermomechanical properties inherent to d-metal carbides and
∗
Corresponding author. Tel.: +55 1231599911; fax: +55 1231533006.
E-mail address: jefferson@demar.eel.usp.br (A.J.S. Machado).
0
038-1098/$ - see front matter ꢀc 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2007.09.028

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A.D. Bortolozo et al. / Solid State Communications 144 (2007) 419–421
Within this scenario we report in this work the observation
of superconductivity in the Ti2InC compound which was
prepared under ambient pressure. The results revealed
that the compound superconducts below 3.1 K. Sharp
superconducting transitions in both transport and magnetization
measurements are observed. The results suggest that the so-
called nanolaminates M2AX compound may represent a new
class of superconducting material.
2
. Experimental procedure
The samples were prepared using mixtures of graphite
(<325 mesh), Ti (<325 mesh), and In powders of high
purity in the Ti2InC stoichiometry. The mixture of powders
2
were compacted in square shape of 10 × 10 mm and
Fig. 1. Experimental (top) and simulated (bottom) X-ray diffractograms for a
◦
2
mm in thickness, sealed in quartz ampoules, and placed
sample prepared at 800 C. The results show only peaks of the Ti InC phase in
2
◦
agreement with the hexagonal structure.
in a tubular furnace at 800 C for 24 h under ambient
pressure. After this heat treatment, the samples were ground
and homogenized in agate mortar, compacted again in the
same dimensions mentioned before, and heat treated at the
same temperature additionally for 96 h. Some samples were
heat treated at higher temperatures in order to study the
effect of the temperature on the formation of the Ti2InC
phase. The samples were characterized by X-ray diffraction
in a Shimadzu diffractometer (model XRD 6000) using Ni
filter Cu Kα radiation. The microstructures of the samples
were observed by Scanning Electron Microscopy (Leo Zeiss
equipment). Electrical resistance as a function of temperature
were performed by using the conventional four-point probe
method in the interval between 2.0 and 300 K in an Oxford
Instruments (MagLab EXA-9 T). Magnetization measurements
(
magnetization versus temperature and versus applied magnetic
◦
Fig. 2. X-ray diffraction for the sample prepared at 900 C. The results show a
field) were carried out in a 5 T SQUID magnetometer from
Quantum Design.
decomposition of the sample in In, Ti C and Ti InC.
2
2
morphology with high porosity due to low heat treatment
temperature and consequently poor densification (Fig. 3). The
Energy Dispersive Spectroscopy (EDS) analysis reveals that
the stoichiometric ratio is in agreement with the nominal
composition without traces of segregated phases. The X-ray and
microstructure results allow us to say unambiguously that this
sample is single phase.
The electrical resistance as a function of temperature
measured between 2.0 K and 9.0 K on the same sample
is shown in Fig. 4(a). The sharp transition at 3.1 K with
transition broadening of ∼0.2 K can be observed in the
measurement. In the inset of this figure is displayed the same
measurement between 2.0 K and 30.0 K. In Fig. 4(b) is shown
the magnetization versus temperature measured in the Field
Cooled (FC) procedure. Essentially it is observed that the same
transition temperature is displayed in the electrical resistance
measurement.
3
. Results and discussion
Typical X-ray diffractograms, displayed in the upper part
◦
of Fig. 1, for the samples heat treated at 800 C show that
they are single phase. The simulated diffractogram (bottom of
Fig. 1) agrees with the experimental results and demonstrates
that the single phase samples have hexagonal structure in the
˚
space group P63/mmc with lattice parameters a = 3.132 A
˚
and c = 14.06 A. The lattice parameters are in good agreement
with those reported previously by Jeitschko et al. [6]. For the
P63/mmc structure, Ti, In, and C atoms occupy the 4f, 2d,
and 2a positions, respectively. The layered structure consists
of three types of slabs, two Ti–In–Ti and one Ti–C–Ti, which
obey the sequence C–Ti–In–Ti–C. Furthermore, it is important
to emphasize that there are no Ti and In atoms placed below or
above the carbon atoms along the c-axis.
◦
For samples heat treated above 800 C, the following phases
The M(H) curve performed at 2.0 K is displayed in Fig. 5
which clearly reveals the signature of type-II superconductivity.
These results reveal bulk superconductivity of the Ti2InC
compound which supports the idea of the existence of a
new class of layered superconducting materials with M2AX
composition.
are observed under thermodynamic equilibrium: In, Ti2C and
Ti2InC, which suggests that the Ti2InC compound decomposes
by a peritectic or peritectoid reaction at higher temperatures
under ambient pressure (see Fig. 2).
Based on the microstructure analysis performed in a sample
◦
heat treated at 800 C, it is possible to observe a homogeneous

A.D. Bortolozo et al. / Solid State Communications 144 (2007) 419–421
421
Fig. 5. Magnetization as a function of applied magnetic field at 2.0 K for a
Fig. 3. Micrograph of the Ti InC sample related to Fig. 1 revealing single phase
2
microstructure. The spots with different contrast are due to the difference in
topography.
Ti InC single phase. The result clearly demonstrates the existence of mixed
2
state or type-II superconductivity.
dependent on the synthesis method. These characteristics are
due to the difficulty in preparing dense samples in this family
of materials under ambient pressure. Along with Mo2GaC,
Nb2SC, Nb2SnC and Nb2AsC compounds [8–11] the results
reported here suggest a new class of superconducting materials
which crystallize in an H-phase with Cr2AlC structure.
Acknowledgements
The authors are grateful to FAPESP for the financial
support through grant numbers 04/13267-6 and 2005/01257-
9. We are also grateful to Dr. R. R. da Silva and A. de
Campos (Instituto de F ´ı sica “Gleb Wataghin”. Universidade
Estadual de Campinas – UNICAMP, Brazil) for carrying out
the magnetization measurements.
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. Conclusion
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The compound Ti2InC with layered structure exhibits zero
resistivity and diamagnetism below 3.1 K with bulk type-II
superconductivity. The superconducting behavior is extremely
(
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