Letter to editor: J. Sup. & Novel Mag. (2012)
Vacuum encapsulated synthesis of 11.5 K NbC superconductor
Rajveer Jha and V. P. S.Awana*
Quantum Phenomena and Application Division, National Physical Laboratory (CSIR)
Dr. K. S. Krishnan Road, New Delhi-110012, India
Bulk polycrystalline NbC samples are synthesized through solid state reaction route in an
evacuated sealed quartz tube. Studied NbC samples are crystallized in NaCl-type cubic structure
with space group Fm-3m. To control cell parameters and minute un-reacted phases, different
samples are synthesized with various heat treatments. Finally phase pure NbC is achieved. The
grain size of the as systemized material being seen from SEM (scanning electron microscopy) is
non-uniform of around 3-10µm size. Crystal structure and lattice parameters of samples have
been calculated by Rietveld analysis of room temperature X-ray powder diffraction data. The
lattice parameter increases with synthesis temperature and scales with superconducting transition
temperature (Tc). Both AC and DC magnetization exhibited highest Tc at around 11.5 K for an
NbC sample with lattice parameter a = 4.471 Ǻ. The lower critical field (Hc1) and irreversibility
field (Hirr) measured at 3 K are around 250 Oe and 4.5 kOe respectively. The upper critical field
(Hc2) being determined from in-field AC susceptibility measurements is 7.8 kOe and 11.7 kOe
with 50% and 90% diamagnetism criteria, respectively.
Keywords: A. NbC; B. Crystal Structure; C. Magnetization; D. Upper critical field.
*Corresponding Author: Dr. V.P.S. Awana, (E-mail: firstname.lastname@example.org)
Senior Scientist, NPL, New Delhi-12, India
Web page- www.freewebs.com/vpsawana/
Tel. +91-11-45609357, Fax. +91-11-45609310
The race for the discovery of new superconductors got a boost in early 1986 with
invention of cuprate high Tc superconductors (HTSc) . Further, when the momentum was a bit
slowed down, in year 2002 the discovery of MgB2  steamed the race once again. More
recently, the scientists are once again puzzled with the invention of pnictide superconductors .
Interestingly, before cuprates , the superconductivity for over 75 years was mostly confined to
the electron-phonon mediated interactions . On materials science front itself, most of exotic
superconductors have appeared only after the discovery of cuprates . In fact after discovery of
superconductivity in Cuprates  and Fe based pnictides , one wonders as if, still more
surprises are yet to come from periodic table.
In this short communication, we focus on an inter-metallic superconducting compound
i.e., NbC, on which there are only few scant reports [5-9] in literature. Our interest on this
compound is two fold, first though it was discovered long back in 1964 itself , the same is yet
least studied superconductor. Second, the solid state chemistry of different allotropes of Carbon
in any crystalline compound had been of immense interest in last couple of decades not only for
chemists but to physicists as well. Couple of examples in this regards are Fullerenes , Carbon
Nano Tubes (CNT) [11,] and Graphene . We synthesized the intermetallic Nb based Carbide
NbC compound via a simple vacuum encapsulation technique. The vacuum encapsulation
technique based high temperature synthesis of compounds recently gained popularity after
discovery of Fe based pnictide superconductors. The superconducting critical temperatures of
superconducting intermetallic compounds can be empirically related to parameters such as
crystal structure, composition, free electron concentration, and lattice parameter [6-8]. It is
difficult to synthesis NbC because transition metal react at high temperature. Niobium Carbide is
a refractory ceramic with a high melting point (3600°C), excellent mechanical properties, and
low electrical resistance . Additionally, NaCl-type transition metal carbides MC (M =V, Ti,
Nb, Ta, Hf and Zr) are of fundamental importance in solid-state science and technology [5-9].
Some researchers have reported the unit-cell parameter and crystal structure of these Carbides [6,
8]. Seemingly, the cell parameters and Carbon content in NbC play deciding role in deterring the
superconducting transition temperature (Tc) . Actually the lattice parameter depends upon the
C content of the NbC system as it possesses wide region of homogeneity (from NbC1.00 to
NbC0.72) [13, 14]. The disordered state of the non-stoichiometric NbCx has B1 structure. Neutron
diffraction studies showed that with the C content of 0.81 to 0.88 ordered phases of M6C5-type
are formed in NbC system . Furthermore, the atomic displacement parameters of the
Carbides have rarely been reported. Cubic NbC is a superconductor below 12 K [5-9]. In other
words, innovative approaches are needed to synthesize NbC with controlled
microstructures/defects so that enhanced superconducting properties, reduced electrical
resistivity in the normal state at low temperatures, and improved mechanical strengths can be
accomplished. From a technological point of view, this material is interesting, because of its
possible application to induce superconductivity in Carbon nanotube junctions . Here we
report the synthesis temperature optimization and host of superconducting properties for the
stoichiometric NbC. We optimized the synthesis temperature for NbC and achieved a single
phase compound without any detectable impurity within the resolution limit of X-ray powder
diffraction. The resultant compound is superconducting at 11.5 K.
All the studied polycrystalline NbC samples were prepared through single step solid-state
reaction route via vacuum encapsulation technique. High purity (~99.8%) Nb, and nano-C in
their stoichiometric amount were weighed, mixed and ground thoroughly using mortar and
pestle. The mixed powders were palletized and vacuum-sealed (10-4 Torr) in a quartz tube. These
sealed quartz ampoules were placed in box furnace and heat treated at (a) 1150oC for 24 hours,
(b) 1200oC for 24 hours and (c) at 1250oC for 24 hours each. Finally furnace was allowed to cool
down to room temperature naturally. The crystal structure was analyzed by the powder X-ray
diffraction patterns at room temperature using Rigaku X-ray diffractometer with Cu-K radiation.
The scanning electron microscopy (SEM) image of the sample (synthesized at 1250°C) was
taken on a ZEISS-EVO MA-10 scanning electron microscope. The magnetic (DC and AC
magnetization) measurements are carried out on the physical property measurement system
(PPMS-14T) from Quantum Design-USA.
III. Results and discussions
Figure 1 shows the room temperature XRD patterns of various NbC samples being
synthesized at different temperatures. The XRD patterns of 1150°C, 1200°C and 1250°C
processed samples are shown one over the other in Fig. 1. The XRD data are fitted using
Rietveld refinement (FullProf Version). It is clear that samples are phase pure and crystallized in
cubic structure with space group Fm-3m. Coordinate positions for the atoms are Nb: 1a (0, 0, 0),
and C: 1b (0.5, 0.5, 0.5). We observed that with an increase in sintering temperature from 1150
°C to 1250°C, the lattice parameter is increased monotonically from 4.468 Å to 4.471 Å, see
Table 1. For further higher temperature synthesized samples, the increase in lattice parameter
was not observed; also the phase purity of the compound also gets compromised. Hence we show
the XRD data only for up to 1250oC synthesized sample.
Figure 2 represents the DC magnetization (M-T) plots for samples synthesized at various
temperatures. Both the Zero-field-cooled (ZFC) and field-cooled (FC) data are shown. The
superconducting transition onset Tc is found to be 9.09 K, 11.0 K, and 11.5 K respectively for
1150°C, 1200°C, and 1250°C synthesized samples. The 1200°C sample exhibited a two step
kind of transition in ZFC magnetization. It seems samples possess both the 1150°C, and 1250°C
phase within. Inset of Fig. 2 depicts the Tc vs lattice parameter variation plot for variously
synthesized NbC samples. It can be seen that with increasing lattice parameter the Tc is
increasing monotonically. We synthesized NbC for higher temperatures as well but no further
increase in lattice parameter is observed, also phase purity of compound gets compromised. The
increase in Tc of NbC with its c-parameter is in agreement with an earlier report . It is reported
that with increase of synthesis temperature the Carbon content in the crystal structure of NbC
increases [15, 16], similar to MgCNi3 [17, 18]. Since Carbon is a light element, so it is difficult
to extract the exact Carbon content by XRD refinement. The exact content of Carbon can be
determined through Neutron diffraction pattern, as done for MgCNi3 . The dependence of Tc
with Carbon content is reported earlier [15, 16]. On the basis of these reports the real Carbon
content in the sample having Tc = 11.5 K (synthesized at 1250oC) can be estimated to be ~ 0.99.
The C content in the samples synthesized at lower temperatures (1150oC and 1200oC) decreases
and thus the Tc. Decrease in both the electron-phonon interaction parameter and the un-
renormalized density of electron states causes this rapid decrease in Tc with decreasing C
concentration . The structural changes occurring in nonstoichiometric (NbCx) as a result of the
ordering of Carbon atoms and vacancies lead to a reconstruction of their electron and phonon
spectra. Earlier it is observed that the ordering of Carbon atoms and vacancies results in
decreased magnetic susceptibility with an increase in the lattice parameter of the B1 basic
structure . In any case we found that best/optimum Tc of around 11.5 K is obtained for 1250
oC samples. Neither lower (<1250oC) nor higher (> 1250oC) temperature synthesis could help in
further improving upon the observed Tc of 11.5 K. Hence now on words we focus on
superconductivity characterization optimum Tc (11.5 K) sample, which is synthesized at 1250oC.
Figure 3(a) shows the DC magnetization of the phase pure NbC sample (Synthesized at
1250°C) in both field cooled (FC) and zero field cooled (ZFC) situations at 10 Oe field. The
sample shows bulk superconductivity with an onset temperature (Tc onset) of 11.5 K. Figure 3(b)
and 3(c) showing the real and imaginary part of AC magnetization for the same. It is clear that
both DC and AC susceptibility magnetization exhibit superconducting transition below 11.5K.
The imaginary part of AC susceptibility exhibit clear single peak at around 11 K. In case of
superconductors, the real part of susceptibility depicts diamagnetic shielding of the sample, while
imaginary part indicates the hysteric losses due to vortex motion. Maximum loss takes place
when the magnetic flux lines just penetrate the centre of the sample and is indicated by the peak
in the magnetization curve at a particular temperature called as peak temperature. The single
sharp peak in imaginary AC susceptibility indicates the better coupling of the grains in the
studied NbC. As far as DC susceptibility and real part of AC susceptibility are concerned, we
define the superconducting transition onset temperature Tc at a temperature where finite change
of diamagnetic moment takes place. The Tc onset for the presently studied bulk NbC is at 11.5 K
in both DC and AC magnetization. There seems to be large irreversibility in the DC
magnetization plot, indicating towards good pinning. This compound seemingly possesses no
grain boundary contributions. This explains its single intra-grain transition with the possibility of
large irreversibility field.
Figure 4 represents the AC magnetization of the same (1250 0C synthesized) sample,
confirming the bulk superconductivity at around 11.5 K. Further, AC magnetic susceptibility
measurements have been done at 333 Hz and varying amplitude of 3-15 Oe. With change in
amplitude from 3-15 Oe, the imaginary part peak height is increased along with increased
diamagnetism in real part of AC susceptibility. This is usual for a superconductor. The
interesting part is that the imaginary part peak position temperature (11.5 K) is not changed at all
with increase in AC amplitude. This shows that the superconducting grains are well coupled and
hence nearly no grain boundary contribution in the superconductivity of NbC. To check the grain
connectivity and morphology of the sample we have made the Scanning Electron Microscopy
(SEM) study. Figure 5 represent the SEM image of the sample synthesized at 1250oC. Well
connected grains can be seen and/or grain connectivity is such that the grain boundary
contribution is almost zero. It is clear from AC susceptibility (imaginary part) results that either
the grain boundaries are non-existent in NbC or transparent to the current. This is similar to that
as observed in case of MgCNi3 .
Figure 6, shows that real part of AC susceptibility being measured applied DC bias field
of up to 30 KOe, and its inset depicts the isothermal magnetization (M-H) plot at 3 K in high
field range of up to 20 kOe in four quadrants for optimized (1250oC) NbC. The lower critical
field (Hc1) and irreversibility field (Hirr) are around 250 Oe and 4.5 kOe, respectively, at 3 K, see
inset Fig. 6. The upper critical field (Hc2) being determined from in-field AC susceptibility
measurements, is 7.78 kOe and 11.73 kOe with 50% and 90% diamagnetism criteria,
respectively. NbC belongs to the dirty-limit superconductor . The value of Hc2 being
measured from the in field AC susceptibility measurements is close to the paramagnetic limit HP
= 1.84Tc. Thus Zeeman pair breaking mechanism will not be effective in this case. It is clear that
irreversibility field (Hirr) and upper critical field (Hc2) both are of the same order of magnitude.
Vacuum encapsulate NbC samples are synthesized and found to crystallize in cubic
structure with space group Fm-3m. Increase in lattice parameter is observed with increase in
synthesis temperature which leads to increase in C content in the system and thus the enhanced
Tc. An optimized Tc of 11.5 K is observed for the 1250°C synthesized NbC sample. AC
magnetization with field and SEM image indicated good grain connectivity for studied NbC
sample. The upper critical field (Hc2) being determined from in-field AC susceptibility
measurements, is above 11.73 kOe with 90% diamagnetism criteria. It is clear that we succeeded
in synthesis of single phase 11.5 K NbC superconductor by quartz vacuum encapsulation in a
The authors are grateful for the encouragement and support from Prof. R. C. Budhani
(Director NPL) for his motivation and discussions. Rajveer Jha would like to thank the CSIR for
providing the SRF scholarship to pursue his Ph.D. This work is also financially supported by
Department of Science and Technology (DST-SERC) New Delhi, India.
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Table 1: Variation of Lattice parameters and transition temperature of NbC samples
with synthesis temperature.
1150 4.468 (2) 9.09 3.27
1200 4.469 (3) 11.0 3.28
1250 4.470 (2) 11.5 3.94
Figure 1 Rietveld fitted XRD pattern of NbC samples with space group Fm-3m synthesized at different temperatures
(1150°C, 1200°C, and 1250°C).
Figure 2 DC magnetization in ZFC (zero-field-cooled) and FC (field-cooled) situation for all samples at 10 Oe.
Figure 3 (a) DC magnetic susceptibility M(T) in ZFC and FC situations at 10 Oe of NbC, (b) real part of AC
susceptibility, measure at 333 Hz and 10 Oe of NbC and (c) imaginary part of AC susceptibility, measured at 333
Hz and 10 Oe of NbC, synthesized at (1250°C).
Figure 4 AC magnetic susceptibility in both real (M’) and imaginary (M) situations at fixed frequency of 333 Hz
and varying amplitudes of 3-15 Oe for NbC Synthesized at (1250°C).
Figure 5 SEM image of NbC synthesized at 1250°C, shows well connected grains.
Figure 6 Isothermal magnetization (MH) for real part of AC susceptibility (M') with applied field up to 30 kOe at 3
K for NbC Synthesized at (1250°C) the upper critical field (Hc2) is marked. Inset shows expanded MH plots at 3 K
in the low field range of up to 10 kOe in four quadrants for NbC.
SG: F m-3m
SG: F m-3m
SG: F m-3m
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