New synthesis route of PbMo6S8 superconducting Chevrel phase from ultrafine precursor mixtures: II. PbS, Mo6S8, and Mo powders

Article abstract of DOI:10.1016/S0025-5408(97)00247-X

We have used a new ultrafine precursor, the Mo₆S₈ binary sulfide, and ultrafine PbS and Mo powders (approximately 0.05-0.5 μm) to prepare an original mixture for PbMo₆S₈ synthesis. The reaction of PbMo₆S₈ formation occurs in a two-step process, at 600 and 800 °C, leading to a approximately 0.5 μm grain size. χ_ac measurements exhibit good superconducting quality (T_c≈14 K) of the resulting PbMo₆S₈ phase. Additionally, we have perfected new synthesis processes of the Ni_{approximately 2}Mo₆S₈ and Li_{approximately 3}Mo₆S₈ ternary Chevrel phases, precursors of the Mo₆S₈ compound. These syntheses are carried out by H₂S gas sulfurization of the liquid or solid route and then reduction by H₂ gas over solid reaction. The Li_{approximately 3}Mo₆S₈ phase was chosen because its cation deintercalation yields the smallest Mo₆S₈ grain size (approximately 0.3 μm).

Full text of DOI:10.1016/S0025-5408(97)00247-X

Materials Research Bulletin, Vol. 33, No. 3, pp. 419–431, 1998
Copyright © 1998 Elsevier Science Ltd
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0025-5408/98 $19.00 ϩ .00
PII S0025-5408(97)00247-X
NEW SYNTHESIS ROUTE OF PbMo6S8 SUPERCONDUCTING CHEVREL
PHASE FROM ULTRAFINE PRECURSOR MIXTURES:
II. PbS, Mo6S8, AND Mo POWDERS
S. Even-Boudjada*, L. Burel, R. Chevrel, and M. Sergent
Laboratoire de Chimie du Solide et Inorganique Moleculaire (LCSIM), UMR CNRS
6511, Universite´ de Rennes I, Avenue du Ge´ne´ral Leclerc, 35042 Rennes Cedex, France
(Refereed)
(Received July 14, 1997; Accepted July 21, 1997)
ABSTRACT
We have used a new ultrafine precursor, the Mo6S8 binary sulfide, and
ultrafine PbS and Mo powders (ϳ0.05–0.5 ␮m) to prepare an original mixture
for PbMo6S8 synthesis. The reaction of PbMo6S8 formation occurs in a
two-step process, at 600 and 800°C, leading to a ϳ0.5 ␮m grain size. ␹
ac
measurements exhibit good superconducting quality (Tc Ϸ 14 K) of the
resulting PbMo6S8 phase. Additionally, we have perfected new synthesis
processes of the Niϳ2Mo6S8 and Liϳ3Mo6S8 ternary Chevrel phases, precur-
sors of the Mo6S8 compound. These syntheses are carried out by H2S gas
sulfurization of the liquid or solid route and then reduction by H2 gas over
solid reaction. The Liϳ3Mo6S8 phase was chosen because its cation deinter-
calation yields the smallest Mo6S8 grain size (ϳ0.3 ␮m). © 1998 Elsevier Science
Ltd
KEYWORDS: A. chalcogenides, B. chemical synthesis, C. electron micros-
copy, D. superconductivity
INTRODUCTION
In Part I of our paper [1], we described the synthesis and the characterization of the
superconducting PbMo6S8 lead Chevrel phase from PbS, MoS2, and Mo ultrafine precursors
mixture (ϳ0.05–0.5 ␮m) in comparison with the same precursors made by the conventional
*To whom correspondence should be addressed.
419

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Vol. 33, No. 3
TABLE 1
Hexagonal and Rhombohedral Unit Cell Parameters of
Chevrel Phase Sulfides [6,7]
Phase
aH (Å)
cH (Å)
VH (Å3)
aR (Å)
␣ (°)
R
Liϳ3Mo6S8
Niϳ2Mo6S8
Mo6S8
9.73
9.51
9.20
10.61
10.24
10.88
869.6
801.0
797.5
6.64
6.46
6.43
94.28
94.70
91.34
route (1–15 ␮m) [1]. From these new synthesis routes (coprecipitation, precipitation, and
gaseous reaction) of precursors powders, we succeeded in making precursor grains with
quasispherical morphology and very small grain size. The resulting PbMo6S8 material keeps
the same morphology grain with ϳ0.5 ␮m size and shows good superconducting qualities
(ϳ14 K). We have already pointed out [2–4] the significance of the Mo6S8 binary as a
precursor in another new PbMo6S8 preparation.
In Part II, we report the synthesis and the characterization of the PbMo6S8 phase from a
new original mixture of PbS, Mo6S8, and Mo ultrafine precursors. This new synthesis route
of PbMo6S8 from that mixture is like a combination of both previous syntheses: one with
PbS, MoS2, and Mo ultrafine powders (route l) [1] and the other one with the PbS and Mo6S8
compounds under gas flow (route 2) [2]. In the new method, we have substituted MoS2 with
Mo6S8 as the new sulfuring agent, with respect to route 1, and we have suppressed the H2 gas
flow with respect to route 2.
The new synthesis of fine PbMo6S8 powder arises from the following reaction:
PbS ϩ 0.875Mo6S8 ϩ 0.75Mo 3 PbMo6S8.
(1)
The preparation of the PbS and Mo ultrafine powders has been presented in our previous
paper [1]. Now, we describe the fabrication of the new ultrafine Mo6S8 powder.
Syntheses and Characterization of Mo6S8 Ultrafine Powder. Contrary to the other binary
molybdenum chalcogenides Mo6Se8 and Mo6Te8, the Mo6S8 binary sulfide cannot be
prepared directly from the elements [5]. Therefore, this metastable phase is generally
obtained from oxidation of the AxMo6S8 ternary phase, where A is a small cation (e.g., Li,
Ni, Cu), by the HCl leaching method, according to eq. 2:
AxnϩMo6S8 ϩ nx(Hϩ⅐ClϪ) 3 Mo6S8 ϩ x( Anϩ⅐nClϪ) ϩ nx/2H21.
(2)
In order to get single-phase Mo6S8 with homogeneous grains smaller than obtained before,
we have looked for new routes in the synthesis of ternary AxnϩMo6S8 compounds. We chose
the Niϳ2Mo6S8 and Liϳ3Mo6S8 phases as precursors of Mo6S8 because nickel and lithium
ions are withdrawn easily and quickly in HCl solution. The deintercalation provides, by the
exploding of grains, very small particles of Mo6S8 powder due to the large variations of the
unit cell rhombohedral angle (ϳ3°) between the binary and ternary phases (Table 1). The
Liϳ3Mo6S8 ternary compound has one of the biggest hexagonal unit cell volumes of the
Chevrel phase sulfides 869.6 Å3, (␣Rϭ 94.28) [6], whereas the Niϳ2Mo6S8 phase has a
hexagonal volume very close to that of the Mo6S8 binary phase, 801.0 Å3 (␣R ϭ 94.70°) vs.
797.5 Å3 (␣R ϭ 91.34°), respectively [7] (Table 1). The large difference in hexagonal volume

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421
FIG. 1
៮
XRD pattern indexed in the hexagonal system (space group R3) of Niϳ2Mo6S8 phase from
the new synthesis process: coprecipitation of Ni and Mo sulfur compounds and reduction (VH
ϭ 804.5(2) Å3).
between the Liϳ3Mo6S8 and Mo6S8 compounds is advantageous in that it allows an even
greater decrease in the grain size of the end product.
New Synthesis Process of the Niϳ2Mo6S8 Ternary Phase. The ternary Niϳ2Mo6S8 (NMS)
phase is generally synthesized by the classical ceramic route from a mixture of Ni, MoS2, and
Mo powders heated at 950°C for 48 h in a sealed evacuated silica tube. We call it the classical
Niϳ2Mo6S8 phase. Several grindings and annealings are usually required to avoid the
presence of MoS2 and Ni3Mo impurities.
Our new synthesis method of NMS phase consists of the coprecipitation of Ni and Mo
sulfur complexes by sulfurization of a solution of nickel nitrate and ammonium heptamo-
lybdate, followed by the reduction of the resulting precipitate [8]. For the first step, we chose
as precursors the materials ammonium heptamolybdate (NH4)6Mo7O24⅐4H2O (purity
Ͼ99.0%) and nitrate nickel Ni(NO3)2⅐6H2O (purity Ͼ99.0%) in atomic proportions Ni/Mo ϭ
1/3. Both precursors are soluble in ammonia solution at room temperature and do not
coprecipitate in the presence of each other.
The sulfurization is realized by bubbling H2S gas in the solution. After 10–15 min, a
precipitated mixture of sulfides [(NH4)2MoS4,NiS] forms. After filtration and drying, the
powder is reduced under H2 gas flow by a gas/solid reaction. From this new synthesis
process, we obtained pure Niϳ2Mo6S8 phase at only 675°C after 24 h of reaction; this is the
minimum reaction temperature ever published for the formation of the Chevrel phase
framework except for that obtained by the inorganic molecular process [9,10]. The lattice
parameters are comparable to the parameters of the ternary phase prepared by the classical
method in a sealed tube (Fig. 1). This new synthesis route of NMS, close to a preindustrial
route, allows a decrease in the fabrication temperature of about 200°C in comparison with the
classical process (laboratory method). This method could be improved by a one-step process
with H2S/H2 gas with sulfurization and reduction occurring simultaneously.
Scanning electron microscopy (SEM) examinations of the Niϳ2Mo6S8 powders show that
the classical phase contains particles of about 5 ␮m in size, recalling the characteristic shape
of Chevrel phase crystals (Fig. 2a). The new fine NMS powder has grains of about 0.5–1 ␮m
in size and a shape that is not well defined (Fig. 2b).

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FIG. 2
SEM images of Niϳ2Mo6S8 ternary phase from the (a) classical route (ϳ5 ␮m) and (b) new
synthesis route (ϳ 0.5-l ␮m).
New Synthesis Process of Liϳ3Mo6S8 Phase. The first synthesis route of ultrafine
Liϳ4Mo6S8 phase uses Li2S and MoS3 binary sulfides under hydrogen gas flow at 900°C for
72 h [2], but this process leads to a ternary compound that sometimes contains impurities
such as MoS2. Therefore, we have looked for a new method of preparation for ultrafine
ternary compound.
Our new LixMo6S8 synthesis route, which was inspired by the thesis of Sergent [11] on
alkaline thiomolybdates, is based on the preparation of the ternary compound, lithium
tetramolybdate oxide, Li2Mo4O13, followed by sulfurization and reduction of this compound
[8]. The choice of this ternary oxide was motivated by the excellent homogeneity of the
metallic mixture, i.e., the good atomic ratio of the lithium and molybdenum metals equivalent
to the metallic proportions occurring in the ternary sulfide Li3Mo6S8.
The Li2Mo4O13 material is prepared from commercial Li2CO3 (Ͼ99.0%) and MoO3
(purity Ͼ99.5%) powders under an air atmosphere at 450–500°C. Several grinding and
annealing stages are required to homogenize the reaction. This ternary oxide is characterized
by X-ray powder diffraction. The sulfurization of the oxide is carried out at 450°C for 1.5 h
under H2S gas flow by a gas/solid reaction process. A reactive mixture of Li2S and MoS2
sulfides is obtained. After the reduction of the powder mixture under hydrogen atmosphere
at 850°C for 48 h, a pure Liϳ3Mo6S8 phase is obtained (Fig. 3a). Flame spectrophotometry

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423
FIG. 3
៮
XRD patterns (Cu K␣1) with hexagonal indexation (space group R3) of (a) Liϳ3Mo6S8 phase
(VH ϭ 872.4(6) Å3); (b) Liϳ1Mo6S8 phase (VH ϭ 813.3(7) Å3); and (c) Mo6S8 binary phase
(VH ϭ 797.1(7) A3).
analysis shows effectively that the powder has the composition Liϳ3Mo6S8 (3Li for lMo6S8)
as expected. This ternary material is very moisture sensitive as reported in the literature [6].
We have observed that a compound such as Liϳ3Mo6S8, after being exposed to air for only
15 min, displays the same lattice parameters as LiϳlMo6S8 (Fig. 3b), and after 1 h, the same
parameters as Mo6S8 (Fig. 3c). This large variation in the crystalline parameters is easily
observed in the (220) and (104) hexagonal hkl diffraction peaks. The explanation for this
phenomena is that the mobility of lithium ions is rather high inside the Mo6S8 unit at room
temperature [6,12] and the oxidation of the Mo6S8 unit takes place. The lithium atoms move
and react at the surface of the channels with the air moisture.

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FIG. 4
SEM image of Liϳ3Mo6S8 phase from the new synthesis process (ϳ20–80 ␮m).
Figure 4 shows the particle morphology of the Liϳ3Mo6S8 phase from this new synthesis
process. The powder grain dimensions of this ternary material are very large (ϳ 20–80 ␮m).
Mo6S8 Binary Sulfide Synthesis from Niϳ2Mo6S8 and Liϳ3Mo6S8 Phases. We prepared
Mo6S8 powder from different ternary phases: classical Niϳ2Mo6S8, fine Niϳ2Mo6S8, and
Liϳ3Mo6S8 powders according to eq. 2. The resulting powder was washed several times with
distilled water to eliminate any trace of chloride and then dried under vacuum. The Mo6S8
binary phase obtained from the new fine ternary phases and the classical ternary phase
displays the same X-ray diffraction (XRD) pattern with the same parameters, and no impurity
is detected (Fig. 3c).
SEM images of Mo6S8 powder from these ternary phases show that the grain size
decreases from 1–5 ␮m for classical NMS (Fig. 5a) to 0.5–1 ␮m for fine NMS (Fig. 5b). The
large grains of the Liϳ4Mo6S8 phase provide, by deintercalation, very small grains of the
Mo6S8 binary compound (ϳ 0.3–0.5 ␮m), as viewed in Figure 5c. This can be explained by
a very strong reactivity of the lithium phase with HCl acid solution and by the large variation
in hexagonal volume between the ternary and the binary sulfide that we have emphasized
previously. Thus, the shape of the Mo6S8 particles recalls that of the ternary compound,
except for the Liϳ3Mo6S8 phase.
Laser granulometry measurements were performed on Mo6S8 (from Liϳ3Mo6S8) powder,
according to the dispersing conditions already reported [1]. The grain size distribution in
volume percent vs. diameter of particle (␮m) ranges mainly between 0.123 and 3.206 ␮m (%
volume ϭ 82.58%; mean diameter dm ϭ 0.619 ␮m). However, the result in percent number
presents a grain size repartition ranging from 0.123 to 1.520 ␮m, corresponding to 99.92%
of particles, with a mean diameter of 0.303 ␮m.
Synthesis and Characterization of PbMo6S8 Phase (PbS, Mo6S8, Mo Ultrafine Precursors).
Until now, we have prepared fine PbMo6S8 powders by only two methods: from a mixture
of PbS, MoS2, and Mo ultrafine precursors, as described in Part I [1] and from a PbS and
Mo6S8 mixture under H2 gas flow [2]. Now we mainly use the new Mo6S8 ultrafine precursor
in a new method of PbMo6S8 preparation. PbMo6S8 phase is prepared according to eq. 1,

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FIG. 5
SEM images of Mo6S8 binary phase from the (a) classical Niϳ2Mo6S8 phase (1–5 ␮m), (b)
new fine Niϳ2Mo6S8 phase (0.5–1 ␮m), and (c) new Liϳ3Mo6S8 compound (0.3–0.5 ␮m).
from an original mixture of ultrafine precursors: lead sulfide PbS, molybdenum, and Mo6S8
(from Liϳ3Mo6S8) powders.
Synthesis of PbMo6S8 Powder. In order to understand the chemical reactions occurring
during the PbMo6S8 phase fabrication from the mixture of PbS, Mo6S8, and Mo powders,
calorimetric measurements were performed from 200 to 900°C at a heating rate of 3°C/h.
About 3 g of mixed precursor powders were introduced into a silica tube sealed under argon
partial pressure.

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FIG. 6
Calorimetry study of PbMo6S8 formation from a mixture of PbS, Mo, and Mo6S8 ultrafine
precursors (heating rate 3°C/h).
Calorimetric analysis of the thermal reaction shows three exothermic peaks with only two
thermal effects (Fig. 6). The first effect between initial temperature Ti ϭ 410°C and final
temperature Tf ϭ 570°C shows a dissipated energy about Ϫ60 J/g with two maxima at Tp ϭ
475°C and Tp ϭ 540°C. The last anomaly at Tp ϭ 630°C from Ti ϭ 600°C to Tf ϭ 645°C
has a very weak intensity. Based on these calorimetric results, we used X-ray powder
diffraction to investigate the chemical reactions in a sealed evacuated silica tube at different
temperatures (i.e., 500, 600, 650, 800, and 850°C) with a ramp of 9 h from room temperature
to stop temperature followed by a 12 h stage duration (Fig. 7). The initial mixture consisted
of PbS ϩ 0.875Mo6S8 ϩ 0.75Mo. Principally, below 400°C, the initial constituents of the
mixture (i.e., the PbS, Mo6S8, and Mo powders) remain. At 500°C, we observe the presence
of PbMo6S8, Mo6S8, PbS, MoS2, and Mo compounds (Fig. 7a).
The formation of PbMo6S8 phase required elemental lead, which is not present in the
mixture. Therefore, we have studied, by calorimetry, the PbS decomposition in the presence
of molybdenum:
2PbS ϩ Mo 3 2Pb ϩ MoS2.
This reaction in a sealed evacuated silica tube takes place at 650°C. In the case of these
experiments, however, we think that this reaction was catalyzed at about 500°C by the
presence of the Mo6S8 binary.
In summary, the PbS transforms largely into lead (87.5%) and sulfur elements; therefore,
the very reactive Pb inserts immediately into Mo6S8 channels to form PbMo6S8. The excess
of sulfur reacts with molybdenum, leading to MoS2 sulfide.
The following reaction occurs in the temperature range of ϳ 500–600°C:
0.875PbS ϩ 0.875Mo6S8 ϩ 0.437Mo 3 0.875PbMo6S8 ϩ 0.437MoS2.
(3)
Our results show definitively that it is the Mo6S8 compound that governs the reaction,
considering the quantities of the constituents. At 600°C, we see the disappearance of Mo6S8

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FIG. 7
XRD investigations of PbMo6S8 formation from a mixture of PbS, Mo6S8, and Mo ultrafine
precursors at different temperatures (a) 500°C), (b) 600°C, (c) 650°C, (d) 800°C, and (e)
៮
850°C. XRD patterns are indexed in the hexagonal system (R3 space group).

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FIG. 8
SEM images of PbMo6S8 phase samples (a) S1 (800°C, 48 h) and (b) S3 (1050°C, 48 h).
phase and the presence of PbMo6S8, PbS, MoS2, and Mo compounds (Fig. 7b). Therefore, the
overall reaction is
1PbS ϩ 0.875Mo6S8 ϩ 0.75Mo 3
0.875PbMo6S8 ϩ 0.437MoS2 ϩ 0.125PbS ϩ 0.313Mo.
(4)
Calorimetric results show two exothermic peaks in the range 400–570°C (Fig. 6). We know
that the insertion of lead into the Mo6S8 compound leads to exothermic behavior [2] as well
as the reaction between PbS and Mo [1]. Hence, beginning at 410°C, we see the decompo-
sition of PbS, accompanied by the insertion of lead into the Mo6S8 phase. But, during the
decomposition of PbS, the sulfur atoms attack the surface of Mo grains, and a thin film of
MoS2 sulfide is formed. This MoS2 thin film in turn protects against a new attack of sulfur
atoms and prevents further decomposition of PbS [1]. The reaction kinetics of PbS decom-
position decreases, and we are obliged to increase the temperature to again effect decom-
position of PbS and lead intercalation (second peak).
At 650°C (Fig. 7c), we observe the presence of lead due to the decomposition of all the
remaining PbS (eq. 4) and the decreasing amount of Mo. The reaction between the resulting
PbS and Mo is
0.125Pb ϩ 0.0625Mo 3 0.125Pb ϩ 0.0625MoS2.
Thus, the components of the mixture are
(5)

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429
FIG. 9
Grain size distribution of PbMo6S8 S3 powder (1050°C, 48 h) from laser granulometer (a) in
% volume and (b) in % number.
0.875PbMo6S8 ϩ 0.5MoS2 ϩ 0.125Pb ϩ 0.25Mo.
We have detected elemental lead because all of the Mo6S8 phase has already reacted.
Furthermore, we have confirmation of the reaction (eq. 5) by calorimetry (Fig. 6). Indeed, a
third thermal effect is shown in the range of 600–650°C. This effect is exothermic, as
observed from classical powders [1]. The intensity of the peak is very weak in comparison
with the peak intensities exhibited from classical precursors, due to the very low molar
coefficients of the constituents of the reaction (eq. 5).
At 800°C, we observe the lowering of Pb, MoS2, and Mo peak intensities (Fig. 7d). At
850°C, the PbMo6S8 formation is complete. Thus, between 650 and 850°C, the last reaction
of the mixture is
0.125Pb ϩ 0.5MoS2 ϩ 0.25Mo 3 0.125PbMo6S8.
(6)
In the calorimetric pattern, we notice that above 800°C we do not detect the endothermic
peak for the PbMo6S8 formation (eq. 6) due to the very small proportions of reagents. The
endothermic peak with a molar composition is already extremely weak, 4 J/g [1].
Characterization of PbMo6S8 Powders. Three PbMo6S8 samples (S1, S2, and S3) were
prepared. Pellets of the ultrafine PbS, Mo, and Mo6S8 powders mixture were placed in
alumina crucibles in a quartz tube, which was then sealed under vacuum. The syntheses were
carried out at 800°C (S1), 900°C (S2), and 1050°C (S3) for 48 h. XRD patterns of these three
samples revealed pure PbMo6S8 phase in all cases, including at 800°C. The hexagonal
volumes have been determined from these patterns and all are close to 840 Å3.
From SEM observations, the micrographs of samples S1 (800°C, 48 h; see Fig. 8a) and S3
(1050°C, 48 h) (Fig. 8b) are very different. Sample S3 shows grains (ϳ0.5–1 ␮m) with the

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FIG. 10
␹
vs. T under 3 mGauss of three fine PbMo6S8 samples.
ac
characteristic shape of Chevrel phase. Indeed, we know that the PbMo6S8 formation results
from the addition of two reactions, at 600°C and then at 800°C. We note that the behavior
at these temperatures is very different from that at 1050°C. For the S3 sample, we see the
formation of small grains at 600° and 800°C, followed by the growth of particles with well
defined shapes; this behavior is opposite that of sample S1 (800°C). At 800°C, we see only
the formation of PbMo6S8 grains but no increase in the grain size.
Laser granulometry measurements on PbMo6S8 powder synthesized at 1050°C for 48 h
exhibit two maxima in the grain size repartition histogram (% volume) (Fig. 9a): the first
between 0.195 and 1.149 ␮m (63.87%; dm ϭ 0.490 ␮m) and the second between 1.149 and
4.656 ␮m (36.13%; dm ϭ 2.508 ␮m). In % number result (Fig. 9b), the second peak
disappears (0.41%), corresponding to few large particles, and the remaining peak (0.105–
1.149 ␮m) represents the real distribution of the grain size (99.59%; dm ϭ 0.416 ␮m).
The critical temperature, Tc(onset), was determined from ac susceptibility measurements
under a 3 mGauss field (Fig. 10 and Table 2). We see that the Tc of sample S2 (14 K) is
higher than that of sample S1 (13.2 K) due to the increasing temperature, from 800 to 900°C,
which allows maximum homogenization and a growth of grains. On the other hand, the
lowering of Tc for sample S3 (13.6 K) already indicates incipient contamination at the
temperature of 1050°C under our experimental conditions. Furthermore, this lower Tc (13.6
K) corresponds to a minimum hexagonal volume for the three samples (838 Å3).
TABLE 2
Superconducting Critical
Temperature, Tc(onset)
,
of three PMS Samples
Heat treatment
Tc(onset) (K)
800°C, 48 h
900°, 48 h
1050°C, 48 h
13.2
14.0
13.6

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CONCLUSION
We have perfected a new mixture of ultrafine precursors for PbMo6S8 synthesis (eq. 1). In
comparison with the mixture of PbS, Mo, and MoS2 constituents [1], we use the metastable
Mo6S8 binary phase in the new mixture instead of MoS2 sulfide (eq. 6). This choice allows
a decrease in the proportion of molybdenum in the new mixture; it changes from 2.5 (ref. [1];
eq. 1) to 0.75 (eq. 1). This is an advantage because of the extreme hardness of elemental Mo,
especially during the drawing of superconducting wire containing the precursor mixture. The
additional significance of this choice is that the reaction with Mo6S8 is exothermic.
Ultrafine PbS, Mo, and Mo6S8 precursors, with their quasispherical morphology and small
grain size, increase the reactivity of the mixture and thus decrease the fabrication temperature
of PbMo6S8 phase. The PbMo6S8 compound is obtained from a two-step process at 600 and
800°C, the limit in temperature for industrial wire insulation.
The ac susceptibility results show a Tc(onset) equal to 14 K for sample S2 heated at 900°C
for 48 h, indicating a high superconducting quality of PbMo6S8 grains with a diameter of
about 0.5–1 ␮m.
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