New synthesis route of PbMo6S8 superconducting Chevrel phase from ultrafine precursor mixtures: I. PbS, MoS2 and Mo powders

Article abstract of DOI:10.1016/S0025-5408(97)00229-8

Fine powders of PbMo₆S₈ Chevrel phase (approximately 0.5 μm) obtained from new ultrafine precursor powders present excellent intrinsic superconducting properties, such as a critical temperature, T_c, of about 14 K. These results are due to ultrafine quasi-spherical PbS, MoS₂, and Mo precursor powders (0.05-0.5 μm) of lead Chevrel phase synthesized by soft chemistry methods (precipitation, co-precipitation). Thus, a better reactivity between the grains of the mixture allows a decrease in the synthesis temperature of the resulting PbMo₆S₈ phase (800 °C instead of 950-1000 °C) required for the PbMo_6SS₈ synthesis from the same precursor powders made by the classical ceramic route. These new Chevrel phase powders have very fine homogeneous grains with high potential surface reactivity, giving improved grain boundaries that are able to carry high current densities.

Full text of DOI:10.1016/S0025-5408(97)00229-8

Materials Research Bulletin, Vol. 33, No. 2, pp. 237–252, 1998
Copyright © 1998 Elsevier Science Ltd
Printed in the USA. All rights reserved
0
025-5408/98 $19.00 ϩ .00
PII S0025-5408(97)00229-8
NEW SYNTHESIS ROUTE OF PbMo S SUPERCONDUCTING CHEVREL
6
8
PHASE FROM ULTRAFINE PRECURSOR MIXTURES: I. PbS, MoS2
AND Mo POWDERS
Sylvie Even-Boudjada*, Laurence Burel, Roger Chevrel, and Marcel Sergent
Laboratoire de Chimie du Solide et Inorganique Mol e´ culaire (LCSIM), UMR CNRS
511, Universit e´ de Rennes I, Avenue du G e´ n e´ ral Leclerc, 35042 Rennes Cedex, France
6
(Refereed)
(Received June 30, 1997; Accepted July 2, 1997)
ABSTRACT
Fine powders of PbMo S Chevrel phase (ϳ0.5 ␮m) obtained from new
6
8
ultrafine precursor powders present excellent intrinsic superconducting prop-
erties, such as a critical temperature, T , of about 14 K. These results are due
c
to ultrafine quasi-spherical PbS, MoS , and Mo precursor powders (0.05–0.5
2
␮m) of lead Chevrel phase synthesized by soft chemistry methods (precipi-
tation, co-precipitation). Thus, a better reactivity between the grains of the
mixture allows a decrease in the synthesis temperature of the resulting
PbMo S phase (800°C instead of 950-1000°C) required for the PbMo S
6
8
6 8
synthesis from the same precursor powders made by the classical ceramic
route. These new Chevrel phase powders have very fine homogeneous grains
with high potential surface reactivity, giving improved grain boundaries that
are able to carry high current densities. © 1998 Elsevier Science Ltd
KEYWORDS: A. chalcogenides, B. chemical synthesis, C. electron micros-
copy, D. superconductivity
INTRODUCTION
The ternary molybdenum chalcogenides of the general formula M Mo X (M ϭ 3d element,
x
6 8
rare earth, Pb, Sn, etc., 1 Յ x Յ 4, and X ϭ S, Se, Te) represent a wide variety of compounds
well known as Chevrel phases [1]. These materials crystallize in black microcrystalline
*
To whom correspondence should be addressed.
2
37

2
38
S. EVEN-BOUDJADA et al.
Vol. 33, No. 2
powders. Some of them have outstanding superconducting properties, for example, PbMo6S8
compound (T : 14.5 K and Bc (0) Ϸ 70 T [2,3]).
c
2
The so-called classical ceramic synthesis of PbMo S (PMS) is carried out at high
6
8
temperature in sealed evacuated silica tubes. A mixture of either Pb, Mo, and S elements or
PbS, Mo, and MoS or Mo S binary sulfides in good proportions is introduced into pellets
2
2 3
or powders. Until now, PMS has generally been prepared at about 950-1000°C from PbS,
Mo, and MoS powders. These starting materials are synthesized from mixtures of elements
2
and give grain sizes of about 1–15 ␮m with different morphologies, such as sheets for MoS2
grains. We call them classical precursors. The grain size of the resulting PMS powders is
about 5–10 ␮m.
Our objective is to lower the PMS grain size (ϳ0.5–1 ␮m) by about one order of
magnitude of that previously obtained in order to have an excellent homogeneity of the
physical properties of the PMS powder grains as the intrinsic superconducting properties (Tc,
Bc ) and to have a better connectivity between grains to improve the transport properties
2
(
e.g., J , critical current density).
c
Already a new synthesis method [4,5] using soft chemistry under gas flow has allowed
ultrafine PMS powder grains to be obtained. In this paper, we describe new synthesis
methods for obtaining PMS ultrafine precursor powders and the PMS phase one, but without
gas flow.
EXPERIMENTAL AND RESULTS
The powders were handled under inert atmosphere (argon gas) to avoid oxygen contamina-
tion, which degrades the physical properties, for instance, the T superconducting tempera-
c
ture of the resulting PMS phase [6,7].
Powder specimens were examined by X-ray diffractometry (XRD) using Cu K␣ radiation
1
to identify the phases. The correction of diffraction angles was carried out using silicon as the
powder standard. The grain size and the morphology of the powders were observed by
scanning electron microscopy (SEM). The particle size distribution of the samples was
determined by Fraunhofer diffraction, using a polarization intensity differential scattering
system on a laser granulometer (Coulter LS230). The powders were dispersed in distilled
water with a sodium polyphosphate dispersing agent and subjected to ultrasonics for 15 min.
The results for particle distribution are represented in % volume or % number vs. the
logarithmic scale of the diameter (␮m) of the grain. Thus, the % volume corresponds to the
volume percent occupied for each granulometric class, and the % number (deduced from the
%
volume result) corresponds to the number percent of grains that have the same diameter.
A sample suspension containing a few large particles and many small ones will have a very
important % volume repartition for small and large diameters, but it will show only a very
high % number distribution for small diameters. We define d as the mean diameter of the
m
one repartition peak.
Calorimetric measurements were performed with a high temperature differential micro-
calorimeter from 200 to 1000°C in a sealed (under partial N pressure) silica tube. On a
2
graph, we define T as the initial temperature of the peak, T as the final temperature, and T
p
i
f
as the temperature of the peak maximum. The superconducting transition temperature,
Tc(onset) is measured by an AC susceptometer under a 3 mgauss AC field.

Vol. 33, No. 2
LEAD MOLYBDENUM SULFIDE
239
FIG. 1
SEM images of PbS powder prepared by (a) classical ceramic route (5–10 ␮m) and (b) new
precipitated route (0.5–1.0 ␮m). Scalemarker in (a) represents 1 ␮m.
Synthesis and Characterization of Ultrafine Precursor Powders of PbMo6S8
In this section, we describe new processes for the synthesis of ultrafine precursors of the lead
Chevrel phase: PbS, MoS , and Mo powders [8].
2
Ultrafine PbS powder. Until now, classical PbS powder has been prepared from lead
(purity: 99.9999%) and sulfur (99.999%) elements by liquid reaction to about 350°C in a
sealed evacuated glass tube. Large broken cubic-shaped crystalline gallena grains are ob-
served, but the grain size of classical PbS powder is about 5–10 ␮m, according to SEM
observations after sieving below 45 ␮m (Fig. 1a).
First, in the synthesis of ultrafine PbS powder, aqueous solutions of Pb(NO ) lead nitrate
3
2
(
99.5%) and CS(NH ) thiourea (99.0%) are prepared [9]. Second, both solutions are added
2 2

2
40
S. EVEN-BOUDJADA et al.
Vol. 33, No. 2
simultaneously to a bath of sodium hydroxide (0.75 M). The resulting precipitate is filtered,
washed with hot diluted HCl acid and then several times with distilled water, and finally dried
under vacuum. PbS powder with a good ratio, 1:1, is obtained. PbS powder grains synthe-
sized by precipitation present a quasi-cubic morphology and a grain size smaller than that of
PbS powders prepared by the classical method [(0.5–1.0 ␮m vs. 5–10 ␮m), see Figure 1b].
PbS powder presents a particle size distribution in % volume situated mainly between
0
.598 and 18.86 ␮m (95.61%; d ϭ 5.22 ␮m). For the % number result, the repartition
m
principally ranges between 0.04 and 0.598 ␮m (99.73%; d ϭ 0.085 ␮m). Therefore, we can
m
say that the suspension contains many small grains but only a few large ones.
The cubic parameters (with standard deviation) for both PbS starting materials, calculated
from the XRD pattern indexing, are very close: for classical PbS powder acubic is 5.937(7) Å,
for ultrafine PbS powder it is 5.926(7) Å.
Ultrafine MoS powder. MoS , molybdenum disulfide, is a natural mineral called molyb-
2
2
denite. It can be prepared by direct synthesis from elements in a sealed evacuated quartz tube.
We synthesized MoS binary component by the classical method from commercial molyb-
2
denum (99.94%) and sulfur (99.999%) at about 720°C with the first heat treatment at 400°C.
The molybdenum powder is pretreated under H gas flow at 700°C. This classical preparation
2
yields particles with a “flower” morphology. MoS is not ground because of its lamellar
2
nature; therefore, the average grain size is about 15 ␮m or less, after sieving at 45 ␮m (Fig.
2
a).
To avoid the high chemical vapor pressure of sulfur, which favors the formation of large
grains, we have perfected a low temperature MoS preparation method that leads to ultrafine
2
particles. This synthesis can be described as a three-step process. The first step consists of
dissolving (NH ) Mo O ⅐4H O ammonium heptamolybdate (99.0%) into an ammonia so-
4
6
7
24
2
lution, giving ammonium monomolybdate. Thus, the sulfurization is performed by bubbling
H S gas into the solution [10]. Large, dark-red rectangular rod-shaped crystals of
2
(
NH ) MoS ammonium thiomolybdate grow into ammonia hydrate. In the second step, after
4 2 4
filtering, this compound is dissolved in water. By adding HCl aqueous acid into the solution,
a gray-black ultrafine powder of amorphous MoS is quickly precipitated. The powder is
3
washed with distilled water to eliminate all traces of chloride and dried under vacuum. The
last step corresponds with the formation of MoS disulfide by decomposition of MoS
2
3
trisulfide at 350°C under hydrogen or nitrogen gas flow. The decomposition reaction yields
ultrafine MoS grains isomorphous to MoS particles. They keep the same quasi-spherical
2
3
morphology as that of the initial grains. Thus, the grain size of this new MoS powder is
2
about 0.1–0.5 ␮m (Fig. 2b), which is smaller than the size observed for classical MoS2
powder.
The particle size distribution histogram in % volume of MoS ultrafine powder is displayed
2
in Figure 3. We observed two maxima peaks: the first one between 0.04 and 0.0926 ␮m
(
42.93%; d ϭ 0.053 ␮m) and the second one between 0.162 and 0.657 ␮m (57.07%; d ϭ
m
m
0
.334 ␮m). In the inset in Figure 3, we present the repartition histogram in % number (first
peak: 0.04–0.0926 ␮m: 99.40%; dm ϭ 0.050 ␮m). We observe that the second peak has
nearly disappeared (0.60%). Other measurements under changing dispersion conditions let us
conclude that the grains of 0.162–0.657 ␮m diameter are, in fact, an agglomerate of small
grains linked by electrostatic forces of the van deer Waals type. The laser granulometry
characterization elicits the SEM observation.
The XRD pattern for ultrafine MoS synthesized by thermal decomposition of MoS
2
3

Vol. 33, No. 2
LEAD MOLYBDENUM SULFIDE
241
FIG. 2
SEM images of MoS : (a) classical powder (Յ15 ␮m) and (b) ultrafine powder arising from
2
MoS decomposition (0.1–0.5 ␮m). Scalemarker in (b) represents 100 nm.
3
exhibits a few broad peaks (2␪: ϳ14°, ϳ33°, ϳ39°, and ϳ58°), indicating a poorly
crystallized compound (Fig. 4). Actually, the HREM observation of MoS particles exhibits
2
many narrow domains composed of ordered fringes (Fig. 5). These fringes characterize the
lamellar nature of the MoS compound, especially the Mo planes: the distance between two
2
black fringes corresponds to 6.15 Å, equivalent to half of the c-hexagonal MoS parameter.
2
Ultrafine Mo powder. Generally, commercial molybdenum powder (99.94%; 3d magnetic
impurities Ͻ50 ppm) is produced from molybdenum oxides, ammonium molybdates, or
molybdenite. But the grain size (1–5 ␮m) is greater than that of the other ultrafine precursors
(
PbS, MoS ) by about one order of magnitude (Fig. 6).
2
We have developed a new fabrication process for preparing molybdenum powder with a

2
42
S. EVEN-BOUDJADA et al.
Vol. 33, No. 2
FIG. 3
Particle size distribution in % volume (laser granulometry) of MoS ultrafine powder. Inset:
2
the distribution in % number.
grain size as small as that of the other ultrafine powders. In this method, MoS ultrafine
3
powder is reduced under H gas flow at 925°C.
2
Depending on the temperature, 925 or 945°C, for Mo synthesis, the grain size is different,
but the morphology is always quasi-spherical. We have obtained an ultrafine network of
FIG. 4
XRD pattern indexed in the hexagonal system of a poorly crystallized MoS ultrafine powder.
2

Vol. 33, No. 2
LEAD MOLYBDENUM SULFIDE
243
FIG. 5
HREM image of an MoS ultrafine particle edge: view of narrow disordered domains of
2
ordered fringes.
weakly interconnected particles resembling peanuts (Fig. 7). After a “manual breaking”, the
grain size is about 0.3–0.5 ␮m for powder synthesized at 925°C (Table 1 and Fig. 8) and
about 1 ␮m for that synthesized at 945°C.
The grain-size distribution (%vol) of molybdenum ultrafine powder presents two peaks:
the first one between 0.214 and 1.261 ␮m (44.62%; dm ϭ 0.585 ␮m) and the second one
between 1.261 and 4.241 ␮m (55.38%; dm ϭ 2.411 ␮m) with a global mean diameter of
1
.282 ␮m. The result in % number shows only one peak, between 0.214 and 1.261 ␮m
FIG. 6
SEM image of commercial Mo powder (1–5 ␮m).

2
44
S. EVEN-BOUDJADA et al.
Vol. 33, No. 2
FIG. 7
SEM images of ultrafine interconnected Mo particles from MoS reduction (H ) at (a) 925°C
3
2
and (b) 945°C.
(
98.55%; d ϭ 0.443 ␮m), indicating that the sample contains mainly small particles of these
m
diameters.
The cubic lattice parameter, acubic, for ultrafine Mo powder is comparable to that for the
commercial product: 3.150(1) Å vs. 3.147(2) Å, respectively.
Synthesis and Characterization of PbMo S Phase (PbS, MoS , and Mo Ultrafine
6
8
2
Precursors)
Synthesis of PMS. We have tried to understand the synthesis process of PMS compounds
from the mixture of PbS, MoS , and Mo powders according to eq. 1, by examining the
2
calorimetric measurements:

Vol. 33, No. 2
LEAD MOLYBDENUM SULFIDE
245
TABLE 1
Comparative Grain Size (SEM Observations) Between Classical
and Ultrafine Precursor Powders and Mean Diameter, dm,
(% Number) of Ultrafine Powders (Laser
Granulometry Measurements)
Classical or
commercial
Ultrafine
Precursor powders
grain size (␮m)
grain size (␮m)
dm (␮m)
PbS
MoS2
Mo
5–10
1–15
1–5
0.5–1
0.1–0.5
0.3–0.5
0.085
0.05
0.443
PbS ϩ 3.5 MoS ϩ 2.5 Mo 3 PbMo S
8
(1)
2
6
Calorimetric measurements were performed up to 1000°C (heating rate: 8°C/h) on a mixture
of classical powders. The mixture was made in an accurate order, depending on the different
morphologies and grain sizes of PbS, MoS , and Mo classical powders: first MoS , then PbS,
2
2
and finally Mo powders. We note that this choice corresponds, in descending order, to the
volumic quantities of starting materials.
In Figure 9, we observe two thermal transitions: an exothermic effect between T ϭ 580°C
i
and T ϭ 785°C with a maximum at T ϭ 750°C and an endothermic peak at T ϭ 910°C
f
p
p
between T ϭ 880°C and T ϭ 940°C. These two anomalies indicate that two different
i
f
reactions occur during the PMS phase synthesis. These results together with XRD investi-
gations allow us to attribute these two calorimetric peaks.
The exothermic effect is related to the following reaction between lead sulfide and
molybdenum:
FIG. 8
SEM image of an ultrafine Mo particle (from MoS reduction at 925°C) after a “manual
3
breaking” (0.3–0.5 ␮m). Scalemarker represents 1 ␮m.

2
46
S. EVEN-BOUDJADA et al.
Vol. 33, No. 2
FIG. 9
Calorimetric study of PbMo S formation from a mixture of classical PbS, MoS , and Mo
6
8
2
powders at an 8°C/h heating rate. Inset: a study of the first exothermic peak at 3°C/h heating
rate.
PbS ϩ 0.5 Mo 3 Pb ϩ 0.5 MoS2
(2)
The dissipated energy for this exothermic reaction is about Ϫ100 J/g. This result is confirmed
by another calorimetric study performed with only the (PbS ϩ 0.5Mo) mixture at the same
heating rate. The resulting powders are a mixture of lead metal and MoS disulfide charac-
2
terized by X-ray powder diffraction. The peak shape of the first transition is wide and can be
modified with the heating rate (inset, Fig. 9). The presence of two peaks during this
exothermic reaction (inset, Fig. 9) is due to the low heating rate (3°C/h) and can be explained
by an easy chemical attack on the surface of all the Mo grains by sulfur vapor arising from
the decomposition of PbS grains. At the end of this reaction, of course, many grains still have
molybdenum cores. The reactivity of these grains is lowered due to this thin layer of MoS2
at the Mo grain surface, which prevents the sulfurization. In consequence of this inertness,
the sulfur pressure increases. With increasing temperature, this peel cracks and a second
attack against the Mo grain cores takes place.
At high temperature (T ϭ 910°C), the endothermic anomaly represents the reaction of
p
PMS synthesis (eq. 3); the PMS formation is made with an enthalpy of 4 J/g.
Pb ϩ 4 MoS ϩ 2 Mo 3 PbMo S
8
(3)
2
6
From classical precursors mixture. The XRD study of the PMS fabrication from these
precursors shows that the compound begins to form at 800°C after 36 h of reaction. To
increase the formation kinetic, the temperature was raised to, and kept at, 950°C for 24 h,
leading to the formation of PMS pure phase. We think these differences of temperature with

Vol. 33, No. 2
LEAD MOLYBDENUM SULFIDE
247
FIG. 10
XRD patterns indexed in the hexagonal system (R3) of PbMo S samples obtained at
6
8
7
50°C/24 h plus 800°C/36 h from a mixture of (a) ultrafine precursor powders and (b)
classical powders.
calorimetry results can be explained by the very high heating rate (ϳ150°C/h ϵ ϳ a ramp
of 5 h to final temperature) in the PMS synthesis process. The energy dissipated by the
intermediate exothermic reaction (eq. 2) can be used to form the PMS phase at lower
temperature, 800°C (furnace temperature). On the other hand, for calorimetry, the heating
rate is very low (8°C/h), which corresponds to a ramp of about 3 days to final temperature;
thus, the dissipated energy for the first reaction has no effect on the PMS fabrication.
From ultrafine precursors mixture. The same mixture of components (PbS, MoS , and
2
Mo) as used for the classical powders is used here but in the state of ultrafine powders.
The XRD study shows that the PbMo S phase is completely formed at 800°C after 18 h
6
8
of reaction. For a comparative investigation of the classical and the ultrafine precursor
syntheses, we have carried out the same heat treatments (750°C/24 h ϩ 800°C/36 h) with the
same amounts of powder in sealed evacuated silica tubes.
First, we chose the 750°C/24 h pretreatment based on the previous calorimetric result
concerning the intermediate reaction between PbS and Mo components. We selected the
second heat treatment, 800°C/36 h, because it was attributed to the start of PMS formation
from classical powders, as viewed from the XRD investigation.
The XRD pattern obtained from ultrafine precursors (Fig. 10a), corresponds to a pure PMS
phase, whereas the XRD pattern observed from the classical powders (Fig. 10b) reveals an
incomplete reaction: a mixture of about 50% PMS with the presence of unreacted Pb, MoS2,
and Mo. Lead element arises from the intermediate reaction (eq. 2).

2
48
S. EVEN-BOUDJADA et al.
Vol. 33, No. 2
FIG. 11
SEM images of PbMo S (1000°C/24 h) from mixture of (a) classical precursors (ϳ5 ␮m)
6
8
and (b) fine precursors (ϳ 0.5–1.0 ␮m).
Physical characterization and properties of PbMo S phase. PMS phase samples from
6
8
PbS, MoS , and Mo mixed classical and ultrafine precursor powders were uniaxially pressed
2
under Ar atmosphere and heated at 1000°C for 24 h in sealed evacuated quartz tubes. XRD
patterns reveal pure PMS phase in the samples of both precursor powders.
The grain size of the classical PMS powders, estimated from SEM observations, is about
5
␮
␮m (Fig. 11a). In comparison, the grain size of the ultrafine PMS powders is about 0.5–1
m, i.e., one order of magnitude smaller (Fig. 11b).
The grain-size distribution (%vol) of classical PMS powders, based on laser granulometry
measurements, mainly ranges from 1.149 to 24.95 ␮m [96.35%; d ϭ 7.232 ␮m; (Fig. 12a)].
m
In % number, this repartition is between 1.149 and 24.95 ␮m (99.99%; dm ϭ 3.189 ␮m),

Vol. 33, No. 2
LEAD MOLYBDENUM SULFIDE
249
FIG. 12
Grain-size distributions of fine and classical PMS powder (a) in % volume and (b) in %
number.
indicating that whole particles have grain sizes in this range (Fig. 12b). These values are
summarized in Table 2.
For ultrafine PMS powders, we observe two grain-size distributions in % volume: the first
peak is between 0.178 and 1.047 ␮m (61.29%; dm ϭ 0.477 ␮m) corresponding to small
particles, and the second peak is between 1.047 and 4.656 ␮m (38.71%; dm ϭ 2.10 ␮m),
corresponding to large particles. On the other hand, for % number distribution, one peak is
displayed between 0.178 and 1.047 ␮m (99.33%; dm ϭ 0.384 ␮m), and the second peak,
between 1.047 and 4.656 ␮m, is very weak in intensity, corresponding to 0.67 % number of
particles (Fig. 12b). Thus, the PMS sample contains almost only small grains in the range of
0
.178 and 1.047 ␮m diameter (Table 2).
As seen in Table 2, the fine PbMo S powder from a mixture of ultrafine precursors
6
8
presents a mean grain size one order of magnitude smaller than that of classical PMS powder.

2
50
S. EVEN-BOUDJADA et al.
Vol. 33, No. 2
TABLE 2
Comparative Grain Size Distributions Between Classical and Fine
PbMo S Powders from Laser Granulometry Measurements (see Fig. 12)
6
8
Volume
dm (␮m)
Number
dm (␮m)
Range
diameter (␮m)
Powder
%
%
Classical PMS
1.149–24.95
96.35
3.65
61.29
38.71
7.232
36.45
0.477
2.10
99.99
0.01
99.33
0.67
3.189
32.73
0.384
1.806
2
4.95–76.43
0.178–1.047
.047–4.656
Fine PMS
1
dm: mean diameter.
These granulometric results are very close to the dimensions estimated from SEM observa-
tions.
The PbMo S Chevrel phase crystallizes in a hexagonal/rhombohedral system with space
6
8
៮
group R3. By X-rays powder diffraction, we determined the hexagonal parameters of the unit
cell, and by inductive susceptibility measurements, we determined the superconducting
critical temperature, Tc(onset), for both samples (Fig. 13). The results are summarized in Table
3
.
In Chevrel phases, there is a relationship between superconducting critical temperature and
hexagonal volume of PMS phase [11]: T increases when the volume rises. Here, we observe
c
the same phenomena: the highest T corresponds with the highest volume.
c
The new PMS synthesis process for ultrafine precursor powders gives the highest T , 14
c
K. As mentioned previously, both samples were subjected to the same heat treatment. In PMS
phase, the highest volume corresponds to 1:6:8 stoichiometry without defects on lead and
sulfur and without sulfur substitution by oxygen atoms, for instance. Here, it seems that our
fine PMS phase leads to such a situation. Furthermore, the sharp transitions at the onset of
FIG. 13
␹AC vs. T under 3 mgauss for both fine and classical PbMo S samples.
6
8

Vol. 33, No. 2
LEAD MOLYBDENUM SULFIDE
TABLE 3
251
Hexagonal Unit Cell Parameters, Mean Diameter (% Number), and Tc(onset)
of PMS Prepared at 1000°C/24 h from Both Classical and Ultrafine
Precursor Mixtures
3
PMS phase
aH (Å)
cH (Å)
VH (Å )
dm (␮m)
Tc(onset) (K)
Classical sample
Ultrafine sample
9.209(5)
9.197(3)
11.412(4)
11.478(2)
838.1(7)
840.8(2)
3.2
0.38
13.0
14.0
the curve, observed in Figure 13, indicate an excellent homogeneity of PMS grains in both
cases.
CONCLUSION
The new synthesis processes for preparing precursor powders by precipitation and/or by a
first-step liquid route allow us to obtain ultrafine quasi-spherical particles (0.05–0.5 ␮m).
The grain size of the ultrafine precursor powders is one order of magnitude smaller,
compared with the grain size of classical precursor powders, which are much more largely
scattered [1–15 ␮m; (Table 1)]. In addition, the precursor syntheses for PbS and MoS are
2
performed at room temperature by liquid route for PbS and at 350°C for MoS sulfide. The
2
temperatures required for the fabrication of these precursors are much lower than those
required for the classical precursors.
Very ultrafine precursor grains lead to a high surface area, allowing for a higher reactivity
related to an easier diffusion between the powders, thus lowering the synthesis temperature
of the PMS phase by about 100–150°C. Furthermore, the PMS compound from the mixture
of ultrafine precursors exhibits a Tc(onset) of 14 K, characteristic of an excellent supercon-
ducting quality of grain cores.
Before testing the current density transport capacity of these powders in future wires, new
M(H) magnetization (Jc(calc)) and ␹AC vs. T at different alternative field measurements on
highly densified bulk pellets will be performed to check the extrinsic properties of the new
fine PMS powders, i.e., improvement of the connectivity between the particles and the
lowering of the sintering temperature of these PMS grains.
ACKNOWLEDGMENTS
The authors thank M.P. Crosnier-Lopez, Y. Laligant, and R. Retoux of the Laboratoire des
Fluorures, UPRES-A CNRS 6010, University Le Mans for their assistance with high
resolution transmission electron microscopy, and J.M. Maufras of the I.U.T Lorient for
calorimetric measurements.
REFERENCES
1
2
.
.
R. Chevrel, M. Sergent, and J. Prigent, J. Solid State Chem. 3, 515 (1971).
B.T. Matthias, M. Marezio, E. Corenzwit, A.S. Cooper, and H.F. Barz, Science 175, 1465 (1972).

2
52
S. EVEN-BOUDJADA et al.
Vol. 33, No. 2
3
4
5
.
.
.
Ø. Fischer, M. Decroux, S. Roth, R. Chevrel, and M. Sergent, J. Phys. C: Solid State Phys. 8, L
74 (1975).
S. Even-Boudjada, L. Burel, V. Bouquet, R. Chevrel, M. Sergent, J.C. Jegaden, J. Cors, and M.
Decroux, Mater. Sci. Forum 152–153, 205 (1994).
P. Rabiller, M. Rabiller-Baudry, S. Even-Boudjada, L. Burel, R. Chevrel, M. Sergent, M.
Decroux, J. Cors, and J.L. Maufras, Mater. Res. Bull. 29, 567 (1994).
D.G. Hinks, J.D. Jorgensen, and H.L. Li, Phys. Rev. Lett. 51, 1911 (1983).
P. Selvam, D. Cattani, J. Cors, M. Decroux, P. Niedermann, S. Ritter, Ø. Fisher, P. Rabiller, R.
Chevrel, L. Burel, and M. Sergent, Mater. Res. Bull. 26, 1151 (1991).
S. Even-Boudjada, Th e` se d’Universit e´ , Rennes, France (1994).
4
6
7
.
.
8
9
.
.
H. Rahnamai, H.J. Gray, and J.N. Zemel, Thin Solid Films 69, 374 (1980).
1
1
0. J.C. Bernard and G. Tridot, Bull. Soc. Chem. 5, 810 (1961).
1. Ø. Fischer, Appl. Phys. 16, 1 (1978).