Effect of tantalum on phase formation in the Fe-S system

Article abstract of DOI:10.1134/S0036023607090252

Phase formation in the Fe_{1 - x} -Ta _x -S system with 0 < x < 0.5 has been studied at temperatures up to 1273 K using X-ray diffraction. When the constituent elements are heated to 823 K, troilite (Tr), pyrrhotites (PoI, PoII, and Po

Full text of DOI:10.1134/S0036023607090252

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2007, Vol. 52, No. 9, pp. 1459–1463. © Pleiades Publishing, Inc., 2007.
Original Russian Text © A.D. Vershinin, V.P. Mar’evich, V.M. Chumarev, E.N. Selivanov, 2007, published in Zhurnal Neorganicheskoi Khimii, 2007, Vol. 52, No. 9,
pp. 1557−1561.
PHYSICOCHEMICAL ANALYSIS
OF INORGANIC SYSTEMS
Effect of Tantalum on Phase Formation in the Fe–S System
A. D. Vershinin, V. P. Mar’evich, V. M. Chumarev, and E. N. Selivanov
Institute of Metallurgy, Ural Division, Russian Academy of Sciences, Yekaterinburg, Russia
Received August 17, 2006
Abstract—Phase formation in the Fe1 – x–Tax–S system with 0 < x < 0.5 has been studied at temperatures up
to 1273 K using X-ray diffraction. When the constituent elements are heated to 823 K, troilite (Tr), pyrrhotites
(PoI, PoII, and Pom), and pyrite (FeS2) are formed. Tantalum at these temperatures only insignificantly dissolves
in iron sulfides. In the range 823–1273 K, tantalum reacts with pyrrhotites PoII and Pom and pyrite to yield PoI,
FeTa3S6, and α-Fe. The solubility limit of tantalum in PoI is near Fe0.98Ta0.02S. The initiation temperature of
the reaction between troilite and tantalum producing FeTa3S6 and α-Fe is 873 K. The unit cell parameters of
tantalum change at 500 K, presumably due to the dissolution of iron and possibly sulfur (iron sulfide).
DOI: 10.1134/S0036023607090252
Phase formation in the Fe–Ta–S system is interest-
ing to both scientists and engineers [1, 2] for elucidat-
ing how refractory elements affect pyrrhotite polymor-
phism and tantalum partition. According to work [1],
tantalum withstands the attack of sulfur until 973 K;
therefore, pyrrhotites Fe1 − xS are the most likely initial
products formed by mixtures of iron, tantalum, and sul-
fur on heating. According to work [2], the ampoule syn-
thesis of tantalum sulfides is possible at 873 K.
The closeness of the Ä and ë parameters of the PoI
and PoII phases [3] implies that they belong to the same
solid solution, whose region at 273 K on one side bor-
ders the Tr + PoI two-phase region (stoichiometric iron
monosulfide or Tr plus PoI), and on the other side, the
PoII + Pom region (PoII plus Pom). Troilite FeS and
monoclinic pyrrhotite Fe7S8 are commonly treated as
stoichiometric compounds that exist within narrow ranges
of composition. The Fe7S8 stability range at 293 K is
46.6–46.9 at % Fe (0.12 ≤ x ≤ 0.13) according to work
[10]. According to works [11, 12], the extent of the
Fe7S8 stability region is at most 0.01 at % Fe (∆ı =
0.005); the stability temperature is at least 543 K [12].
In natural assemblages [3], hexagonal PoI is coherently
associated with troilite and PoII is associated with mono-
clinic pyrrhotite.
The stability of pyrrhotites, variations of the solubi-
lity limits of metals in them, and the decay of solid
solutions upon cooling were discussed in works [3–8].
According to the works cited, phase equilibria between
pyrrhotites and refractory metals, such as platinum,
rhodium, and palladium, are significantly affected by
the composition and temperature [4–8], and minor ele-
ments effect the properties of pyrrhotites, in particular,
the paramagnetic-to-ferrimagnetic transition [3].
In this work, our goals were to explore how tantalum
influences phase formation in the Fe–S system and to
quantify the compositions of the phases formed upon
the synthesis of sulfides of the Fe–Ta–S system.
Iron monosulfide Fe1 − xS as an individual phase
exists with 0 ≤ ı ≤ 0.125 (from FeS to Fe7S8). It crystal-
lizes in three NiAs-type phases. When the vacancy con-
centration is ı = 0–0.06, a superstructure is formed,
which differs from the base structure in that it has a
symmetry center as a result of the displacements of iron
and sulfur atoms from the positions normal for
NiAs-type structures (PoI, the Bertaux structure,
EXPERIMENTAL
A DRON-2.0 diffractometer (ëÓäα radiation,
graphite monochromator in the reflected beam) was
used in the experiments. Semiconductor grade germa-
nium was used as the internal standard [9]. The unit cell
parameters of iron sulfides were determined by least-
squares fits using five to nine reflections. The error in
the unit cell parameters was 0.0002 nm for the para-
meter A and 0.0004 nm for the parameter C. FeTa3S6
was in addition monitored by scanning (h00), (hh0),
and (00l) reflections with 0.01° steps and the accumu-
lation time equal to 300 s. The error in the parameters
in this case was 0.0001 nm. The density ρ of single-
‡ = Ä 3 , Ò = 2ë, where Ä and ë are the parameters of
the base structure). Another phase (iron monosulfide,
PoII) exists with 0.06 ≤ ı ≥ 0.10. The third iron sulfide
phase (Pom) is a vacancy superstructure in the iron sub-
lattice [3, 9] with a monoclinic distortion (β = 90.4°).
The concentration range of the third phase is 0.1 < x ≤
0.125 [9].
1459

1460
VERSHININ et al.
Table 1. Characteristics of (1 – x)Fe · xTa · S phases that are formed upon heating to 823 K
Unit cell parameters and volume
Phase
ρ, g/cm3
Sample no.
x
composition
A, nm
C, nm
V × 103, nm3
calcd.
obs
1
2
0.00
0.02
Tr
0.3443
0.3444
0.5872
0.5877
60.4
59.4
4.85
4.87
4.82
4.87
PoI
PoII
PoI
PoII
PoI
PoII
(Ta)
PoII
Ta
3
4
0.04
0.06
0.344
0.345
0.587
0.576
59.4
0.3451
0.3451
0.3451
0.5757
0.5757
0.5755
0.5715
0.5700
59.4
59.4
58.8
58.4
58.4
5.21
5.21
5.21
5
6
7
8
0.10
0.125
0.20
0.30
5.21
PoII
Ta
PoII
Ta
0.3448
0.3308
0.3440
Pom
Ta
FeS2
Pom
Ta
9
0.40
0.50
0.3440
0.3306
0.5700
FeS2
Ta
10
FeS2
phase samples was calculated from X-ray diffraction annealing of an equimolar mixture of iron and sulfur in
an evacuated silica glass ampoule at 1273 K.
data as descried in [10] and measured by hydrostatic
weighing (in m-xylene).
RESULTS AND DISCUSSION
Samples were synthesized by the ampoule method
[13, 14]. Mixtures of carbonyl iron (reagent grade), ele-
mentary sulfur (high purity grade), and tantalum
(99.9%+) were pelletized and transferred to ampoules,
which were evacuated to 10–2 Pa and sealed off. Sam-
ples contained in Pyrex glass ampoules were heated to
823 K for 60 days; in silica glass ampoules, samples
were brought to 1273 K for 20 days. The rise in tempe-
rature alternated with furnace cooling every 10–20 K.
In the products of heating to 823 K of (1 − x)Fe · xTa · S
mixtures with 0 < x < 0.3, pyrrhotite and remnant unre-
acted tantalum are identified; in the heating products of
0 < x < 0.1 mixtures, PoI + PoII mixtures are found
(Table 1). As the substitution of tantalum for iron in the
batch progresses, PoII is produced until monoclinic
pyrrhotite Pom is formed. In the range 0.3 < x < 0.5,
monoclinic pyrrhotite coexists with pyrite. Figure 1
illustrates the effect of tantalum on the intensity of
X-ray reflections from the identified phases.
In experiments intended to study the reaction of troilite
(Tr) with tantalum, reagent mixtures (FeS : Ta = 2 : 1)
were pelletized and then isothermally heated in silica
The measured density of sample 2 is 4.87 g/cm3,
ampoules. Troilite was synthesized by a long-term which is in satisfactory agreement only with the calcu-
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 9 2007

EFFECT OF TANTALUM ON PHASE FORMATION IN THE Fe–S SYSTEM
1461
lated density of the solid solution (ss) Fe0.99Ta0.01S. The
1
unit cell parameters of Fe0.98S are as follows [9]: Ä =
0.3446 nm, ë = 0.5840 nm. The measured density of
sample 5 satisfactorily agrees with the value calculated
for Fe0.90Ta0.06S ss. From this, the existence of the two-
phase region can be explained by the incomplete solu-
bility of tantalum in PoI and PoII.
Additional experiments (Table 2, Fig. 2) show that
during the heat treatment of 0.94Fe · xTa · S batches
(samples 1–3) and (0.98 – ı)Fe · 0.02Ta · S batches
(samples 4–7), the ratio between the PoI- and PoII-
based solid solutions changes in response to the varia-
tion in ı adequate to the variation in the vacancy density
in the cationic sublattice of iron sulfide. The PoI pro-
portion decreases and the PoII proportion increases in
the reaction product with increasing ı.
2
3
4
5
6
Sample no.
1
2
3
4
It follows that tantalum partially dissolves during
the formation of the pyrrhotite phase in the presence of
tantalum. The results of our experiments imply that the
tantalum solubility in PoII is higher than in PoI. Tanta-
lum dissolution requires that extra vacancies appear,
which are random in the existence region of PoII. The
above reasons expand the PoII concentration region to
ı = 0.2. Vacancy ordering in pyrrhotite until its mono-
clinic distortion lies within 0.2 < x < 0.3.
5
6
7
8
9
Phase formation in (1 – ı)Fe · xTa · S mixtures dur-
ing nonisothermal heating to 1273 K occurs in two
stages. During the first stage (until 823 K), pyrrhotites
(PoI, PoII, Pom) and pyrite are formed. During the se-
cond stage (at 873–1273 K), PoII, Pom, and pyrite react
with tantalum to yield tantalum-saturated PoI, FeTa3S6,
and metallic iron. The PoI proportion decreases and the
FeTa3S6 proportion increases upon the reaction of tan-
10
22
24 θ, deg
Fig. 1. Fragments of X-ray diffraction patterns for the pro-
ducts of the heat treatment at 823 K of (1 – ı)Fe · xTa · S
mixtures (for ı, see Fig. 1): (1) Tr, (2) PoI, (3) PoII, (4) Po ,
m
(5) FeS , and (6) Ta.
2
(a)
(b)
Sample no.
4
Sample no.
1
5
6
2
7
3
θ, deg
θ, deg
25
23
23
25
Fig. 2. Fragments of X-ray diffraction patterns for pyrrhotites (a) 0.94Fe · xTa · S and (b) 0.94Fe · 0.02í‡ · S after heating to 823 K
(samples 1–7 in Table 2).
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 9 2007

1462
VERSHININ et al.
Table 2. Effect of the Fe : S : Ta ratio on the characteristics of the phases that are formed upon heating to 823 K
Batch composition,
Unit cell parameters and volume
ρ, g/cm3
at %
Product phase
composition
Sample no.
x
Fe
S
Ta
A, nm
C, nm
V × 103, nm3 calcd.
obs
1
2
3
48.5
48.0
47.0
51.5
51.0
50.0
–
0.00
0.02
0.06
PoI
0.3448
0.3453
–
0.5851
0.5768
–
60.2
59.6
–
4.66
4.92
–
4.63
4.76
–
1.0
3.0
PoII
PoI
PoII
Ta
4
5
49.0
48.5
50.0
50.5
1.0
1.0
0.00
0.02
PoI
0.3444
0.5877
60.4
–
4.87
–
–
–
PoII
PoI
0.3440
0.3450
0.3453
0.3453
0.5870
0.5760
0.5768
0.5767
PoII
PoII
PoII
6
7
48.0
47.4
51.0
51.5
1.0
1.0
0.04
00.6
59.6
59.5
4.92
4.86
4.76
4.75
talum with PoII and Pom (Table 3). Concurrently, tanta-
lum dissolves in PoI to reach the composition
Fe0.98Ta0.02S (Table 3), whose X-ray density is 4.98 g/cm3
and whose measured density is 4.93 g/cm3. Thus, for all
ı ≥ 0.05 in (1 – ı)Fe · ıTa · S mixtures, the substitution
of tantalum for iron produces FeTa3S6.
The reaction between troilite and tantalum to yield
FeTa3S6 in a stoichiometric mixture is detected upon
heating to 873 K. The FeTa3S6 proportion increases
noticeably with a rise in the annealing temperature.
Metallic iron is detected in the annealing products at
973 K. As a result of 3-h heat treatment at 1123 K, the
reagents are fully consumed in FeTa3S6 formation. The
reaction of troilite with tantalum changes the lattices of
the reagents, more strongly for tantalum than for troi-
lite. The unit cell parameter of cubic tantalum changes
starting with annealing at 500 K; the Ä and ë parame-
ters of hexagonal troilite change starting with annealing
at 873 K. Thus, the reaction between troilite and tanta-
lum starts with their mutual solubility.
I/I0%
80
1
3
60
To summarize, we have found that reactions in
(1 – ı)Fe · xTa · S samples with 0 < x < 0.5 occur in
stages. Initially, up to 823 K, pyrrhotites PoI, PoII, and
Pom and pyrite FeS2 are formed. Tantalum partially dis-
solves in pyrrhotites. An incomplete solubility of tanta-
lum in PoI and PoII has been found. Tantalum widens
the existence limits of PoII; the PoI existence field
shifts toward the troilite phase.
2
40
20
The second reaction stage lies in the range
823−1273 K, where tantalum is reactive. PoI reaches
the composition Fe0.98Ta0.02S, while PoII and Pom, as
well as pyrite, react with tantalum to form PoI,
FeTa3S6, and α-Fe. The initiation temperature of the
reaction between troilite and tantalum to yield FeTa3S6
is 873 K.
0
0.1
0.3
x
Fig. 3. Relative intensity I/I , % and unit cell parameters of
(1) Fe, (2) FeTa S , and (3) PoI phases vs. x in (1 – ı)Fe ·
xTa · S mixtures after heating to 1273 K.
0
3
6
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 9 2007

EFFECT OF TANTALUM ON PHASE FORMATION IN THE Fe–S SYSTEM
1463
Table 3. Characteristics of phases after heating of (1 – x)Fe · xTa · S mixtures to 1273 K
x
Phase composition A, nm C, nm V, nm3
x
Phase composition A, nm C, nm V, nm3
0.02
0.05
ss of tantalum in PoI 0.3444 0.5875 0.0603
ss of tantalum in PoI 0.3444 0.5875 0.0603
0.25
ss of tantalum in PoI 0.3444 0.5875 0.0603
FeTa3S6
0.5736 1.2341 0.3516
0.5737 1.2336 0.3516
0.5738 1.2328 0.3515
0.5739 1.2329 0.3516
FeTa3S6
Fe
0.5737 1.2333 0.3515
Fe
0.30
0.40
0.45
ss of tantalum in PoI
0.10
0.15
0.20
ss of tantalum in PoI 0.3444 0.5875 0.0603
FeTa3S6
FeTa3S6
Fe
0.5737 1.2333 0.3515
Fe
ss of tantalum in PoI
ss of tantalum in PoI 0.3444 0.5875 0.0603
FeTa3S6
FeTa3S6
Fe
0.5738 1.2328 0.3515
Fe
ss of tantalum in PoI
ss of tantalum in PoI 0.3444 0.5875 0.0603
FeTa3S6
Fe
FeTa3S6
Fe
0.5738 1.2328 0.3512
Table 4. Variations in the phase composition and unit cell parameters of troilite and tantalum during heating 2FeS + Ta mixtures
Annealing schedule
Unit cell parameters, nm
Phase composition of products
Ta
FeS
T, K
τ, h
a
A
C
273
500
–
FeS, Ta
0.3308
0.3316
0.3326
0.3318
0.3325
–
0.3447
0.3450
0.3444
0.3453
0.3457
–
0.5867
0.5844
0.5871
0.5882
0.5875
–
24
24
10
10
3
FeS, Ta
673
FeS, Ta
873
FeS, Ta, FeTa3S6
FeS, Ta, FeTa3S6, α-Fe
FeTa3S6, α-Fe
973
1123
7. V.A. Bryukvin, N. M. Pavlyuchenko, L. I. Blokhina, et al.,
Metally, No. 6, 37 (1997).
ACKNOWLEDGMENTS
This work was supported by the Russian Foundation
for Basic Research, project no. 05-03-32156.
8. V. Raghavan, J. Phase Equil. 19 (3), 275 (1998).
9. E. B. Blankova, S. N. Skryabina, and Yu. E. Turkhan,
Physics of Metals and Their Compounds (Ural State
Univ., 1976), issue No. 4, p. 3.
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