Chemical methods of dispergation of metallic phases

Article abstract of DOI:10.1007/s11172-011-0193-9

The results of a cycle of works on the chemical dispergation of intermetallic compounds and alloys were summarized and analyzed. The chemical dispergation occurs in a hydrogen or ammonia atmosphere in the temperature range from 20 to 500 °C and a pressure of 0.5 - 2.0 MPa. The phase transformations were studied. The conditions suitable for the production of polymetallic phase powders of various degrees of dispersity were determined.

Full text of DOI:10.1007/s11172-011-0193-9

Russian Chemical Bulletin, International Edition, Vol. 60, No. 7, pp. 1252—1260, July, 2011
1252
Chemical methods of dispergation of metallic phases
B. P. Tarasov, E. E. Fokina, and V. N. Fokin
Institute of the Problems of Chemical Physics, Russian Academy of Sciences,
1 prosp. Akad. Semenova, 142432 Chernogolovka, Moscow Region, Russian Federation.
Fax: +7 (496) 522 1743. Eꢀmail: fvn@icp.ac.ru
The results of a cycle of works on the chemical dispergation of intermetallic compounds
and alloys were summarized and analyzed. The chemical dispergation occurs in a hydrogen or
ammonia atmosphere in the temperature range from 20 to 500 °C and a pressure of 0.5—2.0 MPa.
The phase transformations were studied. The conditions suitable for the production of polymeꢀ
tallic phase powders of various degrees of dispersity were determined.
Key words: intermetallic compound, alloy, hydrogen, ammonia, dispergation, metallic
powders.
Introduction
sorbed hydrogen is removed, the starting material transꢀ
fers into the dispersed state. This process is conventionally
named the method of hydride dispergation.6,7 This disperꢀ
gation method is also accompanied by the reduction of
surface oxides, which increases the purity of the material
and, hence, improves its characteristics.
Another chemical method of dispergation (ammonia
method) is based on the chemical interaction of metals
and alloys with gaseous ammonia as a source of hydrogen
and nitrogen formed at elevated temperatures.8 As comꢀ
pared to the hydride method, the method of ammonia
dispergation makes it possible to produce metallic powꢀ
ders with a smaller particle size, which achieves, in several
cases, the nanometer range.
Powders of intermetallic compounds and alloys with
varying dispersion are needed, as a rule, in many areas of
modern industry dealing with the use of polymetallic sysꢀ
tems. Among the latter are materials for permanent magꢀ
nets and metallohydride electrodes, catalysts, absorbents
for storage of hydrogen in the bound state, etc. Therefore,
the develoment of preparative procedures useful to maꢀ
nufacture these powders by mechanical, physical, and
chemical methods becomes very significant.1 Chemical
methods1—3 have a number of advantages over other methꢀ
ods due to which they are especially attractive and deꢀ
manded. These include: simple technical accomplishment;
a low energy capacity; a possibility to control the process
of particle formation; and the preparation of powders with
the nonꢀoxidized surface, without plastic deformation, and
with the particle shape optimum for further use.
In this work, an attempt is made to summarize the
results of studies on the production of polymetallic powders
by the methods of hydride and ammonia dispergation and
compare possibilities and features of these methods.
A chemical method for the synthesis of intermetalꢀ
lic compound powders is, for example, the method
of the preparation of mixed basic carbonate
La2Ni10(CO3)8(OH)10•(H2O)x followed by its reduction
with hydrogen and calcium hydride at 1000 °C.4 Thus preꢀ
pared powder of LaNi5 with an average particle size of
∼220 nm has a uniform chemical and phase composition
and its properties of hydrogen adsorption are nearly indisꢀ
tinguishable from the properties of the intermetallic comꢀ
pound prepared by smelting.
The study of the chemical reaction of the compact
metal with hydrogen of various purity and activity showed5
that the insertion of hydrogen atoms in the crystalline
lattice of the material changed its volume by 10—25%.
Internal strains developed in this case reach values conꢀ
siderably exceeding the breaking point of the material, due
to which it disintegrates to particular particles. Once abꢀ
Experimental
Alloys and intermetallic compounds were prepared by smeltꢀ
ing the charge composed of the corresponding metals with
the purity >99.9% in a furnace with an inexpensive tungsten
electrode in highꢀpurity argon followed by annealing treatment
of alloy prills at 800 °C for 2 weeks.
Alloys were purified from the oxide film by the mechanical
method and hydrogenated in the compact or coarseꢀcrystalline
form at temperatures from 20 to 350 °C with highꢀpurity hydroꢀ
gen at the initial pressure 0.5—2 MPa.
For the ammonia treatment, the alloy prills purified from
the oxide film were disintegrated in a titanium mortar and the
powder fraction with the particle size smaller than 100 μm was
used in experiments (except for experiments, where the particle
size of the alloy is specially indicated). The specific surface area
of this fraction was 0.04 m2 g–1
.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1228—1236, July, 2011.
1066ꢀ5285/11/6007ꢀ1252 © 2011 Springer Science+Business Media, Inc.

Chemical dispergation of metallic phases
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 7, July, 2011
1253
The interaction of the prepared powders with ammonia (the
soꢀcalled hydronitriding process) was carried out in the presence
of activator NH4Cl, which was added in an amount of 10 wt.%
of the weight of the alloy introduced in the reaction, in the
temperature range 100—500 °C at the initial ammonia pressure
0.5—0.8 MPa.
The multiple repetition of procedures of hydrogenation or
hydronitriding of the alloys and the subsequent dehydrogenation
of the hydride or hydridonitride phases formed made it possible
to obtain polymetallic powders with the required particle size.
The products of hydronitriding of metals contained chemiꢀ
sorbed ammonia preventing their oxidation and dehydrogenaꢀ
tion, which allowed one to work with the prepared powders in
air. According to the date of differential thermal analysis, chemiꢀ
sorbed ammonia is removed at 80—120 °C.
The composition of the starting alloys and products of the
ammonia treatment was monitored by chemical and Xꢀray phase
analyses. For all substances, the calculated parameters of the
crystalline lattices coincided with those presented in the ASTM
database.
The experimental procedures, analytical methods, and acꢀ
curacy of determination of physical magnitudes are described in
detail.9,10 Magnetization curves of the studied samples were meaꢀ
sured with a PARC Mꢀ4500 vibrational magnetometer.
tion 3dꢀmetals (ScFe2, CeNi3, CeCo3, LaNi5, SmCo5,
etc.),7 for the dispergation or embrittlement of the magꢀ
netic materials based on the alloys from the systems
Sm—Co (SmCo5 and Sm2Co17 with additives of Gd, Zr,
Cu, and Fe) and Nd—Fe—B 9 and of the procatalysts for
synthesis of carbon nanostructures (La—Ni, Mg—Ni,
Y—Ni, ...).13,14
Based on results of Xꢀray phase and granulometric
analyses, measurement of specific surface area, optical and
electron microscopy, various variants of the mechanism
were considered and established, and optimum modes for
dispersion of intermetallic compounds were determined.
The specific surface area (Ssp) of powders of selected
intermetallic compounds after five cycles hydrogenation—
dehydrogenation is given below.
Alloy
YFe2
Ssp/m2 g–1
0.15
Alloy
Ssp/m2 g–1
0.31
CeCo3
LaNi5
YCo5
NdCo5
SmCo5
Sm2Co17
ScFe2
ScCo2
ScNi2
CeNi3
YCo3
0.75
0.46
0.24
0.33
0.29
0.36
0.27
0.27
0.27
0.05
The multiple repetition of procedures on the hydrogeꢀ
nation of intermetallic compounds followed by the dehyꢀ
drogenation of the hydride phases formed makes it possiꢀ
ble to obtain powders with various particle sizes. The disꢀ
perse analysis of the powder of SmCo5 subjected to one
cycle of hydrogen treatment showed that the particle size
of the obtained product did not exceed 40 μm and deꢀ
creases to 10 μm after 8—10 cycles.9 The results of Xꢀray
phase analysis and electron probe Xꢀray microanalysis inꢀ
dicate that the formed powders are singleꢀphase. As can be
seen from the aboveꢀpresented data, the specific surface of
the intermetallic powders obtained after five hydrogenaꢀ
tion—dehydrogenation cycles increases by an order of
Method of hydride dispergation of metallic phases
The possibility of wide use of the method of hydride
dispergation appeared after highꢀpurity hydrogen accuꢀ
mulators were put in practice.9,11
The interaction of hydrogen with intermetallic comꢀ
pounds ABn can proceed via the reversible reaction
ABn (c) + x H2
ABnH2x (d),
(1)
where c and d are the compact and dispergated samples,
respectively; P1 > P2; and T2 > T1. However, in several
cases, the reactions of complete (2) or partial decomposiꢀ
tion (3) of the intermetallic compound with hydrogen are
thermodynamically more favorable
magnitude, being 0.2—0.5 m2 g–1
The alloys based on Sm2Co17 of compositions
Sm(Co0.671Cu0.080Fe0.222Zr0.025 8.92 or Sm0.68Gd0.17Zr0.15
(Co0.7Cu0.07Fe0.23 6.6 absorb hydrogen at 20—200 °C for
.
)
ꢀ
)
ABn (c) + m H2
(AH2m + n B) (d),
(2)
several hours, and the second cycle ends already within
∼30 min. The hydrogenation products contain less than
0.5 wt.% hydrogen and are embrittled, but the powders in
the finely dispersed state cannot be obtained even after
20 hydrogenation—dehydrogenation cycles. Therefore, for
the further use of materials based on R2Co17 and subjected
to the treatment with hydrogen, the embrittled samples
should additionally be disintegrated in an inert or hydroꢀ
gen atmosphere using other methods.
The ingots of the alloys Nd19Fe13B8, TbNd17Co20Fe55B8,
and several other (system Nd—Fe—B), preliminarily deꢀ
gassed by heating for 1 h in a vacuum of ∼1 Pa at 300 °C,
are hydrogenated without an induction period to form the
hydride phase containing 0.4—0.7 wt.% hydrogen. Their
cell volume increases by 2—5%, which induces a signifꢀ
2 ABn (c) + (m + x) H2
(AH2m + AByH2x + (2n – y) B) (d),
(3)
where y > n, and x << m.12
Understandably, only reaction (1), which is reversible
and spontaneous at the direct contact of the reactants, is
used for the lowꢀtemperature dispergation of the intermeꢀ
tallic compounds and alloys, and reactions (2) and (3) can
be used for the preparation of the corresponding homogeꢀ
neous mixtures (A + B and A + ABy + B).
We successfully used the method of hydride dispergaꢀ
tion for the disintegration of the intermetallic compounds
formed by scandium, yttrium, and rareꢀearth and transiꢀ

1254
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 7, July, 2011
Tarasov et al.
icant embrittlement of the samples. After discharging an
excessive pressure, these samples, as a rule, contain
0.2—0.3 wt.% hydrogen that, according to the gravimetric
analysis data, is evolved in one stage at 100—160 °C.
Thus, unlike mechanical milling, the method of hyꢀ
dride dispergation makes it possible to obtain metallic powꢀ
ders with the nonꢀoxidized surface, without coldꢀhardenꢀ
ings and traces of plastic deformation, and with the partiꢀ
cle shape optimum for further operation. However, the
particle size of the compounds and alloys dispergated by
this method is 1—10 μm, which restricts, in several cases,
the application of some of them for the production of
materials with specified properties. Even after tens of hyꢀ
dride dispergation cycles, the specific surface area of the
formed into nanosized powders of the corresponding metꢀ
als by dehydrogenation in vacuo (down to ∼10–3 Torr) at
temperatures ∼100 °C lower than the decomposition temꢀ
peratures (to prevent or minimize metal caking on heatꢀ
ing). The estimation of the average particle size performed
in the approximation of the particles to a ball shape gave
the following values: 85 nm for ZrH2, 60 nm for ZrN, and
30—50 nm for VN.
The scientific data obtained at the first stage of investiꢀ
gation were used for the study of the possibility of using
ammonia for dispergation of polymetallic phases, viz., inꢀ
termetallic compounds and alloys.
Ammonia dispergation of intermetallic compounds
products ranges from 0.1 to 0.7 m2 g–1 7
.
The interaction of ammonia with the intermetallic
compounds of various structural types at 100—150 °C afꢀ
fords the corresponding hydride phases, whose composiꢀ
tion and some properties are listed in Table 2.16—24 The
obtained hydride phases are most frequently solid soluꢀ
tions of hydrogen in the crystalline lattice, whereas with
the temperature increase to 200 °C and higher, in particuꢀ
lar cases, insignificant amounts of nitrogen are inserted in
the metallic lattice (<0.1 at. N/ABn) to form hydridoꢀ
nitride phases.
The products of ammonia treatment carried out at
100—200 °C are highly dispersed powders with a develꢀ
oped specific surface, which exceeds by several orders
of magnitude the specific surface of the starting polymeꢀ
tallic phases.
The behavior of intermetallic compounds CeT2 (T is
a metal of the iron triad) is anomalous compared to other
intermetallic compounds of the AB2 type: at 150 °C in an
ammonia atmosphere compounds CeFe2, CeCo2, and
CeNi2 undergo disproportionation with the formation of
the hydride phase of the new intermetallic compound
Ce2T17Hx and cerium hydride and nitride.25 As will be
shown below, the disproportionation also occurs in other
ABn—NH3 systems but at higher temperatures.
Method of ammonia dispergation of metallic phases
In our recent experiments extensive data on the relaꢀ
tionships between the temperature, size of the initial parꢀ
ticles, cycling (multiple repetition of hydrogenation—deꢀ
hydrogenation procedures), and duration of the treatment
in the chemical transformation of individual metals in
ammonia have been accumulated. The study was primariꢀ
ly aimed at establishing the possibility of preparation of
metal hydrides and nitrides in the ultradispersed state.
This work resulted in metal hydrides and nitrides and their
mixtures with a specified particle size (from microcrystalꢀ
line to ultradispersed) and revealed some specific features
of the processes that occur in the systems M—NH3.15 The
conditions for the preparation of binary compounds of
some metals are presented in Table 1.
The treatment of the metallic powders with ammonia
under the conditions indicated in Table 1 gives metal hyꢀ
drides of a certain composition, as a rule, with a developed
specific surface. The metal hydrides are further transꢀ
Table 1. Conditions for the synthesis of metal hydrides and nitrꢀ
ides with ammonia
The treatment of the intermetallic compounds with
ammonia at 450—500 °C results in the complete decomꢀ
position of matrices of the starting intermetallic comꢀ
pounds and in the formation of homogeneous highly disꢀ
persed compositions consisting of the hydride and nitride
phases of metal A and the phase of 3dꢀtransition metal B.
In these compositions, the particle size is tenths fractions
of μm, as a rule. It should be mentioned that the formaꢀ
tion of metal nitrides ANx is preceded by the reaction of
the metal with hydrogen to form hydride of the correꢀ
sponding metal AHx.15 This fact explained the particularꢀ
ly observed coexistence of hydride and nitride phases of
metals in the products of ammonia treatment.
Metal Metallic
particle
Conditions
for synthesis
Reaction
product
size/μm
T/°C
t/h
Composition Ssp/m2 g–1
Ti
Zr
10
50
50
50
250
250
400
350
400
450
550
350
500
500
450
50
55
55
36
36
34
36
60
50
60
30
TiH2
TiH2
TiN
ZrH1,2
ZrH2
ZrH2
ZrN
28.9
57.4
53.5
1.8
12.9
2.5
50
100
50
3.8
V
100
50
VHx
19.6
10.7
9.1
VN
VN
The experimental results of the hydronitriding of the
intermetallic compounds leading to various chemical and
phase transformations are summarized in Table 3.
100
100—200
Mg
Mg3N2
5.4

Chemical dispergation of metallic phases
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 7, July, 2011
1255
Table 2. Characteristics of the products of the interaction of intermetallic compounds with ammonia at 100—250 °C
Starting intermetallic
compound
T/°C
Phase
composition
Ssp
Particle size
Reference
16
/m2 g–1
/μm
Ti3Al
100
150
200
100
150
200
250
100
150
200
150
150
150
200
150
150
200
150
200
250
150
150
200
250
Ti3AlH2.8
Ti3AlH3.0
Ti3AlH3.0N<0.1
Mg2NiHx
0.1
0.1
0.2
14.7
25.5
4.5
4.3
0.5
0.4
0.4
7.4
6.8
0.3
1.4
1.3
3.4
2.5
1.9
7.8
4.3
19.0
3.4
2.8
4.2
14.1
14.1
7.1
0.12
0.07
0.22
0.25
3.1
3.9
3.9
0.15
0.15
4.0
0.86
0.56
0.20
0.27
0.40
0.11
0.20
0.04
0.25
0.28
0.20
Mg2Ni
TiFe
17
18
16
Mg2NiHx + MgH2
TiFeH0.1 + TiFeH2Nx
TiFeH0.1 + TiFeH2Nx
TiAlH0.7
TiAl
TiAlH1.5
TiAlH1.9N<0.1
ScFe2H1.5
ScNi2H1.0
SmCo5H0.3
ScFe2
ScNi2
SmCo5
19
19
20, 21
SmCo5H0.3Ny
LaNi5H0.5
LaNi5
Sm2Co17
20
21
Sm2Co17HxNy
Sm2Co17HxNy
Sm2Fe17HxNy
Sm2Fe17HxNy
Sm2Fe17HxNy
SmFe11TiHxNy
Nd2Fe14BHxNy
Nd2Fe14BHxNy
Nd2Fe14BHxNy
Sm2Fe17
22
SmFe11Ti
23
24
Nd2Fe14
B
It turned out that intermetallic compound Ti3Al is staꢀ
ble in an ammonia atmosphere at all temperatures of the
studied temperature range.16 As can be seen from the reꢀ
sults of hydronitriding of Ti3Al given in Table 4, the reacꢀ
tion with ammonia begins at 100 °C (sample 1) and conꢀ
tinues with the further temperature increase to 150 °C
(sample 2) to form hydride phases containing no nitrogen.
As the temperature of ammonia treatment further increasꢀ
es, the hydride phase begins to accumulate certain amounts
of nitrogen, which results in an increase in the specific
surface of the obtained phases and, accordingly, in a deꢀ
crease in the particle size. According to the thermal anaꢀ
lysis data, from the samples obtained at 100—250 °C hyꢀ
drogen is completely evolved with the exothermic effect at
270—280 °C.
Reactions (4)—(6) make it possible to distinctly monꢀ
itor how increasing temperature affects the process of the
stageꢀbyꢀstage chemical and/or phase transformation of
the starting intermetallic compound ABn into products of
its complete or partial decomposition (AN, B, AB2). In
addition, a specific role of ammonia in these processes,
which lies in the degradation of the metallic matrix, can
be stressed. Certainly, the reactions presented do not deꢀ
scribe all processes that occur during the treatment of the
intermetallic compound with ammonia in the presence of
a promoter. For example, as already mentioned, the forꢀ
mation of metal nitrides is accompanied by, or saying
more exactly, preceded by the appearance of metal hyꢀ
drides. The corresponding peaks in the diffraction patꢀ
terns of the reaction products are often overlap each other
and, therefore, it is not always possible to conclude unamꢀ
biguously about the presence of the hydride phase in the
products. In some cases, the starting intermetallic comꢀ
pound is resistant to ammonia (for example, Ti3Al) or
disproportionates to form a new intermetallic compound.
Finally, it should specially be noted that in some intermeꢀ
tallic compounds metal A is chemically active. As menꢀ
tioned above, these are intermetallic compounds CeT2,
but the compounds based on samarium (discussed below)
and magnesium, Mg2Ni, are of special interest.
The determination of the temperature dependence of
the hydronitriding products of the intermetallic comꢀ
pounds (see Table 3) allows one to describe the processes
occurring in the ABn—NH3 systems by reactions (4)—(6).
ABn + NH3
ABn + NH3
ABn + NH3
ABnHx
(4)
(5)
(6)
ABnHxNy
The chemical activity of magnesium favors an enꢀ
hanced reactivity shown by the intermetallic compound.
AN + n B (AB2)

1256
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 7, July, 2011
Tarasov et al.
Table 3. Chemical and phase transformations of the intermetallic compounds in NH3 at a pressure of 1.5 MPa at various temꢀ
peratures
Intermetallic
compound
T/°C
Reaction products
Intermetallic
compound
T/°C
Reaction products
Ti3Al
100—150
200—500
100
Ti3AlHx
Ti3AlHxNy
Mg2NiHx
SmCo5
150
200
250—350
SmCo5H0,3
SmCo5HxNy
SmCo5HxNy +
Mg2Ni
150
Mg2NiHx + MgH2
Mg(NH2)2 + Ni
Mg3N2 + Ni
TiFeH0.1 + TiFeH2Ny
TiFeH2Ny + TiN + γꢀFe4N
TiFe2 + TiN + αꢀFe + Fe3N
TiAlHx
TiAlHxNy
TiAl2 + TiNy
ScFe2H1.5
+ Sm2Co7HxNy + αꢀCo
SmN + Cocube
Sm2Co17HxNy
Sm2Co17HxNy + SmCo3HxNy
SmCo3HxNy +
+ Sm2Co17HxNy + Cocube
SmN + Cocube
Sm2Fe17Hx
Sm2Fe17HxNy
Sm2Fe17HxNy + SmFe3Hx +
+ αꢀFe + γꢀFe4N
200—300
350—500
200—250
350—400
500
100—150
200—350
400—500
150
450—500
200
Sm2Co17
Sm2Fe17
SmFe11Ti
TiFe
TiAl
250
300
500
150
200—250
350—400
ScFe2
ScNi2
200—350
500
Amorphous to Xꢀrays
αꢀFe + ScN0.87 + ScH2
ScNi2H1.0
500
150
αꢀFe + SmN
SmFe11TiHxNy
200
300—350
500
Amorphous to Xꢀrays
Ni + ScNx + ScH2
Ce2T17Hx + CeHx + CeN
200—250
SmFe11TiHxNy + SmFe2Hx +
+ TiFe2 + αꢀFe
SmFe2Hx + SmHx + TiFe2 +
+ αꢀFe + γꢀFe4N
SmN + TiFe2 + αꢀFe +
+ γꢀFe4N + TiH2
Nd2Fe14BHxNy
Nd2Fe14BHxNy +
+ γꢀFe4N + αꢀFe
Nd2Fe14BHxNy + γꢀFe4N +
+ αꢀFe + NdN + BFe2 + BFe3
CeT2
150
300—350
400—450
(T – Fe,
Co, Ni)
CeFe2
CeNi2
CeCo2
300—500
250—500
300—400
CeHx + CeN + αꢀFe + Fe4N
CeHx + CeN + Ni
CeHx + Ce2Co17Hx +
+ CeN + αꢀCo + Cocube
LaNi5H0.5
Nd2Fe14
B
150—250
300
LaNi5
150
200—350
400—450
400—500
LaNi5Hx + X phase + Ni
LaN + X phase + Ni
For instance, a series of compounds with different strucꢀ
tures and, in particular, magnesium amide Mg(NH2)2,
which is far from polymetallic and metallic phases, is obꢀ
served in the products of Mg2Ni hydronitriding with the
nature of the product changing with temperature.17 The
formation of amide is due to the decomposition of tetraꢀ
hydride Mg2NiH4, which was formed at 150—200 °C, with
the further temperature increase. However, the tetraꢀ
hydride is involved not only in the decomposition but also
in the competitive disproportionation reaction to form the
new intermetallic compound MgNi2. Nickel and magneꢀ
sium nitride Mg3N2 are the final decomposition products
at 500 °C.
Table 4. Conditions and results of hydronitriding of the intermeꢀ
tallic compound Ti3Al
Ammonia dispergation of alloys
It is well known that alloys of the system Ti—Al are
promising as light structural materials with high strength,
thermal stability, and resistance to oxidation. However,
alloys with a high titanium content are considerably plasꢀ
tic, which is a reason for the appearance of coldꢀhardenꢀ
ings and amorphization and oxidation of the particle surꢀ
face upon mechanical disintegration of the cast material.
Therefore, it is reasonable to use chemical methods of
disintegration for trituration of alloys of the Ti—Al system
to form nanocrystalline materials. Since the interaction of
the intermetallic compounds from this system of compoꢀ
sitions TiAl and Ti3Al with ammonia was studied,16 it
Sample Treatment conditions
Composition
of product
Particle
size/μm
T/°C
t/h
1
2
3
4
5
6
7
8
9
100
150
200
250
300
350
400
450
500
30
32
30
30
28
28
30
30
30
Ti3AlH2.8
Ti3AlH3.0
14.1
14.1
7.1
2.8
1.1
0.8
0.4
0.25
0.2
Ti3AlH3.0N<0.1
Ti3AlH2.0N<0.1
Ti3AlH0.3N0.10
Ti3AlH0.5N0.15
Ti3AlH0.6N0.25
Ti3AlH0.7N0.25
Ti3AlH2.8N0.35

Chemical dispergation of metallic phases
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 7, July, 2011
1257
Table 5. Specific surface areas of the products resulted from the interaction of alloys with
ammonia at various temperatures
T/°C
Specific surface area/m2 g–1
Ti28
Ti94Al6
Ti91Al9
Ti75Al25 Ti67Al33 Ti60Al40 Ti50Al50
(Ti15.7Al)
(Ti10.1Al)
(Ti3Al)
(Ti2Al)
(Ti1.5Al)
(TiAl)
100
150
200
250
300
350
400
450
500
—
—
—
28.8
60.7
49.5
50.1
—
—
5.8
8.5
8.7
11.3
35.6
52.0
78.8
50.5
—
0.4
0.8
1.2
1.5
1.7
48.9
50.5
60.0
0.1
0.1
0.2
0.5
0.8
1.8
3.6
5.7
8.8
0.1
0.2
0.2
0.5
0.7
1.2
3.3
5.4
7.3
0.4
0.2
0.2
0.6
0.9
1.4
1.3
3.9
17.8
0.5
0.4
0.4
0.5
5.7
3.1
2.8
3.7
1.5
—
seemed interesting to obtain and compare the data on
dispergation and possible transformations of other alloys
from this system in ammonia at 100—500 °C.
Powders of the titanium alloys with aluminum Ti94Al6
and Ti91Al9 (formula units Ti15.7Al and Ti10.1Al, respecꢀ
tively),10 Ti67Al33 and Ti60Al40 (Ti2Al and Ti1.5Al)26 were
chosen as objects of the study.
The alloys Ti15.7Al and Ti10.1Al are solid solutions with
an aluminum content in αꢀtitanium of 6 and 9 at.%. It
should be mentioned that processes associated with the
transformations of aluminum itself or aluminumꢀcontainꢀ
ing compounds were not identified because of the low
content of aluminum in the alloys Ti15.7Al and Ti10.1Al.
Therefore, in case of these alloys we practically studied
only the behavior of αꢀTi in ammonia at various tempeꢀ
ratures but taking into account some specific features reꢀ
lated to the presence of the aluminumꢀcontaining phase in
the alloys. The hydrogenation of αꢀTi with ammonia
commences at 250—300 °C to form titanium hydride, and with
the further temperature increase to 400—500 °C it transꢀ
forms into titanium nitride. The processes that occur in the
systems Ti15.7Al—NH3 and Ti10.1Al—NH3 are different
and, hence, the compositions of the formed products vary
(the specific surface area of the products is given in Table 5,
and the compositions and particle sizes are listed in Table 6).
This difference is the result of the influence of different
amounts of aluminum that is present in the alloys.10
Note the formation and coexistence of different phases
of titanium dihydride, cubic and tetragonal, in the prodꢀ
ucts of hydronitriding of the Ti15.7Al alloy at 300—450 °C.
No aluminumꢀcontaining compounds appear in the
reaction products of the Ti10.1Al—NH3 system in the whole
temperature range, but the diffraction patterns of the hyꢀ
dronitriding products of the alloy Ti15.7Al, obtained at
450—500 °C, show the signals characteristic of aluminum
nitride AlN.
It turned out that the presence of small amounts of
aluminum is not favorable for the hydrogenation and niꢀ
triding of titanium and moreover it inhibits these processes.
For example, a comparison of the results of hydronitriding
of αꢀTi and solid solutions of aluminum in titanium shows
that an additive of 6—9 at.% Al to titanium increases the
temperatures needed for the hydrogenation of titanium
(by 100 °C) and for the formation of titanium nitride whereꢀ
by the products formed show nearly the same particle size.
According to the Xꢀray phase analysis data, the melted
alloys Ti2Al and Ti1.5Al were twoꢀphase and consisted of
the intermetallic compounds Ti3Al and TiAl in various
Table 6. Composition and dispersity of the hydronitriding products of alloys Ti—Al
Starting
alloy
Composition after
hydronitriding
Hydronitriding
temperature/°C
Particle size
/μm
Ti50Al50 (TiAl)
Ti75Al25 (Ti3Al)
Ti60Al40 (Ti1.5Al)
Ti67Al33 (Ti2Al)
Ti91Al9 (Ti10.1Al)
Ti94Al6 (Ti15.7Al)
Ti91Al9 (Ti10.1Al)
Ti94Al6 (Ti15.7Al)
TiAlH0.4N0.2
Ti3AlH0.45N0.35
Ti3AlHxNy + TiAlHxNy
Ti3AlHxNy + TiAlHxNy
300
0.3
0.25—0.15
0.3—0.6
0.3—0.6
(35—45)•10–3
(35—45)•10–3
(15—20)•10–3
(15—20)•10–3
450—500
350—400
350—400
350—400
350—400
450—500
450—500
TiH2
TiH2
TiN
TiN

1258
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 7, July, 2011
Tarasov et al.
ratios.26 Therefore, the hydronitriding "Ti2Al" and "Ti1.5Al"
in the temperature range 100—400 °C turned out to be
similar to the interaction of intermetallic compounds with
ammonia. In addition, intermetallic compound TiAl sufꢀ
fers amorphization as the heating temperature increases
from 100 to 400 °C.27 The treatment of the studied alloys
with ammonia at 450—500 °C results in the partial deꢀ
struction of crystalline lattices of the intermetallic comꢀ
pounds that compose the alloys. Titanium, which is formed
due to the destruction of the crystalline matrix at these
temperatures, reacts with hydrogen evolved upon ammoꢀ
nia decomposition or with ammonia to form titanium diꢀ
hydride, the amount of which increases with an increase
in the reaction temperature from 450 to 500 °C. Titanium
hydride is simultaneously transformed into titanium
nitride. This transformation is accompanied by the forꢀ
mation of intermediate hydridonitride phases of varying
compositions. In addition, the reaction products include
intermetallic compounds TiAl3 and TiAl2, which do not
react with hydrogen. These compounds are the products
of destruction of the metallic lattice of intermetallic phasꢀ
es composing the initial alloys. As a result, the complicatꢀ
ed composition of the reaction products of the alloys with
ammonia at 450—500 °C can be explained. It should be
mentioned that, according to the chemical analysis data,
the products of hydronitriding performed at 450—500 °C
contain a considerable amount of nitrogen: up to 8.6 and
3.9 wt.% for the systems Ti1.5Al—NH3 and Ti2Al—NH3,
respectively. These data confirm the results of Xꢀray phase
studies of the formation and accumulation of the nitride
phases in the reaction products.
The values of the specific surface area of the hydroꢀ
nitriding products for all alloys of the Ti—Al systems studꢀ
ied at 150—500 °C and (for comparison) for titanium acꢀ
cording to the published data28 are listed in Table 6. They
indicate the monotonic growth and a significant increase
(by orders of magnitude) in the specific surface area of the
products with an increase in the hydronitriding temperaꢀ
ture. The increase in the specific surface area corresponds
to a further disintegration of the particles during the
hydronitriding of the alloy powder due to the chemical transꢀ
formations of the particles with the formation of products
with a higher dispersity. In fact, the estimation of the
average particle size assuming their spherical shape showed
high dispersity of the powders (for the corresponding data,
see Table 6).
ammonia, is necessary for the preparation of highly disꢀ
persed or nanosized powders of the alloys Ti1.5Al and Ti2Al.
The powders obtained at a hydronitriding temperature of
350—400 °C are highly dispersed (particle size 0.3—0.6 μm).
The powders formed at 500 °C, especially those of Ti1.5Al,
have a developed specific surface area exceeding that of
the starting alloys by two—three orders of magnitude, but
they represent a mixture of various substances.
Ammonia dispergation of magnetic materials
It is especially important for the manufacturing of perꢀ
manent magnets to find easily preparative methods of disꢀ
integration of cast materials that are also suitable for inꢀ
dustrial applications. The initial materials for the producꢀ
tion of the permanent magnets are alloys based on saꢀ
marium and iron and especially alloys of the system
RE —Fe—B (RE is a rareꢀearth element) characterized by
a high plasticity. These alloys are based on the correspondꢀ
ing intermetallic compounds. Therefore, for the preparaꢀ
tion of highly dispersed powders, we studies possibilities of
using the method of ammonia dispergation at various temꢀ
peratures for intermetallic compounds SmCo5,20,21
Sm2Co17,21 Sm2Fe17,22 SmFe11Ti,23 and Nd2Fe14B.24 In
addition, the treatment of polymetallic compositions with
ammonia results in the insertion of 3.0—3.5 nitrogen atoms
per unit cell of the crystalline lattice of intermetallic comꢀ
pounds. This insertion increases the interatomic distances
Fe—Fe and thus improves the characteristics of the
exchange interaction of "magnetic" iron atoms.29 Accordꢀ
ingly, the process of insertion makes it possible to obtain
a materials with improved magnetic properties. Some of
the relevant results are summarized in Tables 2 and 3.
The reaction of intermetallic compound SmCo5 with
ammonia at 150—200 °C affords solid solutions of hydroꢀ
gen and nitrogen in the metallic matrix of the intermetalꢀ
lic compound.21 At 250 °C during the hydronitriding of
SmCo5, the intermetallic compound begins to decompose
within 26 h to evolve metallic cobalt and form the hydriꢀ
donitride phase with a smaller content of cobalt of the
composition Sm2Co7HxNy. The specific surface area of
the reaction products achieves a value of 6.5 m2 g–1, which
is several tens of times higher than the specific surface area
of the products obtained by the method of hydride disꢀ
pergation. The estimation of the average particle size
(∼110 nm) confirmed the formation of a highly dispersed
material. A similar process is characteristic of the hydroꢀ
nitriding of Sm2Co17 under the same conditions.21 The
particle size of the powder is ∼0.3 μm. In addition, acꢀ
cording to the Xꢀray phase analysis data, the reaction products
of both intermetallic compounds, SmCo5 and Sm2Co17,
with ammonia contain trace amounts of hydridonitride
phases. These phases are based on new intermetallic comꢀ
pounds Sm2Co7 and SmCo3, respectively, and they are
the products of decomposition of the starting intermetalꢀ
As should be expected, the presented values of specific
surface for the products of treatment of titanium and alloy
Ti15.7Al containing a small amount of aluminum with amꢀ
monia at 350—400 °C coincide (see Table 5).
It can be seen that the products of hydronitriding of
the alloys Ti1.5Al and Ti2Al obtained on heating to 250 °C
have a low specific surface area indicating the embrittleꢀ
ment of the particle surface only. An additional highꢀenꢀ
ergy treatment, for example, in planetaryꢀtype ball mills in

Chemical dispergation of metallic phases
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 7, July, 2011
1259
lic compounds caused by the combined insertion of small
amounts of nitrogen and hydrogen in the metallic matrix.
It is known that in the range of particle sizes lower
than micrometers intermetallic compound SmCo5 shows
a correlation between two variables: the coercive force of
the intermetallic compound increases with a decrease in
the particle size. The measurement of the magnetic charꢀ
acteristics did not show the presence of the initial phase in the
products of reaction of SmCo5 with ammonia at a temperꢀ
ature of 250 °C corresponding, as already indicated, to the
maximum dispergation (Ssp = 6.5 m2 g–1). Indeed, the
presence of strong peaks related to the phases SmCo5HxNy
and Sm2Co7HxNy in the diffraction patterns yet doesn´t
indicate that the sample contains no small particles of
SmCo5 amorphous to Xꢀrays. However, the samples of
SmCo5 amorphous to Xꢀrays, which were obtained by millꢀ
ing in an inert atmosphere, retain a high coercive force
(H ≥ 0.4 T).30 The values of Н for the products of the reaction
of SmCo5 with ammonia at 250 °C are 0.006—0.007 T.
The hydronitriding of compound Sm2Fe17 at 200—250 °C
for 24 h yielded hydridonitride containing 1.0—2.5 wt.%
nitrogen with a developed specific surface area and a parꢀ
ticle size of 0.1—0.2 μm. The composition of the products
obtained by the treatment of this compound with ammoꢀ
nia at other temperatures is presented in Table 3.
According to the published data,31 correct measureꢀ
ments of the coercive force H and saturation magnetizaꢀ
tion σ for the samples Sm2Fe17 can be performed only in
fairly high fields. Our measurements of the hysteresis loop
at –1 T ≤ H ≤ 1 T made it possible to emphasize this
specific feature of intermetallic compound Sm2Fe17 and
to show the character of changes in its magnetic properꢀ
ties upon the ammonia treatment.
As for the system SmFe11Ti—NH3 (see Table 3), it
should be mentioned that the specific surface area of the
nitriding products SmFe11Ti obtained at 150—200 °C (Ssp =
= 19—25 m2 g–1, particle size 40—50 nm) is higher than
that of the hydrogenation product of the same compound
(Ssp = 0.4 m2 g–1 for SmFe11TiH1).
appreciable increase in the ratio c/a compared to this valꢀ
ue for the starting sample. As known, hydrogen and nitroꢀ
gen atoms occupy different sites in the lattice of intermeꢀ
tallic compound Nd2Fe14B. Hydrogenation results in
a decrease in the c/a ratio. The effect from the insertion of
nitrogen in the lattice is stronger and, hence, the c/a ratio
for samples 12 and 13 (Table 7) is higher than that for the
starting intermetallic compound.
The magnetic properties of intermetallic compound
Nd2Fe14B, its hydride phase, and the reaction products
with ammonia at 150 and 200 °C are characterized in Table 7.
It was found that the magnetic properties of samples 12
and 13 are different from analogous characteristics of both
the initial intermetallic compound and its hydride phase. The
measurements were performed in the fields –1 T ≤ H ≤ 1 T
far from the saturation fields. Nevertheless, it can be noted
that the insertion of hydrogen atoms results in an increase
in the magnetization value in an external field of 1 T (σ1),
whereas the insertion of nitrogen atoms decreases this valꢀ
ue. In the case of sample 12, the effect from the insertion
of hydrogen atoms is stronger, which resulted in an inꢀ
creased value of σ1. On the contrary, in the case of sample
13, the effect from inserted nitrogen atoms is stronger and
the value of σ1 was found to be lower than that for the
starting intermetallic compound. These data confirm the
known fact that the introduction of nitrogen increases the
magnetic characteristics of intermetallic compounds.29
No clear explanation can be so far suggested for the
presented values of magnetic characteristics of the subꢀ
stances, the starting materials for the production of perꢀ
manent magnets. However, these values clearly indicate the
retention and improvement of the magnetic properties of
the substances subjected to the ammonia treatment (see,
e.g., the data in Table 7). Since it was established that the
use of ammonia for the dispergation of magnetic materials
makes it possible to produce highly dispersed powders or
powders with the nanometer particle size (see Table 2),
the conclusion can be made that they can be used for the
production of permanent magnets. The works in this diꢀ
rection are in progress, and testing of pilot samples are
currently under way.
The measurements of the coercive force showed that
the treatment of SmFe11Ti with ammonia afforded the
nanosized powder SmFe11TiNxHy with the value Н(1T) =
= 0.0212 T, which makes it a more convenient object for
technological procedures than the starting powder SmFe11Ti.
The interaction of Nd2Fe14B with NH3 (see Table 3)
already at 150 °C (24 h) produces the crystalline and
a highly dispersed hydridonitride phase Nd2Fe14BHxNy
(a = 8.905 Å, c = 12.36 Å, nitrogen content 2.3 wt.%, avꢀ
erage particle size 0.25 μm), which starts to decompose at
250 °C in ammonia to form trace amounts of metallic
αꢀiron. The nitrogen content increases to 5.6 wt.%, and
the particle size is 0.25 μm, which is sufficient for the
preparation of permanent magnets.
Table 7. Ratio c/a, its change (Δ), saturation magnetization in
an external field of 1 T (σ ), and coercive force (H) for the
1
studied samples Nd2Fe14
B
Sample
c/a
Δ
σ
H•104
/T
1
/emu g–1
Nd2Fe14B (10)
Nd2Fe14BHx (11) 1.3798
Nd2Fe14BHxNy
(150 °C) (12)
Nd2Fe14BHxNy
(200 °C) (13)
1.3838
0
77.6
113.5
91.5
131
117
139
–0.0040
+0.0042
1.3880
1.3911
+0.0073
66.4
123
Note that an indirect evidence of the incorporation of
nitrogen atoms in the intermetallic compound lattice is an

1260
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 7, July, 2011
Tarasov et al.
12. V. A. Yartys´, V. V. Burnasheva, K. N. Semenenko, Usp.
Khim., 1983, 52, 529 [Russ. Chem. Rev. (Engl. Transl.), 1983, 52].
13. B. P. Tarasov, V. E. Muradyan, Y. M. Shul´ga, E. P. Krinichꢀ
naya, N. S. Kuyunko, O. N. Efimov, E. D. Obraztsova, D. V.
Schur, J. P. Maehlen, V. A. Yartys, H.ꢀJ. Lai, Carbon, 2003,
41, 1357.
14. A. A. Volodin, P. V. Fursikov, Yu. A. Kasumov, I. I. Khodos,
B. P. Tarasov, Izv. Akad. Nauk, Ser. Khim., 2005, 2210 [Russ.
Chem. Bull., Int. Ed., 2005, 54, 2281].
15. V. N. Fokin, E. E. Fokina, B. P. Tarasov, Zh. Neorg. Khim.,
2010, 55, 1628 [Russ. J. Inorg. Chem. (Engl. Transl.), 2010,
55, 1536].
16. V. N. Fokin, E. E. Fokina, B. P. Tarasov, Zh. Obshch. Khim.,
2008, 78, 888 [Russ. J. Gen. Chem. (Engl. Transl.), 2008,
78, 1118].
Conclusion
The hydride dispergation based on the reversible interꢀ
action of polymetallic systems with hydrogen is a promisꢀ
ing method for the preparation of highly dispersed powꢀ
ders of metallic phases or their hydrides.
Depending on the treatment temperature, the reaction
of polymetallic systems with ammonia, viz., ammonia disꢀ
pergation, can easily be used to prepare nanodispersed
powders of metallic phases, hydrides, hydronitrides, and
nitrides of metals and metallic phases, whereas highly disꢀ
persed powders of the metallic phases are obtained after
the removal of hydrogen from hydrides.
The use of ammonia instead of hydrogen for the disꢀ
pergation of metallic phases makes it possible to obtain
highly dispersed powders or powders with the nanometer
particle size without using preliminary activation, without
adherence to strict requirements imposed on the purity of
hydrogen used, and to rigid restrictions on the composiꢀ
tion of the alloy during its melting. The prepared powders
containing some amounts of hydrogen and nitrogen are
suitable as the starting materials for the production of
pressed items, which is well known from the literature
(see, e.g., Ref. 29). Mixtures of powders obtained at high
temperatures of hydronitriding are utilizable for the proꢀ
duction of diverse composite materials.
17. V. N. Fokin, E. E. Fokina, B. P. Tarasov, Neorg. Mater., 2009,
45, 926 [Inorgan. Materials (Engl. Transl.), 2009, 45, 859].
18. V. N. Fokin, E. E. Fokina, I. I. Korobov, B. P. Tarasov,
Neorg. Mater., 2008, 44, 184 [Inorgan. Materials (Engl. Transl.),
2008, 44, 142].
19. S. P. Shilkin, V. N. Fokin, E. E. Fokina, B. P. Tarasov, I. I.
Korobov, Zh. Obshch. Khim., 2000, 70, 399 [Russ. J. Gen.
Chem. (Engl. Transl.), 2000, 70, 371].
20. V. N. Fokin, Yu. M. Shul´ga, B. P. Tarasov, E. E. Fokina,
I. I. Korobov, A. G. Burlakova, S. P. Shilkin, Zh. Obshch.
Khim., 2004, 74, 1765 [Russ. J. Gen. Chem. (Engl. Transl.),
2004, 74, 1641].
21. V. N. Fokin, S. P. Shilkin, B. P. Tarasov, E. E. Fokina, I. I.
Korobov, A. G. Burlakova, Yu. M. Shul´ga, Zh. Neorg. Khim.,
2004, 49, 17 [Russ. J. Inorg. Chem. (Engl. Transl.), 2004, 49, 13].
22. V. N. Fokin, Yu. M. Shul´ga, B. P. Tarasov, E. E. Fokina, I. I.
Korobov, A. G. Burlakova, S. P. Shilkin, Neorg. Mater., 2004,
40, 580 [Inorgan. Materials (Engl. Transl.), 2004, 40, 497].
23. V. N. Fokin, Yu. M. Shul´ga, B. P. Tarasov, E. E. Fokina,
I. I. Korobov, A. G. Burlakova, S. P. Shilkin, Zh. Obshch.
Khim., 2005, 75, 881 [Russ. J. Gen. Chem. (Engl. Transl.),
2005, 75, 831].
24. V. N. Fokin, Yu. M. Shul´ga, B. P. Tarasov, E. E. Fokina,
I. I. Korobov, A. G. Burlakova, S. P. Shilkin, Zh. Neorg.
Khim., 2005, 50, 1061 [Russ. J. Inorg. Chem. (Engl. Transl.),
2005, 50, 975].
25. V. N. Fokin, E. E. Fokina, I. I. Korobov, B. P. Tarasov, S. P.
Shilkin, Zh. Neorg. Khim., 2001, 46, 587 [Russ. J. Inorg.
Chem. (Engl. Transl.), 2001, 46, 511].
References
1. R. A. Andrievskii, Usp. Khim., 1994, 63, 431 [Russ. Chem.
Rev. (Engl. Transl.), 1994, 63].
2. I. V. Tananaev, V. B. Fedorov, E. G. Kalashnikov, Usp. Khim.,
1987, 56, 193 [Russ. Chem. Rev. (Engl. Transl.), 1987, 56].
3. I. I. Kornilov, Titan [Titanium], Moscow, Nauka, 1963,
123 pp. (in Russian).
4. B. P. Tarasov, A. G. Burlakova, L. S. Volkova, N. G. Mozgiꢀ
na, Z. A. Rumynskaya, Yu. M. Shul´ga, S. P. Shilkin,
Zh. Neorg. Khim., 1997, 42, 209 [Russ. J. Inorg. Chem.
(Engl. Transl.), 1997, 42].
5. K. N. Semenenko, V. V. Burnasheva, Vest. Mos. Gos. Univ.,
Ser. 2. Khimiya, 1977, 18, 618 [Vestn. Mosk. Univ., Ser. Khim.
(Engl. Transl.), 1977, 18].
6. V. N. Verbetskii, A. A. Salamova, K. N. Semenenko, Izv.
Akad. Nauk SSSR, Ser. Metally [Bull. Acad. Sci. USSR,
Ser. Met.], 1989, 196 (in Russian).
7. V. N. Fokin, B. P. Tarasov, I. I. Korobov, S. P. Shilkin,
Koord. Khim., 1992, 18, 526 [Russ. J. Coord. Chem.
(Engl. Transl.), 1992, 18].
8. V. N. Fokin, S. P. Shilkin, E. E. Fokina, B. P. Tarasov, I. I.
Korobov, Zh. Neorg. Khim., 1999, 44, 885 [Russ. J. Inorg.
Chem. (Engl. Transl.), 1999, 44, 823].
9. V. N. Fokin, E. E. Fokina, S. P. Shilkin, Zh. Prikl. Khim., 1994,
67, 1372 [Russ. J. Appl. Chem. (Engl. Transl.), 1994, 67, 1207].
10. V. N. Fokin, E. E. Fokina, B. P. Tarasov, Neorg. Mater., 2010,
46, 1438 [Inorgan. Materials (Engl. Transl.), 2010, 46, 1304].
11. V. N. Fokin, S. L. Troitskaya, E. E. Fokina, S. P. Shilkin,
Zh. Neorg. Khim., 1991, 36, 299 [Russ. J. Inorg. Chem.
(Engl. Transl.), 1991, 36].
26. V. N. Fokin, E. E. Fokina, I. I. Korobov, B. P. Tarasov, Int.
J. Hydrogen Energy, 2011, 36, 1217.
27. K. Hashi, K. Ishikawa, K. Suzuki, K. Aoki, J. Alloys Comp.,
2002, 330—332, 547.
28. V. N. Fokin, S. P. Shilkin, E. E. Fokina, N. G. Mozgina, Zh.
Obshch. Khim., 1997, 87, 705 [Russ. J. Gen. Chem. (Engl. Transl.),
1997, 87].
29. O. V. Kravchenko, V. V. Burnasheva, A. D. Terekhov, K. N.
Semenenko, Zh. Obshch. Khim., 1998, 68, 705 [Russ. J. Gen.
Chem. (Engl. Transl.), 1998, 68].
30. M. L. Kahn, J.ꢀL. Bobet, F. Weill, B. Chevalier, J. Alloys
Comp., 2002, 334, 285.
31. J. M. D. Coey, H. Sun, D. P. F. Hurley, J. Magn. Magn.
Mat., 1991, 101, 310.
Received March 4, 2011;
in revised form June 1, 2011