Controlled synthesis of lead magnesium tantalate

Article abstract of DOI:10.1134/S0036023614120195

The results of elaborating a method for controlled synthesis of the ferroelectric phase of lead magnesium tantalate having the perovskite structure and distinguished by high phase homogeneity are reported.

Full text of DOI:10.1134/S0036023614120195

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2014, Vol. 59, No. 12, pp. 1411–1416. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © E.E. Nikishina, E.N. Lebedeva, D.V. Drobot, 2014, published in Zhurnal Neorganicheskoi Khimii, 2014, Vol. 59, No. 12, pp. 1660–1664.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Controlled Synthesis of Lead Magnesium Tantalate
E. E. Nikishina, E. N. Lebedeva, and D. V. Drobot
Moscow State University of Fine Chemical Technologies, pr. Vernadskogo 86, Moscow, 119571 Russia
eꢀmail: helena_nick@mail.ru
Received January 22, 2014
Abstract—The results of elaborating a method for controlled synthesis of the ferroelectric phase of lead magꢀ
nesium tantalate having the perovskite structure and distinguished by high phase homogeneity are reported.
DOI: 10.1134/S0036023614120195
The success on the hiꢀtech market is largely deterꢀ
This study was aimed at elaborating a method for
mined by advances in the elaboration of methods for preparing lead magnesium tantalate that would have
controlled synthesis (in terms of both chemical and the perovskite structure and contain no impurity pyroꢀ
phase compositions) of new functional materials and chlore phase.
optimization of their properties. Many novel concepts
in electronic engineering, acoustical engineering,
OBJECTS AND METHODS
communicating control and automation systems, as
well as medical engineering are associated with ferroꢀ
magnetic ceramics.
Tantalum pentachloride of high purity grade (OAO
SMW, Russia) was used in this study.
Materials based on complex oxides of niobium and
tantalum belong to the family of dielectric materials
widely used in the aforementioned fields. These eleꢀ
ments are often distinguished by offꢀstoichiometry
resulting in the emergence of various lattice defects
that have a significant effect on ferromagnetic, elecꢀ
troꢀoptical, and nonlinear optical properties [1, 2].
The content of tantalum pentoxide in low hydrated
tantalum hydroxide was determined gravimetrically by
calcining the hydroxide to obtain Ta2O5 at 800–
900°C.
The concentration of chloride ions was determined
argentometrically using the Volhard method [4].
In the 1960s, Smolenski and Agranovskaya [3]
demonstrated the feasibility to produce oxygenꢀbearing
compounds of complex composition with perovskite
2+ 5+
Magnesium and lead acetates were selected
because they are distinguished by good solubility in
water (Mg(CH3COO)2, 61.0 g/100 g H2O (15
°C)
Pb(CH3COO)2, 55.2 g/100 g H2O (25 С)) and easy
°
structure
. Their lattice contains metal catꢀ
PbB1 3B2 3O3
removal of volatile products from the mixture as CO
and СО2 [5].
ions with different formal oxidation states at positions
B. This class includes cadmiumꢀ, cobaltꢀ, nickelꢀ,
scandiumꢀ, zincꢀ, and magnoniobates and lead magnoꢀ
tantalate. Lead magnesium tantalate PbMg1/3Ta2/3O3
Magnesium and lead acetates were prepared by disꢀ
solving the corresponding oxide in acetic acid (~ 80%):
(PMT) is an exemplary material belonging to this
family and exhibiting a number of unique dielectric
properties, namely:
—a diffuse phase transition giving rise to a very
high and wide dielectric peak at the Curie point; and
—permittivity dependent on electromagnetic field
frequency (the relaxation effect).
The main challenge in the preparation of singleꢀ
phase lead magnesium tantalate with perovskite strucꢀ
ture is the formation of an impurity phase having the
pyrochlore crystal structure (which crystallizes in
cubic unit cells, just as perovskite). The presence of
MО + 2CH3COOH
→
M(CH3COO)2
⋅
n
H2O + H2O
(1)
(M = Mg, Pb).
The reaction is exothermic. The resulting solution
was concentrated until saturation and acetate crystalꢀ
lization. To remove excess acid from the salt, magneꢀ
sium (lead) acetate was recrystallized from aqueous
solution. Magnesium (lead) acetate crystals were sepꢀ
arated from the mother liquor by filtration and airꢀ
dried until constant weight.
Magnesium and lead acetates were isolated as crysꢀ
this phase considerably reduces the permittivity of tal hydrates with composition M(CH3COO)2
lead magnotantalate. (M = Mg, Pb).
⋅
4.5H2O
1411

1412
NIKISHINA et al.
Magnesium and lead concentrations in acetate
solutions were determined complexonometrically [6].
α
=
Δ
/
c
in, %,
(3)
(4)
Δ
=
cin
–
cafter sorption, mol/L,
It was found by titrating solutions containing both
magnesium and lead that the simultaneous presence of
lead and magnesium in an acetate solution did not
affect the accuracy of measuring lead concentration in
the solution but had a significant effect on determinaꢀ
tion of magnesium concentration. The masking agent,
unithiol CH2SH–CH–SH–CH2SO3Na, was used to
eliminate the effect of lead on the accuracy of measurꢀ
ing magnesium content in an acetate solution [7].
where cin is concentration of magnesium (lead) soluꢀ
tion before sorption (mol/L) and after sorption is concenꢀ
tration of magnesium (lead) solution after sorption
(mol/L).
The sorption on low hydrated tantalum hydroxide
was calculated using the formula
c
Г =
is the solution volume (mL) and
the sorbent (g).
Δ
V
/
m
, mmol/g,
(5)
where
V
m
is weight of
The final and intermediate products were identiꢀ
fied by Xꢀray diffraction analysis on a Rigaku D/maxꢀ
C Xꢀray diffractometer (Cu
monochromator). The range of angles
K radiation, Ni filter, Si
α
EXPERIMENTAL
5
° ≤
2
θ ≤ 80
°
A method for producing low hydrated tantalum
hydroxide having high pentoxide contents has been
elaborated [8].
The method is based on a heterophase reaction
between tantalum pentachloride and ammonia soluꢀ
tion that allows one to obtain a material that would
have good filterability, low impurity content, wellꢀdevelꢀ
oped surface (150–220 m2/g), and high sorption capacity
with respect to cations of Group II–IV metals.
with steps of 0.02
°
was used to determine the unit cell
parameters.
Differential thermal analysis was conducted in air
in a temperature range of 20–900 using platinum–
°C
rhodium thermocouples. Four thermoanalytical
curves were recorded on a Qꢀ1500 D derivatograph
(PaulikꢀPaulikꢀErdey, MOM) simultaneously, namely
the differential curve (DTA), temperature curve (T),
differential thermogravimetric curve (DTG), and
integral thermogravimetric curve (TG), at a heating
rate of 10 K/min using Al2O3 as the reference. The
sample weight was 100–300 mg.
Low hydrated tantalum hydroxide with the general
formula TaO0.5–2.0(OH)1–4 mH2O is a fluffy white powꢀ
⋅
der. The content of chloride ion is less than 0.05 wt %;
the content of tantalum pentoxide is 80.0 0.1 wt %.
The particleꢀsize analysis of the synthesized low
hydrated tantalum hydroxide demonstrated that the
The infrared absorption spectra were recorded on a
Specord M 80 IR spectrometer in the frequency range
of 4000–400 cm–1 using tablets containing an anaꢀ
lyzed substance compacted with KBr or Nujol mulls.
share of particles of ~1 m in size exceeds 80%.
µ
The physicochemical properties of low hydrated
tantalum hydroxide were described in [9, 10]. The
sequence of phase transitions of low hydrated tantaꢀ
lum hydroxide during thermolysis can be described
using the scheme
The particleꢀsize analysis (calculation of the partiꢀ
cle size distribution function) was conducted on an LS
laser analyzer (Beckman Coulter).
CALCULATIONS
Low hydrated tantalum hydroxide
240°C
The relative content of the perovskite phase in the
samples under study was determined using the intensiꢀ
ties of the major peaks of the phases having the perovsꢀ
kite (110) and pyrochlore (222) structure using the
formula
–H O
2
amorphous TaOp(OH)m
,
m < 3
(6)
710°C
900°C
TTꢀTa2O5
hexagonal
TꢀTa2O5.
orthorhombic
The physicochemical characterization of magneꢀ
sium and lead acetates by DTA/DTG, Xꢀray powder
diffration, and IR spectroscopy made it possible to
suggest a general scheme for thermal decomposition of
magnesium and lead acetates:
%perovskite
I
perovskite
(2)
110
(
)
=
,
I
pyrochlore + I
perovskite
110
222
(
)
(
)
where I(110) perovskite and I(222) pyrochlore are the
intensities of peak (110) of lead magnesium tantalate
having the perovskite structure and of the peak (222)
of pyrochlore structure.
M(CH3COO)2 ⋅ 4.5H2O
80–100
°
C M(CH3COO)2 3H2O C M(CH3COO)2
⋅
200°
(7)
The degree of absorption ( , %) of magnesium and
α
360–370
°
C MO, M = Mg, Pb.
300°C
MCO3
lead cations by the solid phase of low hydrated tantaꢀ
lum hydroxide from the solution was calculated using
the formulas
Sorption of Pb2+ and Mg2+ cations by low hydrated
tantalum hydroxide from acetate solutions was carried
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 59 No. 12 2014

CONTROLLED SYNTHESIS OF LEAD MAGNESIUM TANTALATE
1413
Table 1. Sorption of Mg2+ by low hydrated tantalum hydroxide containing 80.0 0.1 wt % Ta2O5
Concentration
of the initial
Mg(CH3COO)2
solution, cin, mol/L
Concentration
of the Mg(CH3COO)2
solution after sorption,
Changes in concentraꢀ Degree of absorption
Sorption value
mol/g
Γ
,
tion of the solution
of low hydrated
after sorption,
Δ
, mol/L tantalum hydroxide
c
after sorption, mol/L
0.106
0.215
0.380
0.100
0.193
0.340
0.006
0.022
0.039
5.66
10.2
10.3
0.12
0.44
0.78
Table 2. Sorption of Pb2+ by low hydrated tantalum hydroxide containing 80.0 0.1 wt % Ta2O5
Concentration
of the initial
Pb(CH3COO)2
Concentration
of the Pb(CH3COO)2
solution after sorption,
Changes in concentraꢀ Degree of absorption
Sorption value
mol/g
Γ
,
tion of the solution
of low hydrated
after sorption,
Δ
, mol/L tantalum hydroxide
solution,
c
in, mol/L
c
after sorption, mol/L
0.110
0.195
0.297
0.340
0.028
0.089
0.190
0.226
0.082
0.107
0.107
0.114
74.4
54.6
36.0
33.4
1.64
2.13
2.14
2.27
out under static conditions at room temperature. The hydroxide and an almost complete absorption of lead
cations (αmax
Lead and magnesium acetate solutions in the conꢀ
centration range of = 0.229–0.310 mol/L and
~ 75%).
concentrations of lead and magnesium ions in the iniꢀ
tial acetate solutions were 0.110–0.340 and 0.106–
0.380 mol/L, respectively. The S : L ratio was 1 : 20.
cPb2+
= 0.106–0.180 mol/L were used to prepare lead
The data on magnesium and lead sorption by low
hydrated tantalum hydroxide are summarized in
Tables 1 and 2. When the ratio between magnesium
(lead) oxide and low hydrated tantalum hydroxide S : L =
1 : 20, the latter vigorously absorbs Mg2+ (Pb2+) ions
cMg2+
magnesium tantalate. The weight of the low hydrated
tantalum hydroxide sample was 0.5 g. The required
volume of magnesium and lead acetate solutions was
then calculated based on the desired PbO : MgO :
from the solution and saturation is achieved as soon as Ta2O5 molar ratio (Table 3). The effect of the order in
after 30–40 min.
which magnesium and lead solutions were added to
the low hydrated tantalum hydroxide and the effect of
magnesium and lead excess on the preparation of sinꢀ
gleꢀphase samples were studied. Magnesium acetate
was taken in excess of 4 and 10 mol % with respect to
MgO and lead acetate was taken in excess of 2 mol %
with respect to PbO, which was added together with
lead acetate solution.
As opposed to magnesium cations, lead cations are
almost completely absorbed by low hydrated tantalum
hydroxide. The high degree of absorption of lead can
be attributed to the species in which magnesium and
lead cations occur in the solution. Magnesium occurs
in aqueous solution as an aqua cation containing six
water molecules, [Mg(H2O)6]2+ [11]. At pH 3–4, lead
is occurs in solution as Pb2+ or as hydrated species
PbOH+ [12]. According to [13], magnesium cation
can be absorbed from solution as an aqua cation. Since
its size is larger than that of lead cation, this fact can be
used to explain the low degree of absorption of magneꢀ
In all cases, the formation of lead magnesium tanꢀ
talate having the perovskite structure was observed at
temperatures as low as 650°C. However, pyrochlore
lead magnesium tantalate PbMg1/3Ta2/3O3 and a certain
amount of unreacted individual lead and/or magnesium
tantalates were detected in addition to the stoichiometric
sium cations (< 11%) by low hydrated tantalum lead magnesium tantalate Pb1.83Ta1.71Mg0.29O6.39. The
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 59 No. 12 2014

1414
NIKISHINA et al.
I/I0, %
100
[110]
80
60
40
20
[211]
[200]
[220]
[310]
[111]
40
[100]
20
0
10
30
50
60
70
80
, deg
2
θ
Xꢀray diffraction pattern of lead magnesium tantalate having the perovskite structure.
individual perovskite lead magnesium tantalate (free
of impurity phases) can be synthesized only at 850°C.
(1) acetate solutions are added consecutively (magꢀ
nesium acetate solution is added first, followed by
addition of lead acetate solution);
(2) synthesis is performed at 850 5°С.
RESULTS AND DISCUSSION
Magnesium and/or lead excess in the initial acetate
solution does not yield a singleꢀphase sample having
the perovskite structure.
Table 4 summarizes the data on synthesis condiꢀ
tions of lead magnesium tantalate and their effect on
the phase composition of the resulting product. Synꢀ
thesis was carried out in air for 15–20 h.
The advantages of the proposed method over the
commonly used methods of solidꢀphase synthesis and
coprecipitation of components from solutions are the
The optimal conditions for preparing perovskite
lead magnesium tantalate PbMg1/3Ta2/3O3 have been
determined to be the following:
reduced synthesis temperature (to 850°С) and the
increased phase homogeneity of the target product.
Furthermore, there is no stage comprising the refineꢀ
Table 3. Calculated values for the synthesis of lead magnesium tantalate
Calculated volume
of acetate solutions V, mL
2+
cM , mol/L
Molar ratio PbO : MgO : Ta2O5
Pb2+
Mg2+
Pb2+
8.78
Mg2+
3 : 1 : 1
0.310
0.279
0.229
0.297
0.297
0.180
0.180
0.180
0.180
0.106
4.97
4.97
5.17
5.47
8.90
3 : 1 : 1
9.76
11.9
9.17
9.35
3 : 1 : 1 + 4 mol % MgO
3 : 1 : 1 + 10 mol % MgO
3 : 1 : 1 + 4 mol % MgO + 2 mol % PbO
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 59 No. 12 2014

CONTROLLED SYNTHESIS OF LEAD MAGNESIUM TANTALATE
1415
Table 4. Effect of synthesis conditions on the phase composition of the resulting lead magnesium tantalate
Phase composition (XRD)
and yield of the product
Composition
Synthesis procedure
T, °C
PMT
PMT
Mg and Pb acetate solutions
were added simultaneously
530
850
Pyrochloreꢀ100%
(a = 10.58 Å)
Pyrochlore, magnesium tantaꢀ
late Mg4Ta2O9
Magnesium acetate solution was
added first followed by addition
of lead acetate solution
700
800
850
Perovskite, pyrochlore, lead
tantalate PbTa2O6
Perovskiteꢀ95%,
Pyrochloreꢀ5%
Perovskiteꢀ100% (figure)
(a = 4.038 0.005 Å)
PMT + 4 mol % MgO
PMT + 10 mol % MgO
Same
850
Perovskiteꢀ77%,
Pyrochloreꢀ23%
Pyrochloreꢀ100%
1100
(a = 10.58 Å)
''
''
850
Pyrochloreꢀ100%
= 10.58 Å)
Pyrochloreꢀ100%
= 10.58 Å)
(a
1100
(a
PMT
+ 4 mol % MgO
+ 2 mol % PbO
850
Pyrochloreꢀ100%
= 10.58 Å)
(a
ment of the initial reagents, which eliminates the losses loss of individuality: absorption of Mg2+ ions is supꢀ
and contributes to purity of the resulting ceramics.
pressed.
(4) Experiments intended to prepare lead magneꢀ
sium tantalate using low hydrated tantalum hydroxide
and magnesium and lead acetate solutions by hetꢀ
erophase reaction have been carried out. The factors
affecting the composition of the phases synthesized
have been recognized to be as follows: offꢀstoichiomeꢀ
try, the order in which magnesium and lead acetates
are added, and heatꢀtreatment temperature. The optiꢀ
mal conditions of heterophase synthesis of complexꢀ
oxide phases with composition Pb1.83Ta1.71Mg0.29O6.39
(pyrochlore) and PbMg1/3Ta2/3O3 (perovskite) have
been determined.
CONCLUSIONS
(1) The low hydrated tantalum hydroxide with penꢀ
toxide content of 80.0 0.1 wt % was prepared using
the heterophase reaction. The scheme of thermal
decomposition of low hydrated tantalum hydroxide in
air has been determined.
(2) The sequence of stages of thermal decomposiꢀ
tion of magnesium and lead acetates has been eluciꢀ
dated. Magnesium and lead acetates were shown to
decompose giving rise to oxides at 365
°C (for magneꢀ
sium acetate) and 370 (for lead acetate).
°
C
(5) Singleꢀphase lead magnesium tantalate having
the perovskite structure (
obtained.
а = 4.038 0.005 Å) has been
(3) The sorption properties of low hydrated tantaꢀ
lum hydroxide have been studied. Sorption of Mg2+ and
Pb2+ ions was carried out from acetate solutions with
concentrations 0.106–0.380 and 0.110–0.340 mol/L,
respectively, under static conditions. In the selected range
of concentrations of initial solutions, the maximum
degree of absorption of Pb2+ cations by low hydrated tanꢀ
talum hydroxide is ~75% (cin = 0.110 mol/L) and
the maximum degree of absorption of Mg2+ cations
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≤
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Translated by D. Terpilovskaya
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 59 No. 12 2014