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Formation of lead magnesium niobate perovskite from MgNb2O6 and Pb3Nb2O8 precursors
Materials Research Bulletin, Vol. 32. No. 12, pp. 1643-1649, 1997 Copyright 0 1997 Elsevier Science Ltd Printed in the USA. All rights reserved 0025.5408/97 $17.00 + .oo
Pergamon
PI1 SOOZS-5408(97)00154-Z
FORMATION
Materials
OF LEAD MAGNESIUM NIOBATE PEROVSKITE MgNb,O, AND Pb,Nb,08 PRECURSORS
Chemistry
K. Sreedbar* and A. Mitra Division, National Chemical Laboratory,
(Received
February
FROM
Pune 411 008, India
(Refereed) 24, 1997; Accepted April 18, 1997)
ABSTRACT The formation of Pb,MgNb,O, (PMN) perovskite by the reaction of PbO with columbite type MgNb,O, precursor as well as M&OH), with Pb,Nb,O, precursor is investigated. It has been shown that pure PMN perovskite can be made by reacting PbO with phase-pure MgNb,O, at about 700°C without adding excess MgO or PbO. Evidence for the formation of an intermediate Mg-containing pyrochlore phase, which coexists with PMN perovskite. is found when this reaction is carried out at low temperatures (500-650°C). In contrast, no significant reaction has been observed when Pb,Nb,O, is reacted with Mg(OH), between 500 and 675°C suggesting that the activation energy for the reaction between PbO and MgNb,O, to form the PMN pyrochlore/ perovskite phase is less than that for the reaction of MgO with Pb,Nb,O,. 0 1997 Elsevirr Science Ltd KEYWORDS: synthesis
A. ceramics,
A. electronic
materials,
A. oxides, B. chemical
INTRODUCTION Lead-based perovskite-type oxides such as Pb(M,,,Nb,,)O, (M = Mg, Fe, Zn, etc.) [l], known as relaxor ferroelectrics, are emerging as potential candidates for multilayer ceramic capacitors and other applications [2-41. However, these compounds are difficult to synthesize and process, compared to BaTiO,-based systems, to get reliable dielectric characteristics. It is known that the conventional method of synthesizing these compounds, by reacting
*To whom correspondence
should be addressed. 1643
1644
K. SREEDHAR
01 trl.
Vol. 32, No. 12
individual oxides, generally results in varying amounts of a pyrochlore phase, along with the required perovskite phase ]2.3]. Alternative methods of synthesis. by which the pyrochlore phase can almost completely be eliminated. have been developed by Swarty et al. 151 and Guha and Anderson ]6J for the compound Pb(Mg,,,Nb,,,)O, (PMN). In the tirst method. MgO and Nb,O, are initially made to react to form the MgNb,O, precursor havin g the columbite structure. This precursor is subsequently reacted with a stoichiometric amount of PbO at about 800°C to obtain nearly phase-pure perovskite PMN. Further. it has been shown that by adding excess MgO and/or PbO to the precursor. the pyrochlore phase can be eliminated completely (2.3.7-91. In the second method. PbO and Nb,O, are reacted to form the Pb,Nbl_O, precursor. which is subsequently reacted with MgO to form the PMN perovskite 161. In this case. it has been shown that the formation of the pyrochlore phase can be further suppressed by the addition of excess MgO (2-S%) during synthesis. Although the pyrochlore phase can be eliminated by the above reactions by adding excess MgO. the excess MgO segregates in the grain as well as in the grain boundary [9], which could significantly affect the dielectric properties. In this work. the formation of PMN perovskite by the reaction of PbO with phase-pure MgNb,O, as well as Mg(OH), with Pb,Nb,O, precursors have been studied in a temperature range of 450-750°C. because the formation and stability of various phases by the above reactions at low temperatures (T XC700°C) have not yet been established. Experimental Synthesis of PMN was carried out by a two-stage process using MgNb,O, and Pb,Nb,O, precursors. The MgNb,O, precursor was prepared by reacting I : 1 molar ratios of Mg(NO,), and Nb,O, (Aldrich, 99.9%) in the temperature range 900-I 100°C for 4 to 5 days with a few intermittent grindings. The Pb,Nb,O, precursor was prepared by reacting a stoichiometric amount of PbO and Nb?O, between 700-750°C’ for 2 days. Synthesis of PMN was carried out by reacting these precursors with a stoichiometric amount of PbO or Mg(OH), in a closed alumina crucible at 4SO-750°C. In all cases. the reactants were hand ground under acetone. using an agate mortar and pestle. The total duration of grinding varied between -2 h for the preparation of MgNb,O, and --I h for the preparation of Pb,Nb,O, and PMN. All of the compounds were characterized by powder X-ray diffraction (XRD) using a Philips 1730 X-ray diffractometer with Cu Kcx radiation.
RESULTS
AND DISCUSSION
When MgO and Nb,O, are reacted with I: I stoichiometry. a minor amount (-2%) of Mg,Nb?O, and unreacted Nb20, phases are sometimes observed along with the columbitetype phase [ 10, I 11. The PMN formed by reacting such a precursor with PbO shows a minor amount (-5%) of pyrochlore phase, besides the required perovskite phase. However, by reacting a stoichiometric amount of Mg(NO,)? and Nb,O, at 1000°C for 2 days and at 1100°C for an additional 3 or 4 days with several intermittent grindings. a phase-pure MgNbZO, without Mg,Nb*O, or NblO, was formed [IO] (Fig. la). All the reflections in the XRD pattern were indexed on an orthorhombic unit cell with lattice parameters (I = 14.14 A, h = 5.68 A, and c = 5.02 A in the space group Pht. The reaction of PbO with the phase-pure MgNb,O, precursor in a 3: I molar ratio was carried out at various temperatures. At 450°C. no significant reaction between lead oxide and
LEAD MAGNESIUM
Vol. 32, No. 12
NIOBATE
cb)
0 0
a
0
0
L_L
I l
l
L I ,
1
50
I
40 t
28 ( Cu-K,,
30
2
>
1
FIG. 1 The X-ray diffraction pattern of MgNb,O, (a) and the pattern obtained by its reaction with 3PbO at 600°C (b) and 650°C (c), showing PbO (A), PMN perovskite (0), PMN pyrochlore (a), and MgNb,O, (a) phases. MgNb,O, occurred, but oxidation of PbO to Pb,O, was detected by XRD. When samples were heated to 500-550°C the PMN pyrochlore and PMN perovskite phases were formed as well as tetragonal (T) and orthorhombic (0) forms of PbO, and MgNb,O,. At 6OO”C, the XRD pattern shows the presence of PMN perovskite and PMN pyrochlore, as well as a minor
K. SREEDHAR c’t(I/
1646
Vol. 32, No. 12
40
50 -
2’3
30
(G-K&
FIG. I! The X-ray diffraction pattern of phase pure PMN perovskite MgNb,O, and 3PbO at 700°C.
obtained
by the reaction
of
amount of unreacted 0-PbO and MgNb,O, phases (Fig. I b). When further heating the sample to 650°C. the reaction between PbO and MgNb,O, was completed and the XRD pattern shows a mixture of PMN pyrochlore and perovskite phases (Fig. I c). The pyrochlore phase that is formed is cubic having lattice parameter (I - 10.60 A and is expected to have a stoichiometry of Pb,(Nb,. xMgx)O, o with \ 0.67 and 6 - 1.0. However, at high temperatures, complete transformation of this pyrochlore phase to PMN perovskite was not found, suggesting a significant deviation from the above stoichiometry. The sequence of reaction between PbO and MgNb?O, at low temperatures could be written as 550°C .?PbO(O)
+ MgNb,O,
) Pb;O,
PbO (0 + T) + MgNb?O(, + Pb3MgNbJOq
t MgNb?O,
-
(Per) + Pb,Nb,_.,Mg,0,__8
(Pyr)
600-650°C f PblMgNb,O,
(Per) + Pb?Nb_
,MgJL8
(Or)
(1)
However, when the reaction is carried out in a preheated furnace at about 700°C for 12 h with a few intermittent grindings, a completely phase-pure cubic PMN perovskite (a - 4.04 A) was obtained without any pyrochlore phase (Fig. 2).
Vol. 32, No. 12
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TABLE 1 of Various Phases Observed in the XRD by the Reaction 3PbO at Various Temperatures
Summary
Temperature (“C) 450 550 600 650 700 750
of MgNb,O,
Time (h)
Products
24 24 24 24 12 12
Pb,O,, MgNW, PbO(0 + T), MgNb,O,, PMN (Pyr)“, PMN (Pedh PbO(O), MgNb,O,, PMN (Pyr), PMN (Per) PMN (Pyr), PMN (Per) PMN (Per) PMN (Per)
and
“Pyr = Pyrochlore. “Per = Perovskite.
- 700°C 3PbO + MgNbzO,-
Pbi(MgNbJOg
(Per)
(2)
The different phases obtained by the reaction of PbO and MgNb,O, at various temperatures are summarized in Table 1. The Pb,Nb,Os precursor was prepared by reacting PbO with Nb,O, in a 3:l molar ratio at -750°C. The X-ray difiaction pattern of this phase is similar to that reported by Guha and Anderson [6], although a minor Pb deficiency is known to exist for this phase leading to the formula Pb,JJb,07,s [ 121. Hence, the stoichiometry of this phase Pb,Nb,O, is only nominal. In Figure 3, the XRD patterns of the product obtained by the reaction between Pb,Nb,O, and Mg(OH), at various temperatures are shown. As shown in Figure 3a, no significant reaction between Pb,Nb,Os and Mg(OH), occurred up to 65O”C, and the pattern is similar to that of the starting Pb,Nb,08 and unreacted MgO. At 7OO”C, a minor amount of PMN perovskite phase is formed without any evidence of an intermediate PMN pyrochlore phase (Fig. 3b). When the sample is further heated to 75O”C, a significant increase in the reaction kinetics leading to an increased amount of perovskite phase was observed with a concomitant decrease in the amount of Pb,Nb,O, phase (Fig. 3~). However, the X-ray diffraction pattern of the pyrochlore phase formed after the reaction at 750°C is significantly different from that of the starting Pb,Nb,O, pyrochlore. This suggests that incorporation of MgO into the Pb,Nb,O, pyrochlore could occur, leading to the formation of a Mg-containing pyrochlore phase Pb,(Nb,_,Mg,)O,_s along with the Pb,(MgNb,)O, perovskite [9,13]. The sequence of reaction in this case can be written as T < 650°C Pb,Nb,O, T -
+ Mg(OH)?-
No significant
reaction
+ PMN perovskite
(minor)
700°C
-
Pb,Nbz08
(major)
T - 750°C -
PMN perovskite
(major)
+ PMN pyrochlore
(minor)
(3)
By using a low-temperature sol-gel route, Chaput et al. [14] have shown that the PMN perovskite is formed through a Mg-containing lead niobate pyrochlore phase. They have
K. SKEEDHAK
I648
Vol. 32. No. 12
<‘t trl
(a)
tb)
1
I
50
40 --28
I
30
i 1
(h-K,) FIG.
.i
The X-ray diffraction pattern of the sample obtained by the reaction of Pb,Nb,O, and Mg(OH), at 650°C showing Pb,%_O, and MgO (V) (a); at 700°C showing Pb,Nb,O,, PMN perovskite (0) and MgO (V) (b); and at 750°C showing PMN perovskite (0) and PMN pyrochlom (0) phases (c). proposed that the progressive insertion of MgO into Pb_?(Nb,.~3Mgx)05.33+x (0.0 < x < 0.66) is the mechanism for the PMN pyrochlore phase formation at low temperatures. However, in the low-temperature (500-600°C) solid-state reaction, an intermediate PMN pyrochlore phase has been formed by the reaction of PbO and MgNb,O, and not by the reaction of
Vol. 32, No. 12
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1649
Pb,Nb,O, and MgO. In the latter case, no significant reaction has been observed up to 675°C. It thus appears that the activation barrier for the reaction between Pb,Nb,O, and MgO is significantly higher than that of the reaction between PbO and MgNb,O,. The relatively high activation energy for the reaction between Mg(OH), and the precursor having nominal stoichiometry Pb,Nb,O, can be rationalized in terms of the block structure of Pb,,,Nb,O,,, reported by Bernotat-Wulf and Hoffmann [ 121. In the block structure of the [Pb,.5Nb206.5] pyrochlore blocks are separated by additional PbO layers, Pb,.sNb,O,.s, and the compound has no B-site vacancy. However, the compound is expected to have two potential sites, viz., interblock and intrablock, having differing reactivity toward MgO. The reaction of MgO with the former site could lead to “reactive condensation” of the blocks to form PMN perovskite, which requires less activation energy than the intrablock reaction, where the Nb-O-Nb-O-Nb. . . octahedral network has to be broken first for its reaction with MgO to form the perovskite. In conclusion, it has been shown that pure PMN perovskite can be formed by the reaction of PbO with phase-pure MgNb,O, precursor at about 700°C without adding excess MgO or PbO. Evidence for the formation of an intermediate PMN pyrochlore phase, which coexists with the PMN perovskite phase, is found when PbO is reacted with phase-pure MgNb,O, precursor at low temperatures (500-650°C). However, no such PMN pyrochlore phase formation or significant reaction has been detected when Mg(OH), is reacted with Pb,Nb,O, precursor between 500 and 675°C suggesting that the activation energy for the formation of PMN pyrochlore/perovskite phase by the reaction of PbO with MgNb,O, is less than that for the reaction of MgO with Pb,Nb,O,.
REFERENCES
2. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
G.A. Smolenskii and A.I. Agranovskaya, Sov. Phys. Tech. Phys. 3, 1380 (1958). M. Lejeune and J.P. Boilot, Am. Cerum. Sot. Bull. 65(4), 679 (1986). S.L. Swartz, T.R. Shrout, W.A. Schulze, and L.E. Cross, J. Am. Ceram. Sot. 67(S), 311 (1984). Y. Yamashita, Am. Cerum. Sot. Bull. 73(8). 74 (1994). S.L. Swartz and T.R. Shrout, Muter. Res. Bull. 17, 1245 (1982). J.P. Guha and H.U. Anderson, J. Am. Ceram. Sot. 69, C-287 (1986). M. Lejeune and J.P. Boilot, Mater. Rex Bull. 20, 493 (1985). H.C. Wang and W.A. Schulze, .I. Am. Cemm. Sot. 73, 825 (1990). E. Goo, T. Yamamoto, and K. Okazaki, J. Am. Cerum. Sot. 69, C-188 (1986). D. Saha, A. Sen, and H.S. Maiti, .I. Mat. Sci. Lett. 13, 723 (1994). P.A. Joy and K. Sreedhar, J. Am. Cerum. Sot., in press. H. Bemotat-Wulf and W. Hoffmann, Z. Kristallogr. 164, 129 (1983). J. Chen, A. Gorton, H.M. Chan, and M.P. Harmer, .I. Am. Cerum. Sot. 69, C-303 (1986). F. Chaput, J.P. Boilot, M. Lejeune, R. Papiernik, and L.G. Pfalzgraf, J. Am. Ceram. Sot. 72, 135.5 (1989).
Pergamon
PI1 SOOZS-5408(97)00154-Z
FORMATION
Materials
OF LEAD MAGNESIUM NIOBATE PEROVSKITE MgNb,O, AND Pb,Nb,08 PRECURSORS
Chemistry
K. Sreedbar* and A. Mitra Division, National Chemical Laboratory,
(Received
February
FROM
Pune 411 008, India
(Refereed) 24, 1997; Accepted April 18, 1997)
ABSTRACT The formation of Pb,MgNb,O, (PMN) perovskite by the reaction of PbO with columbite type MgNb,O, precursor as well as M&OH), with Pb,Nb,O, precursor is investigated. It has been shown that pure PMN perovskite can be made by reacting PbO with phase-pure MgNb,O, at about 700°C without adding excess MgO or PbO. Evidence for the formation of an intermediate Mg-containing pyrochlore phase, which coexists with PMN perovskite. is found when this reaction is carried out at low temperatures (500-650°C). In contrast, no significant reaction has been observed when Pb,Nb,O, is reacted with Mg(OH), between 500 and 675°C suggesting that the activation energy for the reaction between PbO and MgNb,O, to form the PMN pyrochlore/ perovskite phase is less than that for the reaction of MgO with Pb,Nb,O,. 0 1997 Elsevirr Science Ltd KEYWORDS: synthesis
A. ceramics,
A. electronic
materials,
A. oxides, B. chemical
INTRODUCTION Lead-based perovskite-type oxides such as Pb(M,,,Nb,,)O, (M = Mg, Fe, Zn, etc.) [l], known as relaxor ferroelectrics, are emerging as potential candidates for multilayer ceramic capacitors and other applications [2-41. However, these compounds are difficult to synthesize and process, compared to BaTiO,-based systems, to get reliable dielectric characteristics. It is known that the conventional method of synthesizing these compounds, by reacting
*To whom correspondence
should be addressed. 1643
1644
K. SREEDHAR
01 trl.
Vol. 32, No. 12
individual oxides, generally results in varying amounts of a pyrochlore phase, along with the required perovskite phase ]2.3]. Alternative methods of synthesis. by which the pyrochlore phase can almost completely be eliminated. have been developed by Swarty et al. 151 and Guha and Anderson ]6J for the compound Pb(Mg,,,Nb,,,)O, (PMN). In the tirst method. MgO and Nb,O, are initially made to react to form the MgNb,O, precursor havin g the columbite structure. This precursor is subsequently reacted with a stoichiometric amount of PbO at about 800°C to obtain nearly phase-pure perovskite PMN. Further. it has been shown that by adding excess MgO and/or PbO to the precursor. the pyrochlore phase can be eliminated completely (2.3.7-91. In the second method. PbO and Nb,O, are reacted to form the Pb,Nbl_O, precursor. which is subsequently reacted with MgO to form the PMN perovskite 161. In this case. it has been shown that the formation of the pyrochlore phase can be further suppressed by the addition of excess MgO (2-S%) during synthesis. Although the pyrochlore phase can be eliminated by the above reactions by adding excess MgO. the excess MgO segregates in the grain as well as in the grain boundary [9], which could significantly affect the dielectric properties. In this work. the formation of PMN perovskite by the reaction of PbO with phase-pure MgNb,O, as well as Mg(OH), with Pb,Nb,O, precursors have been studied in a temperature range of 450-750°C. because the formation and stability of various phases by the above reactions at low temperatures (T XC700°C) have not yet been established. Experimental Synthesis of PMN was carried out by a two-stage process using MgNb,O, and Pb,Nb,O, precursors. The MgNb,O, precursor was prepared by reacting I : 1 molar ratios of Mg(NO,), and Nb,O, (Aldrich, 99.9%) in the temperature range 900-I 100°C for 4 to 5 days with a few intermittent grindings. The Pb,Nb,O, precursor was prepared by reacting a stoichiometric amount of PbO and Nb?O, between 700-750°C’ for 2 days. Synthesis of PMN was carried out by reacting these precursors with a stoichiometric amount of PbO or Mg(OH), in a closed alumina crucible at 4SO-750°C. In all cases. the reactants were hand ground under acetone. using an agate mortar and pestle. The total duration of grinding varied between -2 h for the preparation of MgNb,O, and --I h for the preparation of Pb,Nb,O, and PMN. All of the compounds were characterized by powder X-ray diffraction (XRD) using a Philips 1730 X-ray diffractometer with Cu Kcx radiation.
RESULTS
AND DISCUSSION
When MgO and Nb,O, are reacted with I: I stoichiometry. a minor amount (-2%) of Mg,Nb?O, and unreacted Nb20, phases are sometimes observed along with the columbitetype phase [ 10, I 11. The PMN formed by reacting such a precursor with PbO shows a minor amount (-5%) of pyrochlore phase, besides the required perovskite phase. However, by reacting a stoichiometric amount of Mg(NO,)? and Nb,O, at 1000°C for 2 days and at 1100°C for an additional 3 or 4 days with several intermittent grindings. a phase-pure MgNbZO, without Mg,Nb*O, or NblO, was formed [IO] (Fig. la). All the reflections in the XRD pattern were indexed on an orthorhombic unit cell with lattice parameters (I = 14.14 A, h = 5.68 A, and c = 5.02 A in the space group Pht. The reaction of PbO with the phase-pure MgNb,O, precursor in a 3: I molar ratio was carried out at various temperatures. At 450°C. no significant reaction between lead oxide and
LEAD MAGNESIUM
Vol. 32, No. 12
NIOBATE
cb)
0 0
a
0
0
L_L
I l
l
L I ,
1
50
I
40 t
28 ( Cu-K,,
30
2
>
1
FIG. 1 The X-ray diffraction pattern of MgNb,O, (a) and the pattern obtained by its reaction with 3PbO at 600°C (b) and 650°C (c), showing PbO (A), PMN perovskite (0), PMN pyrochlore (a), and MgNb,O, (a) phases. MgNb,O, occurred, but oxidation of PbO to Pb,O, was detected by XRD. When samples were heated to 500-550°C the PMN pyrochlore and PMN perovskite phases were formed as well as tetragonal (T) and orthorhombic (0) forms of PbO, and MgNb,O,. At 6OO”C, the XRD pattern shows the presence of PMN perovskite and PMN pyrochlore, as well as a minor
K. SREEDHAR c’t(I/
1646
Vol. 32, No. 12
40
50 -
2’3
30
(G-K&
FIG. I! The X-ray diffraction pattern of phase pure PMN perovskite MgNb,O, and 3PbO at 700°C.
obtained
by the reaction
of
amount of unreacted 0-PbO and MgNb,O, phases (Fig. I b). When further heating the sample to 650°C. the reaction between PbO and MgNb,O, was completed and the XRD pattern shows a mixture of PMN pyrochlore and perovskite phases (Fig. I c). The pyrochlore phase that is formed is cubic having lattice parameter (I - 10.60 A and is expected to have a stoichiometry of Pb,(Nb,. xMgx)O, o with \ 0.67 and 6 - 1.0. However, at high temperatures, complete transformation of this pyrochlore phase to PMN perovskite was not found, suggesting a significant deviation from the above stoichiometry. The sequence of reaction between PbO and MgNb?O, at low temperatures could be written as 550°C .?PbO(O)
+ MgNb,O,
) Pb;O,
PbO (0 + T) + MgNb?O(, + Pb3MgNbJOq
t MgNb?O,
-
(Per) + Pb,Nb,_.,Mg,0,__8
(Pyr)
600-650°C f PblMgNb,O,
(Per) + Pb?Nb_
,MgJL8
(Or)
(1)
However, when the reaction is carried out in a preheated furnace at about 700°C for 12 h with a few intermittent grindings, a completely phase-pure cubic PMN perovskite (a - 4.04 A) was obtained without any pyrochlore phase (Fig. 2).
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TABLE 1 of Various Phases Observed in the XRD by the Reaction 3PbO at Various Temperatures
Summary
Temperature (“C) 450 550 600 650 700 750
of MgNb,O,
Time (h)
Products
24 24 24 24 12 12
Pb,O,, MgNW, PbO(0 + T), MgNb,O,, PMN (Pyr)“, PMN (Pedh PbO(O), MgNb,O,, PMN (Pyr), PMN (Per) PMN (Pyr), PMN (Per) PMN (Per) PMN (Per)
and
“Pyr = Pyrochlore. “Per = Perovskite.
- 700°C 3PbO + MgNbzO,-
Pbi(MgNbJOg
(Per)
(2)
The different phases obtained by the reaction of PbO and MgNb,O, at various temperatures are summarized in Table 1. The Pb,Nb,Os precursor was prepared by reacting PbO with Nb,O, in a 3:l molar ratio at -750°C. The X-ray difiaction pattern of this phase is similar to that reported by Guha and Anderson [6], although a minor Pb deficiency is known to exist for this phase leading to the formula Pb,JJb,07,s [ 121. Hence, the stoichiometry of this phase Pb,Nb,O, is only nominal. In Figure 3, the XRD patterns of the product obtained by the reaction between Pb,Nb,O, and Mg(OH), at various temperatures are shown. As shown in Figure 3a, no significant reaction between Pb,Nb,Os and Mg(OH), occurred up to 65O”C, and the pattern is similar to that of the starting Pb,Nb,08 and unreacted MgO. At 7OO”C, a minor amount of PMN perovskite phase is formed without any evidence of an intermediate PMN pyrochlore phase (Fig. 3b). When the sample is further heated to 75O”C, a significant increase in the reaction kinetics leading to an increased amount of perovskite phase was observed with a concomitant decrease in the amount of Pb,Nb,O, phase (Fig. 3~). However, the X-ray diffraction pattern of the pyrochlore phase formed after the reaction at 750°C is significantly different from that of the starting Pb,Nb,O, pyrochlore. This suggests that incorporation of MgO into the Pb,Nb,O, pyrochlore could occur, leading to the formation of a Mg-containing pyrochlore phase Pb,(Nb,_,Mg,)O,_s along with the Pb,(MgNb,)O, perovskite [9,13]. The sequence of reaction in this case can be written as T < 650°C Pb,Nb,O, T -
+ Mg(OH)?-
No significant
reaction
+ PMN perovskite
(minor)
700°C
-
Pb,Nbz08
(major)
T - 750°C -
PMN perovskite
(major)
+ PMN pyrochlore
(minor)
(3)
By using a low-temperature sol-gel route, Chaput et al. [14] have shown that the PMN perovskite is formed through a Mg-containing lead niobate pyrochlore phase. They have
K. SKEEDHAK
I648
Vol. 32. No. 12
<‘t trl
(a)
tb)
1
I
50
40 --28
I
30
i 1
(h-K,) FIG.
.i
The X-ray diffraction pattern of the sample obtained by the reaction of Pb,Nb,O, and Mg(OH), at 650°C showing Pb,%_O, and MgO (V) (a); at 700°C showing Pb,Nb,O,, PMN perovskite (0) and MgO (V) (b); and at 750°C showing PMN perovskite (0) and PMN pyrochlom (0) phases (c). proposed that the progressive insertion of MgO into Pb_?(Nb,.~3Mgx)05.33+x (0.0 < x < 0.66) is the mechanism for the PMN pyrochlore phase formation at low temperatures. However, in the low-temperature (500-600°C) solid-state reaction, an intermediate PMN pyrochlore phase has been formed by the reaction of PbO and MgNb,O, and not by the reaction of
Vol. 32, No. 12
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NIOBATE
1649
Pb,Nb,O, and MgO. In the latter case, no significant reaction has been observed up to 675°C. It thus appears that the activation barrier for the reaction between Pb,Nb,O, and MgO is significantly higher than that of the reaction between PbO and MgNb,O,. The relatively high activation energy for the reaction between Mg(OH), and the precursor having nominal stoichiometry Pb,Nb,O, can be rationalized in terms of the block structure of Pb,,,Nb,O,,, reported by Bernotat-Wulf and Hoffmann [ 121. In the block structure of the [Pb,.5Nb206.5] pyrochlore blocks are separated by additional PbO layers, Pb,.sNb,O,.s, and the compound has no B-site vacancy. However, the compound is expected to have two potential sites, viz., interblock and intrablock, having differing reactivity toward MgO. The reaction of MgO with the former site could lead to “reactive condensation” of the blocks to form PMN perovskite, which requires less activation energy than the intrablock reaction, where the Nb-O-Nb-O-Nb. . . octahedral network has to be broken first for its reaction with MgO to form the perovskite. In conclusion, it has been shown that pure PMN perovskite can be formed by the reaction of PbO with phase-pure MgNb,O, precursor at about 700°C without adding excess MgO or PbO. Evidence for the formation of an intermediate PMN pyrochlore phase, which coexists with the PMN perovskite phase, is found when PbO is reacted with phase-pure MgNb,O, precursor at low temperatures (500-650°C). However, no such PMN pyrochlore phase formation or significant reaction has been detected when Mg(OH), is reacted with Pb,Nb,O, precursor between 500 and 675°C suggesting that the activation energy for the formation of PMN pyrochlore/perovskite phase by the reaction of PbO with MgNb,O, is less than that for the reaction of MgO with Pb,Nb,O,.
REFERENCES
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