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Characterization of γ-BaCO3
Solid State lonics 32/33 (1989) 394-397 North-Holland, Amsterdam
CHARACTERIZATION OF 6-BaCO3 Tadashi N I S H I N O Alusashi Institute (~fTechnology. 1-28 Tamazutsumi. Setagaya, Tokyo. Japan Received 21 June 1988: accepted for publication 4 July 1988
When a mixed powder of BaCO~ with a small amount of BaSO4 is heated at around the transition temperature of Ba('O~ for several minutes and followed by quenching, a new cD'stalline metastable phase (6-BaCO3) is obtained. The paper is concerned with the experimental results for the formation, property, chemical composition and crystal analysis of the g-phase.
1. Introduction
2. Experimental 2.1. Starting materials and calcination
BaCOn(y) transforms enantiotropically to ct- and [~-phases with representing clear thermal deflections on a D T A curve, such as 8 2 0 ("
yBaCO3
~
{a r a g o n i t e t y p e )
tt-BaCO3 ( calcite type }
9(~<) ( '
13-BaCO+
The starting materials with extremely high purity were dried at 110° C, carefully weighed on an analytical balance to give desired composition, and mixed thoroughly in an agate mortar with pestle. The mixed samples (ca. 500 rag) were heated under different conditions ( T ° C, t min ) and quenched abruptly by immersion in liquid nitrogen.
S r C Q (~,) transforms, analogously with BaCO3, to a-phase at about 930°C. A m o n g these phases, the high-temperature p o l y m o r p h (s) cannot be obtained at room t e m p e r a t u r e even if they are quenched into liquid nitrogen owing to abrupt transition to y-phase. However, a new crystalline metastable phase. named g-phase of BaCO3 or SrCO3, is obtained by heating mixed powders of each carbonate with a few mole% o f a sulfate or chromate (such as BaSO4 or SrCrO4) near the ~,-ct phase transition temperature and followed by quenching in liquid nitrogen [ 1 ]. The formation of the g-phase m a y be attributed to the partial replacement o f CO3- ion in the host lattice with S O l - or C r O ] - having larger ionic radius. The present paper is concerned with the characterization of the g-phase ( m a i n l y for g-BaCO3) introducing the following subjects: (1) F o r m a t i o n process, (2) metastability, ( 3 ) chemical composition and (4) crystal structure.
2.2. IdentliBcation Identification of g-phase was carried out by X-ray diffraction m e t h o d as an ordinary means. Phase change process of g-BaCO3 during heating was exa m i n e d by thermal analysis and X R D equipped with a furnace and temperature controller.
2.3. Chemical analysis The a m o u n t o f BaSO4 incorporated in g-BaCO3 was dissolved by treating with an aqueous suspension with a strongly acidic ion exchange resin, H-form (Dowex 50Wx4, 200-400 mesh, abr. H - R ) . The a m o u n t o f released SO4- caused by cation exchange reaction between H + and Ba -~+ was determined using ion chromatography (Dionex, IC 10) with a calibration curve in the concentration range from 5 to 30 p p m SO4.
3 95
72 Nishino /Characterization qf S-BaCO ¢
3. Results and discussion
1.0 ¢,_ 4-
3.1. Formation process
Analytical evaluation of the amount of the formed 6-phase was established by measuring the diffracted line intensity at 26.6°C of Bragg's angle (20). Fig. 1 illustrates X-ray diffraction patterns of three modifications of BaCO~ in which the 6-phase was obtained by heat-treatment of mixed powders of BaCO3 with 10 mol% BaSO4 at 820°C for 30 min and followed by quenching. Phase transformation of the carbonate is essential for the formation of the 6-phase. The 6-phase cannot be obtained for CaCO3 because of the absence of such reversible transformation. BaCO3 and SrCO3 transform abruptly to the aphase at 810 and 926 oC, respectively, during heating ( 10 ° C / m i n ) but its transition temperature decrease at around 750 and 840°C respectively, owing to the incorporation of an additive ( B a S O 4 ) tO form a solid solution. The result obtained by DTA was confirmed by high-temperature XRD method. The phase present at higher temperature was identified by XRD as a slightly expanded a-phase rather than 6-phase which indicates that the 6-phase generate as a distorted orphase during quenching process. In fig. 2, the fraction of the formed 8-BaCO3 are plotted against heating temperature (for 30 min) and time for the mixture of BaCO3 with 10 mol% BaSO4 [2]. It can be seen from fig. 2 that the formation of 6BaCO3 depends markedly on the heating temperaJ
I
'
I
.{- BaCO3
b
20
}]
i0c-aaC03 at 830°C
[h
g" BaI03
30
~/fS";:
*'820°C o f / /
4500 0.5
t-
¢.-
I
X 700
800
o
Heating Temperature(C)
,
Z
0
10 20 30
Heating Time(min)
Fig. 2. Semiquantitative formation curves of 6-BaCO~ as a function of heating condition for the mixture of 0.9BaCO~ + 0.1 BaSO4. Dotted line in the left figure indicates the variation of mean diameter of residual BaSO4 after leaching with diluted HCI.
ture and is accomplished for a few minutes at a higher temperature. As a general rule, solid state reactions are governed by a diffusion process through the reaction product, and the rate of diffusion for anion groups is very much lower than that of cations. In view of the rapid formation of 6-BaCO3, fluctuation owing to the phase transition of the carbonate triggers off the diffusion to incorporate SO4 ion into the host lattice. Changes of mean diameter of the residual BaSO4 after soaking in a dilute HC1 solution is indicated by a dotted line which will be discussed latter. 3.2. Metastability
F-
.
i_J L_ i
840°C
,,¢~ 800 1.(3
40
50
Cu Ka (20)
Fig. 1. X-ray diffraction patterns.
Metastability of the 6-phase formed by a partial displacement between anions of different size leads to rearrangement of ions to give a stable phase by reheating or by contacting with water. A sequence of structure changes during heating is represented in fig. 3. [3]. Similarly to exothermal deflection around 650°C on DTA curve, 8-BaCO3 transformed to T-BaCO3 with extruding incorporated B a S O 4 , followed by transformation to a-phase with reincorporating BaSO4. Quite similar phenomenon was observed for 6-SRC03 at 750°C (6-'/) and at 900°C (y-a).
396
T. Nishino / Characterization q/'5-Ba('O ,
20
25
30
35
40
45
Cu Kot (20)
Fig. 3. Phase change of 5-BaCO, to y- and a-phases during heating (the numbers in parenthesis on the weak peaks are for the Miller indices of BaSO4extruded out of~-BaCO~. The 6-y phase transition occurs gradually by absorbing water vapor in humid environment. Furthermore, an abrupt exothermal peak (ca.9.0 cal/g) due to the transition was observed by dipping 8BaCO3 powders in water using twin-type microcalorimeter. The resulting powders were consisted of fine acicular ]t-BaCO3 crystals cleavaged in [0 0 1 ] direction and a small amount of tiny BaSO4 crystals.
cedure of ~-BaCO3 is based on the following facts: ( 1 ) By treatment with dilute HC1 solution, the host carbonate dissolves completely, while the incorporated BaSO4 remains as extremely fine particles (ca. 60 nm) as shown in fig. 2. (2) When the starting amount of B a S Q exceeds the solubility limit in eq. ( 1 ), these unreacted BaSO4 particles grow to ca. 400 nm in mean diameter by sintering at a high temperature. (3) The fine BaSO4 particles derived from 6BaCO3 are able to dissolve in H - R containing aqueous suspension with liberating of corresponding amounts of SO4 ion, whereas the solubility behaviour can be employed to distinguish between the fine and coarse BaSO4 powders which is useful to determine the x value. The experimental result is shown in fig. 4, in which the relation between the amount of BaSO4 substituted in the 5 lattice and that added in the starting mixture. All mixed samples with different amount of BaSO4 were heated at 820°C for 3 h and quenched. The calcined sample (100-150 mg) thus obtained and 4 g of H - R were suspended together in 3 ml of 1M-HC1 aqueous solution, filtered, washed and analyzed for the released SO4 quantitatively using ion chromatograph after adjustment to 100 ml in a volumetric flask. It can be seen from fig. 4 that BaSO4 added in the range of 0-10 tool% can be perfectly exchanged with BaCO3 but for more than 10 mol%, unreacted BaSO4 was found, viz., the chemical formula of 6-BaCO3 can be represented as Ba (CO3) o~o(SO4) o~o [ 4 ].
3.3. C h e m i c a l composition
Formation reaction of 5-BaCO3 can be represented by the following equation when BaSO4 is used as an additive;
d110 -69 m ~ 100
@
9 0 - -
( 1-x)BaSO3+xBaS04
0
=Ba(CO3) l_x(SO4)x •
._~
80
--
(1)
"0
Disappearance method by XRD will be applicable to determine indirectly the x value in eq. (1) but some instrumental error is unavoidable. In this study, a unique direct analytical method was used to determine chemical composition of the 8-phase. An important characteristic of the analytical pro-
.~
40
--
U_
5
10 15 BaSO 4 mol%
Fig. 4. Plotsof BaSO4solubility (exchangedBaSO4/added BaSO4 (%) against added BaSO4.
397
T. Nishino / Characterization of 6-BaC03 Table 1 Crystal data for selected BaCO3 and CaCO3 modifications.
Orthorhombic
~-BaCO 3 (Witherite) BaCO3
Hexagonal
~-BaC03 (830%)
6.4301 Z=4, Pmcn
" --5.-31
a 17--b94 I5.72A c
4.94
S-BaC03 (New metastable)
.z=2, P2 ~/,m
Z=3, R3m
Aragonite CaCO 3
Monoclinic
cacaO0
Calcite a
C
4;990 I~:oo~
Z=4, Pmcn
3. 4. Crystal structure The analysis was carried out in collaboration with Prof. Mizutani and his colleague [5 ]. Powder X-ray diffraction data of the ~i-BaCO3 were obtained by step scanning technique with the step size of 0.02 ° and a fixed counting time of 10 s in the 20 range 4-100 ° . Correction for the systematic angular error was done using Si powders as the external standard. Visser's indexing program was used to find the unit cell of the 6-phase. Crystal data obtained for ~-BaCO3)o.9o(SO4)0.Jo are given in table 1, in which the selected polymorphs of BaCO3 and CaCO3 are given for comparison. As shown in table 1, the structure of 5-BaCO3 is closely related to that of CaCO3 (II), a high-pressure metastable phase.
6.334
Z=6, R3m
10 .9 ° Z=4, P21/c
The [3 angles of 5-BaCO3 and C a C O 3 (II) are the same within a standard deviation, and the a, b and c axes of 6-BaCO3 essentially correspond to a, b and c / 2 o f CaCO3. Slight increase in axes lengths is due to the size effect of cations with different ionic radii.
References [ 1 ] T. Nishino and S. Nishiyama, Reactivity of solids (Chapman and Hall, London, 1972) p. 766. [2] T. Nishino, J. Am. Ceram. Soc. 70 (1987) C-162. [ 3 ] T. Nishino, A. Dosho, M. Nagai and T. Sakurai, Gypsum and Lime 202 (1986) 137. [41 T. Nishino, T. Sakurai and Y. Moriyoshi, J. Ceram. Assoc. Japan 94 (1986) 268. [5] T. Nishino, T. Sakurai, N. Ishizawa, N, Mizutani and M. Kato, J. Solid State Chem. 69 (1987) 24.
CHARACTERIZATION OF 6-BaCO3 Tadashi N I S H I N O Alusashi Institute (~fTechnology. 1-28 Tamazutsumi. Setagaya, Tokyo. Japan Received 21 June 1988: accepted for publication 4 July 1988
When a mixed powder of BaCO~ with a small amount of BaSO4 is heated at around the transition temperature of Ba('O~ for several minutes and followed by quenching, a new cD'stalline metastable phase (6-BaCO3) is obtained. The paper is concerned with the experimental results for the formation, property, chemical composition and crystal analysis of the g-phase.
1. Introduction
2. Experimental 2.1. Starting materials and calcination
BaCOn(y) transforms enantiotropically to ct- and [~-phases with representing clear thermal deflections on a D T A curve, such as 8 2 0 ("
yBaCO3
~
{a r a g o n i t e t y p e )
tt-BaCO3 ( calcite type }
9(~<) ( '
13-BaCO+
The starting materials with extremely high purity were dried at 110° C, carefully weighed on an analytical balance to give desired composition, and mixed thoroughly in an agate mortar with pestle. The mixed samples (ca. 500 rag) were heated under different conditions ( T ° C, t min ) and quenched abruptly by immersion in liquid nitrogen.
S r C Q (~,) transforms, analogously with BaCO3, to a-phase at about 930°C. A m o n g these phases, the high-temperature p o l y m o r p h (s) cannot be obtained at room t e m p e r a t u r e even if they are quenched into liquid nitrogen owing to abrupt transition to y-phase. However, a new crystalline metastable phase. named g-phase of BaCO3 or SrCO3, is obtained by heating mixed powders of each carbonate with a few mole% o f a sulfate or chromate (such as BaSO4 or SrCrO4) near the ~,-ct phase transition temperature and followed by quenching in liquid nitrogen [ 1 ]. The formation of the g-phase m a y be attributed to the partial replacement o f CO3- ion in the host lattice with S O l - or C r O ] - having larger ionic radius. The present paper is concerned with the characterization of the g-phase ( m a i n l y for g-BaCO3) introducing the following subjects: (1) F o r m a t i o n process, (2) metastability, ( 3 ) chemical composition and (4) crystal structure.
2.2. IdentliBcation Identification of g-phase was carried out by X-ray diffraction m e t h o d as an ordinary means. Phase change process of g-BaCO3 during heating was exa m i n e d by thermal analysis and X R D equipped with a furnace and temperature controller.
2.3. Chemical analysis The a m o u n t o f BaSO4 incorporated in g-BaCO3 was dissolved by treating with an aqueous suspension with a strongly acidic ion exchange resin, H-form (Dowex 50Wx4, 200-400 mesh, abr. H - R ) . The a m o u n t o f released SO4- caused by cation exchange reaction between H + and Ba -~+ was determined using ion chromatography (Dionex, IC 10) with a calibration curve in the concentration range from 5 to 30 p p m SO4.
3 95
72 Nishino /Characterization qf S-BaCO ¢
3. Results and discussion
1.0 ¢,_ 4-
3.1. Formation process
Analytical evaluation of the amount of the formed 6-phase was established by measuring the diffracted line intensity at 26.6°C of Bragg's angle (20). Fig. 1 illustrates X-ray diffraction patterns of three modifications of BaCO~ in which the 6-phase was obtained by heat-treatment of mixed powders of BaCO3 with 10 mol% BaSO4 at 820°C for 30 min and followed by quenching. Phase transformation of the carbonate is essential for the formation of the 6-phase. The 6-phase cannot be obtained for CaCO3 because of the absence of such reversible transformation. BaCO3 and SrCO3 transform abruptly to the aphase at 810 and 926 oC, respectively, during heating ( 10 ° C / m i n ) but its transition temperature decrease at around 750 and 840°C respectively, owing to the incorporation of an additive ( B a S O 4 ) tO form a solid solution. The result obtained by DTA was confirmed by high-temperature XRD method. The phase present at higher temperature was identified by XRD as a slightly expanded a-phase rather than 6-phase which indicates that the 6-phase generate as a distorted orphase during quenching process. In fig. 2, the fraction of the formed 8-BaCO3 are plotted against heating temperature (for 30 min) and time for the mixture of BaCO3 with 10 mol% BaSO4 [2]. It can be seen from fig. 2 that the formation of 6BaCO3 depends markedly on the heating temperaJ
I
'
I
.{- BaCO3
b
20
}]
i0c-aaC03 at 830°C
[h
g" BaI03
30
~/fS";:
*'820°C o f / /
4500 0.5
t-
¢.-
I
X 700
800
o
Heating Temperature(C)
,
Z
0
10 20 30
Heating Time(min)
Fig. 2. Semiquantitative formation curves of 6-BaCO~ as a function of heating condition for the mixture of 0.9BaCO~ + 0.1 BaSO4. Dotted line in the left figure indicates the variation of mean diameter of residual BaSO4 after leaching with diluted HCI.
ture and is accomplished for a few minutes at a higher temperature. As a general rule, solid state reactions are governed by a diffusion process through the reaction product, and the rate of diffusion for anion groups is very much lower than that of cations. In view of the rapid formation of 6-BaCO3, fluctuation owing to the phase transition of the carbonate triggers off the diffusion to incorporate SO4 ion into the host lattice. Changes of mean diameter of the residual BaSO4 after soaking in a dilute HC1 solution is indicated by a dotted line which will be discussed latter. 3.2. Metastability
F-
.
i_J L_ i
840°C
,,¢~ 800 1.(3
40
50
Cu Ka (20)
Fig. 1. X-ray diffraction patterns.
Metastability of the 6-phase formed by a partial displacement between anions of different size leads to rearrangement of ions to give a stable phase by reheating or by contacting with water. A sequence of structure changes during heating is represented in fig. 3. [3]. Similarly to exothermal deflection around 650°C on DTA curve, 8-BaCO3 transformed to T-BaCO3 with extruding incorporated B a S O 4 , followed by transformation to a-phase with reincorporating BaSO4. Quite similar phenomenon was observed for 6-SRC03 at 750°C (6-'/) and at 900°C (y-a).
396
T. Nishino / Characterization q/'5-Ba('O ,
20
25
30
35
40
45
Cu Kot (20)
Fig. 3. Phase change of 5-BaCO, to y- and a-phases during heating (the numbers in parenthesis on the weak peaks are for the Miller indices of BaSO4extruded out of~-BaCO~. The 6-y phase transition occurs gradually by absorbing water vapor in humid environment. Furthermore, an abrupt exothermal peak (ca.9.0 cal/g) due to the transition was observed by dipping 8BaCO3 powders in water using twin-type microcalorimeter. The resulting powders were consisted of fine acicular ]t-BaCO3 crystals cleavaged in [0 0 1 ] direction and a small amount of tiny BaSO4 crystals.
cedure of ~-BaCO3 is based on the following facts: ( 1 ) By treatment with dilute HC1 solution, the host carbonate dissolves completely, while the incorporated BaSO4 remains as extremely fine particles (ca. 60 nm) as shown in fig. 2. (2) When the starting amount of B a S Q exceeds the solubility limit in eq. ( 1 ), these unreacted BaSO4 particles grow to ca. 400 nm in mean diameter by sintering at a high temperature. (3) The fine BaSO4 particles derived from 6BaCO3 are able to dissolve in H - R containing aqueous suspension with liberating of corresponding amounts of SO4 ion, whereas the solubility behaviour can be employed to distinguish between the fine and coarse BaSO4 powders which is useful to determine the x value. The experimental result is shown in fig. 4, in which the relation between the amount of BaSO4 substituted in the 5 lattice and that added in the starting mixture. All mixed samples with different amount of BaSO4 were heated at 820°C for 3 h and quenched. The calcined sample (100-150 mg) thus obtained and 4 g of H - R were suspended together in 3 ml of 1M-HC1 aqueous solution, filtered, washed and analyzed for the released SO4 quantitatively using ion chromatograph after adjustment to 100 ml in a volumetric flask. It can be seen from fig. 4 that BaSO4 added in the range of 0-10 tool% can be perfectly exchanged with BaCO3 but for more than 10 mol%, unreacted BaSO4 was found, viz., the chemical formula of 6-BaCO3 can be represented as Ba (CO3) o~o(SO4) o~o [ 4 ].
3.3. C h e m i c a l composition
Formation reaction of 5-BaCO3 can be represented by the following equation when BaSO4 is used as an additive;
d110 -69 m ~ 100
@
9 0 - -
( 1-x)BaSO3+xBaS04
0
=Ba(CO3) l_x(SO4)x •
._~
80
--
(1)
"0
Disappearance method by XRD will be applicable to determine indirectly the x value in eq. (1) but some instrumental error is unavoidable. In this study, a unique direct analytical method was used to determine chemical composition of the 8-phase. An important characteristic of the analytical pro-
.~
40
--
U_
5
10 15 BaSO 4 mol%
Fig. 4. Plotsof BaSO4solubility (exchangedBaSO4/added BaSO4 (%) against added BaSO4.
397
T. Nishino / Characterization of 6-BaC03 Table 1 Crystal data for selected BaCO3 and CaCO3 modifications.
Orthorhombic
~-BaCO 3 (Witherite) BaCO3
Hexagonal
~-BaC03 (830%)
6.4301 Z=4, Pmcn
" --5.-31
a 17--b94 I5.72A c
4.94
S-BaC03 (New metastable)
.z=2, P2 ~/,m
Z=3, R3m
Aragonite CaCO 3
Monoclinic
cacaO0
Calcite a
C
4;990 I~:oo~
Z=4, Pmcn
3. 4. Crystal structure The analysis was carried out in collaboration with Prof. Mizutani and his colleague [5 ]. Powder X-ray diffraction data of the ~i-BaCO3 were obtained by step scanning technique with the step size of 0.02 ° and a fixed counting time of 10 s in the 20 range 4-100 ° . Correction for the systematic angular error was done using Si powders as the external standard. Visser's indexing program was used to find the unit cell of the 6-phase. Crystal data obtained for ~-BaCO3)o.9o(SO4)0.Jo are given in table 1, in which the selected polymorphs of BaCO3 and CaCO3 are given for comparison. As shown in table 1, the structure of 5-BaCO3 is closely related to that of CaCO3 (II), a high-pressure metastable phase.
6.334
Z=6, R3m
10 .9 ° Z=4, P21/c
The [3 angles of 5-BaCO3 and C a C O 3 (II) are the same within a standard deviation, and the a, b and c axes of 6-BaCO3 essentially correspond to a, b and c / 2 o f CaCO3. Slight increase in axes lengths is due to the size effect of cations with different ionic radii.
References [ 1 ] T. Nishino and S. Nishiyama, Reactivity of solids (Chapman and Hall, London, 1972) p. 766. [2] T. Nishino, J. Am. Ceram. Soc. 70 (1987) C-162. [ 3 ] T. Nishino, A. Dosho, M. Nagai and T. Sakurai, Gypsum and Lime 202 (1986) 137. [41 T. Nishino, T. Sakurai and Y. Moriyoshi, J. Ceram. Assoc. Japan 94 (1986) 268. [5] T. Nishino, T. Sakurai, N. Ishizawa, N, Mizutani and M. Kato, J. Solid State Chem. 69 (1987) 24.