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Thermal stability and properties of Bi2Sr3−yCayCu2Ox
Physica C 243 (1995) 139-144
ELSEVIER
Thermal stability and properties of Bi2Sr3_yCayCu20 x s. Chernyaev a,b,c, j. Hauck a,., A. Mozhaev b, K. Bickmann a, H. Altenburg c a Institutfiir FestkOrperforschung, KFA Jiilich, Jiilich, Germany b Department of Chemistry, Moscow State Universi~, Moscow, Russian Federation c FH Miinster, FB Chemieingenieurwesen, Steinfurt, Germany Received 15 December 1994
Abstract Bi2Sr3_yCayCu20~.can be obtained at 1 _
1. Introduction
2. Experimental
The oxygen content of high-To superconductors influences the structural and superconducting properties. The Bi2Sr3_yCavCu20 x (2212) phase exhibits a narrower range of oxygen nonstoichiometry (x) as compared with the YBa2Cu3Ox system, but a variation of the cationic content (y) [ 1,2]. Therefore the investigation of correlations between properties and composition is more complicated in this case. The P - T - x diagrams for a fixed cationic composition (y) [ 3,4 ] showed that the oxygen content (x) increases with decreasing temperature (T) at constant oxygen partial pressure (P) and with increasing oxygen pressure at constant temperature. But the dependence of the transition temperature (To) on the oxygen content and the cationic nonstoichiometry is controversial [1,2,5,6], because of an instability below ~700°C. Therefore the phase relations of this nonstoichiometric compound and the variation of T~ values were studied in some detail.
We investigated 2212 samples with different calcium (1.0 < y < 2.0) and oxygen content. These samples were synthesized from nitrate solutions by a spray drying technique and annealed at 815-820°C in air. The initial samples were treated at different temperatures and oxygen partial pressures before quenching to obtain a homogeneous oxygen content. Samples with a decreased oxygen content ( x < 8.04) were obtained by reduction with hydrogen gas at 250-310°C. The oxygen content of different samples was determined by iodometry and the transition temperature by low-temperature resistivity and AC measurements. The X-ray analysis was performed with a "Stoe" powder diffractometer (Germany). TGA curves were obtained with the thermogravimetric analyzer "951 of DuPont Instruments" (USA) by heating in different gas mixtures with a constant rate of 2 K/min. A similar treatment was performed in a furnace to investigate the phase composition of samples after different steps of decomposition. The samples were quenched and analyzed by X-ray analysis.
* Corresponding author. 0921-4534/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI0921-4534 (95) 00016-X
140
S. Chernyaev et al. I Physica C 243 (1995) 139-144
3. Results
C, p m ~3100
5 5 0 a, pm
y=1.75
i
The X-ray analysis o f synthesized samples showed single phases in the range 1.0 < y < 1.75. The lattice parameter c decreases with increasing calcium content and stays practically constant at y > 1.75 (Fig. 1). Ca rich samples (y > 1.75) contain some (Sr,Ca)2CuO3, CuO and probably some Bi2Sr2CuO6 ÷ z. The lattice parameter c decreases and the parameter a increases slightly with the reduction o f the oxygen content x as shown for y = 1 in Fig. 2. The linear approximation shows a major influence of the c lattice parameter on the calcium content y and smaller effects on the oxygen content x. W e have
3080
"L..t
s4s
3060
i
540
-
--
--
3040
I /
535 L 1.0
.
. . 1.2
. 1.4
i 1.6
~ 1.8
3020 2.0
Y
a = - l y - 3 x + 5 6 3 . 2 pm for 7 . 9 < x < 8.3
Fig. 1. Lattice parameters a and c of the tetragonal unit cell of Bi2Sr3_yCayCu20~at x = 8.23 and different y.
and c= -30y+
5 5 0 a, pm
c, n m 3 . 1 0
545
540
3.04
535 - -7.85
- -
7.95
8.05
8.15
8.25
3.02
8.35
X
Fig. 2. Lattice parameters a and c of Bi2Sr3_yCayCu2Oxaty = 1 and different x values. T~-,n K
Oxygen content x 8.4
92~..~1..
~
y-1.75 i \
!
t
82 -,t
77 72 1.2
'
'
1.4
1.6
Y Fig. 3. Tc values and oxygen content x of differenty values.
Low-temperature resistivity measurements of quenched samples showed about constant Tc = 9 1 - 9 3 K for samples w i t h y = 1 and 8.15 < x < 8.25 and lower Tc values at increased y or decreased x similar to what was observed by Majewski [ 1 ] and Bock and Preisler [ 5 ]. The transition temperatures and oxygen nonstoichiometry of samples with different y annealed at 815 oC in air are given in Fig. 3. The critical temperature decreases to 75-77 K at increasing calcium content y. The oxygen content varies between 8.22 and 8.24, which is contrary to previous results [6]. The T~ values are also reduced, if samples are cooled slowly from 800, 830 and 860°C (Table 1). Investigations in the temperature range 600-700° C confirmed a partial eutectoid decomposition of 2212 samples. The AC susceptibility measurements of annealed samples showed T °"~t values decreased by 5 - 7 K even after Table 1 Transition temperature To, onset and oxygen content of samples annealed at T~, in air and subsequentquenching or slowly cooling
87
82'
19x+ 2964 pm for 1 < y < 1.75.
:
8 1
1.8
Bi2Sr3_yCayCu20 x at
Ta,, (°C) Quenched samples 800 860 Slowly cooled samples 800 830 860
T °" (K)
Oxygen content
96 93
8.22 8.19
77 76.1 76.1
8.18 -
S. Chernyaev et al. / Physica C 243 (1995) 139-144
c, nm a.lo~,=
"~nn, K n 1,100
3.09~ ~
before annealing
a.osI ~ 1
1.2
. . . . . 1.4
1.6
=
Iro
1.8
2
Y Fig. 4. Lattice parameter c of Bi2Sr3_yCavCu20, before (Fig. 1) and after annealing for 24 h at 650°C in air.
141
short annealing. Samples treated during 24 h exhibit lattice parameters c close to the y-- 1.75 compound and the decreased transition temperatures of this composition (Figs. 3 and 4). This can be explained by a partial low-temperature decomposition and redistribution of strontium and calcium cations in the remaining 2-1.25-1.75-2 compound. At first the decomposition products form on the surfaces of the samples. A further annealing allowed us to identify Bi2Sr2CuO6+ ~ and (Ca,Sr)CuO2 by Xray analysis. The data also allow a correlation between the transition temperature and the lattice parameter c or To, y and x at x < 8.22. We have T°"(K) = - 1868 +0.64c [ p m ] , T°"(K) = 1 4 - 19y+ 12x.
mass,
%
Ar/CI"I30H/
104 102
Ar
100 ! 98 96
y-1 y-1.25 y-1.5
94 92
0
100
L
I
200 300 400 temperature, °C
500
600
Fig. 5. Thermogravimetry for the reaction of Bi2Sr3_yCayCtl2Os.3 (y= 1, 1.25 and 1.5) with H2 gas, CH3OH, CH4 and At"at a heating rate of 2 K/rain.
102
mass, %
lOO Bi2Sra
98
"
yCayCU207~ .
96
94 0-15Bi20~*(3-y)Sr(OH)2÷yCaO*2Cu÷I.7Bi / 92 90
0
(3-y)SrO+yCaO÷2Bi+2Cu ~ ~. . . . 100
200
300
400
500
600
temperature, °C
Fig. 6. Reactionof Bi~Sr3_yCayCu2Oxwith H2 at each step of the TGA experiment.
According to data of Idemoto and Fueki [ 3,9 ], the 2212 phase has about 11-12% of Bi(V) at temperatures above 800 °C, while the 2-2-0-1 decomposition products have a decreased Bi(V) content. So, the partial reduction of Bi(V) during the decomposition process is responsible for the small oxygen loss detected by chemical analysis: a value of x = 8.18 was obtained at slow cooling from 800°C instead of x = 8.19 and 8.22 at quenching (Table 1). The Bi2Sr3_ yCayCu2Oxsamples with different y values show a different behavior at the reaction with hydrogen gas and a similar behavior in Ar, Ar/CH4 or Ar/CH3OH gas mixtures (Fig. 5). The gases are inert as pure Ar gas below ~ 200°C. Hydrogen gas reduces the oxygen content at increased temperature. CH3OH is oxidized to formic acid, which reacts to Bi formate BiO2(HCO) at ~ 300°C and decomposes to Bi203, Bi, Cu and CaCO3 above ~450°C. The mass is also increased at the decomposition of CI-L at formation of carbon, CaCO3 and Bi203. The thermogravimetric analysis reveals some different steps of mass loss at the reaction with hydrogen gas (Fig. 6). The first step at 290-310°C corresponds to an oxygen removal to x = 7 . 9 without decomposition. The samples are decomposed at increased temperatures to Sr(OH)2, CaO, Cu, Bi and some Bi203. Samples with larger y values yield a larger weight loss because of the decreased amount of Sr(OH)2 as demonstrated in Fig. 5. The final values of formal oxygen content at complete reduction of Bi203 to Bi would be x = 5, 4.75, 4.5
142
S. Chernyaevet al. / Physica C 243 (1995) 139-144
Cu +
Co 2+ , Bi3+
2:2,
oO 22 o o.50.51
o o', m',,
0 0.5o.51
l ]
22 0 8
o:,,
'
,m
24
0 0,4]0.4
1
,OOl I 9 ) t l , m1,m 1.00.t 0.2 I.B1 0.21.81 0 2 J / o _ ( 2 1.90.11 )
~i , ;,
I ] ~(o2)/b~=
l-
-4
Bi S r C a C u
500
7.0
,
7.5
2 2 1 2
-~,',,',/oo.4o.4 ~f
[,oo
2 1 7 0.3 00.5 0.5
8.0 oxygen content x
1 [ 1
500
Fig. 7. PseudobinaryphasediagramMOxwithM = BizSr2CaCu2(22-1-2) at differentoxygenpartial pressures (dashed lines) and the ratios Bi/Sr/Ca/Cu of the differentdecompositionproducts(Table 2). for y = 1.0, 1.25, ! .5, respectively. The experimental values x=5.4, 5.15 and 4.9 at the second step are increased by 0.4 because of some remaining Bi203 (Figs. 5 and 6), which is reduced in a third step at = 500°C with simultaneous decomposition of Sr(OH)2. The relative change of formal oxygen content A x - 0.25 is in a good agreement with the change of calcium content in the three samples with y = 1.0, 1.25 and 1.5, respectively.
4. Pseudobinary phase diagram
The phase relations of Bi2Sr3_ yCayCuzOx vary with temperature T and oxygen partial pressure P. The variation of the oxygen content with temperature and oxygen partial pressure can be plotted in a pseudobinary phase diagram MOx, e.g. with M=Bi2Sr2CaCu2 at y = 1 (Fig. 7). The P - T - x data at y--- 1 [4], the T, y and T-Bi2Sr2CuO6 + z-(Ca,Sr) CuOz phase diagrams in air [1] and the P, T phase diagram aty = 1 [7] were used to construct the pseudobinary phase diagram for the Bi2Sr2CaCu2Ox composition (2-2-1-2). The compositions of decomposition products Biz(Sr,Ca)306 (2-2-1-0), Bi2Sr2CuO6+z (2-1.9-0.1-1), (Sr,Ca)zCuO3 (0-0.2-1.8-1), (Sr,Ca)CuOz (0-0.50.5-1 ), (Sr,Ca)14Cu24041 (0-8-6-24), CuO (0-0-0-1 ), CuzO (0-0-0-2) and (Bi,Sr,Ca)4CuOx melt as e.g. (2.2-1.4-0.4-1, m) were approximated from experimental data [ 1,7 ], the SrO-CaO-CuO and SrO-CaO-
Bi203 phase diagrams [1]. The decomposition products can be plotted in a pseudoternary triangle of Bi, Cu and a solid solution of Sr and Ca in different phases (Fig. 8). Three phases are required by the phase rule with the 2-2-1-2 composition within the triangle of these phases. Table 2 lists the sequence of different phases at variation of the oxygen content x, temperature T and oxygen partial pressure P. Only one phase changes at each step. Four phases are in equilibrium at intermediate x values, five phases at the constant T and P values of the eutectoid or peritectoid reactions as e.g. 2-2-1-2, 0-0.2-1.8-1, 2.2-1.7-0.1-1, m, 0-0.5-0.5-1 and 2-2-1-0 at 883°C and p ( O 2 ) = 7 . 1 X 10 -2 bar [7]. These values [ 7 ], however, vary at different Ca content y. DTA experiments show decreased temperatures at increased y. The oxygen content of most decomposition products can be calculated from the different amounts of reaction products (Table 2). The oxygen content of Bi2(Sr,Ca)2CuO6+~ phase 2-1.9-0.1-1 was obtained from the data of 2.12-1.86-0-1.02 phase [ 9 ]. The oxygen content of phase assemblages containing the melt was determined by thermogravimetric analyses in Oz/ Nz gas mixtures. The lower limit corresponds to a melt containing monovalent Cu. (Ca,Sr) 1_yCuO2, y = 0.2 with some trivalent Cu instead of (Ca,Sr)CuO2 with divalent Cu can be expected at high oxygen partial pressures [ 10,11 ]. Also bismutates containing pentavalent Bi as e.g. (Ca,Sr)l.2Bio.803 (0.8-0.8-0.4-0) instead of Bi2(Ca,Sr)306 2-2-1-0 with trivalent Bi should be formed at high oxygen pressures [ 12]. The pseudobinary phase diagram shows the possibility of congruent melting of the 2-2-1-2 compound at high oxygen partial pressures or a peritectic decomposition to (Ca,Sr) 1_yCuO2, Ca, Sr, Bi oxides containing Bi 5÷ and melt. Some relations had to be varied to be in agreement with the phase rule and the closely related pseudobinary phase diagrams of the 2-2-0-1 and 2-22-3 phases [ 13]. 5. Conclusion
The influence of oxygen and calcium content on structural and superconducting properties of 2-2-1-2 phase was studied. The correlation between lattice parameter c and transition temperature is established. The Tc value of the 2-2-1-2 compound is reduced during slow cooling or low-temperature treatment
S. Chernyaev et al. / Physica C 243 (1995) 139-144 Cu
Cu
X=7 ~ 0 O O 2
~ 0002 Cu20
Cu20
08624
/ Bi
~8624
/
//~150.51
il j
/W
2 190.1 1 ~
x/cuo
"
21.90.1 I ;
2210
Ca,Sr
Bi
/
/
/
/
/
~ l , Ca,Sr
,
Cu
X= 7
Cu20
/
~8624
V86.,
'l~O 0'50"51
/
/
TM
2210
Cu
/'
143
/
• 2223~00.21.81 2"21'40'4 l'm-. ~
12f
21.90.1
~, \
•
Bi ~
\,,
\ ,=
2210
~,a,or
Bi "
"
~ 2210
'i
Ca,SF
Fig. 8. Sequence of different decomposition products of Bi2Sr2CaCu2Ox (2-2-1-2) at increasing oxygen content x = 7, 7.25, 7.54 and 7.6, respectively ( Table 2).
The pseudobinary phase diagram of Fig. 7 includes previous measurements [ 1-7 ] and the results concerning the oxygen content and the decomposition products of the present investigation. The decomposition prod-
because of the partial decomposition and increasing calcium content in the residual 2-2-1-2 phase. The quenching after high-temperature annealing is suggested to avoid this deterioration.
Table 2 Oxygen content x, ratios Bi/Sr/Ca/Cu (Figs. 7 and 8) and quantity a, b, c of different decomposition products of Bi2Sr2CaCu2Ox (2-2-1-2) x
a
Bi
Sr
Ca
7 7.25 7.54 7.6 7.9 8.~ (8.97 8.~ 8.2 8.15 7.75
0 1 0.9 0.8 0.55 0.7 0.7 0.89 0.79 1 1
2 2 2.2 2.2 2.2 2.2 2.2 2 2 2 2
1.9 1.9 1.7 1.5 1.4 1.45 1.45 1.5 1.7 1.5 1.9
0.1 0.1 0.1 0.3 0.4 0.35 0.35 0.5 0.3 0.5 0.1
Cu
b
Bi
Sr
Ca
Cu
c
Bi
Sr
Ca
Cu
1 0.25 0.8 1.2 0.06 1.3 1.3 1.11 0.79 1 0.25
0 0 0 0 0 0 0 0 0 0 0
0 0 0.5 0.5 8 0.4 0.4 0.4 0.5 0.5 0
0 0 0.5 0.5 6 0.4 0.4 0.4 0.5 0.5 0
2 2 1 1 24 1 1 1 1 1 2
1 0.5 0.3 0.12 0.39 0.23 0.58 0.11 0.21 0 0.5
2 0 0 2 2 2 0.8 2 2 0 0
2 0.2 0.2 2 2 2 0.8 2 1.25 0.2 0.2
1 1.8 1.8 1 1 1 0.4 1 1.75 1.8 1.8
0 1 1 0 0 0 0) 0 2 1 1
144
s. Chernyaev et al. /Physica C 243 (1995) 139-144
ucts at high o x y g e n c o n t e n t x, high temperature T and high oxygen partial pressure P are qualitative, because of the lack o f data. The present data, however, indicate ( 1 ) the instability of 2-2-1-2 below ~ 7 0 0 ° C , (2) the possibility of c o n g r u e n t m e l t i n g at high oxygen partial pressure and (3) the conditions for the formation of stable 2-2-1-2 c o m p o u n d s with defined o x y g e n content x.
Acknowledgements The authors w o u l d like to thank the D A A D (Deutscher A k a d e m i s c h e r Austauschdienst) for financial support of this work and Dr. Freiburg ( K F A Jiilich) for the use o f the " S t o e " diffractometer.
References [ 1] P. Majewski, Adv. Mater. 6 (1994) 460.
[2] S.V. Chernyaev, M.M. Kudra and A.P. Mozhaev, Rus. J. Inorg. Chem. 38 (1993) 529. [3] Y. ldemoto and K. Fueki, Physica C 168 (1990) 167. [4] T. Schweizer, R. Mailer, P. Bohac and L.J. Gauckler, in: Proc. 3rd Conf. Europ. Ceram. Soc., Madrid, 1993 (Elsevier, London, 1993) p. 611. [5] J. Bock and E. Preisler, Supercond. Sci. Technol. 6 (1993) 215. [61 P. Majewski, H.-L. Su and F. Aldinger, Physica C 229 (1994) 12. 17] J.L. MacManus-Driscoll, J. Bravman, R.J. Savoy, G. Gorman and R.B. Beyers, J. Am. Ceram. Soc. 77 (1994) 2305. [ 8 ] M. Kato, W. Ito, Y. Koike, T. Noji and Y. Saito, Physica C 226 (1994) 243. [9] Y. ldemoto and K. Fueki, Physica C 190 (1992) 502. [10] K.K. Singh, D.E. Morris and A.P.B. Sinha, Physica C 231 (1994) 377. [ 11] S. Adachi, H. Yamauchi, S. Tanaka and N. M6ri, J. Supercond. 7 (1994) 55. [ 12] F. Abbattista, C. Brisi, D. Mazza and M. Vallino, Mater. Res. Bull. 26 (1991) 107. [ 13] J. Hanck, S.V. Chernyaev and K. Mika, in preparation.
ELSEVIER
Thermal stability and properties of Bi2Sr3_yCayCu20 x s. Chernyaev a,b,c, j. Hauck a,., A. Mozhaev b, K. Bickmann a, H. Altenburg c a Institutfiir FestkOrperforschung, KFA Jiilich, Jiilich, Germany b Department of Chemistry, Moscow State Universi~, Moscow, Russian Federation c FH Miinster, FB Chemieingenieurwesen, Steinfurt, Germany Received 15 December 1994
Abstract Bi2Sr3_yCayCu20~.can be obtained at 1 _
1. Introduction
2. Experimental
The oxygen content of high-To superconductors influences the structural and superconducting properties. The Bi2Sr3_yCavCu20 x (2212) phase exhibits a narrower range of oxygen nonstoichiometry (x) as compared with the YBa2Cu3Ox system, but a variation of the cationic content (y) [ 1,2]. Therefore the investigation of correlations between properties and composition is more complicated in this case. The P - T - x diagrams for a fixed cationic composition (y) [ 3,4 ] showed that the oxygen content (x) increases with decreasing temperature (T) at constant oxygen partial pressure (P) and with increasing oxygen pressure at constant temperature. But the dependence of the transition temperature (To) on the oxygen content and the cationic nonstoichiometry is controversial [1,2,5,6], because of an instability below ~700°C. Therefore the phase relations of this nonstoichiometric compound and the variation of T~ values were studied in some detail.
We investigated 2212 samples with different calcium (1.0 < y < 2.0) and oxygen content. These samples were synthesized from nitrate solutions by a spray drying technique and annealed at 815-820°C in air. The initial samples were treated at different temperatures and oxygen partial pressures before quenching to obtain a homogeneous oxygen content. Samples with a decreased oxygen content ( x < 8.04) were obtained by reduction with hydrogen gas at 250-310°C. The oxygen content of different samples was determined by iodometry and the transition temperature by low-temperature resistivity and AC measurements. The X-ray analysis was performed with a "Stoe" powder diffractometer (Germany). TGA curves were obtained with the thermogravimetric analyzer "951 of DuPont Instruments" (USA) by heating in different gas mixtures with a constant rate of 2 K/min. A similar treatment was performed in a furnace to investigate the phase composition of samples after different steps of decomposition. The samples were quenched and analyzed by X-ray analysis.
* Corresponding author. 0921-4534/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI0921-4534 (95) 00016-X
140
S. Chernyaev et al. I Physica C 243 (1995) 139-144
3. Results
C, p m ~3100
5 5 0 a, pm
y=1.75
i
The X-ray analysis o f synthesized samples showed single phases in the range 1.0 < y < 1.75. The lattice parameter c decreases with increasing calcium content and stays practically constant at y > 1.75 (Fig. 1). Ca rich samples (y > 1.75) contain some (Sr,Ca)2CuO3, CuO and probably some Bi2Sr2CuO6 ÷ z. The lattice parameter c decreases and the parameter a increases slightly with the reduction o f the oxygen content x as shown for y = 1 in Fig. 2. The linear approximation shows a major influence of the c lattice parameter on the calcium content y and smaller effects on the oxygen content x. W e have
3080
"L..t
s4s
3060
i
540
-
--
--
3040
I /
535 L 1.0
.
. . 1.2
. 1.4
i 1.6
~ 1.8
3020 2.0
Y
a = - l y - 3 x + 5 6 3 . 2 pm for 7 . 9 < x < 8.3
Fig. 1. Lattice parameters a and c of the tetragonal unit cell of Bi2Sr3_yCayCu20~at x = 8.23 and different y.
and c= -30y+
5 5 0 a, pm
c, n m 3 . 1 0
545
540
3.04
535 - -7.85
- -
7.95
8.05
8.15
8.25
3.02
8.35
X
Fig. 2. Lattice parameters a and c of Bi2Sr3_yCayCu2Oxaty = 1 and different x values. T~-,n K
Oxygen content x 8.4
92~..~1..
~
y-1.75 i \
!
t
82 -,t
77 72 1.2
'
'
1.4
1.6
Y Fig. 3. Tc values and oxygen content x of differenty values.
Low-temperature resistivity measurements of quenched samples showed about constant Tc = 9 1 - 9 3 K for samples w i t h y = 1 and 8.15 < x < 8.25 and lower Tc values at increased y or decreased x similar to what was observed by Majewski [ 1 ] and Bock and Preisler [ 5 ]. The transition temperatures and oxygen nonstoichiometry of samples with different y annealed at 815 oC in air are given in Fig. 3. The critical temperature decreases to 75-77 K at increasing calcium content y. The oxygen content varies between 8.22 and 8.24, which is contrary to previous results [6]. The T~ values are also reduced, if samples are cooled slowly from 800, 830 and 860°C (Table 1). Investigations in the temperature range 600-700° C confirmed a partial eutectoid decomposition of 2212 samples. The AC susceptibility measurements of annealed samples showed T °"~t values decreased by 5 - 7 K even after Table 1 Transition temperature To, onset and oxygen content of samples annealed at T~, in air and subsequentquenching or slowly cooling
87
82'
19x+ 2964 pm for 1 < y < 1.75.
:
8 1
1.8
Bi2Sr3_yCayCu20 x at
Ta,, (°C) Quenched samples 800 860 Slowly cooled samples 800 830 860
T °" (K)
Oxygen content
96 93
8.22 8.19
77 76.1 76.1
8.18 -
S. Chernyaev et al. / Physica C 243 (1995) 139-144
c, nm a.lo~,=
"~nn, K n 1,100
3.09~ ~
before annealing
a.osI ~ 1
1.2
. . . . . 1.4
1.6
=
Iro
1.8
2
Y Fig. 4. Lattice parameter c of Bi2Sr3_yCavCu20, before (Fig. 1) and after annealing for 24 h at 650°C in air.
141
short annealing. Samples treated during 24 h exhibit lattice parameters c close to the y-- 1.75 compound and the decreased transition temperatures of this composition (Figs. 3 and 4). This can be explained by a partial low-temperature decomposition and redistribution of strontium and calcium cations in the remaining 2-1.25-1.75-2 compound. At first the decomposition products form on the surfaces of the samples. A further annealing allowed us to identify Bi2Sr2CuO6+ ~ and (Ca,Sr)CuO2 by Xray analysis. The data also allow a correlation between the transition temperature and the lattice parameter c or To, y and x at x < 8.22. We have T°"(K) = - 1868 +0.64c [ p m ] , T°"(K) = 1 4 - 19y+ 12x.
mass,
%
Ar/CI"I30H/
104 102
Ar
100 ! 98 96
y-1 y-1.25 y-1.5
94 92
0
100
L
I
200 300 400 temperature, °C
500
600
Fig. 5. Thermogravimetry for the reaction of Bi2Sr3_yCayCtl2Os.3 (y= 1, 1.25 and 1.5) with H2 gas, CH3OH, CH4 and At"at a heating rate of 2 K/rain.
102
mass, %
lOO Bi2Sra
98
"
yCayCU207~ .
96
94 0-15Bi20~*(3-y)Sr(OH)2÷yCaO*2Cu÷I.7Bi / 92 90
0
(3-y)SrO+yCaO÷2Bi+2Cu ~ ~. . . . 100
200
300
400
500
600
temperature, °C
Fig. 6. Reactionof Bi~Sr3_yCayCu2Oxwith H2 at each step of the TGA experiment.
According to data of Idemoto and Fueki [ 3,9 ], the 2212 phase has about 11-12% of Bi(V) at temperatures above 800 °C, while the 2-2-0-1 decomposition products have a decreased Bi(V) content. So, the partial reduction of Bi(V) during the decomposition process is responsible for the small oxygen loss detected by chemical analysis: a value of x = 8.18 was obtained at slow cooling from 800°C instead of x = 8.19 and 8.22 at quenching (Table 1). The Bi2Sr3_ yCayCu2Oxsamples with different y values show a different behavior at the reaction with hydrogen gas and a similar behavior in Ar, Ar/CH4 or Ar/CH3OH gas mixtures (Fig. 5). The gases are inert as pure Ar gas below ~ 200°C. Hydrogen gas reduces the oxygen content at increased temperature. CH3OH is oxidized to formic acid, which reacts to Bi formate BiO2(HCO) at ~ 300°C and decomposes to Bi203, Bi, Cu and CaCO3 above ~450°C. The mass is also increased at the decomposition of CI-L at formation of carbon, CaCO3 and Bi203. The thermogravimetric analysis reveals some different steps of mass loss at the reaction with hydrogen gas (Fig. 6). The first step at 290-310°C corresponds to an oxygen removal to x = 7 . 9 without decomposition. The samples are decomposed at increased temperatures to Sr(OH)2, CaO, Cu, Bi and some Bi203. Samples with larger y values yield a larger weight loss because of the decreased amount of Sr(OH)2 as demonstrated in Fig. 5. The final values of formal oxygen content at complete reduction of Bi203 to Bi would be x = 5, 4.75, 4.5
142
S. Chernyaevet al. / Physica C 243 (1995) 139-144
Cu +
Co 2+ , Bi3+
2:2,
oO 22 o o.50.51
o o', m',,
0 0.5o.51
l ]
22 0 8
o:,,
'
,m
24
0 0,4]0.4
1
,OOl I 9 ) t l , m1,m 1.00.t 0.2 I.B1 0.21.81 0 2 J / o _ ( 2 1.90.11 )
~i , ;,
I ] ~(o2)/b~=
l-
-4
Bi S r C a C u
500
7.0
,
7.5
2 2 1 2
-~,',,',/oo.4o.4 ~f
[,oo
2 1 7 0.3 00.5 0.5
8.0 oxygen content x
1 [ 1
500
Fig. 7. PseudobinaryphasediagramMOxwithM = BizSr2CaCu2(22-1-2) at differentoxygenpartial pressures (dashed lines) and the ratios Bi/Sr/Ca/Cu of the differentdecompositionproducts(Table 2). for y = 1.0, 1.25, ! .5, respectively. The experimental values x=5.4, 5.15 and 4.9 at the second step are increased by 0.4 because of some remaining Bi203 (Figs. 5 and 6), which is reduced in a third step at = 500°C with simultaneous decomposition of Sr(OH)2. The relative change of formal oxygen content A x - 0.25 is in a good agreement with the change of calcium content in the three samples with y = 1.0, 1.25 and 1.5, respectively.
4. Pseudobinary phase diagram
The phase relations of Bi2Sr3_ yCayCuzOx vary with temperature T and oxygen partial pressure P. The variation of the oxygen content with temperature and oxygen partial pressure can be plotted in a pseudobinary phase diagram MOx, e.g. with M=Bi2Sr2CaCu2 at y = 1 (Fig. 7). The P - T - x data at y--- 1 [4], the T, y and T-Bi2Sr2CuO6 + z-(Ca,Sr) CuOz phase diagrams in air [1] and the P, T phase diagram aty = 1 [7] were used to construct the pseudobinary phase diagram for the Bi2Sr2CaCu2Ox composition (2-2-1-2). The compositions of decomposition products Biz(Sr,Ca)306 (2-2-1-0), Bi2Sr2CuO6+z (2-1.9-0.1-1), (Sr,Ca)zCuO3 (0-0.2-1.8-1), (Sr,Ca)CuOz (0-0.50.5-1 ), (Sr,Ca)14Cu24041 (0-8-6-24), CuO (0-0-0-1 ), CuzO (0-0-0-2) and (Bi,Sr,Ca)4CuOx melt as e.g. (2.2-1.4-0.4-1, m) were approximated from experimental data [ 1,7 ], the SrO-CaO-CuO and SrO-CaO-
Bi203 phase diagrams [1]. The decomposition products can be plotted in a pseudoternary triangle of Bi, Cu and a solid solution of Sr and Ca in different phases (Fig. 8). Three phases are required by the phase rule with the 2-2-1-2 composition within the triangle of these phases. Table 2 lists the sequence of different phases at variation of the oxygen content x, temperature T and oxygen partial pressure P. Only one phase changes at each step. Four phases are in equilibrium at intermediate x values, five phases at the constant T and P values of the eutectoid or peritectoid reactions as e.g. 2-2-1-2, 0-0.2-1.8-1, 2.2-1.7-0.1-1, m, 0-0.5-0.5-1 and 2-2-1-0 at 883°C and p ( O 2 ) = 7 . 1 X 10 -2 bar [7]. These values [ 7 ], however, vary at different Ca content y. DTA experiments show decreased temperatures at increased y. The oxygen content of most decomposition products can be calculated from the different amounts of reaction products (Table 2). The oxygen content of Bi2(Sr,Ca)2CuO6+~ phase 2-1.9-0.1-1 was obtained from the data of 2.12-1.86-0-1.02 phase [ 9 ]. The oxygen content of phase assemblages containing the melt was determined by thermogravimetric analyses in Oz/ Nz gas mixtures. The lower limit corresponds to a melt containing monovalent Cu. (Ca,Sr) 1_yCuO2, y = 0.2 with some trivalent Cu instead of (Ca,Sr)CuO2 with divalent Cu can be expected at high oxygen partial pressures [ 10,11 ]. Also bismutates containing pentavalent Bi as e.g. (Ca,Sr)l.2Bio.803 (0.8-0.8-0.4-0) instead of Bi2(Ca,Sr)306 2-2-1-0 with trivalent Bi should be formed at high oxygen pressures [ 12]. The pseudobinary phase diagram shows the possibility of congruent melting of the 2-2-1-2 compound at high oxygen partial pressures or a peritectic decomposition to (Ca,Sr) 1_yCuO2, Ca, Sr, Bi oxides containing Bi 5÷ and melt. Some relations had to be varied to be in agreement with the phase rule and the closely related pseudobinary phase diagrams of the 2-2-0-1 and 2-22-3 phases [ 13]. 5. Conclusion
The influence of oxygen and calcium content on structural and superconducting properties of 2-2-1-2 phase was studied. The correlation between lattice parameter c and transition temperature is established. The Tc value of the 2-2-1-2 compound is reduced during slow cooling or low-temperature treatment
S. Chernyaev et al. / Physica C 243 (1995) 139-144 Cu
Cu
X=7 ~ 0 O O 2
~ 0002 Cu20
Cu20
08624
/ Bi
~8624
/
//~150.51
il j
/W
2 190.1 1 ~
x/cuo
"
21.90.1 I ;
2210
Ca,Sr
Bi
/
/
/
/
/
~ l , Ca,Sr
,
Cu
X= 7
Cu20
/
~8624
V86.,
'l~O 0'50"51
/
/
TM
2210
Cu
/'
143
/
• 2223~00.21.81 2"21'40'4 l'm-. ~
12f
21.90.1
~, \
•
Bi ~
\,,
\ ,=
2210
~,a,or
Bi "
"
~ 2210
'i
Ca,SF
Fig. 8. Sequence of different decomposition products of Bi2Sr2CaCu2Ox (2-2-1-2) at increasing oxygen content x = 7, 7.25, 7.54 and 7.6, respectively ( Table 2).
The pseudobinary phase diagram of Fig. 7 includes previous measurements [ 1-7 ] and the results concerning the oxygen content and the decomposition products of the present investigation. The decomposition prod-
because of the partial decomposition and increasing calcium content in the residual 2-2-1-2 phase. The quenching after high-temperature annealing is suggested to avoid this deterioration.
Table 2 Oxygen content x, ratios Bi/Sr/Ca/Cu (Figs. 7 and 8) and quantity a, b, c of different decomposition products of Bi2Sr2CaCu2Ox (2-2-1-2) x
a
Bi
Sr
Ca
7 7.25 7.54 7.6 7.9 8.~ (8.97 8.~ 8.2 8.15 7.75
0 1 0.9 0.8 0.55 0.7 0.7 0.89 0.79 1 1
2 2 2.2 2.2 2.2 2.2 2.2 2 2 2 2
1.9 1.9 1.7 1.5 1.4 1.45 1.45 1.5 1.7 1.5 1.9
0.1 0.1 0.1 0.3 0.4 0.35 0.35 0.5 0.3 0.5 0.1
Cu
b
Bi
Sr
Ca
Cu
c
Bi
Sr
Ca
Cu
1 0.25 0.8 1.2 0.06 1.3 1.3 1.11 0.79 1 0.25
0 0 0 0 0 0 0 0 0 0 0
0 0 0.5 0.5 8 0.4 0.4 0.4 0.5 0.5 0
0 0 0.5 0.5 6 0.4 0.4 0.4 0.5 0.5 0
2 2 1 1 24 1 1 1 1 1 2
1 0.5 0.3 0.12 0.39 0.23 0.58 0.11 0.21 0 0.5
2 0 0 2 2 2 0.8 2 2 0 0
2 0.2 0.2 2 2 2 0.8 2 1.25 0.2 0.2
1 1.8 1.8 1 1 1 0.4 1 1.75 1.8 1.8
0 1 1 0 0 0 0) 0 2 1 1
144
s. Chernyaev et al. /Physica C 243 (1995) 139-144
ucts at high o x y g e n c o n t e n t x, high temperature T and high oxygen partial pressure P are qualitative, because of the lack o f data. The present data, however, indicate ( 1 ) the instability of 2-2-1-2 below ~ 7 0 0 ° C , (2) the possibility of c o n g r u e n t m e l t i n g at high oxygen partial pressure and (3) the conditions for the formation of stable 2-2-1-2 c o m p o u n d s with defined o x y g e n content x.
Acknowledgements The authors w o u l d like to thank the D A A D (Deutscher A k a d e m i s c h e r Austauschdienst) for financial support of this work and Dr. Freiburg ( K F A Jiilich) for the use o f the " S t o e " diffractometer.
References [ 1] P. Majewski, Adv. Mater. 6 (1994) 460.
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