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Cerium distribution coefficient and its dependence on oxygen partial pressure in Nd2−xCexCuO4 single crystal growth
PHYSICA
Physica C 218 (1993) 437-442 North-Holland
Cerium distribution coefficient and its dependence on oxygen partial pressure in Nd2_xCexCuO4 single crystal growth K. G a m a y u n o v a,b, I. Tanaka a and H. Kojima ~ a Institute of Inorganic Synthesis, Yamanashi University, Miyamae 7, Kofu 400, Japan b General Physics Institute of Russian Academy of Science, Vavilov st., 38, Moscow 117942, Russian Federation
Received 20 September 1993
A strong dependence of the cerium distribution coefficient (Kc~) on oxygen partial pressure P[O2] has been shown in direct 0.2 a r m TSFZ experiments by changing the oxygen pressure during Nd2_xCe~CuO4single crystal growth. The ratio of K°~. l a l m /Kc~ has been found to be equal to ~ 0.62. Absolute values of Kc~measured from zone compositions have been found to be about 2.42, 3.37 and 9.06 at P[O2 ] = 0.1, 0.2 and 2.0 atm, respectively. Our experiment shows good agreement between measured and calculated Kc~ values. We have shown that the quite large difference in Kc~ obtained in a number of investigations is explained by a strong dependence of Kc~ on P[O2]. We have also shown that a reason for this dependence is the variable valence of Ce. Cerium effectively incorporates in a Nd2_,CexCuO4single crystal in the form of Ce+4 ion.
1. Introduction
2. Pre-experimental analysis
The discovery in 1989 o f the first electron-doped high-temperature superconductor Nd2_xCexCuO4 has induced interest in the single crystal growth o f this solid solution. Many published papers have dealt with measurements o f physical properties carried out on single crystals. However, data on the process o f Nd2_xCe~CuO4 single crystal growth, especially with respect to the dopant distribution coefficient, are rather contradictory. The dopant distribution coefficient is one of the most important physical properties for single crystal growth. It defines the growth conditions for obtaining crystals o f the required properties. Published results on the cerium distribution coefficient Kce in N C C O are presented in table 1 [ 1-9 ]. Even a fluent examination o f the data shows a strong variation o f Kc~ and its dependence on growth conditions. The present work is aimed at defining the reason o f strong variations o f the cerium distribution coefficient Kc~ at Nd2_xCexCuO4 single crystal growth.
Special investigations on Kce have not been carried out except in ref. [ 8 ]. We base our analysis on this work so a brief description is necessary. Spontaneous crystallization from a nonstoichiometric melt using a Pt crucible has been used as a method for Nd2_xCexCuO4 single crystal growth. Measurements o f crystal compositions have been done by E P M A and the connecting equation between cerium concentration in a crystal ( X ) and the composition o f the used melt has been suggested: X = O . 2 5 - O . 1 8 / [ CeO2]m - O . 1 / [Nd203 ]m
+ 1.7/[CeO2 ] m" [Nd203 ]m ,
( 1)
where [CeO2]m is the cerium oxide concentration in the melt (mol.%), [Nd203]m is the n e o d y m i u m oxide concentration in the melt (mol.%). All measurements of X have been done on a crystal surface that was in contact with atmosphere. It has been found that Ce is incorporated by the crystal only up to a depth of about 10 ~tm. The other part o f the crystal is free from Ce (at least X < 0 . 0 2 ) . Crystal nucleation has been taken place in the center o f the melt surface. Then the crystal has grown as a
0921-4534/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
438
K. Gamayunov et al. /Cerium distribution coefficient in Ndz_xCexCu04
Table 1 Data on Ce distribution coefficient in Nd2_xCexCu04 Initial charge
Growth conditions
X melt/crystal
8.4% Nd203-6.0% CEO2-85.6% CuO
Pt crucible, air atm., cooling from 1320°C at 1-3 K/h A1203 crucible, N2 atm., cooling from 1250°C at 3 K/h
(0.28-0.36) / (0.14-0.18)
50% ( Nd203 + CeO2) + 50% Cu20 (outside of the region of NCCO primary crystallization)
70% C u O - ( 3 0 - y ) % Nd203-Y% CeO2 (outside of the region of NCCO primary crystallization) Feed rod of stoichiometric composition, solvent 85% CuO ( 1 ) Nd203 [ (2y)/2 ] + yCeO2 + 3 times stoich. CuO (2) The same but + 5 times stoich. CuO Stoichiometric composition + 50 wt.% of CuO as a flux
Feed rod of stoichiometric composition, solvent 85% CuO Compositions inside of the region of NCCO primary crystallization 16.4% Nd203-2.7% CEO2-80.9% CuO 16.7% Nd203-2.1% CEO2-81.2% CuO
Kce
Ref. [ 1]
0.5
(0.06) / (0.14, 0.16 ) (0.10)/(0.15, 0.19) ( 0.15 ) / (0.12-0.20) (0.18)/(0.18, 0.20) (0.20) / (0.14-0.18 ) (0.25)/(0.16,0.22) (0.35)/(0.18,0.21) (0.095-0.19)/ (0.00-0.22) for every initial composition
impossible to count
[2 ]
impossible to count
[3]
(0.15)/(0.15)
1
(0.40)/(0.12)
0.3
(0.29)/(0.14) (0.33)/(0.16)
0.5 0.5
(0.025)/(0.025) (0.05)/(0.11) (0.10)/(0.18) (0.13)/(0.21) (0.18)/(0.23) (0.26)/(0.23)
1.0 2.2 1.8 1.6 1.3 0.89
(0.02)/(0.15)
7.5
Pt crucible, air arm., cooling from 1250 ° C at 1-4 K/h
see fig. 1 and eq.(1)
seefig. 1 andeq. (1)
[8]
Pt crucible, air atm., cooling rate 0.5 K/h
(0.15)/(0.20)
1.33
[9]
(0.12)/(0.15) (coveredbyalid)
1.2
A1203 crucible, air atm., cooling from 1250°C at 5 K/h, growth in cavities TSFZ, 0.5-I.0 mm/h, Ar/O2=9/1 atm, P[O2] =0.1 atm Pt crucible, air atm., cooling from 1500 ° C at 6 K/h A1203 crucible, air arm., cooling from 1300 °C at 6 K / h A1203 crucible., air atm., cooling from 1270°C at 2 K/h, growth on the bottom of crucible
TSFZ, 0.5-1.0 ram/h, P[O2] =2 atm, d=6 mm, 1=50 mm
t h i n plate because o f t h e large difference o f the growth rates II a n d _L to c-axis. T h e t h i c k n e s s o f a b o u t 10 g m c o r r e s p o n d s to a m o m e n t w h e n t h e m e l t surface b e c o m e s c o v e r e d by t h e g r o w i n g crystal. W e will explain the C e b e h a v i o r later.
[41
[51
[6]
[7]
S o m e results f r o m table 1 c a n n o t b e t a k e n u n d e r c o n s i d e r a t i o n for c a l c u l a t i o n o f Kce b e c a u s e the m e l t c o m p o s i t i o n s lie o u t s i d e the region o f Nd2_xCexCuO4 p r i m a r y crystallization [2,3]. A typical f e a t u r e for such c r y s t a l l i z a t i o n is that X fluctuates f r o m crystal
439
K. Gamayunov et al. / Cerium distribution coefficient in Nd2_xCexCuO~
to crystal within the range 0-0.23 independently on the initial composition. Results o f TSFZ give only indirect information about Kc~ because different solvent compositions give the same X value just after completion o f the relaxation processes on the initial stage of crystallization. Also compositional stability o f the molten zone cannot be assured. However, we can compare solvent compositions from refs. [ 7 ] and [4 ] because usually they are chosen with the aim to minimize variations in dopant concentration on an initial stage o f crystallization. The ratio o f X and Xr~ (subindex " m " belongs to the Ce concentration in the melt) gives 7.5 and 1 respectively for the data o f the mentioned works. The valuation o f Kce is not absolutely correct but the large difference o f counted values leads us to suggest strong dependence of Kce on oxygen partial pressure P [ O 2 ] that was about 2 and 0.1 atm respectively. We can see from table 1 and fig. 1 that a good agreement o f the data ofrefs. [8] and [6] exists. Essentially for the high Ce concentration region, the dependences of Kc~ versus melt composition are qualitatively the same. The data in ref. [ 9 ] are in good agreement with those of ref. [ 8 ] too. The best agreement is found for the melt composition with Xm=0.20. We see that Kce decreased for the composition with Xm = 0.15 instead o f increasing in accordance with the general tendency given by eq. ( 1 ). But the growth conditions have not been the same. In the case o f Xm= 0.15 the crucible with the melt has been covered by a lid. Such a change in experimental conditions leads to a difficulty in gas exchange between melt and atmosphere and causes some reduction o f partial oxygen pressure in using atmosphere over a melt surface. So we have another indirect evidence for the possible dependence of Kce on P[O2].
Results ofrefs. [ 1,5 ] are rather different from those examined above. First of all the fact that Kce< 1 attracts our attention. We could not find a suitable explanation for such a strong difference in Kce values. C o m m o n features for both cases are (1) high Ce concentration in the melt ( [CeOE]m> 5mo1.%) and (2) change in the type o f crystallization from threedimensional to two-dimensional. Let us return to the examination o f Ce incorporation in Nd2_xCexCu04. Some results give indirect
N d 2 0 3 . %mol 13 ~J[ ] ,~
I1 0.2
15 O]75
-k=~
0.6
17
19
,: ~
~
~
I ,
1.0 1.4 1.8 -6 E
2.2 [
c~2.6
._
_
30----
-
I OI3
1.0
[
I
I.:
3.4 3.8
i
i
x~0.23
4.2
i
!
',
i
~
i
I
i
:
i
I
'
'
Ii
i
:
l
l
i
4.6 5.0 5.4 5.8
' _
Fig. 1. Concentration field of used melt compositions for Nd2_xCexCuO4 single crystal growth (copper oxide concentra-
tion is 100% - Y~( [Nd203] + [CeOz] ) ). Marks inside the field: (11) ref. [8], (tq), ref. [9], ( 0 ) refs. [4,7], ( ~ ) ref. [1] and (0) ref. [ 5 ]; numbers near the marks correspond to the K~ value; thin lines correspond to melt compositions that give an equal crystal composition X=0.13, 0.18 and 0.23; thick lines correspond to melt compositions that provide an equal Ce distribution coefficient Kce= 1, 2, 3, 4 and 5. evidence that Kce depends on oxygen partial pressure P[O2] and increases with increasing P[O2]. We think that the reason for such a behavior is an altered valence o f Ce. Usually Ce exists in the form o f a 4 + ion but sometimes in the form of a 3 + ion. High temperature leads to reduction of ion valence. So dynamic equilibrium Ce 4+ ~ Ce 3+ exists in Ce-containing solutions. Obviously it must depend on P[O2]. The formal oxidation number o f Ce in Nd2_xCexCu04 is about + 3.85 [10]. If we assume that Ce incorporates in the Nd2_xCexCuO4 crystal only in the Ce 4+ form the Kce dependence on P[O2] becomes understandable. The support o f dynamic e q u i l i b r i u m C e 4+ ~ C e 3+ demands the chemical reaction Ce 3+ + 102 = C e 4+ + ½0 2- .
(2)
440
K. Gamayunov et al. / Cerium distribution coefficient in Nde_xCe~CuO,
Exchange of oxygen between melt and atmosphere is necessary accordingly. Let us review the data [ 1-9 ] from the point of view of our previous assumption. Effective Ce incorporation in the Nd2_~CexCuO4 crystal [8] has been taken place up to the moment when the melt becomes screened from atmosphere by the growing crystal. The screening stops the exchange of oxygen between melt and atmosphere. So the chemical reaction (2) becomes impossible and Kc, falls down from about K c , = 2 - 6 to Kce< 1. The same situation has taken place in ref. [ 9 ]. The covering of the crucible by a lid stopped the free oxygen exchange. The Kc, fall was not so large as in the previous case because the oxygen exchange was reduced only. Validity of our assumption about Kc, dependence on P[O2] can be checked by a direct experiment if oxygen pressure will be changed during Nd2_xCe~CuO4 single crystal growth.
3. Experiment and discussion We have carried out our experiment on TSFZ as described in ref. [ 11 ]. A feed rod of Ndl.s4sCeo.155CuO4 60 m m in length and 7 m m in diameter has been used. The growth rate was about 0.6 m m / h . Oxygen partial pressure has been changed from 0.2 atm (air) to 0.1 atm (mixture of 10°/0 0 2 Jr- 900/0 N 2 ). Also we have carried out experiments changing the oxygen pressure as 2 a t m ~ 1 atm (oxygen flow)-+0.2 atm (air). But such changes in oxygen pressure induced a cessation of single crystal growth. Reduction of oxygen pressure in the N d - C e - C u - O system decreases the liquidus temperature. That changes also the zone composition. The changes in zone composition for jumps of oxygen pressure from 2 to 1 atm and from 1 to 0.2 atm are too large and cut off single crystal growth. The change of oxygen pressure from 0.2 to 0.1 atm did not induce a cessation of single crystal growth although it also changed the zone composition. The crystal has grown continuously. The cerium distribution along the growth axis is presented in fig. 2. The measurements have been carried out by EPMA (JEOL-8600-M).
I I m m E n 0.18
~ 0.12 ,~i~ ~ 0.10 0.08 1.2
2.4 Distancealonggrowthaxis, mm
Fig. 2. Ceriumdistribution in Nd2_xCexCuO4singlecrystalsalong the growth axis in the area of changed oxygenpressure from 0.2 to 0.1 arm. We can see that the oxygen pressure reduction has changed the Ce concentration in the single crystal from 0.145 to 0.09. (Disagreement between Ce concentration in the growing crystal X = 0.145 and that in the feed rod X = 0.155 is subject of a separate investigation). A distance of about 1.2 m m has been necessary to increase the cerium concentration up to the normal level X = 0.155. The dopant distribution in a single crystal grown by TSFZ is described by Cs = C f - ( C f - K C o ) e x p ( - K Z / L ) ,
(3)
where Cs is the dopant concentration in a single crystal, Cf the dopant concentration in a feed rod, Co initial dopant concentration in a molten zone, L the zone length, Z the flowing coordinate and K the dopant distribution coefficient. If Kce depends on P[O2] then C~ must be changed after a jump of oxygen pressure and relaxes to C , = Cf. Parameters of relaxation are Co=Xr, before the jump and a new value of Kce. The cerium concentration in the crystal just after the jump becomes equal to X = K c ~ = . The dopant distribution gives a possibility for fitting and direct determination of K using (3). Unfortunately our data are too rough for such a fitting. We can only determine the relative change of Kc, as K ~ I atm/rd0.2/"~ Ce atm
= X 0-2 atrn/x°'l arm=0.09/0.145 =0.62 .
(4)
Of course, the change of oxygen pressure has changed the zone composition and accordingly Kce.
K. Gamayunov et el. / Cerium distribution coefficient in Nd2_xCexCuO~
It is easy to show from the phase diagram that the increase of Ce concentration induced by the j u m p of oxygen pressure does not exceed 10%. The corresponding change of Kcc must therefore be about of 5%. So we can conclude that Kce has been changed by the j u m p of oxygen pressure because Kc, has a strong dependence on P [ 02 ]. The absolute value of the dopant distribution coefficient can be determined by a comparison of crystal and molten zone compositions. But it is not so simple to determine the dopant concentration in a molten zone for TSFZ. We have carried out three additional experiments at P [ O 2 ] = 0 . 1 , 0.2 and 2.0 atm in order to determine the absolute values of Kcc. The molten zone has been spilled and quenched at the end of each experiment. The conditions of such quenching are not hard enough to prevent compositional segregation in a drop volume. So we have analyzed compositions of fine crystallites just near the quenching surface and found that they were equal to about Arm= 0.060; 0.043 and 0.016 respectively. These values can be accepted as the cerium concentration in the molten zone with high accuracy because the high cooling rate in this region makes K, fr= 1. Determined in this way the values of Kc, at P[O2] =0.1, 0.2 and 2.0 atm are about 2.42, 3.37 and 9.06 respectively. Let us analyze the obtained Kc, using eq. ( 1 ). Its value is valid for P[O2] =0.2 atm. If we assume that the ratio (Nd + C e ) / C u has not been changed during crystal growth, then the melt with Xm=0.043 must give a cerium concentration in the crystal of X=0.145 and Kc~=3.45. This is in fine agreement with the value obtained from the analysis of the quenched zone (Kc¢= 3.37) and a cerium concentration in the single crystal (X=0.145). If we calculate in the same way the X and Kc, for a composition with Am= 0.060 then K c , = 2.95. The experimentally obtained value is K c , = 2.42 as it must be if Kc, depended on P[O2]. At the end of the discussion we can make a supposition about the results of refs. [ 1,5 ]. One is that Ce 4+ and Ce 3+ have a different distribution coefficient. The distribution coefficient of Ce 4+ ions follows from eq. ( 1 ) and falls down with increasing Ce concentration in a melt. That for C e 3 + is stable and
441
is equal to ~ 0.5. So, Ce incorporation is determined by Kc,,+ at low Ce concentrations and by Kc~3+ at high Ce concentrations in a melt. This is a supposition only but if it is right, a good way to decrease a concentration supercooling by choosing Kc~ = 1 could be realized.
4. Summary A strong dependence of Kce on oxygen partial pressure has been shown in direct TSFZ experiments by changing the oxygen pressure during single crystal growth. The ratio o f K ~ 2 atm/K°~ arm has been found to be equal to ~ 0.62. Absolute values of Kc, measured from zone composition have been found to be ~2.42; 3.37 and 9.06 for P [ O 2 ] = 0 . 1 ; 0.2 and 2.0 atm respectively. Our experiment shows good agreement between measured values of Kce and those calculated from eq. ( 1 ). We have shown that the quite large difference in Kc~, obtained in a number of investigations, is explained by a hard dependence of Kce o n P [ O 2 ] . We have also shown that the reason of the dependence is the variable valence of Ce. Cerium effectively incorporates in a Nd2_xCexCuO4 single crystal in the form of a Ce 4+ ion.
Acknowledgement K G acknowledges the Japanese Society for the Promotion of Science for the possibility to carry out the present investigation under grant ID No. 93083.
References [ 1] L. Zhang, J.Z. Liu, M.D. Lan, P. Klavinsand R.N. Shelton, Physica C 174 ( 1991 ) 431. [2] W. Sadowski,H. Hagemann,M. Francois, H. Bill, M. Peter, E. Walkerand K. Yvon, Physica C 170 (1990) 103. [ 3 ] S. Pinol, J. Fontcuberta, C. Miravitlles and D.McK. Paul, Physica C 165 (1990) 265. [4] K. Oka and H. Unoki, Jpn. J. Appl. Phys. 28 (1989) L937. [5] J.-M. Tarascon, E. Wang, L.H. Greene, B.G. Bagley,G.W. Hull, S.M. D'Egidio, P.F. Miceli, Z.Z. Wang, T.W. Jing, G. Clayhold, D. Brawnerand N.P. Ong, Phys.Rev.B 40 (1989) 4494. [ 6 ] J.L. Peng, Z.Y. Li and R.L. Greene, Physica C 177 ( 1991 ) 79.
442
K. Gamayunov et a L / Cerium distribution coefficient in Nde_xCexCu04
[7]H. Kojima and I. Tanaka, JJAP Ser. Mechanisms of Superconductivity 7 (1992) 76. [8]K.V. Gamayunov, A.L. Ivanov, V.V. Osiko and V.M. Tatarintsev, Sverhprovodimost: Fiz. Khim. Tehn. 4 ( 1991 ) 2404 [in Russian].
[ 9 ] A. Cassanho, D.R. Gabbe and H.P. Jenssen, J. Cryst. Growth 96 (1989) 999. [ 10] Y. Idemoto, K. Fueki and T. Shinbo, Physics C 166 (1990) 513. [ 11 ] I. Tanaka, K. Yamane and H. Kojima, J. Cryst. Growth 96 (1989) 711.
Physica C 218 (1993) 437-442 North-Holland
Cerium distribution coefficient and its dependence on oxygen partial pressure in Nd2_xCexCuO4 single crystal growth K. G a m a y u n o v a,b, I. Tanaka a and H. Kojima ~ a Institute of Inorganic Synthesis, Yamanashi University, Miyamae 7, Kofu 400, Japan b General Physics Institute of Russian Academy of Science, Vavilov st., 38, Moscow 117942, Russian Federation
Received 20 September 1993
A strong dependence of the cerium distribution coefficient (Kc~) on oxygen partial pressure P[O2] has been shown in direct 0.2 a r m TSFZ experiments by changing the oxygen pressure during Nd2_xCe~CuO4single crystal growth. The ratio of K°~. l a l m /Kc~ has been found to be equal to ~ 0.62. Absolute values of Kc~measured from zone compositions have been found to be about 2.42, 3.37 and 9.06 at P[O2 ] = 0.1, 0.2 and 2.0 atm, respectively. Our experiment shows good agreement between measured and calculated Kc~ values. We have shown that the quite large difference in Kc~ obtained in a number of investigations is explained by a strong dependence of Kc~ on P[O2]. We have also shown that a reason for this dependence is the variable valence of Ce. Cerium effectively incorporates in a Nd2_,CexCuO4single crystal in the form of Ce+4 ion.
1. Introduction
2. Pre-experimental analysis
The discovery in 1989 o f the first electron-doped high-temperature superconductor Nd2_xCexCuO4 has induced interest in the single crystal growth o f this solid solution. Many published papers have dealt with measurements o f physical properties carried out on single crystals. However, data on the process o f Nd2_xCe~CuO4 single crystal growth, especially with respect to the dopant distribution coefficient, are rather contradictory. The dopant distribution coefficient is one of the most important physical properties for single crystal growth. It defines the growth conditions for obtaining crystals o f the required properties. Published results on the cerium distribution coefficient Kce in N C C O are presented in table 1 [ 1-9 ]. Even a fluent examination o f the data shows a strong variation o f Kc~ and its dependence on growth conditions. The present work is aimed at defining the reason o f strong variations o f the cerium distribution coefficient Kc~ at Nd2_xCexCuO4 single crystal growth.
Special investigations on Kce have not been carried out except in ref. [ 8 ]. We base our analysis on this work so a brief description is necessary. Spontaneous crystallization from a nonstoichiometric melt using a Pt crucible has been used as a method for Nd2_xCexCuO4 single crystal growth. Measurements o f crystal compositions have been done by E P M A and the connecting equation between cerium concentration in a crystal ( X ) and the composition o f the used melt has been suggested: X = O . 2 5 - O . 1 8 / [ CeO2]m - O . 1 / [Nd203 ]m
+ 1.7/[CeO2 ] m" [Nd203 ]m ,
( 1)
where [CeO2]m is the cerium oxide concentration in the melt (mol.%), [Nd203]m is the n e o d y m i u m oxide concentration in the melt (mol.%). All measurements of X have been done on a crystal surface that was in contact with atmosphere. It has been found that Ce is incorporated by the crystal only up to a depth of about 10 ~tm. The other part o f the crystal is free from Ce (at least X < 0 . 0 2 ) . Crystal nucleation has been taken place in the center o f the melt surface. Then the crystal has grown as a
0921-4534/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
438
K. Gamayunov et al. /Cerium distribution coefficient in Ndz_xCexCu04
Table 1 Data on Ce distribution coefficient in Nd2_xCexCu04 Initial charge
Growth conditions
X melt/crystal
8.4% Nd203-6.0% CEO2-85.6% CuO
Pt crucible, air atm., cooling from 1320°C at 1-3 K/h A1203 crucible, N2 atm., cooling from 1250°C at 3 K/h
(0.28-0.36) / (0.14-0.18)
50% ( Nd203 + CeO2) + 50% Cu20 (outside of the region of NCCO primary crystallization)
70% C u O - ( 3 0 - y ) % Nd203-Y% CeO2 (outside of the region of NCCO primary crystallization) Feed rod of stoichiometric composition, solvent 85% CuO ( 1 ) Nd203 [ (2y)/2 ] + yCeO2 + 3 times stoich. CuO (2) The same but + 5 times stoich. CuO Stoichiometric composition + 50 wt.% of CuO as a flux
Feed rod of stoichiometric composition, solvent 85% CuO Compositions inside of the region of NCCO primary crystallization 16.4% Nd203-2.7% CEO2-80.9% CuO 16.7% Nd203-2.1% CEO2-81.2% CuO
Kce
Ref. [ 1]
0.5
(0.06) / (0.14, 0.16 ) (0.10)/(0.15, 0.19) ( 0.15 ) / (0.12-0.20) (0.18)/(0.18, 0.20) (0.20) / (0.14-0.18 ) (0.25)/(0.16,0.22) (0.35)/(0.18,0.21) (0.095-0.19)/ (0.00-0.22) for every initial composition
impossible to count
[2 ]
impossible to count
[3]
(0.15)/(0.15)
1
(0.40)/(0.12)
0.3
(0.29)/(0.14) (0.33)/(0.16)
0.5 0.5
(0.025)/(0.025) (0.05)/(0.11) (0.10)/(0.18) (0.13)/(0.21) (0.18)/(0.23) (0.26)/(0.23)
1.0 2.2 1.8 1.6 1.3 0.89
(0.02)/(0.15)
7.5
Pt crucible, air arm., cooling from 1250 ° C at 1-4 K/h
see fig. 1 and eq.(1)
seefig. 1 andeq. (1)
[8]
Pt crucible, air atm., cooling rate 0.5 K/h
(0.15)/(0.20)
1.33
[9]
(0.12)/(0.15) (coveredbyalid)
1.2
A1203 crucible, air atm., cooling from 1250°C at 5 K/h, growth in cavities TSFZ, 0.5-I.0 mm/h, Ar/O2=9/1 atm, P[O2] =0.1 atm Pt crucible, air atm., cooling from 1500 ° C at 6 K/h A1203 crucible, air arm., cooling from 1300 °C at 6 K / h A1203 crucible., air atm., cooling from 1270°C at 2 K/h, growth on the bottom of crucible
TSFZ, 0.5-1.0 ram/h, P[O2] =2 atm, d=6 mm, 1=50 mm
t h i n plate because o f t h e large difference o f the growth rates II a n d _L to c-axis. T h e t h i c k n e s s o f a b o u t 10 g m c o r r e s p o n d s to a m o m e n t w h e n t h e m e l t surface b e c o m e s c o v e r e d by t h e g r o w i n g crystal. W e will explain the C e b e h a v i o r later.
[41
[51
[6]
[7]
S o m e results f r o m table 1 c a n n o t b e t a k e n u n d e r c o n s i d e r a t i o n for c a l c u l a t i o n o f Kce b e c a u s e the m e l t c o m p o s i t i o n s lie o u t s i d e the region o f Nd2_xCexCuO4 p r i m a r y crystallization [2,3]. A typical f e a t u r e for such c r y s t a l l i z a t i o n is that X fluctuates f r o m crystal
439
K. Gamayunov et al. / Cerium distribution coefficient in Nd2_xCexCuO~
to crystal within the range 0-0.23 independently on the initial composition. Results o f TSFZ give only indirect information about Kc~ because different solvent compositions give the same X value just after completion o f the relaxation processes on the initial stage of crystallization. Also compositional stability o f the molten zone cannot be assured. However, we can compare solvent compositions from refs. [ 7 ] and [4 ] because usually they are chosen with the aim to minimize variations in dopant concentration on an initial stage o f crystallization. The ratio o f X and Xr~ (subindex " m " belongs to the Ce concentration in the melt) gives 7.5 and 1 respectively for the data o f the mentioned works. The valuation o f Kce is not absolutely correct but the large difference o f counted values leads us to suggest strong dependence of Kce on oxygen partial pressure P [ O 2 ] that was about 2 and 0.1 atm respectively. We can see from table 1 and fig. 1 that a good agreement o f the data ofrefs. [8] and [6] exists. Essentially for the high Ce concentration region, the dependences of Kc~ versus melt composition are qualitatively the same. The data in ref. [ 9 ] are in good agreement with those of ref. [ 8 ] too. The best agreement is found for the melt composition with Xm=0.20. We see that Kce decreased for the composition with Xm = 0.15 instead o f increasing in accordance with the general tendency given by eq. ( 1 ). But the growth conditions have not been the same. In the case o f Xm= 0.15 the crucible with the melt has been covered by a lid. Such a change in experimental conditions leads to a difficulty in gas exchange between melt and atmosphere and causes some reduction o f partial oxygen pressure in using atmosphere over a melt surface. So we have another indirect evidence for the possible dependence of Kce on P[O2].
Results ofrefs. [ 1,5 ] are rather different from those examined above. First of all the fact that Kce< 1 attracts our attention. We could not find a suitable explanation for such a strong difference in Kce values. C o m m o n features for both cases are (1) high Ce concentration in the melt ( [CeOE]m> 5mo1.%) and (2) change in the type o f crystallization from threedimensional to two-dimensional. Let us return to the examination o f Ce incorporation in Nd2_xCexCu04. Some results give indirect
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tion is 100% - Y~( [Nd203] + [CeOz] ) ). Marks inside the field: (11) ref. [8], (tq), ref. [9], ( 0 ) refs. [4,7], ( ~ ) ref. [1] and (0) ref. [ 5 ]; numbers near the marks correspond to the K~ value; thin lines correspond to melt compositions that give an equal crystal composition X=0.13, 0.18 and 0.23; thick lines correspond to melt compositions that provide an equal Ce distribution coefficient Kce= 1, 2, 3, 4 and 5. evidence that Kce depends on oxygen partial pressure P[O2] and increases with increasing P[O2]. We think that the reason for such a behavior is an altered valence o f Ce. Usually Ce exists in the form o f a 4 + ion but sometimes in the form of a 3 + ion. High temperature leads to reduction of ion valence. So dynamic equilibrium Ce 4+ ~ Ce 3+ exists in Ce-containing solutions. Obviously it must depend on P[O2]. The formal oxidation number o f Ce in Nd2_xCexCu04 is about + 3.85 [10]. If we assume that Ce incorporates in the Nd2_xCexCuO4 crystal only in the Ce 4+ form the Kce dependence on P[O2] becomes understandable. The support o f dynamic e q u i l i b r i u m C e 4+ ~ C e 3+ demands the chemical reaction Ce 3+ + 102 = C e 4+ + ½0 2- .
(2)
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K. Gamayunov et al. / Cerium distribution coefficient in Nde_xCe~CuO,
Exchange of oxygen between melt and atmosphere is necessary accordingly. Let us review the data [ 1-9 ] from the point of view of our previous assumption. Effective Ce incorporation in the Nd2_~CexCuO4 crystal [8] has been taken place up to the moment when the melt becomes screened from atmosphere by the growing crystal. The screening stops the exchange of oxygen between melt and atmosphere. So the chemical reaction (2) becomes impossible and Kc, falls down from about K c , = 2 - 6 to Kce< 1. The same situation has taken place in ref. [ 9 ]. The covering of the crucible by a lid stopped the free oxygen exchange. The Kc, fall was not so large as in the previous case because the oxygen exchange was reduced only. Validity of our assumption about Kc, dependence on P[O2] can be checked by a direct experiment if oxygen pressure will be changed during Nd2_xCe~CuO4 single crystal growth.
3. Experiment and discussion We have carried out our experiment on TSFZ as described in ref. [ 11 ]. A feed rod of Ndl.s4sCeo.155CuO4 60 m m in length and 7 m m in diameter has been used. The growth rate was about 0.6 m m / h . Oxygen partial pressure has been changed from 0.2 atm (air) to 0.1 atm (mixture of 10°/0 0 2 Jr- 900/0 N 2 ). Also we have carried out experiments changing the oxygen pressure as 2 a t m ~ 1 atm (oxygen flow)-+0.2 atm (air). But such changes in oxygen pressure induced a cessation of single crystal growth. Reduction of oxygen pressure in the N d - C e - C u - O system decreases the liquidus temperature. That changes also the zone composition. The changes in zone composition for jumps of oxygen pressure from 2 to 1 atm and from 1 to 0.2 atm are too large and cut off single crystal growth. The change of oxygen pressure from 0.2 to 0.1 atm did not induce a cessation of single crystal growth although it also changed the zone composition. The crystal has grown continuously. The cerium distribution along the growth axis is presented in fig. 2. The measurements have been carried out by EPMA (JEOL-8600-M).
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Fig. 2. Ceriumdistribution in Nd2_xCexCuO4singlecrystalsalong the growth axis in the area of changed oxygenpressure from 0.2 to 0.1 arm. We can see that the oxygen pressure reduction has changed the Ce concentration in the single crystal from 0.145 to 0.09. (Disagreement between Ce concentration in the growing crystal X = 0.145 and that in the feed rod X = 0.155 is subject of a separate investigation). A distance of about 1.2 m m has been necessary to increase the cerium concentration up to the normal level X = 0.155. The dopant distribution in a single crystal grown by TSFZ is described by Cs = C f - ( C f - K C o ) e x p ( - K Z / L ) ,
(3)
where Cs is the dopant concentration in a single crystal, Cf the dopant concentration in a feed rod, Co initial dopant concentration in a molten zone, L the zone length, Z the flowing coordinate and K the dopant distribution coefficient. If Kce depends on P[O2] then C~ must be changed after a jump of oxygen pressure and relaxes to C , = Cf. Parameters of relaxation are Co=Xr, before the jump and a new value of Kce. The cerium concentration in the crystal just after the jump becomes equal to X = K c ~ = . The dopant distribution gives a possibility for fitting and direct determination of K using (3). Unfortunately our data are too rough for such a fitting. We can only determine the relative change of Kc, as K ~ I atm/rd0.2/"~ Ce atm
= X 0-2 atrn/x°'l arm=0.09/0.145 =0.62 .
(4)
Of course, the change of oxygen pressure has changed the zone composition and accordingly Kce.
K. Gamayunov et el. / Cerium distribution coefficient in Nd2_xCexCuO~
It is easy to show from the phase diagram that the increase of Ce concentration induced by the j u m p of oxygen pressure does not exceed 10%. The corresponding change of Kcc must therefore be about of 5%. So we can conclude that Kce has been changed by the j u m p of oxygen pressure because Kc, has a strong dependence on P [ 02 ]. The absolute value of the dopant distribution coefficient can be determined by a comparison of crystal and molten zone compositions. But it is not so simple to determine the dopant concentration in a molten zone for TSFZ. We have carried out three additional experiments at P [ O 2 ] = 0 . 1 , 0.2 and 2.0 atm in order to determine the absolute values of Kcc. The molten zone has been spilled and quenched at the end of each experiment. The conditions of such quenching are not hard enough to prevent compositional segregation in a drop volume. So we have analyzed compositions of fine crystallites just near the quenching surface and found that they were equal to about Arm= 0.060; 0.043 and 0.016 respectively. These values can be accepted as the cerium concentration in the molten zone with high accuracy because the high cooling rate in this region makes K, fr= 1. Determined in this way the values of Kc, at P[O2] =0.1, 0.2 and 2.0 atm are about 2.42, 3.37 and 9.06 respectively. Let us analyze the obtained Kc, using eq. ( 1 ). Its value is valid for P[O2] =0.2 atm. If we assume that the ratio (Nd + C e ) / C u has not been changed during crystal growth, then the melt with Xm=0.043 must give a cerium concentration in the crystal of X=0.145 and Kc~=3.45. This is in fine agreement with the value obtained from the analysis of the quenched zone (Kc¢= 3.37) and a cerium concentration in the single crystal (X=0.145). If we calculate in the same way the X and Kc, for a composition with Am= 0.060 then K c , = 2.95. The experimentally obtained value is K c , = 2.42 as it must be if Kc, depended on P[O2]. At the end of the discussion we can make a supposition about the results of refs. [ 1,5 ]. One is that Ce 4+ and Ce 3+ have a different distribution coefficient. The distribution coefficient of Ce 4+ ions follows from eq. ( 1 ) and falls down with increasing Ce concentration in a melt. That for C e 3 + is stable and
441
is equal to ~ 0.5. So, Ce incorporation is determined by Kc,,+ at low Ce concentrations and by Kc~3+ at high Ce concentrations in a melt. This is a supposition only but if it is right, a good way to decrease a concentration supercooling by choosing Kc~ = 1 could be realized.
4. Summary A strong dependence of Kce on oxygen partial pressure has been shown in direct TSFZ experiments by changing the oxygen pressure during single crystal growth. The ratio o f K ~ 2 atm/K°~ arm has been found to be equal to ~ 0.62. Absolute values of Kc, measured from zone composition have been found to be ~2.42; 3.37 and 9.06 for P [ O 2 ] = 0 . 1 ; 0.2 and 2.0 atm respectively. Our experiment shows good agreement between measured values of Kce and those calculated from eq. ( 1 ). We have shown that the quite large difference in Kc~, obtained in a number of investigations, is explained by a hard dependence of Kce o n P [ O 2 ] . We have also shown that the reason of the dependence is the variable valence of Ce. Cerium effectively incorporates in a Nd2_xCexCuO4 single crystal in the form of a Ce 4+ ion.
Acknowledgement K G acknowledges the Japanese Society for the Promotion of Science for the possibility to carry out the present investigation under grant ID No. 93083.
References [ 1] L. Zhang, J.Z. Liu, M.D. Lan, P. Klavinsand R.N. Shelton, Physica C 174 ( 1991 ) 431. [2] W. Sadowski,H. Hagemann,M. Francois, H. Bill, M. Peter, E. Walkerand K. Yvon, Physica C 170 (1990) 103. [ 3 ] S. Pinol, J. Fontcuberta, C. Miravitlles and D.McK. Paul, Physica C 165 (1990) 265. [4] K. Oka and H. Unoki, Jpn. J. Appl. Phys. 28 (1989) L937. [5] J.-M. Tarascon, E. Wang, L.H. Greene, B.G. Bagley,G.W. Hull, S.M. D'Egidio, P.F. Miceli, Z.Z. Wang, T.W. Jing, G. Clayhold, D. Brawnerand N.P. Ong, Phys.Rev.B 40 (1989) 4494. [ 6 ] J.L. Peng, Z.Y. Li and R.L. Greene, Physica C 177 ( 1991 ) 79.
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K. Gamayunov et a L / Cerium distribution coefficient in Nde_xCexCu04
[7]H. Kojima and I. Tanaka, JJAP Ser. Mechanisms of Superconductivity 7 (1992) 76. [8]K.V. Gamayunov, A.L. Ivanov, V.V. Osiko and V.M. Tatarintsev, Sverhprovodimost: Fiz. Khim. Tehn. 4 ( 1991 ) 2404 [in Russian].
[ 9 ] A. Cassanho, D.R. Gabbe and H.P. Jenssen, J. Cryst. Growth 96 (1989) 999. [ 10] Y. Idemoto, K. Fueki and T. Shinbo, Physics C 166 (1990) 513. [ 11 ] I. Tanaka, K. Yamane and H. Kojima, J. Cryst. Growth 96 (1989) 711.