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Single crystalline whiskers of La1-xSrxCuO3-y
0038-1098/90 $3.00 + .00 Pergamon Press plc
Solid State Communications, Vol. 76, No. 5, pp. 725-726, 1990. Printed in Great Britain.
S I N G L E C R Y S T A L L I N E W H I S K E R S OF Lal_xSrxCuO 3 y K.V. Gamayunov, A.L. Ivanov, V.V. Osiko, V.M. Tatarintsev and A.I. Chernov General Physics Institute of Academy Sciences of the USSR, Vavilov Street 38, 117942 Moscow, USSR
(Received 28 June 1990 by V.M. Agranovich) La~_.~SrxCuO3 _y single crystals in the whiskers-like form were obtained by spontanous crystallization method from the CuO flux. Strontium distribution coefficient has been found to be k = 0.6. It has been shown that the Lal_xSrxCuO3_ ~, solid solution region (under equilibrium growth conditions) is spread to x = 0.
1. I N T R O D U C T I O N IT IS W E L L known that HTS was found by Bednorz and Mfiller on the ceramic samples in the L a - B a - C u O system. It has been found soon that Sr (instead Ba) improves the T, in this system. Although technically Laz_zSr=fuO4 has been superseded as a superconductor by YBa2Cu307, its interest for research remains. Investigations gave some interesting facts for this material: T,. in the single crystals is lower than in the ceramics of the same composition; Sr has nonstatisticai distribution on the lanthanum sites; distribution coefficient is not constant and depends on the Sr concentration in the melt and some others [1, 2]. The first report about other solid solution La~ xSrxCuO3_y was published two years ago and the first report on the single crystals o f the same composition was published some times later [3]. It was reported on the superconducting properties at the temperature lower than 40 K in the La~. ~SrxCuO3_ ), ceramic samples [4]. Results of the X-ray diffraction measurements were published in [5]. La~ _xSrxCuO3_y has the tetragonal unit cell and the lattice parameters are a = 10.851(1)./k and c = 3.8623(4)A (for x = 0.2). In this work we found the new morphology type o f the Lal_xSrxCuO3_y single crystals and showed that the region of the Laj_~SrxCuO3_y solid solution (under equilibrium growth conditions) is spread up to x=0.
2. E X P E R I M E N T A L La~ xZrxfuO3_y single crystals in the whiskerslike form were obtained by spontaneous crystallization method from the CuO flux. We used vertical furnace with resistivity heating and grew crystals in the platinum crucibles. More detailed description is given in our previous works [1, 2].
Lal _xSrxCuO3_y single crystals were produced at the temperature gradient in the melt region about 1-5 K cm-l and with the cooling rate 0.4-0.8 K h -I . CuO concentration in the melt was approximately 85-90mo1% for [Sr]/[La] = 0: 1/9. The initial temperature of the process was 1150-1180 ° C in accordance to the initial composition. Slow cooling was carried out to 1005 ° C, i.e. eutectic temperature on the LazO3-CuO phase diagram. 3. R E S U L T S A N D DISCUSSION La2_~ S e z f u O 4 single crystals in the form o f plates and pyramids with the sizes up to 10 × 10 × 1 and 8 x 8 x 8 mm 3 were obtained for each initial composition (results of the dependence of physical-chemical properties on the process parameters will be published in [8]). We have found a lot of whiskers-like single crystals frozen into the melt together with La 2_ =Sr=fuO4 single crystals. This crystals had length up to 15 mm and cross-section up to 0.5 × 0.5 mm 2. Data on the electron-probe microanalysis showed that the composition of this crystals corresponds to Lal _ xSrxCuO3-y. X-ray powder analysis indicates this phase too. Strontium concentration in the doped crystals was x = 0.06. Strontium distribution coefficient is approximately 0.6. This value is higher than that in the single crystals of the La2 =Sr:CuO4 solid solution. As-grown crystals did not have superconducting properties above 4.2 K. Data on the La,_ ~SrxCuO 3 y solid solution are limited by several papers. It was shown that the solid solution region has limits 0.20 < x < 0.25 for ceramics [5]. Later other authors gave limits 0.16 < x < 0.24 [6]. We pointed out the appearance of this phase as an impurity of second phase in the La2 :Sr=CuO 4 crystals (in accordance to the results of X-ray powder diffraction analysis) [7].
725
726
SINGLE CRYSTALLINE WHISKERS OF La~_~SrxCuO3_y ,3_,o
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Vol. 76, No. 5
between the La2_~SrzCu04 solid solution region and the figurative points of the initial melt composition is not finally clear. It seems that the practically equilibrium conditions are required for crystallization of the Lal_xSrxCuO3_y. The fact that Lal_xSrxCuO3_y is crystallized only when the cooling rate is lower than 1 K h - l supports this hypothesis. Simultaneous single crystal growth of the Lal_xSrxCuO3_y and La2_zSrz CuO4 may be explained, we think, by the deviation of the growth conditions to the equilibrium as is required for La~_xSrxCuO3_y single phase crystallization. 4. CONCLUSION
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Fig. 1. La2_zSr.CuOa and Lal _xSrxCu03solid solution regions on the La-Sr-Cu-O phase diagram. (a) La2_.Sr:CuO4 solid solution region, (b) La~_~SrxCuO3 solid solution region, (c) initial compositions of the melts in our experiments.
Single crystalline whiskers of the Lal _xSrxCuO3_y were obtained. Strontium distribution coefficients has been found to be k = 0.6. It has been shown that the Laj _xSr~CuO3 y solid solution region (under equilibrium growth conditions) is spread to x = 0. REFERENCES
La~ __xSrxCuO3 y plate-like single crystals (instead 1. of La2 -Sr:CuO4 single crystals) with sizes up to 8 x 8 x 0.5 mm 3 have been grown by spontaneous crystallization with cooling rate lower than 1 K h I [3]. 2. Unfortunately the data on composition and superconducting properties of these crystals are absent in this report. 3. We must mention that La~ xSrxCuO3 y single crystals with x = 0 have been grown. It means that 4. lower limit of the solid solution region under equilibrium growth conditions is spread to x = 0. That must be reflected on the phase diagram (Fig. 1). 5. We think that the question of superconducting properties of the La, xSr,.CuO3_y remains open. Superconductivity may appear under special growth 6. conditions or after certain post-growth treatment, it is possible because this material has perovskite structure and the oxygen cencentration can be changed. For 7. example, ceramics, which are obtained in air, have approximately 0.07 extra oxygen atoms per formula 8. unit [5] if all Cu atoms have 2 + formal valency. The question of how La2_=Sr:CuO4 can crystallize if the Laj ,.Sr,CuO3 , solid solution region is situated
V.G. Veselago, K.V. Gamajunov, V.I. Zorya, A.L. lvanov, V.V. Osiko, V.M. Tatarintsev, V.A. Fradkov, M.A. Chernikov & A.I. Chernov, Supercond. S. & T. 3, 121 (1990). V.I. Simonov, L.A. Muradian, R.A. Tamazian, K.V. Gamajunov, V.V. Osiko & V.M. Tatarintsv, Physica C at press (1990). C. Chen, B.E. Watts, B.M. Wanklyn & P. Thomas, SolidState Commun. 66, 611 (1988). L. Haupt, J.-W. Schunemaann, K. Barner, U. Sondermann, B. Rager, R.M. Abdelouhab, R. Braunstein & S. Dong, Solid State Commun. 72, 1093 (1989). N. Murayama, S. Sakaguchi, F. Wakai, E. Sudo, A. Tsuzuki & Y. Torii, Jap. J. Appl. Phys. 27, L55 (1988). D.M. De Leeuw, C.A.H.A. Mutsaers G.P.J. Geelen & C. Langereis, J. Solid State Chem. 80, 276 (1989). V.V. Voronov, K.V. Gamajunov, V.M. Ivanovskaya, O.Y. Morozova, V.V. Osiko & V.M. Tatarintsev, Prepreint IOFAN, M. (1990). K.V. Gamajunov, A.L. Ivanov, V.V. Osiko, V.M. Tatarintsev, A.I. Chernov, V.I. Zorya, Y. Yakovets, V.V. Vornov & O.Y. Morozova, Submitted to Supercond. S. & T.
Solid State Communications, Vol. 76, No. 5, pp. 725-726, 1990. Printed in Great Britain.
S I N G L E C R Y S T A L L I N E W H I S K E R S OF Lal_xSrxCuO 3 y K.V. Gamayunov, A.L. Ivanov, V.V. Osiko, V.M. Tatarintsev and A.I. Chernov General Physics Institute of Academy Sciences of the USSR, Vavilov Street 38, 117942 Moscow, USSR
(Received 28 June 1990 by V.M. Agranovich) La~_.~SrxCuO3 _y single crystals in the whiskers-like form were obtained by spontanous crystallization method from the CuO flux. Strontium distribution coefficient has been found to be k = 0.6. It has been shown that the Lal_xSrxCuO3_ ~, solid solution region (under equilibrium growth conditions) is spread to x = 0.
1. I N T R O D U C T I O N IT IS W E L L known that HTS was found by Bednorz and Mfiller on the ceramic samples in the L a - B a - C u O system. It has been found soon that Sr (instead Ba) improves the T, in this system. Although technically Laz_zSr=fuO4 has been superseded as a superconductor by YBa2Cu307, its interest for research remains. Investigations gave some interesting facts for this material: T,. in the single crystals is lower than in the ceramics of the same composition; Sr has nonstatisticai distribution on the lanthanum sites; distribution coefficient is not constant and depends on the Sr concentration in the melt and some others [1, 2]. The first report about other solid solution La~ xSrxCuO3_y was published two years ago and the first report on the single crystals o f the same composition was published some times later [3]. It was reported on the superconducting properties at the temperature lower than 40 K in the La~. ~SrxCuO3_ ), ceramic samples [4]. Results of the X-ray diffraction measurements were published in [5]. La~ _xSrxCuO3_y has the tetragonal unit cell and the lattice parameters are a = 10.851(1)./k and c = 3.8623(4)A (for x = 0.2). In this work we found the new morphology type o f the Lal_xSrxCuO3_y single crystals and showed that the region of the Laj_~SrxCuO3_y solid solution (under equilibrium growth conditions) is spread up to x=0.
2. E X P E R I M E N T A L La~ xZrxfuO3_y single crystals in the whiskerslike form were obtained by spontaneous crystallization method from the CuO flux. We used vertical furnace with resistivity heating and grew crystals in the platinum crucibles. More detailed description is given in our previous works [1, 2].
Lal _xSrxCuO3_y single crystals were produced at the temperature gradient in the melt region about 1-5 K cm-l and with the cooling rate 0.4-0.8 K h -I . CuO concentration in the melt was approximately 85-90mo1% for [Sr]/[La] = 0: 1/9. The initial temperature of the process was 1150-1180 ° C in accordance to the initial composition. Slow cooling was carried out to 1005 ° C, i.e. eutectic temperature on the LazO3-CuO phase diagram. 3. R E S U L T S A N D DISCUSSION La2_~ S e z f u O 4 single crystals in the form o f plates and pyramids with the sizes up to 10 × 10 × 1 and 8 x 8 x 8 mm 3 were obtained for each initial composition (results of the dependence of physical-chemical properties on the process parameters will be published in [8]). We have found a lot of whiskers-like single crystals frozen into the melt together with La 2_ =Sr=fuO4 single crystals. This crystals had length up to 15 mm and cross-section up to 0.5 × 0.5 mm 2. Data on the electron-probe microanalysis showed that the composition of this crystals corresponds to Lal _ xSrxCuO3-y. X-ray powder analysis indicates this phase too. Strontium concentration in the doped crystals was x = 0.06. Strontium distribution coefficient is approximately 0.6. This value is higher than that in the single crystals of the La2 =Sr:CuO4 solid solution. As-grown crystals did not have superconducting properties above 4.2 K. Data on the La,_ ~SrxCuO 3 y solid solution are limited by several papers. It was shown that the solid solution region has limits 0.20 < x < 0.25 for ceramics [5]. Later other authors gave limits 0.16 < x < 0.24 [6]. We pointed out the appearance of this phase as an impurity of second phase in the La2 :Sr=CuO 4 crystals (in accordance to the results of X-ray powder diffraction analysis) [7].
725
726
SINGLE CRYSTALLINE WHISKERS OF La~_~SrxCuO3_y ,3_,o
o
"'.,. el
t
"I ,oooi L%03
2O
4O
]
1
]
60 I I
-,,2oop
!V_,ooo 8O I I
Vol. 76, No. 5
between the La2_~SrzCu04 solid solution region and the figurative points of the initial melt composition is not finally clear. It seems that the practically equilibrium conditions are required for crystallization of the Lal_xSrxCuO3_y. The fact that Lal_xSrxCuO3_y is crystallized only when the cooling rate is lower than 1 K h - l supports this hypothesis. Simultaneous single crystal growth of the Lal_xSrxCuO3_y and La2_zSrz CuO4 may be explained, we think, by the deviation of the growth conditions to the equilibrium as is required for La~_xSrxCuO3_y single phase crystallization. 4. CONCLUSION
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Fig. 1. La2_zSr.CuOa and Lal _xSrxCu03solid solution regions on the La-Sr-Cu-O phase diagram. (a) La2_.Sr:CuO4 solid solution region, (b) La~_~SrxCuO3 solid solution region, (c) initial compositions of the melts in our experiments.
Single crystalline whiskers of the Lal _xSrxCuO3_y were obtained. Strontium distribution coefficients has been found to be k = 0.6. It has been shown that the Laj _xSr~CuO3 y solid solution region (under equilibrium growth conditions) is spread to x = 0. REFERENCES
La~ __xSrxCuO3 y plate-like single crystals (instead 1. of La2 -Sr:CuO4 single crystals) with sizes up to 8 x 8 x 0.5 mm 3 have been grown by spontaneous crystallization with cooling rate lower than 1 K h I [3]. 2. Unfortunately the data on composition and superconducting properties of these crystals are absent in this report. 3. We must mention that La~ xSrxCuO3 y single crystals with x = 0 have been grown. It means that 4. lower limit of the solid solution region under equilibrium growth conditions is spread to x = 0. That must be reflected on the phase diagram (Fig. 1). 5. We think that the question of superconducting properties of the La, xSr,.CuO3_y remains open. Superconductivity may appear under special growth 6. conditions or after certain post-growth treatment, it is possible because this material has perovskite structure and the oxygen cencentration can be changed. For 7. example, ceramics, which are obtained in air, have approximately 0.07 extra oxygen atoms per formula 8. unit [5] if all Cu atoms have 2 + formal valency. The question of how La2_=Sr:CuO4 can crystallize if the Laj ,.Sr,CuO3 , solid solution region is situated
V.G. Veselago, K.V. Gamajunov, V.I. Zorya, A.L. lvanov, V.V. Osiko, V.M. Tatarintsev, V.A. Fradkov, M.A. Chernikov & A.I. Chernov, Supercond. S. & T. 3, 121 (1990). V.I. Simonov, L.A. Muradian, R.A. Tamazian, K.V. Gamajunov, V.V. Osiko & V.M. Tatarintsv, Physica C at press (1990). C. Chen, B.E. Watts, B.M. Wanklyn & P. Thomas, SolidState Commun. 66, 611 (1988). L. Haupt, J.-W. Schunemaann, K. Barner, U. Sondermann, B. Rager, R.M. Abdelouhab, R. Braunstein & S. Dong, Solid State Commun. 72, 1093 (1989). N. Murayama, S. Sakaguchi, F. Wakai, E. Sudo, A. Tsuzuki & Y. Torii, Jap. J. Appl. Phys. 27, L55 (1988). D.M. De Leeuw, C.A.H.A. Mutsaers G.P.J. Geelen & C. Langereis, J. Solid State Chem. 80, 276 (1989). V.V. Voronov, K.V. Gamajunov, V.M. Ivanovskaya, O.Y. Morozova, V.V. Osiko & V.M. Tatarintsev, Prepreint IOFAN, M. (1990). K.V. Gamajunov, A.L. Ivanov, V.V. Osiko, V.M. Tatarintsev, A.I. Chernov, V.I. Zorya, Y. Yakovets, V.V. Vornov & O.Y. Morozova, Submitted to Supercond. S. & T.