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Growth of Ho-123 superconducting single crystals
PHYSICA[
Physica B 194-196 (1994) 2271-2272 North-Holland
Growth of Ho-123 superconducting single crystals H Arabi, D Ciomartan, R H Fenn*, O S Mills~, J W Ross, and D J Sandiford Physics Department, Victoria University of Manchester, Manchester, M 13 9PL, UK *Physics Department, Portsmouth University, Portsmouth, PO1 2DZ, UK #Chemistry Department, Victoria University of Manchester, Manchester, M13 9PL, UK To grow single crystals of HoBa2Cu3074 we first used the citrate gel approach to provide finely divided and homogeneous starting materials and to develop a crystal growing regime in oxygen. Crystals grown on the surface of the melts with compositions 1:4:6 and 1:4:10, were up to 20rag in mass and superconducting with a Tc of 92K. More recently, we have grown crystals from mixtures of either simple or complex oxides of composition 1:111:25 and 1:12:26 as starting materials, under different temperature control so as to produce larger crystals at the bottom of the crucible. These crystals are up to 50mg in mass, with Tes of about 45K, which may be raised close to 90K by later annealing in oxygen. Crystals grown by the first method were always found to be tetragonal. We speculate on the reasons for this. 1. CONDITIONS FOR CRYSTAL GROWTH Figure 1 gives the Differential thermal analysis (DTA) and thermogravimetric analysis (TG) curves of a powder of Ho-123 phase in a typical flux in a cycle of heating and cooling, up to 1200°C. There are two obvious endothermic effects in the heating phase. The first, at 930°C is the melting point of the mixture, and the second at 1030°C corresponds to the decomposition of the 123 phase (incongruent melting). The two exothermic features starting at 940°C and 910°C are related to the crystallization of the 123 phase and the solidification of the flux. Comparable results have been reported for Y-1231.
I DTA~----~ ~
TG
~-
600 I
800 I
I
1000 I
I
I
Figure 1 DTA and TG for Ho-123 and flux A range of similar measurements at different initial compositions and for different temperature ranges, with varying compositions of cation ratios, and in argon as well as oxygen have been carried out. In addition, by quenching from high
temperature followed by XRD identification, we obtained information on the rate at which Ho2BaCuOs (the 211 phase) was formed above 980°C from the decomposition of the 123. It is preferable to work with the eutectic mixture of BaO and CuO (29:71 molar percent) to give the widest range of temperatures for the growth of the 123 crystals from 920°C (the eutectic melting point) to about 1030°C when 123 decomposes into 211, 011, and CuO. This is the temperature range where 123 crystals are grown. 1.1 Citrate gel method The citrate gel approach could give precursor materials which have the following advantages: stoiehiometric control, mixing at the ionic molecular level, control of the morphology of the particles by producing active and uniform micron-sized particles, a reduced chance of invasive impurities since no significant grinding/milling is necessary, and production of a uniform mixture. Single crystals of HoBa2Cu3074 have been obtained in a resultant flux of BaCuO2 and CuO, in an alumina crucible by the citrate gel method. Batches, rich in 123, with compositions of cation ratios (Ho:Ba:Cu) in the range of 1:4:6 to 1:4:10 have yielded the best crystals. Finally, the growth within a pure oxygen environment produces surface developed crystals with high concentrations of oxygen and hence high T¢ values. Our best-growth programme involved heating above the decomposition point of the 123 at 1020-1030°C, soaking on a dwell time to dissolve the 211 phase in the melted flux, then cooling at a rate of 0.3-
0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved S S D I 0921-4526(93)1768-H
2272 0.1°C/hr in flowing oxygen, down to 900°C. A cooling rate of 10°C/hr between 700-400°C, where a well-known tetragonal to orthorhombic transition occurs, was used. This process produced flat-surfaced, shiny crystals of good prismatic habit, which grew at the surface of our melt and were later extracted with a dental drill. They were always accompanied by acicular CuO and BaCuO 2 crystals. The crystals of 123 were typically of mass 5mg with a few as large as 20mg. These crystals were tetragonal and superconducting at 92K.
1.2 Oxide method In the earlier method it is difficult to control the near explosive decomposition of the citrates that occurs at about 240°C. The BaCO3 phase decomposes between 940°C and 1250°C ejecting CO:. In the melts used for single crystal growth, this gas produces small cavities, on which small crystals of 123 can develop. To reduce the number of crystallization centres, the 123:flux ratio in the batch was reduced and BaCO 3 was replaced first by BaO and then by 011. The batches, finely powdered, were put in boats of high density 99 % pure alumina. The furnace used had a temperature gradient, across the boat, of less than 1 °C. The heating rate up to 850°C was 120°C/hr. At this point 123 started to form together with 011 by a sintering process in the solid state. A rate of 50°C/hr was then set to 925°C when the batch melted A rate of 10°C/hr follows, up to 995°C so that 123 is complely formed and a final rate of 5°C/hr follows up to 1030°C so that all 123 decomposes to 211 and other components. A dwell time of three hours allows the 211 to dissolve in the liquid flux, while 012 decomposes, and in this way the viscosity of the melt was reduced. A cooling rate of 4°C/hr at 1030°C is followed by successively slower ones until at 930°C a 0.1°C/hr rate was set. In this range of temperature crystallisation occurs. This slow cooling rate allows a good development of the crystals, by the peritectic reaction in liquid 211 + 011 + CuO = 123. The process of controlled slow cooling stops at 925°C, and higher cooling rates are used down to room temperature. Decantation 3 revealed the crystals grown at the bottom of the crucible. An ultrasound micro-chisel was used to extract the desired crystals from the crucible. Yields of 25 % of crystals larger than 2mm were obtained. Many crystals were around 50mg. The crystals were grown away from the oxygen atmosphere, consequently their Tc was typically
found to be about 45K, which could be raised to 1.0
j ~
0.8 0.6 0.4 0.2
~
~0W ' rl
ted
0.0 i
0
2O
40 6O Temperature/K
i
80
100
Figure 2 Meissner effect at 20MHz 89K on annealing at 900°C in oxygen for 4 hours (see Figure 2), lower than the T c values obtained by the citrate gel method.
2 TETRAGONAL OR ORTHORHOMBIC? XRD has given evidence of orthorthombicity with
(b-a)/a=1% for Ho-123 crystals grown from the purest starting oxides, whereas the citrate gel method always gave tetragonal crystals. Tetragonal crystals were grown from oxides with AI impurities as is also found for Y-1234'5. It is generally accepted that pure Y-123 is orthorhombic, with (b-a)/a of some 2%. Indeed, some properties in the a and b directions show considerable anisotropy ~ which is thought to reflect the preferential occupation by oxygen of sites along the b axis, ie the binding energy of O atoms on the b sites (~b) is greater than at the a sites (~). Tetragonal Ho-123 crystals imply that oxygen is distributed randomly in the a-b plane. The smaller value of (b-a)/a in Ho-123 we ascribe to a smaller value of (E:e~). Our crystal growth via citrate gel in oxygen nucleated 123 crystals at temperatures such that it is probable that k T > (eb-E,). The crystals will have started their growth with an equal occupation of a and b sites, and once in position their ability to diffuse to a site of lower energy will not re-occur. REFERENCES 1 Lindemer T B, Washburn F A, MacDougall C S, Feenstra R, Cavin O B, Physica C178 93 (1991) 2 Arabi H et al, Physica C193 90 (1992) 3 Fischer P, Physica C196 105 (1992) 4 Tarascon J M e t al, Phys Rev B37 7458 (1988) 5 Jiang X, Wochner P, Moss S C, Zschack P, Phys Rev Lett 67 2167 (1991) 6 Friedman T A et al, Phys Rev B42 6217 (1990)
Physica B 194-196 (1994) 2271-2272 North-Holland
Growth of Ho-123 superconducting single crystals H Arabi, D Ciomartan, R H Fenn*, O S Mills~, J W Ross, and D J Sandiford Physics Department, Victoria University of Manchester, Manchester, M 13 9PL, UK *Physics Department, Portsmouth University, Portsmouth, PO1 2DZ, UK #Chemistry Department, Victoria University of Manchester, Manchester, M13 9PL, UK To grow single crystals of HoBa2Cu3074 we first used the citrate gel approach to provide finely divided and homogeneous starting materials and to develop a crystal growing regime in oxygen. Crystals grown on the surface of the melts with compositions 1:4:6 and 1:4:10, were up to 20rag in mass and superconducting with a Tc of 92K. More recently, we have grown crystals from mixtures of either simple or complex oxides of composition 1:111:25 and 1:12:26 as starting materials, under different temperature control so as to produce larger crystals at the bottom of the crucible. These crystals are up to 50mg in mass, with Tes of about 45K, which may be raised close to 90K by later annealing in oxygen. Crystals grown by the first method were always found to be tetragonal. We speculate on the reasons for this. 1. CONDITIONS FOR CRYSTAL GROWTH Figure 1 gives the Differential thermal analysis (DTA) and thermogravimetric analysis (TG) curves of a powder of Ho-123 phase in a typical flux in a cycle of heating and cooling, up to 1200°C. There are two obvious endothermic effects in the heating phase. The first, at 930°C is the melting point of the mixture, and the second at 1030°C corresponds to the decomposition of the 123 phase (incongruent melting). The two exothermic features starting at 940°C and 910°C are related to the crystallization of the 123 phase and the solidification of the flux. Comparable results have been reported for Y-1231.
I DTA~----~ ~
TG
~-
600 I
800 I
I
1000 I
I
I
Figure 1 DTA and TG for Ho-123 and flux A range of similar measurements at different initial compositions and for different temperature ranges, with varying compositions of cation ratios, and in argon as well as oxygen have been carried out. In addition, by quenching from high
temperature followed by XRD identification, we obtained information on the rate at which Ho2BaCuOs (the 211 phase) was formed above 980°C from the decomposition of the 123. It is preferable to work with the eutectic mixture of BaO and CuO (29:71 molar percent) to give the widest range of temperatures for the growth of the 123 crystals from 920°C (the eutectic melting point) to about 1030°C when 123 decomposes into 211, 011, and CuO. This is the temperature range where 123 crystals are grown. 1.1 Citrate gel method The citrate gel approach could give precursor materials which have the following advantages: stoiehiometric control, mixing at the ionic molecular level, control of the morphology of the particles by producing active and uniform micron-sized particles, a reduced chance of invasive impurities since no significant grinding/milling is necessary, and production of a uniform mixture. Single crystals of HoBa2Cu3074 have been obtained in a resultant flux of BaCuO2 and CuO, in an alumina crucible by the citrate gel method. Batches, rich in 123, with compositions of cation ratios (Ho:Ba:Cu) in the range of 1:4:6 to 1:4:10 have yielded the best crystals. Finally, the growth within a pure oxygen environment produces surface developed crystals with high concentrations of oxygen and hence high T¢ values. Our best-growth programme involved heating above the decomposition point of the 123 at 1020-1030°C, soaking on a dwell time to dissolve the 211 phase in the melted flux, then cooling at a rate of 0.3-
0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved S S D I 0921-4526(93)1768-H
2272 0.1°C/hr in flowing oxygen, down to 900°C. A cooling rate of 10°C/hr between 700-400°C, where a well-known tetragonal to orthorhombic transition occurs, was used. This process produced flat-surfaced, shiny crystals of good prismatic habit, which grew at the surface of our melt and were later extracted with a dental drill. They were always accompanied by acicular CuO and BaCuO 2 crystals. The crystals of 123 were typically of mass 5mg with a few as large as 20mg. These crystals were tetragonal and superconducting at 92K.
1.2 Oxide method In the earlier method it is difficult to control the near explosive decomposition of the citrates that occurs at about 240°C. The BaCO3 phase decomposes between 940°C and 1250°C ejecting CO:. In the melts used for single crystal growth, this gas produces small cavities, on which small crystals of 123 can develop. To reduce the number of crystallization centres, the 123:flux ratio in the batch was reduced and BaCO 3 was replaced first by BaO and then by 011. The batches, finely powdered, were put in boats of high density 99 % pure alumina. The furnace used had a temperature gradient, across the boat, of less than 1 °C. The heating rate up to 850°C was 120°C/hr. At this point 123 started to form together with 011 by a sintering process in the solid state. A rate of 50°C/hr was then set to 925°C when the batch melted A rate of 10°C/hr follows, up to 995°C so that 123 is complely formed and a final rate of 5°C/hr follows up to 1030°C so that all 123 decomposes to 211 and other components. A dwell time of three hours allows the 211 to dissolve in the liquid flux, while 012 decomposes, and in this way the viscosity of the melt was reduced. A cooling rate of 4°C/hr at 1030°C is followed by successively slower ones until at 930°C a 0.1°C/hr rate was set. In this range of temperature crystallisation occurs. This slow cooling rate allows a good development of the crystals, by the peritectic reaction in liquid 211 + 011 + CuO = 123. The process of controlled slow cooling stops at 925°C, and higher cooling rates are used down to room temperature. Decantation 3 revealed the crystals grown at the bottom of the crucible. An ultrasound micro-chisel was used to extract the desired crystals from the crucible. Yields of 25 % of crystals larger than 2mm were obtained. Many crystals were around 50mg. The crystals were grown away from the oxygen atmosphere, consequently their Tc was typically
found to be about 45K, which could be raised to 1.0
j ~
0.8 0.6 0.4 0.2
~
~0W ' rl
ted
0.0 i
0
2O
40 6O Temperature/K
i
80
100
Figure 2 Meissner effect at 20MHz 89K on annealing at 900°C in oxygen for 4 hours (see Figure 2), lower than the T c values obtained by the citrate gel method.
2 TETRAGONAL OR ORTHORHOMBIC? XRD has given evidence of orthorthombicity with
(b-a)/a=1% for Ho-123 crystals grown from the purest starting oxides, whereas the citrate gel method always gave tetragonal crystals. Tetragonal crystals were grown from oxides with AI impurities as is also found for Y-1234'5. It is generally accepted that pure Y-123 is orthorhombic, with (b-a)/a of some 2%. Indeed, some properties in the a and b directions show considerable anisotropy ~ which is thought to reflect the preferential occupation by oxygen of sites along the b axis, ie the binding energy of O atoms on the b sites (~b) is greater than at the a sites (~). Tetragonal Ho-123 crystals imply that oxygen is distributed randomly in the a-b plane. The smaller value of (b-a)/a in Ho-123 we ascribe to a smaller value of (E:e~). Our crystal growth via citrate gel in oxygen nucleated 123 crystals at temperatures such that it is probable that k T > (eb-E,). The crystals will have started their growth with an equal occupation of a and b sites, and once in position their ability to diffuse to a site of lower energy will not re-occur. REFERENCES 1 Lindemer T B, Washburn F A, MacDougall C S, Feenstra R, Cavin O B, Physica C178 93 (1991) 2 Arabi H et al, Physica C193 90 (1992) 3 Fischer P, Physica C196 105 (1992) 4 Tarascon J M e t al, Phys Rev B37 7458 (1988) 5 Jiang X, Wochner P, Moss S C, Zschack P, Phys Rev Lett 67 2167 (1991) 6 Friedman T A et al, Phys Rev B42 6217 (1990)