The influence of oxygen on the physical properties of the superconducting series Bi2.1(CaχSr1−χ)n+1Cun+4+δ

Physica C 156 (1988) 629-634 North-Holland, Amsterdam

THE INFLUENCE OF OXYGEN ON THE PHYSICAL PROPERTIES OF THE S U P E R C O N D U C T I N G S E R I E S Bi2.1(CaxSrl-x)n+lCumO2n+4+6 R.G. B U C K L E Y , J.L. T A L L O N , I.W.M. B R O W N *, M.R. P R E S L A N D , N.E. F L O W E R , P.W. G I L B E R D , M. B O W D E N * a n d N.B. M I L E S T O N E * Physics and Engineering Laboratory, Department of Scientific and lndustrial Research, P.O. Box 31313, Lower Hutt, New Zealand Received 22 August 1988 Revised manuscript received 22 September 1988

The sensitivity of Tc to oxygen sorption and the Ca to Sr ratio has been studied for the homologous series Bi2L(Ca,Sr~ _,),,+ ~Cu,O~÷4+~ for n= l, 2 and 3. The n=0 and n = ~ members have been produced as single phase samples but are found to be semiconducting. While single phase n = 2 material has been prepared, the n = l member always displays a small n = 2 contribution to the X-ray diffraction pattern, n = 3 is only found in multiphase samples. Single phase superconducting samples were equilibrated under a fixed partial pressure of oxygen at temperatures between 300 and 850°C and quenched in liquid nitrogen. It is shown that T¢ can be controlled in a reversible and systematic way by varying the oxygen partial pressure and the annealing temperature and consequently the oxygen stoichiometry. The site energy per oxygen sorbed for n=2 is 3.24 eV, only a little less than that for YBa2Cu3OT. However, the volume change on sorption is ten times smaller than that for YBa2CU3OT,and is manifested as a changing c-axis with the a-axis invariant. This can be interpreted as implying that oxygen sorption occurs within the Bi-O layers. T¢ is found to be maximised for a Ca to Sr ratio of 1 to 2 for both n= 1 and 2. Thermal expansion data for n=2 material is also reported to illustrate that oxygen sorption does affect the physical properties of these materials.

1. Introduction Since the first reports o f superconductivity in the B i - C a - S r - C u - O system [ 1 - 3 ] and its T1 analogue [4,5] it is now k n o w n that these systems form a n u m b e r o f distinct phases that constitute an homologous series [6,7]. The c o m p o s i t i o n o f the Bi materials can be described in the single formula [6 ] Bi2t(Ca~Sr,_x),+tCunO2,+4+~ ( n = l , 2, 3). The structure o f this series [ 3,7 ] has been explored but, so far, the d e p e n d e n c e o f the physical properties, in particular Tc, on oxygen stoichiometry is unquantifled and assumed to be weak while the influence o f the Ca to Sr ratio, to our knowledge, has not been reported. The d e p e n d e n c e o f Tc on these p a r a m e t e r s m u s t be u n d e r s t o o d before other physical properties are studied, a p r o b l e m that to date is related to the difficulties in obtaining single phase material. We have successfully p r e p a r e d single phase material for * Chemistry Division.

the n = 2 m e m b e r and report results on the m a x i m isation o f Tc with regard to ( a ) oxygen stoichiometry for three m e m b e r s ( n = 1, 2 a n d 3) a n d ( b ) the Ca to Sr ratio for n = 1 a n d 2. The n = 1 m e m b e r always displayed a small n = 2 c o n t r i b u t i o n a n d the n = 3 m e m b e r was found in multiphase samples only. The study has entailed equilibrating samples u n d e r a fixed oxygen partial pressure at t e m p e r a t u r e between 300 a n d 850°C and quenching into liquid nitrogen. The change in mass a n d the lattice cell p a r a m e t e r s were then m e a s u r e d as a function o f the annealing temperature. F o r the n = 2 m e m b e r the t h e r m a l expansion in b o t h air a n d nitrogen is rep o r t e d as an illustration o f the effect o f oxygen stoichiometry on the physical properties o f this superconductor.

2. Preparation and structure A variety o f p r e p a r a t i o n conditions were tried that in the end were o p t i m i s e d to p r o d u c e single phase

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o f o x y g e n on p r o p e r t i e s o f B i : i ( C a , S r l ,),, + ~Cu,,O:,, + ~ +,~

(n = 2 ) or near single phase (n = 1 ) samples starting from stoichiometric compositions. The main difficulty is that if the reaction occurs at or near the melting point of ~ 870°C, the n = 2 member dominates the products irrespective of starting compositions. An additional factor is that low calcium content tends to favour n = 1 while high calcium promotes n = 3. The x = 0 , n = l member (Bi2SrzCuO6) is readily prepared, although we were unable to prepare the x = 1 composition. Like others, we have found the n = 3 member impossible to prepare as a single phase, but the fraction can be increased when Pb is partially substituted for Bi or if Cu- and Ca-rich non-stoichiometric starting mixtures are used. O p t i m u m reaction temperatures for each o f these materials are summarised in table I. This system has a propensity to form syntactic intergrowths, notably from nonstoichiometric starting mixtures and, also, if the optimum reaction temperatures are not used. This is particularly true for the n = 3 member where small drops in the resistivity at 110 K can be shown by the lack of the appropriate diffraction peaks to be associated with syntactic intergrowths and not the presence of n = 3 crystals. In fact, we have identified single crystals of n = 3 material in only a few instances. X-ray diffraction patterns measured at room temperature for typical preparations are displayed in fig. 1. The n = 1 and n = 2 members can be indexed on tetragonal cells. The lattice parameters are found to depend on the Ca to Sr ratio and are summarised in table II. A member of multiphase samples displayed a sequence o f n = 3 00l peaks consistent with the expected spacing o f 37.2 A [6] although these were Table I Reaction t e m p e r a t u r e s and C a : S r ratios for p r e p a r i n g Bi21 ( S r , C a l _ ,),,+ iCu,,O2,,+4+,~ for n = 0 , 2, 3 a n d ~ . The ideal annealing temperatures for m a x i m u m Tc are also listed. The n = 0 and ~ e n d - m e m b e r s are not superconducting. Reaction temperature

Ca to Sr ratios

A n n e a l i n g temp. ( ° C ) for max. T~

(°c) 0 1 2 3

820 800 860-870 800 950

3:1 t o 0 : l l : l to 0 : 2 1:1 to 1:5 1:1 9:1

500 ~825 300-400 -

weak and often broad. Electron diffraction confirmed the formation of n = 3 crystals but compositions determined by energy-dispersive X-ray analysis ( E D X ) indicated in some cases the presence of intergrowths with n < 3 . Some predominantly n = 1 samples reacted at 800°C also exhibit a sequence of diffraction peaks consistent with a c-axis repeat distance o f approximately 19 A. This is the n = 0 member for which subsequent single phase specimens possessed microcrystals of micaceous morphology, a c-axis dimension contracting systematically with increasing Ca content and a basal plane with hexagonal rather than the square symmetry displayed by n > 0 members. The n = ~ member has been discussed by Siegrist et al. [ 8 ] and is not metallic. Neither the n = 0 nor the n = ~ member loads or unloads oxygen when annealed in air at temperatures between 500 and 800 °C. In summary, single phase or near single phase bulk specimens may be prepared for n = 0, 1, 2 and ~ but we have been able to prepare only a small fraction ( ~ 10%) o f n = 3 from stoichiometric starting mixtures. Impurity phases tend to be small quantities of the other members o f the homologous series.

3. Results and discussion

3.1. Effect of oxygen on T, Figure 2 shows the effect of annealing temperature on the resistivity o f the n = 2 and n = 3 members. We will see that, similar to YBa2Cu307_6 ( Y B C O ) , it is the oxygen stoichiometry which is controlled by these anneals although the sorption of oxygen is a factor o f ten less. Also, again in c o m m o n with YBCO, T, is sensitive to the oxygen content though to a lesser extent. The dependence o f Tc on annealing temperatures is summarised in fig. 3 for n = 1, 2 and 3, where measured Tc values are plotted against the temperature from which a sample was quenched into liquid nitrogen after annealing in air. The n = 2 member was also equilibrated in an atmosphere of 2% oxygen and 98% nitrogen at 1 atmosphere before quenching. The results for different Ca to Sr ratios are also recorded. The significant and reversible shift in the quenching curve to lower temperatures, upon reducing the oxygen content o f the annealing atmosphere by a factor

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of ten, demonstrates that it is oxygen stoichiometry which affects T~ and, in fact, for n = 2 an oxygen partial pressure less than that of air is required to maxTable 1I A summary of the lattice parameters as determined by X-ray diffraction for the n = 1 and 2 members and their dependence on Ca: Sr ratio. The n = 1 members were quenched into liquid nitrogen from 800°C in air and the n = 2 members were similarly quenched from 400°C. The n = 1 samples all displayed small components of other phases so that the compositions are uncertain. n

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imise To. The optimum annealing temperature varies for the different members of the series and depends on the Ca to Sr ratio. These ratios were checked by EDX on microcrystals within single phase material and were found to be close to the starting ratios. It is interesting to note that, in the absence of Ca, T~ for the n = 1 member is as low as 22 K [ 1 ]. Figure 3 also displays a plot of the maximum T~ measured against the number of Cu layers, i.e.n. 3.2. Effect of oxygen on structure For a n = 2 sample, the cell parameters and zero resistance Tc values were determined for different oxygen stoichiometries quenched in from different annealing temperatures and oxygen partial pressures. The a-axis is invariant and only the c-axis changes. Figure 4a shows its relationship to To. The point generated by annealing in an atmosphere of 2% oxygen lies on the same curve though it is obtained,

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of course, by quenching from a different annealing temperature. This clearly demonstrates that oxygen stoichiometry directly controls T~ and the c-axis lattice parameter. As in YBCO the lattice expands with oxygen unloading. The fractional change in oxygen content per formula unit, & can be estimated from the change for a sample quenched into liquid nitrogen from various temperatures. Although small ( < 1 mg), reproducible mass changes were observed if the samples were carefully dired after quenching. It has been previously observed [ 9 ] that the mass of the n = 2 member is sensitive to the annealing atmosphere; however, the explicit effect on T~ has not been reported. In fig. 4b the mass for n = 2 is plotted against the sample volume as determined by X-ray diffraction, with the slope indicating a volume change of 0.47 A3 per oxygen atom sorbed. This is approximately six times smaller than the value for YBCO which is 2.94/k3/ oxygen [10]. Assuming that oxygen sorption is a simple thermally activated process, the calculated binding activation energy from fig. 4c is 0.51 eV. On reducing to its constituent oxides a n = 2 sample that

Fig. 3. Plot of T< against the annealing temperature in air from which a sample was quenched into liquid nitrogen. ( X ) Bi2 ~Ca2SrzCu30,o a multiphase sample; ( O ) Bi2:CaSr~Cu2Og ( + ) Bi2 iCaosSr25CueOs; ( A ) Bie iCaisSrl 5Cu_.Os; ( • ) Bi2 ~CaSDCu2Os quenched from 2% oxygen; ( [] ) Bi_~,CaooTSr, 3,CuOo; ( ~ ) Bi2 ,CaSrCuO6. Inset: plot of the maximum Tc against n, the numer of CuO2 layers.

had been quenched from 800 ° C, the observed mass change indicated that 6 is 0.4. Another estimate for the oxygen binding energy may be obtained from the displacement to lower temperature of the curve of Tc versus quench temperature when the oxygen partial pressure is changed from 21% oxygen (air) to 2% oxygen. We assume that points on the two curves shown in fig. 3 which have the same value of Tc correspond to the same oxygen stoichiometry. For constant composition [10] p 112 e x p ( ~ / k T ) = c o n s t a n t ,

where p is the oxygen partial pressure and ~ is the binding energy per half an oxygen molecule. Let T2, and T2 be the quench temperatures for the same T, at the two oxygen partial pressures respectively. Thus (21/2) ~/2 exp{~ (T£,' - T2-' ) / k } = 1

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and a linear plot of T~-lj versus T~- ' yields the value c = 0 . 6 6 eV from the intercept. Given that the dissociation energy for an O2 molecule is 5.16 eV [ 11 ], the site energy per oxygen is 3.24 eV. This may be compared with the value of 3,73 eV for YBCO [ 10 ]. It is notable that while the site energy for oxygen in

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634

R.G. B u c k l e y et al. / Influence o f oxygen on properties o f Bi : ~(Ca,Sr~

to allow for the control o f the a m b i e n t gas [ 14]. Samples were air-quenched from the sintering temperatures o f 860 °C so that initially they were oxygen deficient. The first run in nitrogen confirms this as it displays the m o n o t o n i c rise in t h e r m a l expansion expected for a simple lattice expansion c o n t r i b u t i o n and does not show any changes due to oxygen unloading. On reheating in air there are three temperature ranges where the thermal expansion deviates from the lattice part a n d indicates that oxygen loading occurs at ~ 600 K a n d ~ 800 K, a n d unloading above 1000 K. On a second scan in air (after slow cooling in air) the t h e r m a l expansion follows the lattice c o n t r i b u t i o n until a p p r o x i m a t e l y 550 K when equilibrium unloading o f oxygen begins to contribute to the expansion. This interpretation is confirmed by the t h e r m a l expansion curve 6, also displayed in fig. 5, which is calculated using the lattice constants in fig. 4a. These d a t a are for a sample equilibrated in air then quenched into liquid nitrogen a n d which, lacking a t h e r m a l contribution, thus exhibits only the c o n t r i b u t i o n from the changing oxygen content. The points lie on a straight line that extrapolates to zero at about 550 K, confirming that appreciable oxygen d e s o r p t i o n does not begin until ~ 550 K. On repeating the nitrogen scan there is a further unloading peak at ~ 650 K due to non-equil i b r i u m oxygen content but the subsequent nitrogen scan displays the lattice c o n t r i b u t i o n only. These results then indicate that oxygen sorption does affect the physical properties o f these materials a n d is reversible in nature.

4. Conclusion In the present study we have observed that Tc for the Bi homologous series can be systematically varied by controlling the oxygen s t o i c h i o m e t r y via the oxygen partial pressure a n d the annealing t e m p e r a ture. The n o n - s u p e r c o n d u c t i n g e n d - m e m b e r s do not

,),, + ~Cu,,O:,, + 4 +,~

load or unload oxygen. Further, it has been shown that the second variable in this system, the Ca to Sr ratio, is i m p o r t a n t in d e t e r m i n i n g the m a x i m u m Tc a n d the o p t i m u m annealing temperature. In comparison with YBCO, the changes in To, the quantity o f oxygen uptake and the v o l u m e change for oxygen sorption are very much smaller. However, the physical properties o f this system are controllable by changing the oxygen stoichiometry while its lower sensitivity to oxygen m a y well prove to be an advantage in processing.

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