Superconducting cuprates of the series Bi,Ca, ~,Ln,Sr,Cu,O, + (Ln = rare earth or V)

Transcription

1 Supercond. Sci. Technol. 3 (1990) Printed in the UK Superconducting cuprates of the series Bi,Ca, ~,Ln,Sr,Cu,O, + (Ln = rare earth or V) C N R RaotS, R Nagarajant, R Vljayaraghavant, N Y Vasanthacharyat, G V Kulkarnlt, G Ranga Raot, A M Umarji9, P Somasundaram9, G N Subbanna9, A R RajuP, A K Sood and N Chandrabhas t Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore , India Materials Research Centre, Indian Institute of Science, Bangalore , India 11 Department of Physics, Indian Institute of Science, Bangalore , India Received 21 February 1990 Abstract. A systematic investigation of the properties of the series Bi,Ca, -xlnxsrzcuz08+d (Ln = rare earth or V) of superconducting cuprates has been carried out by employing a variety of techniques. In these cuprates, the a and b parameters of the unit cell increase with x while the c parameter decreases. These cuprates show interesting superlattice modulation with both 46 and 8b periodicity depending on the rare earth when x = 1.O. This observation suggests that the superlattice modulation has nothing to do with superconductivity, but may be determined by the oxygen stoichiometry. The oxygen excess, 6, increases linearly with x, but the hole concentration shows a maximum around x = 0.25 or 6 v & Interestingly, T, also shows a maximum around x = We have examined the nature of copper by XANES and Cu core level spectra. The relative intensity of the satellite in the Cu(2p) spectrum shows a minimum at x = The Raman band around 630 cm- due to the oxygen atoms in Bi-O layers decreases with increase in x, as does the a parameter. 1. introduction Superconducting bismuth cuprates of the general formula Bi,(Ca,Sr), + 1C~n02n + + have been investigated extensively [l-31. There has been considerable interest in recent months in studying the properties of bismuth cuprates wherein the divalent calcium ion is replaced by a trivalent rare earth or yttrium ion [4-S]. Such substitution would be expected to be accompanied by a change in the carrier concentration. We have carried out a detailed investigation of the structure and properties of several rare earth derivatives belonging to the Bi,Ca, -xlnxsrzcu20b+d series of cuprates where Ln = Pr, Nd, Dy, Y or Yb over a wide range of compositions. We have examined the structure by x-ray diffraction and high-resolution electron microscopy. Besides electrical resistivity and magnetic susceptibility measurements, our study includes measurements of thermopower, x-ray absorption near-edge spectra (XANES), non-resonant microwave absorption, laser Raman spectra and x-ray photoelectron spectra of these materials. The present study has clearly shown that superconductivity in the bismuth cuprates is not related 2 To whom correspondence should be addressed. to the superlattice modulation and that the latter is more likely to be determined by oxygen stoichiometry which also controls the superconductivity. Interestingly, T, in these series of cuprates attains a maximum value around x = 0.25 (or 6 = k 0.025) at which composition the hole concentration is also highest. 2. Experimental details Oxides of the formula Bi,Ca, -xln,sr,cu,o,+, (Ln = rare earth or Y) were prepared by heating appropriate stoichiometric quantities of the oxides and/or carbonates at K for extended periods in air and quenching them to room temperature. Annealing in oxygen or air is avoided to obtain the desired oxygen content in the samples. The oxygen content of the samples was estimated by Fe(I1)-Fe(II1) redox titrations. This enabled us to estimate the hole concentration. X-ray powder diffraction patterns of the cuprates were recorded with a JEOL JDX 8P diffractometer using Cu Ka radiation. Electron microscopy was /90/ $03.50 Q l90 IOP Publishing Ltd

2 Superconducting cuprates of the series Bi,Ca, -rlnrsr2c~z08+d carried out using a JEOL JEM-200CX electron microscope operating at 200 kv. Four-probe DC resistivity and DC magnetic susceptibility measurements were carried out to characterise the superconducting properties. Thermopower was measured on polycrystalline pellets of the characterised samples with respect to Cu as the reference. The contribution of Cu electrodes was subtracted to obtain the absolute thermopower of the samples. Non-resonant microwave absorption measurements were carried out with a Varian ESR spectrometer (9.1 GHz). X-ray absorption spectra were recorded with a Rigaku spectrometer fitted with a 12 kw rotating anode x-ray generator (Cu target) and a Ge (333) monochromatising crystal. X-ray photoelectron spectra were recorded with a VG-ESCAIII Mark 2 instrument. Raman spectra were recorded at room temperature on polycrystalline pellets in a reflection geometry (angle of incidence -70") using the 4880 A line of an Ar+ laser (~50 mw) and a Spex Ramalog spectrometer in the photon-counting mode. 3. Results and discussion All the members of the Bi,Ca,~,Ln,Sr,Cu,O,,, series (Ln = rare earth or Y, 0 x 1.0) possess the 2122 type layered structure of Bi,CaSr,Cu,O,,, with the Amaa space group. In figure 1, we show the powder x-ray diffraction patterns of a few compositions with Dy substitution to illustrate how they all possess the same structure. We could index all the reflections based on an orthorhombic type,unit cell although at low values of x, the a and b parameters are rather close to each other. In figure 2, we show the variation of the unit cell parameters with x in the Bi,Ca, -xlnxsr2cu208+6 series for different rare earth substitutions. We see that both the a and b parameters increase with the increase in x, while the c parameter decreases. The variation of the unit cell volume with composition is shown in figure 2(c). The change in unit cell volume with x depends on the rare earth. When lighter rare earths such as Pr and Nd substitute Ca, the unit cell volume increases with x; the heavier cations such as Y and Yb cause a slight decrease in the unit cell volume. In figure 3, we show the variation of the unit cell parameters with the radius of the rare earth ion. The a and c parameters as well as the unit cell volume increase as the size of the rare earth ion increases. One of the interesting features of these bismuth cuprates is the observation of incommensurate superlattice reflections in the electron diffraction patterns along one of the fundamental axes (b axis). The incommensurate superlattice spacings vary from 4.0b to 4.7b depending on the preparative conditions and the composition. When Bis partially replaced by Pb especiallyin the n = 3 member of the Bi,(Ca, ST),,+ 1Cu,,02n+4 series, a new superlattice modulation close to 8b has been observed [7]. Electron diffraction (SAED) patterns of plate-like crystals of Bi,Sr,YCu,O,+, oxide showed an l I I I l I L C U K ~ 28 Figure 1. Powder x-ray diffraction patterns of members of the Bi,Ca, -rdy,srzcu,o,,, system. approximately square array of strong subcell reflections corresponding to a lattice dimension of 2.7 A. This would correspond to 200 and 020 reflections with a and b unit cell dimension of ~5.4 A. A typical pattern recorded is shown in figure 4. We see both the subcell reflections and the incommensurate superlatticemodulated spots along the b axis. Furthermore, we see fine streaks and extra spots along the other direction as well. After an analysis of the superlattice spacings, we find both 4b and 8b type modulations along the b direction. A SAED pattern recorded from another orientation (301) is shown in figure qb). It also shows the 4b and 8b modulation periods. In Bi,Ca, -,Ln,Sr,Cu,08 +,, we could not see these extra 8b features when x = as observed for x = 1. The SAED patterns of Bi,Sr,DyCu,O,+, were similar to those for Bi,Sr,YCu,O,+,. When Ln = Nd or Pr, however, we see only 4b type modulation in the x = 1 compositions (figure qc)). Another interesting aspect is that there is no change in the morphology of the crystals with complete replacement of Ca by Ln, although the oxides do not exhibit superconductivity. Based on a single-crystal x-ray diffraction study, both 4b and 8b type modulations have been reported recently in Bi,Sr,GdCu,OB+, C81. The occurrence of such modulated structures involves waves of distortion perpendicular to the b axis. 243

3 there C N R Raoetal I a b + 0 Pr e Pr Yb O.u 5 44 e o 5.40k L l I I I I I Pr I I I I I I l m OQ + + I L OA 0 A l I I I I x Figure 2. Compositional variation of (a) the a and b parameters, (b) the c parameter and (c) the unit cell volume in the Bi,Ca, -xlnrsr2cu208+d(ln = Pr, Nd, Dy, Yb and Y) series. In the regular Bi cuprates with Ca, HREM studies [S] have shown the presence of distortion in the Bi-0 layers. From a single crystal x-ray diffraction analysis in the Amaa space group, Gao et al [lo] have reported large modulation amplitudes for Bi and Sr atoms upto 0.40 A along the c and the a axes, whereas for the Cu atoms, the displacement is only along the c axis (0.03 A). Sato and Tamegai [g] report small modulation ampli- tudes of all the metal atoms along the a axis and large amplitudes along the c axis in the case of Bi2Sr2GdC~ This analysis was carried out in Figure 3. Variation of the a and c parameters and of the unit cell volume with the radius of the Ln ion in Bi,Cao,,Ln,,,Sr,Cu,08+~ (Ln = Pr, Dy, Ho, Yb and V). Imcb and I2cb space groups. Zandbergen et al [l l] have proposed several models for the observed modulated structures in Bi cuprates. Concomitant with the replacement of + Ca2 ions + Dy3 is an increase in the oxygen content. For example, redox titrations indicate that when x = 0.5 the excess oxygen is 6 = 0.3. The ionic radii of + Y3 and + Dy3 are same as that of Ca2+ (1.06 A) while those of Pr3+ and Nd3+ are 1.16 A. It appears that while the modulation itself has nothing to do with superconductivity (since the x = 1 samples do not show superconductivity), the modulation seems to be associated with the incorporation of oxygen in the Bi-0 layers. This incorporation is likely to be periodic. Based on the redox titration results, we have obtained the oxygenexcess non-stoichiometry, 6, in a by + Y3 or few members of the Bi2Cal -xlnxsr2cu208+6 series. In figure 5(a), we have plotted x against 6 to show the proportionality between the two. We have calculated the number of moles of holes, nh, or the excess positive charge in the Cu-0 sheets per formula weight of the cuprate from the 6 values. In figure 5(b), we show the variation of the number of holes so calculated with the composition, x. We see that the hole concentration goes X 244

4 Superconducting cuprates of the series Bi,Ca, -xln,sr2cu208+d Figure 4. Selected area diffraction pattern (SAED) of (a) Bi,Sr,LnCu,O,+, where Ln = Y or Dy recorded along the (001) direction; (b) Bi,Sr,LnCu,O,+, (Ln = Y or Dy) recorded from another orientation (301); and (c) Bi,Sr,LnCu,O,+, where Ln = Pr or Nd, recorded along the (001) direction. through a maximum around x = 0.25 in all the systems studied. The 6 value at the maximum is k In figure 6 and 7 we show the temperature variation of normalised the resistance of Bi,Ca, -,Yb,Sr,Cu,O, +d, Bi,Ca, -,Nd,Sr,Cu,O, +d and Bi,Ca, -xdy,sr,cu,o, +d. We see that these cuprates exhibit superconductivity in the composition range 0 < x with the different rarearth substitutions. These systems also traverse a metalinsulator transition in the normal state with the change in composition. In the inset of figure qh) we have shown the magnetic susceptibility data of Bi,Ca, -xyxsr2cu208+d. Both the magnetic susceptibility and electrical resistivity data show that the temperature of onset of superconductivity varies with x, showing a broad maximum around x = The T, values decrease thereafter. We have also obtained the temperatures corresponding to the onset of superconductivity from non-resonant microwave absorption Figure 5. (a) Variation of the oxygen content, 6, with x in Bi,Ca, -,Ln,Sr2Cu208+a (Ln = Pr, Nd, Yb and V). (b) Variation of the hole concentration in the C M sheets with x in Bi,Ca, -xlnrsr2cu208+d(ln = Nd, Yb and V). Uncertainty in nh is indicated. Figure 6. Temperature variation of the normalised resistivity of Bi2Ca, -xyb,sr,cu,o,+,. The inset shows the DC magnetic susceptibility of the Bi,Ca, -xyxsr2cuz08+6 system. 245

5 C N R Rao et a/ 2.6 x=10 1 st h Flgure Q. (a) Variation of the superconducting transition temperature with x in Bi,Ca, -xlnrsr2cu208+d.(b) Plot of the superconducting transition temperature against the hole concentration in Cu-0 sheets. Uncertainty inn,, is indicated. Ill 1 l l T IKI Figure 7. Temperature variation of the normalised resistivity of Bi,Ca, -,Nd,SrzCuz08+d. The inset shows the plot of the normalised resistivity against temperature in the case of Bi,Ca, -xdyxsr,cuz08+g. measurements in a few members of the series (figure 8). The onset temperatures from these measurements show the same trend as those from resistivity and magnetic susceptibility measurements. In figure 9(4 we have plotted the variation of the onset temperature with x to SLOW how the T, goes through a maximum around x = As mentioned earlier, at this value of x, 6 is around It was at this composition that we found the maximum hole concentration ashown in figure 5(b). Thus, the present study establishes that T, is maximum when the hole concentration is maximum in the Bi,Ca, -xlnxsr2cu20s+6 series of cuprates. Accordingly, T, is sensitively proportional to hole concentration as indicated in figure 9(b). In figure 10 we have shown the thermopower data of a few Bi,Ca, -xlnxsr2cu208+d compositions. The parent Bi,CaSr2Cu,08+a (x = 0) shows a small negative thermopower from 300 K down to T,. With the T (K) substitution of Ca by Y or a rare earth, the thermopower becomes increasingly positive. Thus, when x = 0.1, the thermopower is slightly negative at 300 K and becomes positive around 260 K. The positive thermopower then goes to zero at T,. When x = 0.25, however, the thermopower is positive from room temperature down to T,. It is noteworthy that the hole concentration and T, are both maximum when x = We therefore surmise that the magnitude of positive thermopower (up to x = 0.25) reflects the change in hole concentration in these cuprates. The small negative thermopower found in the parent compound (x = 0) could, however, arise as a combined effect of entropic and quasiparticle contributions in the two-band system. The sign and temperature dependence of the thermopower in bismuth and thallium cuprates have been discussed by Rao et a1 [12]..X' x -010 IYI x =O 10 INdI 0 0 X =O.lO(Yb) * 6 x =o 25lYl A X=O2OlDyl b T IKi T IKI Figure 8. Non-resonant microwave absorption of Bi,Ca, -xlnxsr2cu208+d(ln = Yb and V). Figure 10. Temperature variation of the Seebeck coefficient of Bi,Ca, -xlnxsrzcuz08+d (Ln = Nd, Dy, Yb and V). 246

6 Superconducting cuprates of the series Bi,Ca, -,L~,S~,CU,O~+~ CUO x = 0.1 x 0.25 x = 0.50 x=075 cuzo c U0 x = 0.0 x= 0.1 x- 02 I x = 0.5 'CU20 I l l 0 l Energy (ev) Figure 11. Cu K absorption edges of (e) the Bi,Ca, ~,Y,Sr,Cu,O,+, system; (b) the Bi,Ca, -xprxsr2cu20s+6 system. Since superconductivity in the cuprates is related to the electron states of Cu and oxygen in the Cu-0 sheets, we sought we examine a few members of the Bi,Ca, -xlnxsrzcu208+d series by x-ray absorption spectroscopy and x-ray photoelectron spectroscopy. In figure 11 we show the XANES of Bi,Ca, -xlnxsr,cu,08+, (Ln = Pr and Y). The main peak position in the Cu K absorption spectra of the Bi cuprates is intermediate between that of Cu,O and CuO. The after-edge feature generally appearing as a shoulder around 21 ev in most of the cuprates becomes distinct in these bismuth cuprates, getting better defined with the increase in the rare earth concentration. We notice that there is some CU"in these bismuth cuprates as indicated by the presence of measurable intensity in the region of the pre-edge feature around 8993 ev [13] m! v l l I l I Romon shlft (cm") Figure 13. Raman bands of Bi,Ca, -xy,sr2cu20e+a. The Cu 2P3,, core-level spectra of these bismuth cuprates show the main feature (M) around 933 ev due to the d'ol" + d'ol-2 final states and a satellite (S) around 943 ev due to the d9l- final state. We see no feature due to Cu3+, The relative intensity of the 943 ev feature with respect to the main feature at 933 evis a measure of the proportion of the Cuz+ type states present in the cuprates. The ratio, ZdZM, in Bi,Ca, -xyxsr2cu208+b is generally smaller than in CuO which exhibits a value around 0.5. Interestingly, this ratio decreases slightly with the increase in x up to x = 0.25 and increases with a further increase in x (figure 12). This behaviour suggests that when the hole concentration is maximum the proportion of Cuz+ becomes minimum. The I$I, ratio seems to be inversely related to the hole concentration. We have examined the Raman spectra of Bi2Cal -,DyxSr,Cu,08+d and Bi,Ca, -xyxsrzcu208+d. In these cuprates the 631 cm-' mode (figure 13) is due to the oxygen atoms in the Bi-0 layers. We notice that as x in Bi,Ca, -xlnxsrzcu208+d(ln = Dy, Y) increases, this Raman mode frequency decreases as shown in figure 14. Webelieve that this is due to the l T X Figure 12. Variation of the relative intensity of the Cu(2p) satellite, Is//,.,, with composition. The inset shows the plot of is//,., against the hole concentration. 610 i, X Figure 14. Variation of the Raman frequency, W, with x in Bi,Ca, -rlnxsr2cu208+6(dy or V). 247

7 C N R Rao et a/ increase in a (b) parameter with increase in x. The increase in the frequency, Awlo, is larger than the increase in lattice parameter, Aala. It is also to be noted that decrease in W with increase in x is also accompanied by an oxygen excess in the cuprate (see figure 5). Acknowledgments The authors thank the Department of Science and Technology for support of this research. One of the authors (RV) thanks the CSIR for a Fellowship. References [l] Rao C N R and Raveau B 1989 Acc. Chem. Res [2] Ramakrishnan T V and Rao C N R 1989 J. Phys. Chem [3] Sleight A W 1988 Science [4] Manthiram A and Goodenough J B 1988 Appl. Phys. Lett [S] Ganguli A K, Nagarajan R, Nanjundaswamy K S and Rao C N R 1988 Mater. Res. Bull [S] Tarascon J M, Barboux P, Hull G W, Ramesh R, Greene L H, Girond M, Hegde M S and McKinnon W R 1989 Phys. Rev. B [7] Ramesh R, Van Tendaloo G, Thomas G, Green S M and Luo H 1988 Appl. Phys. Lett [S] Sat0 S and Tamegai T 1989 Japan. J. Appl. Phys. at press [S] Hewat E A, Dupuy M, Bordet P, Capponi J J, Chaillout C, Hodeau J L and Maeizio M 1988 Nature [lo] Gao Y, Lee P, Coppens P, Subramanian M A and Sleight A W 1988 Science [ 1 l] Zanderbergen H W, Groen W A, Mijlhoff F C, Van Tandeloo G and Amelinekx S 1988 Physica C [l21 Rao C N R, Ramakrishnan T V and Kumar N 1990 Physica C at press [l31 Vijayaraghavan R, Ganguli A K, Vasanthacharya N Y, Rajumon M K, Kulkarni G U, Sankar G, Sarma D D, Sood A K, Chandrabhas N and Rao C N R 1989 Supercond. Sci. Technol