DSpace at VNU: X-ray photoelectron study of Bi2Sr2CaCu2-xCoxO similar to 8

July 12, 1999 17:34 WSPC/140-IJMPB 0164 International Journal of Modern Physics B, Vol 13, No 13 (1999) 1655–1662 c World Scientific Publishing Company Int J Mod Phys B 1999.13:1655-1662 Downloaded from www.worldscientific.com by UNIVERSITY OF CALIFORNIA @ SAN DIEGO on 06/02/15 For personal use only X-RAY PHOTOELECTRON STUDY OF Bi2 Sr2 CaCu2−x Cox O∼8 N K MAN∗ , KAMLESH KUMARI† , S VENKATESH† , T D HIEN∗ , N K SINH‡ , N X PHUC§ and K B GARG† ∗ International Training Institute for Materials Science, Hanoi, Vietnam of Physics, University of Rajasthan, Jaipur 2-3, Vigyan Bhawan Jaipur 302004, India ‡ Cryogenic Laboratory, National University, Hanoi, Vietnam § Institute of Materials Science, Hanoi, Vietnam † Department Received 21 December 1998 An X-ray photoelectron spectroscopy study has been performed on well characterized Bi2 Sr2 CaCu2−x Cox O∼8 (x = 0, 0.02 and 0.1) samples There is a shift in the Sr binding energy with Co concentration, which is related to the change in Tc This relationship can be understood by the change of hole concentration in the CuO2 planes as a result of Cobalt doping The results of Bi 4f and Co 2p core level spectra are also discussed in detail Introduction The Bi based cuprate superconductors are represented by the general formula Bi2 Sr2 Can−1 Cun O(2n+4)+y , the n = being known as the 80 K (2212 phase) Immense efforts have been made to understand the basic mechanisms that govern the superconducting properties, by virtue of substitutional studies at different cationic sites.1 – The choice of BSCCO as a model superconductor is motivated by its relatively simple structure as both CuO2 planes in this compound are chemically equivalent unlike the YBCO superconductor If Cu is replaced by 3d elements in BSCCO, the substitution affects directly the CuO2 planes Extensive cationic substitutional studies are available for La2−x Srx CuO4 (LSCO), YBa2 Cu3 O7−δ (YBCO) and NdCe-Cu-O system.1 – In contrast, number of studies dealing with the substitution for Cu by 3d elements in BSCCO is smaller A widely observed effect with respect to the incorporation of the magnetic Fe and Co in it is the suppression of the Tc linearly with increasing impurity concentration, the minimum value of Tc being 57 K for 5% Fe doping6 and 45 K for 8% Co doping.7 Our own study on Co doped YBCO system has shown remarkable decrease in Tc and also changes in the microstructural properties with Co concentration.8 We report herein a systematic study of the changes in the core level XPS spectra of Bi2 Sr2 CaCu2−x Cox O∼8 samples with increasing Co concentration 1655 July 12, 1999 17:34 WSPC/140-IJMPB 1656 0164 N K Man et al Int J Mod Phys B 1999.13:1655-1662 Downloaded from www.worldscientific.com by UNIVERSITY OF CALIFORNIA @ SAN DIEGO on 06/02/15 For personal use only Experimental The samples of Bi2 Sr2 CaCu2−x Cox O∼8 (0 < x < 0.2) were prepared by solid state reaction method in a microprocessor controlled furnace (with temperature accuracy of ±1◦ C at 1000◦C) The starting materials were Bi2 O3 , SrCO3 , CaCO3 , CuO and CoO powders with the purity of 4N (> 99.99%) The powders were mixed well in absolute alcohol for one hour and calcined in air at 800◦C for 20 hours The reacted powder was then pulverized, pressed into pellets and sintered at 240◦ C for 100 hours The resistivity of the samples were measured by four probe method in the temperature range 20–300 K, while the near single phase nature was confirmed by powder X-ray diffraction with CuKα radiation Lattice parameters for the various samples were calculated from the recorded patterns with an accuracy of A dcal − dobs = 0.001 ˚ The core level photoemission studies were carried out using an X-ray photoelectron spectrometer (VG Microtech MT 300) The spectra were taken with nonmonochromatic AlKα radiation (1486.6 eV) Calibration of the spectrometer was done using the Ag 3d5/2 peak at 367.9 eV The total instrumental resolution in all the measurements was 0.8 eV Fresh surfaces of the samples needed for the measurements were obtained by repeated scraping of thin layers of the samples in the sample preparation chamber (∼ 10−7 torr) The measurements were made in the main chamber in a vacuum of 10−10 torr The samples showed negligible intensity for C 1s at a binding energy of 285 eV No shift due to charging was observed in any of the lines of different samples The spectra were analyzed using a least square fitting photoemission program known as Rainbow-PC which is especially suited to the analysis of Photoemission and Auger spectroscopy data Its main purpose is to fabricate a theoretical lineshape and adjust it to make it most closely resemble the data The theoretical lineshape is described by its width, energy and intensity and these parameters are adjusted automatically by the program until the error sum of squares between the theory and the data is minimized Reducing the error between the theory and the data is referred to as “optimization” Results and Discussion Figure shows the variation in Tc and the variation of the lattice constants (in inset) with Co doping Both the lattice parameters a and c decrease with increasing Co similar to the result of A Maeda et al.7 Doping Co for Cu decreases Tc with increasing Co concentration The trend of decrease of Tc is close to being linear Since Tc is decreasing on Co doping it may imply that these may be going at the Cu site thereby killing some of the itinerant holes Also, the ionic radii of Co and Cu are almost same thus strengthening the idea that Co may be going at the Cu site X-ray photoemission studies have been carried out on the core level electron states of cobalt, strontium and bismuth in the Bi2 Sr2 CaCu2−x Cox O∼8 system July 12, 1999 17:34 WSPC/140-IJMPB 0164 Int J Mod Phys B 1999.13:1655-1662 Downloaded from www.worldscientific.com by UNIVERSITY OF CALIFORNIA @ SAN DIEGO on 06/02/15 For personal use only X-Ray Photoelectron Study of Bi2 Sr2 CaCu2−x Cox O∼8 1657 Fig Variation of the transition temperature Tc and tetragonal lattice parameters a and c (in inset) as a function of Co doping for Bi2 Sr2 CaCu2−x Cox O∼8 (0 < x < 0.1) series Table Ratios of intensities of LBE and HBE of Sr 3d5/2 and Bi 4f7/2 for different concentrations of Co Sr 3d5/2 Bi 4f7/2 Co conc LBE (%) HBE (%) LBE/HBE LBE (%) HBE (%) LBE/HBE 0% 76.37 23.63 3.23 85.70 14.30 5.99 1% 70.60 29.40 2.40 86.96 13.04 6.66 5% 69.50 30.50 2.27 87.04 13.96 6.72 The deconvoluted spectra of Sr 3d, Bi 4f and the observed spectra of Co 2p are given in the Figs 2, and respectively 3.1 Sr 3d core level Figure shows the core level spectra of Sr 3d for various doping concentrations of Co The spectra show two doublets (Higher Binding Energy and Lower Binding Energy), each corresponding to the transitions from Sr 3d5/2 and Sr 3d3/2 separated by spin-orbit splitting of 1.8 eV The Lower Binding Energy (LBE) doublet arises from Sr atoms at the Sr site (between Bi-O and Cu-O layers) and the Higher Binding Energy (HBE) doublet, from Sr atoms at the Ca site (between adjacent Cu-O layers).9 The HBE and LBE doublets are separated by 1.2 eV The ratios of intensity of LBE peak and of the HBE peak relative to the total intensity for the samples are shown in Table We see that for pure sample Sr going at the Sr site is 76.37% of the total Sr and Sr going at Ca site is 23.63% of the total July 12, 1999 17:34 WSPC/140-IJMPB Int J Mod Phys B 1999.13:1655-1662 Downloaded from www.worldscientific.com by UNIVERSITY OF CALIFORNIA @ SAN DIEGO on 06/02/15 For personal use only 1658 0164 N K Man et al Fig Sr 3d XPS spectra of Bi2 Sr2 CaCu2−x Cox O∼8 for different Co doping concentrations Sr As Co concentration increases, the ratio of LBE component to HBE component decreases which implies that with Co doping more Sr is now going at the Ca site The two peaks due to spin orbit coupling are at ∼ 133 eV representing 3d5/2 and ∼ 135 eV representing 3d3/2 level These results show a shift of 0.12 eV in the binding energy of Sr 3d5/2 as determined from the change in energy of the LBE doublet The LBE contribution comes from the Sr sitting at the Sr site which is in the planes adjacent to the CuO2 planes and therefore may signify a change in the CuO2 planes The core level shifts of alkaline earth ions can be expressed by the sum of changes in (i) chemical potential (ii) the valence charges of the ions concerned July 12, 1999 17:34 WSPC/140-IJMPB 0164 1659 Int J Mod Phys B 1999.13:1655-1662 Downloaded from www.worldscientific.com by UNIVERSITY OF CALIFORNIA @ SAN DIEGO on 06/02/15 For personal use only X-Ray Photoelectron Study of Bi2 Sr2 CaCu2−x Cox O∼8 Fig Bi 4f7/2 XPS spectra of Bi2 Sr2 CaCu2−x Cox O∼8 for different Co doping concentrations (iii) the covalency (iv) the Madelung potential and (v) final state screening.10 According to Nagoshi et al.,11 the contribution from the chemical potential is very small and he rules out the effect of valence charges of ions,12,13 covalency and the final state screening.14 The only factor affecting the core level shifts is the Madelung potential and consequently the hole density It has been reported that Cu-O bonds contract with an increase of hole density in the CuO2 planes.15 The contraction of Cu-O bonds in turn lead to contraction of alkaline earth-oxygen bond and hence July 12, 1999 17:34 WSPC/140-IJMPB Int J Mod Phys B 1999.13:1655-1662 Downloaded from www.worldscientific.com by UNIVERSITY OF CALIFORNIA @ SAN DIEGO on 06/02/15 For personal use only 1660 0164 N K Man et al Fig Co 2p XPS spectra of Bi2 Sr2 CaCu2−x Cox O∼8 for different Co doping concentrations The inset in the figure shows Co 2p XPS spectra of CoO results to an increase of Madelung potential at the alkaline earth ions He also found a good relationship between binding energy and Tc which changes in opposite direction In our case, as we dope Co, Tc decreases, binding energy of Sr increases and lattice parameters decreases but the change in lattice parameter a is very small So, following Nagoshi et al., we can say that as we dope Co for Cu, hole density in CuO2 planes decreases which in turn increases the binding energy of Sr 3d5/2 and consequently affects the Tc adversely The only disagreement is the contraction of Cu-O or Co-O bonds The change in a is so small that it is unlikely to say that it has any effect on the hole density or the Tc However, doping Co is affecting the hole density in CuO2 planes by killing the itinerant holes in them So, its doping is decreasing the hole density and consequently affecting Tc and binding energy of Sr 3d5/2 core level 3.2 Bi 4f core level Figure shows the core level spectra of Bi 4f7/2 for various doping concentrations of Co The Lower Binding Energy (LBE) peak is at 158.5 eV and Higher Binding Energy (HBE) peak is at 159.7 eV The Lower Binding Energy (LBE) peak is due to Bi in the Bi-O sheet and the Higher Binding Energy (HBE) peak is due to Bi between the Bi-O and Cu-O sheets.9 No appreciable shift was observed in the July 12, 1999 17:34 WSPC/140-IJMPB 0164 X-Ray Photoelectron Study of Bi2 Sr2 CaCu2−x Cox O∼8 1661 Int J Mod Phys B 1999.13:1655-1662 Downloaded from www.worldscientific.com by UNIVERSITY OF CALIFORNIA @ SAN DIEGO on 06/02/15 For personal use only binding energy of Bi 4f7/2 level with respect to doping concentration The LBE peak at 158.5 eV confirms Bi to be at oxidation state of 3+ The ratios of intensity of LBE peak and of the BLE peak relative to the total intensity for the samples are shown in Table We see that for pure sample Bi going in the Bi-O layer is 85.7% of the total Bi and Bi going between Bi-O and Cu-O sheets is 14.3% of the total Bi With increasing Co concentration, the ratio of LBE component to HBE component increases which implies that with Co doping more Bi is going in the Bi-O sheets 3.3 Co 2p core level Figure shows the core level spectra of Co 2p for various doping concentrations of Co and core level spectra of CoO (in inset) for comparison The two spectra clearly exhibit two peaks, one arising from Co 2p3/2 at ∼ 780 eV and the other arising from Co 2p1/2 at ∼ 795 eV There is still a controversy on the question of valence of Co Since, the energy for both the peaks matches well, we can conclude that Co exists in 2+ valence state But we not observe a satellite structure in the spectra of 5% Co doped sample Hence, we can not claim Co to be in a valence state of 2+ on this account Conclusion The Bi2 Sr2 CaCu2−x Cox O∼δ (0 < x < 0.1) superconductor has been synthesized and characterized Doping Co for Cu results in decrease of Tc ensuring that Co is going at the Cu site The core level binding energies have been studied and discussed in detail Sr spectra confirms increase in binding energy and Sr fitting shows that Sr atoms becomes more ionic in character resulting in a decrease of Tc Bi spectra shows no shift in binding energy and confirms Bi to be in a valence state of 3+ References A V Narlikar, C V Narasimha Rao and S K Agarwal, in Studies of High Temp Superconductors I, ed A V Narlikar (Nova Science, New York, 1989), p 341 G Hilscher, P Rogl, E Gratz, H Muller and P Fischer, Physica C158, (1988); G Hilscher, S Pollinger, L H Greene, G W Hull and W R McKinnon, Physica C167, 472 (1990) M Tarascon and B G Bagley, in Chemistry of Superconducting Materials, ed T Vanderah (Noyes, Park Ridge, NJ, 1990) E Wang, J M Tarascon, L H Greene, G W Hull and W R McKinnon, Phys Rev B41, 6582 (1990) S Yamagata, K Adachi, M Onoda, H Fujishita, M Sera, Y Ando and M Sato, Solid State Comm 74, 177 (1990) K V R Rao, K B Garg, S K Agarwal, V P S Awana and A V Narlikar, Physica C192, 419 (1992) A Maeda, T Yabe, S Takebayashi, M Hase and K Uchinokura, Phys Rev B41, 4112 (1990) July 12, 1999 17:34 WSPC/140-IJMPB Int J Mod Phys B 1999.13:1655-1662 Downloaded from www.worldscientific.com by UNIVERSITY OF CALIFORNIA @ SAN DIEGO on 06/02/15 For personal use only 1662 0164 N K Man et al N L Saini, B R Sekhar, P Srivastava, K B Garg, P Filip, K Mazenec, J Rusek and P Kuirek, J Mat Sci and Eng B15, (1992) S Kohiki, T Wada, S Kawashima, H Takagi, S Uchida and S Tanaka, Phys Rev B38, 7051 (1988); S Kohiki, T Wada, S Kawashima, H Takagi, S Uchida and S Tanaka, Phys Rev B38, 8868 (1988) 10 M Cardona and L Ley, in Photoemission in Solids I, eds M Cardona and L Ley (Springer-Verlag, Berlin, 1978), p 60 11 M Nagoshi, Y Syono, M Tachiki and Y Fukuda, Phys Rev B51, 9352 (1995) 12 N Nucker et al., Phys Rev B39, 6619 (1989) 13 F J Himpsel et al., Phys Rev B38, 11946 (1988) 14 R P Vasquez et al., J Electron Spectrosc Relat Phenom 57, 317 (1991) 15 M.-H Wangbo et al., Physcia C158, 371 (1989)  ... energy of Bi 4f7/2 level with respect to doping concentration The LBE peak at 1 58. 5 eV confirms Bi to be at oxidation state of 3+ The ratios of intensity of LBE peak and of the BLE peak relative to. .. from Sr atoms at the Ca site (between adjacent Cu-O layers).9 The HBE and LBE doublets are separated by 1.2 eV The ratios of intensity of LBE peak and of the HBE peak relative to the total intensity... use only X-Ray Photoelectron Study of Bi2 Sr2 CaCu2−x Cox O 8 1657 Fig Variation of the transition temperature Tc and tetragonal lattice parameters a and c (in inset) as a function of Co doping