Garg, K B

Kamlesh Kumari, L C

Venkatesh, S

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Indian J. Phys. 73A (6). 783-788 (1999)U P  A- an international journalA core level spectroscopic st«dy on RNI2B2 C ( R  = Y, Er) borocarbidesK am lesh Kumari, S Vcnkatesh'^, L  C Gupta* and K B GargDepaitment of Physics, University of Rajasthan, Jaipur*302 004. India■‘‘Nuclear Science Centre. P O. Box 10502, New Delhi. India*Tata Institute of Fundamental Research. Homi Bhabha Road. Mumbai, IndiaReceived 5 February 1999, accepted 6 July 1999Abstract : Polycrystailine samples of YN12B2C and magnetic ErNi2B2C were prepared by arc-melting technique with a 7  ^ ~15 K Core level spectroscopic studies have been performed in order to see the valence of Y. Er and Ni, The results show that Ni and Er exists in 2-f valence state and Y exists in 3+ valence state.Keywords : Borocarbides. spectroscopic studies, superconductivityPACSNos. : 74.10.. 75.50., 79.60.~iThe quartem ary borocarbide intem ietallic compounds RN12B 2C where R is a rare-earth elem ent exhibit a w ide variety o f physical properties. The structure of these compounds is body-centered tetragonal (space group 14/mmm) with alternating square planar layers of rare-earth carbides and corrugated Ni2B 2 sheets with a unit cell consisting of two formula units [1], their physical properties depending upon the R atom. However, their electronic structure is not 2D  but it is 3D like in case of the conventional superconductors [2,3]. They behave like typc-II superconductors with a small coherence length (-50-1  (X) A). Electronic structure calculations on YNi2B 2C show that the states near the Fermi Level (Ef) are dom inated m ainly by the Ni (3d) character and have a relatively high density of states at Ep and the superconductivity is believed to originate in the Ni2B2 layers [4]. The estimation o f and r ,  w ithin the rigid ion approxim ation indicates that the superconductivity in YNi2B 2C  can be w ell accounted for by the conventional phonon mechanism [4]. The presence o f  the 3d  transition m etal Ni and its layered crystal structure raise intngumg questions about the origin o f superconductivity in them. Hence the study of Ni core leve© 1999IA C S784 Kamlesh Kumari, S Venkatesh, L C Gupta and K B Gargassumes great importance. As is well known, introduction of ferromagnetic elements cause depression of , superconductors with ferromagnetic elements such as Fe, Co or Ni as one of the constituents have I o n v  T^.. In borocarbides also, 7,. has been found to decrease by partial substitution of other transition metals for Ni in Y N i2B 2C  [S*-?] and in ErN i2B 2C  [8]. Further, only a few nickel-containing binary superconductors are known in the literature and their T, are less than 5 K. Also, Ni being a Jahn-Teller ion and known to exhibit mixed valency, makes this study all the more interesting.Here, we report the results of our XPS study on the core levels of Y N i2B 2C  and ErN i2B 2C  polycrystalline samples.Materials were prepared by standard arc-melting techniques. Detailed method of preparation and characterization results have been published elsewhere [9]. The core-level photoemission studies were carried out on an X-ray photoelectron spectrometer (V G  Microtech M T  300). The spectra were taken with non-monochromatic but monoenergctic A1 K a  x-radiation. 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 0.80 cV. Fresh surfaces needed for measurements were obtained by mechanically scraping and sputtering the sample surface in U H V . No shift due to charging was observed in any of the core-level spectra recorded.The resistivity of the samples were measured and their superconducting transition temperature was found to be 15.7 K  for Y N i2B 2C  and 10.5 K  for ErN i2B 2C, for the latter superconductivity coexists with magnetism below 6.5 K  -6 .5  K).The binding energies of the core levels for the two samples Y N i2B 2C  and ErN i2B 2C  are measured and analyzed. The spectia of B l5 and Ni 2p  for both the samples, Y  3d for Y N i2B 2C  and Er 4 d  for ErN i2B 2C  arc shown in the Figures (1 -4 ) respectively.C  Js  co re  l e v e l :After deconvolution of the C  I s  core level spectra, we observed three peaks at energies -2 8 5  eV, -2 8 6  eV, -2 8 7  eV in Y N i2B 2C  and -2 8 5  cV, -28 6  eV, -2 8 9  eV in E rN i2B 2C. The peak at -2 8 5  eV  arises from compounds having predominantly C - C  and C -H  type bonds, the other peaks at -2 8 6  eV and -2 8 7  eV arise from compounds having C - 0  type bonds and the peak at -28 9  eV  arises from compounds having C = 0  type bonds.These peaks are likely to be from the carbon impurity on the surface o f the sample. It is very difficult to remove carbon impurities from the sam ple completely by scraping. However, this can be done by repeated sputtering by Argon ions but since carbon is there in the la ttice  itself, repeated  sputtering changes the stoichiom etry o f  the sam ple and consequently may also affect its properties. Therefore, we are not reporting the carbon spectra in our results.A core level spectroscopic study etc 785B Is core leve l:The m ain  B I s  core level peak is found to b« at 192.7 eV and 192.4 eV for YNijBzC and ErN i2B 2C  respectively (Figure 1). The energy corresponding to pure Boron for B IsFigure 1. XPS spectra of B 1j  for YN12B2C and ErNi2B2C.level is 187.3 eV. The observed energy of B core level appears to also lie close to B core level binding energy in B2O 3 which is -192 -193  eV. Hence, we can infer that this peak may also have contribution from the boron bonded strongly to the surface oxygen. The observed increase in binding energy or strengthening o f bonds of boron is apparent from the chem istry  o f  boron. B ecause o f incom plete octet, repulsions between non-bonding electrons may be som ew hat less than normal, permitting closer approach of bonded atoms. This m ay also  be due to B—O bonds, which have high bond energies because of large electronegativity differences./Ni 2p core level :The background subtracted spectra for YNi2B2C show that the Ni 2p core level binding energy for N i 2 p ^  peak occurs a t 852.9 eV and for Ni 2pM  at 870.35 eV. We have then deconvoluted the spectrum  by creating a window from 848 to 860 eV to hold Ni 2pyz peak w ith its associated satellite structure (at 855.96 eV). After deconvolution, the Ni 2p^ri 73Af6UO786 Kamlesh Kumari, S Venkatesh, L C Gupta and K B Gargpeak is found to be at 8S2.89 eV with its satellite peak at 8SS.96 eV. W hen the d-orbitals are involved in the bond the XPS spectrum shows a structured satellite having bindingFigure 2. XPS spectra of Ni ^  (Ni ^ 3/2 with, its satellite stnicture) for YNijBjCandErNiaBjC.f f l im a . XPSipectraof Y 3dfor YN12B2C (nwdsta).energy higher than the main line [ 10]. Band structure calculations on LuNi^B2C by Pickett et al [2] and on YN ijBzC by Lee et al [4] also suggest strong hybridization between Ni 3d  and B 2p orbital, giving a strong indication o f  its three dimensional character, strongly suggesting Ni to  be in m etallic state. Also, the distance between N i-N i atoms is 2.49 A close to that o f fee  N i metal (2.50 A) K] sugges^ng that Ni is divalent in YNiiB 2C.A core level spectroscopic study etc170 Entrgy fiV)Figure 4. XPS spectra of Er Ad for ErNi2B2C (raw data).Sim ilarly, for E rN i2B 2C, Ni Ip^a peak occurs at 852.8 eV and Ni 2p i/2 occurs at 870.2 eV  and after deconvolution, the Ni 2p^fj found to be at 852.8 eV with itssatellite peak at 855.5 eV. The binding energy clearly corresponds to that of Ni^ '*^  oxidation state. This again shows that Ni exists in its metallic stale only and hence is divalent and does not show any sign of mixed valency.Y 3d and Er 4d core levels :The binding energy o f Y 3d i^2 peak occurs at 158.6 eV which matches well with the binding energy o f  Y 3d5/2 in Y2O3 indicating that the valence of Y is 3+. This is in agreem ent with the work published by Gupta [11J. According to him, all the rare earth ions exist in trivalent state except Ce in these systems.Figure 4 shows the binding energy peak of the XPS spectra of Er 4d. The value of Er 4^ /572 level is 169.4 eV and agrees well with the binding energy of Er Adsn level for elem ental Er. This indicates that Er exists in 2+ state or nearly 2-f state which is not in agreem ent with G upta [11].H ow ever, in both the compounds measurement of Y  Lm and Er Lm edge X A N ES  spectra is likely to be more conclusive on the question of valence state of Y and Er.The core level binding energies have been studied for the quartemary borocarbide samples ErNi^BjC and confinns Y and  Er to be in a valence state o f  3+ and 2+ respectively. The Ni s p e c n  abo# l l» t  Ni slays in 2+ state in both the samples. We can788 Kamlesh Kumari, S Venkatesh, L C Gupta and K B Gargunderstand the electronic structure o f quartem ary borocarbides in detkil by studying the valence o f other rare earths in such systems.AcknowledgmentKBG is thankful to CSIR for Emeritus Scientist fellowship. Also, KK is thankful to UGC, New Delhi for the JRF fellowship.References[IJ T Siegrist, H W Zandgergen, R J Cava, J J Krajewski and W F Peck (Jr.) Nature (London) 367 254 (1994)[2] Warren E Pickett and David J Singh Phys. Rev. Utts. 72 3702 (1994)[3] R Cochconn Physica C228 331 (1994)[4] J I  Lee, T S Zhao. IG  Kim, B I Min and S J Youn Phys, Rev. B50 4030 (1994)[5] L C Gupta et al Physica C235-240 ISO (1994)[6] S L Bud'ko et al Physica C243 183(1995)[7] A K Gangopadhyay et al Physica C246 317(1995)18] P Bonvcile, J A Hodges, C VaaKt. E Alleno, C Godart, L C Gupta, Z Hossain and R Nagarajan Physica C223&224 72(1996)[9] R Nagarajan, Chandan Mazumdar, Zakir Hossain, S K Dhar, K V Gopalakrishnan. L C Gupta, C Godart, B D Padalia and R Vijayaraghvan Phys. Rev. Letts, 72 274 (1994)[10] S Venkatesh, P Snvastava, N L Saini. K B Garg and G L Dwivcdi Phys Stat. Sot (a) 158 545(1996)[11] L C Gupta Physica B223 *  224 56 (1996)

A Core Level Spectroscopic Study on RNi2B2C (R = Y, Er) Borocarbides

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A Core Level Spectroscopic Study on RNi2B2C (R = Y, Er) Borocarbides

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