Mössbauer study of 129I in a high-Tc superconductor YBa2Cu3O6Ix

Physica C 171 (1990) 311-314 North-Holland

M/Sssbauer study of 1291 in a high-Tc superconductor YBa2Cu306Ix Yu.A. Ossipyan, O.V. Zharikov, A.M. Gromov, V.K. Kulakov, R.K. Nikolaev and N.S. Sidorov Institute of Solid State Physics, USSR Ac. Sci., 142432, Chernogolovka, USSR

Yu.S. Grushko, Yu.V. Ganzha, M.F. Kovalev and L.I. Molkanov Leningrad Institute of Nuclear Physics. USSR Ac. Sci., Leningrad, USSR

E.F. Makarov and A.T. Maylybaev Institute of Chemical Physics, USSR Ac. Sci., Leningrad, USSR

Received 16 February 1990 Revised manuscript received 11 September 1990

A MBssbauer study of valence states of iodine has been performed in lz9I-dopedsamples of YBa2Cu306.1 ceramics, demonstrating the superconducting transition with the onset at T¢o-~55 K. Iodine is shown to be mainly present in the form of iodide-ion; insignificant quantities of orthoperiodate-ion are also observed. The isomeric shift magnitude suggests that the iodide-ion charge ( QI) in this system does not exceed - 0.87 ( - 0.87 < Ql < 0 ).

It has been shown recently [ 1 - 3 ] that t r e a t m e n t of tetragonal nonsuperconducting ceramics YBazCu306 in halogen v a p o u r s recovers its superconducting properties. In the work [2] a n u m b e r o f important questions were posed, the answers to which are i m p o r t a n t for elucidating specific features o f the occurrence o f high-temperature superconductivity ( H T S C ) in the halogen-doped Y - B a - C u - O system. In this work we have a t t e m p t e d to d e t e r m i n e charge states o f iodine in this system using M 6 s s b a u e r spectroscopy methods. Samples o f tetragonal n o n s u p e r c o n d u c t i n g ceramics YBa2Cu306.1 were treated in 129I vapours by the technique described in [2,4]. Elementary iodine-129 ( T j / 2 = l . 7 × 1 0 7 , e n r i c h m e n t 94%) was p r e p a r e d by solid-phase o x i d a t i o n o f KI by potassium b i c h r o m a t e and purified by sublimation. After the synthesis the samples were boiled in CCI4 in order to remove the a d s o r b e d iodine. The chemical c o m p o s i t i o n o f a typical I - d o p e d sample corresponded to the formula YBa2Cu306.1Io.96 wherein the content o f 1291 was d e t e r m i n e d by a m e t h o d o f sulphide leaching, described in ref. [5]. It should be noted that mentioned quantity o f iodine (Io.96) is the

top estimate o f its possible content in the Y - B a - C u O - I crystal lattice, since the e m p l o y e d technique determines all the iodine available in the sample, including, for example, possible by-products o f reactions and a m o r p h o u s X-ray-nonsensitive forms. The crystal structure o f i o d i n a t e d samples, as p r o m p t e d by X-ray structural analyses data, corresponds to the orthorhombic phase with the parameters, close to the earlier o b t a i n e d ones [2,4], see fig. 1. N o impurities o f other phases were observed by X-ray methods. The measuring o f the magnetic susceptibility ZAC ( T ) o f 129I-doped samples d e m o n s t r a t e s the occurrence o f the superconducting transition with the onset at Tco-~ 55 K and the transition width ATc~ 15 K. These values also agree well with the earlier o b t a i n e d ones. The M6ssbauer spectra o f the transition 5 / 2 + ~ 7 / 2 + with the energy 27.7 keV in 129I were measured in a transmission geometry. The source and the sample were at T = 4 . 2 K. The sample was m o v e d at a constant acceleration. 5 M g O . ~29mTeO3 ( T1/2 = 3 3 d ) p r e p a r e d according to the technique, described in [ 6 ], was used as a source. The spectrometer was calibrated in real time with the metallic 57Fe spectrum. The spectra were fitted using a least squares tech-

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312

Yu.A. Ossipyan et al. / MOssbauer study

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nique with a sum o f quadrupole multiplets with a line shape described by the transmittance integral. Figure 2 demonstrates, as an example, the spectrum o f the i o d i n a t e d YB2Cu306.11096 sample. The obtained spectra are well described by the sum of subspectra, corresponding to two chemical forms

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of iodine. The spectral parameters of these forms are tabulated in table I. Figure 3 demonstrates the diagram of isomeric shifts for some iodine compounds and for two forms, obtained by us, which are designated 1 and 2. A comparison of the obtained values o f the isomeric shifts for forms 1 and 2 with known systematics of isomeric shifts of 1291compounds shows that the system studied involves the forms I - and IO6-5, corresponding to formal valences of iodine - 1 and + 7. It should be emphasized that the multiplet corresponding to elementary iodine is absent in the spectrum. The spectrum of elementary iodine is strongly split (e2qQ~2300 M H z ) and should be well resolved. The ratio of the areas covered by the spectra of the iodide-ion and the lOg-S-ion is approximately 16:1 at 4.2 K. The area beneath the spectrum for thin absorbent is proportional to nf(T)ao, where n is the number of resonance atoms per sample unit area, f ( T ) is the Debye-Waller factor, and ao is the absorption cross section. In order to determine the quantitative relationship o f the found iodine forms, one has to know their f ( T ) for the system in question. It can also be determined from M6ssbauer spectra. However, it has to be additionally studied.

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Fig. 2. The MSssbauer spectrum of YBa2Cu3061Io.96 sample. T= 4.2 K. Form 1: iodide-ion; form 2: orthoperiodate-ion.

Yu.A. Ossipyan et al. / MOssbauer study of J29Iin YBa;Cuj061x

Table I The parameters of the M6ssbauer spectrum of Y-Ba-Cu-O, doped with iodine. T,l (arb. units ) T~2(arb. units) 61 (mm/s) 62 (mm/s) e2qQ2 (MHz) D (mm/s) M(%)

94.4% 5.6% 0.19 (1) -2.74 (5) 52 (3) 1.07 (2) 0.3 (1)

T.: effectivethickness of the sample. & isomeric shift. e2qQ:constant of the quadrupole splitting. D: experimental line width. M: misfit criterion. Indices 1 and 2 stand for the correspondingspectral forms of iodine (see fig. 2).

-3-

--- N a 3 H 2 1 0 6 --\

Form 2

-2- . . . . K I 0 4

Nevertheless one can use the known values off4.2 K for alkali iodides and Na5IO6 for the estimation. For all alkali iodides f4.2K~0.58-0.64 [7], and for NaslO6f4.2 K~0.9 [8]. Allowing for this fact the ratio of the quantity of iodide to that of IOg-5 becomes 24: 1. So, it is obvious that the IOg-5-ion is present in very small quantities. It should also be noted that the f ( T ) dependence for iodide in the Y - B a - C u - O I system may differ noticeably from the case of individual alkali iodides, and the character of this dependence yields the information on the dynamics of the sublattice, comprising iodine. The population of the valence electronic shell of the atom, dependent on its chemical bond in the given compound, determines an isomeric shift, measured in the M6ssbauer experiment. So, from the isomeric shift magnitude one may judge about the electronic valence configuration and the ion charge. A free iodine atom has a valence shell 5s2p 5, i.e. one 5p-electron smaller than the filled shell of Xe 5s2p 6. The systematics of isomeric shifts in J29I compounds and the Hartree-Fock atomic calculations give the relationship between the value of the isomeric shift and the number of vacancies (holes) in the valence shell of iodine against the Xe shell [9]: 8 = - ( 9 + 1)hs + (1.5+0.1)hp,

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/ .... l~-s iodide of alkaline metals . . . . Form 1

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imi3 12

Ba (I03 )2

313

(1)

where ~ is an isomeric shift in m m / s with respect to the iodide-ion with the shell 5s2p 6, and h~ and hp stand for the number of vacancies in 5s- and 5p-shells of iodine with respect to 5s2p 6. Evidently, the iodine charge is determined by the expression QI = ( h ~ + h p ) -

1.

(2)

By combining (1) and (2), we obtain the dependence of the charge on hp: h0 = 8 / 1 0 . 5 + ( 6 / 7 ) ( Q , + 1 ) .

(3)

_

.... KI03

_

(ram/s) Fig. 3. Diagram of isomeric shifts for some iodine compounds and for two chemical forms of iodine found by us in Y-Ba-CuO-I. Form 1: iodide-ion;form 2: orthoperiodate-ion.

It is seen that for a given isomeric shift value the charge of iodine may be different depending on the values of hp and hs, connected with hp via eq. (1). So, in order to estimate the iodine charge from the isomeric shift magnitude one has to make an assumption about the value of the sp-hybridization of iodine valence electrons. Nevertheless, some limitations on the iodine charge may be obtained from the condition, that hs and hp may assume only pos-

314

Yu.A. Ossipyan et aL /M6ssbauer study 0H291 in YBa2Cu3061x

itive value #. Putting hs> 0 and substituting the value 6 = 0 . 1 9 into ( 1 ) , we obtain the l i m i t a t i o n on hp>0.13, and from the eq. ( 2 ) the value 0>QI>__-0.87. Note, that the charge value is very sensitive to the degree o f the sp-hybridization. So, hs= 0.05 for the given value 6 = 0.19 requires hp = 0.45 and then Q~ ~_ - 0.5. As the iodide-ion charge changes from - 1 to 0, its d i m e n s i o n s have to decrease from the ionic radius (2.16 A ) to the covalent radius ( 1.33 ) [ 10 ]. So, for example, a linear interpolation for the case Q~= - 0 . 5 yields the ion radius 1.74 ~. The experimental a b s o r p t i o n line o f the i o d i d e - i o n is somewhat b r o a d e n e d . However, the o b t a i n e d data do not make it possible to reliably see whether the cause of b r o a d e n i n g is n o n h o m o g e n e i t y o f the nearest surroundings o f the iodide-ion, that leads to distribution with respect to isomeric shifts or, else, it is the unresolved q u a d r u p o l e splitting o f the line. If the latter is assumed, then there exist two possible causes o f the appearance of the electric field gradient ( E F G ) on the ion nucleus: the gradient from charges on the nearest n e i g h b o u r h o o d in the lattice at the s y m m e t r y o f their arrangement lower than the tetrahedral one and the gradient, resultant from the difference o f distribution of 5p valence electrons o f iodine from the spheric one. The e s t i m a t i o n o f the lattice contribution to the E F G on the iodine nucleus in the 01 position in this lattice, believing the Sterneimer antishielding for I - is negligible, m a x i m a l l y gives the value of the q u a d r u p o l e splitting constant ~ 10 M H z with its e x p e r i m e n t a l value of 52 MHz. So, if the cause o f b r o a d e n i n g is splitting, then the latter is due to the covalence or electrical m u l t i p o l a r polarization effects, leading to a smaller p o p u l a t i o n o f the 5pz-orNegative hs and hp imply that the populations of 5s and 5p orbitals are greater than 2 and 6, respectively, in violation Pauli's rule.

bitals o f iodine as c o m p a r e d with 5px,y and decreasing the negative charge o f the iodide-ion and its dimensions. In conclusion we note that the m a i n result of our study is recognition o f the presence in the Y - B a - C u O - I system o f negatively charged iodine atoms, whose charge value is within - 0.87 < QI < 0, the fact being experimentally found from the isomeric shift value.

Acknowledgements The authors express their deep gratitude to G.V. Novikov, L.V. Sipavina a n d A.P. Shibaev for the p e r m i s s i o n to publish their X-ray experimental d a t a o f ~29I-doped samples.

References [ 1] Yu.A. Ossipyan, O.V. Zharikov, N.S. Sidorov, et al., JETF Lett. 48 (1988) 246. [2] Yu.A. Ossipyan, O.V. Zharikov, G.V. Novikov, et al., JETF Lett. 49 (1989) 73. [3] A.G. Klimenko, V.I. Kuznetsov, Ya.Ya. Medikov, et al., Superconductivity 2 ( 1989 ) 5 (in Russian). [4]Yu.A. Ossipyan, O.V. Zharikov, G.V. Novikov, et al., Physica C 159 (1989) 137. [5] Yu.S. Grushko, USSR Patent No. 417007 (26 October 1973). [ 6 ] A.J. Alexandrov, Yu.S. Grushko, E.F. Makarov, K.Y. Mishin and D.A.J. Baltrunas, U.S. Patent 4,004,970, (24 January 1977). [7 ] G.J. Kemerink, N. Ruvi and H. de Waard, J. Phys. C 19 (1986) 4897. [8] M. Pasternak, M. Van der Heyden and G. Langouche, J. Chem. Phys. 80 (1) (1 January 1984). [ 9 ] H. de Waard, M6ssbauer Effect Data Index, eds. J.G. Stevens and V.E. Stevens (Plenum, New York, 1975 ). [ 10] U Pauling and P. Pauling, Chemistry, Moscow (1978) p. 153-154 (in Russian).