Ba2(Cu, Fe)

PHYSICA

Physica C 205 ( 1993 ) 63-77 North-Holland

The complex distribution of iron in the (Y, Ca)Ba2(Cu, Fe)306+y cuprate A M~ssbauer

study

A. R y k o v , V. C a i g n a e r t , N . N g u y e n , A. M a i g n a n , E. S u a r d a n d B. R a v e a u Laboratoire CRISMA T CNRS URA 1318 ISMRA, Universite de Caen, 14050 Caen cedex, France

Received 23 September 1992 Revised manuscript received 28 October 1992

M6ssbauer studies with S7Fe in Yl.zCazBa2Cu3_xFexO6+rfor x=0.09 and 0.24, z=0.09 and 0.24, and y between 0 and 1, allowed, for fixed z and x values, different distributions of iron on the copper sites to be observed according to thermal treatment under different gaseous atmospheres (Ar, air, oxygen). Three types of coordination - a tetrahedral and two pyramidal - are proposed for Fe on Cu ( 1) sites; one of the pyramidal Cu ( 1) sites is compatible with the existence of small iron clusters (Fe209, Fe3Ol3, etc. ). The accomodation oftetrahedral coordination by a fraction of iron atoms induces the transition from orthorhombic to tetragonal symmetry with increasing x. The migration of iron from these sites into the Cu (2) layer restores the orthorhombic symmetry. An increase in the occupancy of Fe on the Cu (2) site leads to a sharp detrimental effect on the critical temperature T¢; this confirms the predominant role of the Cu (2) sites in superconductivity for the "123"-structure.

1. Introduction It is by now well known that the C u - O network plays a crucial role in processes which establish highT¢ superconductivity in YBa2Cu306+y. Substitution o f Fe for Cu in this material is o f special interest owing to the possibility o f elucidating the correspondence between modifications induced in the structure by substituting cations and the changes in superconducting properties. The parameters essential for superconductivity can be ascertained by carrying out M6ssbauer spectroscopy studies on local probe 57Fe. Recent studies o f high-Tc superconductors YBa2Cu3_xFexO6+y ( x = 0 . 0 - 0 . 5 , y = 0 . 9 - 1 . 2 ) have raised many interesting questions about the interrelations between Fe-substituent location in Cu sites, structure and superconducting transition temperature Tc. The Fe-cation distribution over the Cu-sublattice was observed to be affected by the thermal history o f the samples. Some specific sequences o f heat treatments were applied by several groups o f researchers independently; it was shown that they significantly m o d i f i e d the relative iron occupancy on the C u ( l ) sites o f the " C u - O chains" and on the

Cu (2) sites o f the p y r a m i d a l copper layers [ 1-4 ]. Two processes were supposed [ 2 ] to be essential in the redistribution o f iron cations over the Cu-sublattice o f YBa2Cq~_xFexO6 +y. Both the clustering o f Fe in Cu ( 1 ) planes and the migration o f iron from Cu( 1 ) to C u ( 2 ) sites were proposed to induce orthorhombicity after reducing heat treatment. It has been widely accepted that the presence and mutual arrangement o f substituted iron on Cu ( 1 ) sites influences the character o f oxygen ordering in the Cu(1 ) plane. However, only a migration o f Fe atoms from C u ( 1 ) to C u ( 2 ) sites was observed directly by M6ssbauer spectroscopy, since iron location in C u ( 2 ) sites is well defined in M6ssbauer spectra. Another way o f modifying the Fe a t o m distribution in the structure consists in the coupled substitution o f Cu by Fe and o f Y by Ca [5,6 ]. The neutron diffraction study o f these phases [ 5 ] has shown that the migration o f iron from Cu( 1 ) to C u ( 2 ) sites results in a d r a m a t i c decrease o f Tc in the oxygensaturated sample; moreover, this migration or iron has been consistently explained by a significant loss o f oxygen at the level fo Cu ( 1 ) sites during the in-

0921-4534/93/$06.00 © 1993 Elsevier Science Publishers B.V. All fights reserved.

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A. Rykov et al. / Complex distribution olt"e in (Y. ('a)Ba2(( 'u, P~9 ~06 ~

termediate thermal treatment in an argon flow. The different superconducting properties of the " C a Y FeCu"-phases with respect to the "Y-FeCu"-phases were explained by a different distribution of iron atoms by Smith et al. [6]. These authors propose a random distribution of iron atoms in Cu (2) sites in the "'CaY-FeCu" oxide, whereas some clustering of iron on those sites was proposed to occur in YBa2Cu3_xFexO7 [3]. However, the clustering of iron atoms or their random distribution was argued [2,3,6] only from observation of differences in structure or 7~. No direct assignment of M6ssbauer spectra features to clustered iron atoms was given. Moreover, two completely different hypotheses on iron coordinations in the Cu(1) site [2,3] give rise to close conclusions on clustering. Meanwhile, the study of the conditions of cluster formation and a detailed structural characterization of oxides could be the key to a correct interpretation of M6ssbauer spectroscopy on YBa2Cu3_xFe~O6+y. This paper reports on a M6ssbauer spectroscopy investigation of oxides Y~_zCa~BazCu3_xFexO6+,,. We have performed a series of thermal treatments to modify the distribution of iron at several x and : values and to examine its effect on the superconducting properties of the "123" substituted cuprates.

2. Experimental Three starting solid solutions Y~_ zCazBa2Cu3_xFexO6 + ~. (x=0.09, z=0.09; x=0.24, z=0.24; and x=0.09, z = 0 . 2 4 ) were prepared from Y203, BaCO3, CaCO3, CuO and Fe203 by heating in air at 950°C for 60 h with two intermediate grindings. Then the oxides were furnacecooled in air (cooling rate > 4 ° C / m i n ) down to 380°C and annealed in air at 380°C for 90 rain. These starting compounds were then treated in different ways in order to investigate the distribution of iron in the different sites, according to the thermal treatments. For the sake of convenience the samples were labelled according to the three following parameters of the sequence of the thermal treatments: gas atmosphere, temperature and method of cooling. We specify in square brackets the gas by corresponding symbol, while subscripts are used for notation of the method of cooling and temperature of treatment.

Furnace (fast) cooling, low cooling and quenching are denoted by subscripts "'fc", "'sc", "'q", respectively. The annealings at relatively low temperatures (380°C-400~C) are distinguished by subscript "LT". Eight protocols of thermal treatment used in the preparation of the samples studied here arc denoted: [Airr,,AirL-r] - initial air-treatment which produced partially oxygenated samples as described above: [OscOLv] - conventional method of preparation of oxygen-saturated superconducting samples in oxygen flow, involving both high-temperature treatment in 02 and low-temperature annealing in 02; in this work samples were first treated at 980°C in 02 for 24 h, then slow cooled ( v 1~'C/min) to 400°C and annealed at 400°C for 9 h; [Arsc] - annealing at 850"C in flowing Ar gas for 12 h and slow cooling (2°C/rain) down to room temperature; [Afro] - annealing at 850~C in flowing Ar gas for 12 h and fast cooling to room temperature in Ar with the rate 15~C/min: [Ars~OL-r] - treatment [ Ar~ ] followed by saturation in oxygen by annealing at 400°C in 02 gas for 10 h: [Ars~OH-ArLT] - treatment [ArscOH-] followed by partial desoxygenation in Ar at low temperature (400°C) during 8 h: the removal of Ay= -0.41 from oxygen-saturated [ArscOLT]-samples was perfectly controlled in this manipulation; [Airq] - quenching of starting compounds in to liquid N2 from 950°C in air (cooling rate > 10~ C/ min); [AirqOLt] - treatment [Airq] followed by oxygen saturation by anneal at 400°C in 02 gas. Room-temperature M6ssbauer spectra and fullprofile diffraction data were collected after each thermal treatment. The M6ssbauer measurements were carried out by using 50 mCi STCo in Rh as the y-ray source. Calibrations of the isomer shift and the hyperfine field for each source velocity were performed by using the spectra ofct-Fe foil at room temperature. Powder X-ray diffraction data were measured on high-resolution diffractometer SEIFERT (Cu Kay-radiation). No impurity phases were detected in the diffraction patterns. Precise determination of lattice parameters, interatomic distances and evaluation of oxygen content were performed by Rietveld refinements of X-ray diffraction patterns.

A. Rykov et al. / Complexdistribution of Fe in (Y, Ca)Ba2(Cu,Fe)306+y The oxygen content was determined by partial reducing of oxides in Ar/Hz gas mixture at 970°C. An agreement in the range Ay= + 15 between oxygen content evaluated by diffraction method and thermogravimetric data was observed. The superconducting transition temperature was determined from susceptibility data for oxygen-saturated samples in a magnetic field of 5 Oe. The measurements were performed using an AC Lake-Shore susceptometer.

3. Results

3.1. The "compensated" oxides of equal Ca-Fe substitution level Yl_ xCaxBa2Cu3_ ~Fe~O6+y (x=z=O.09; 0.24) 3.1.1. Oxygen-saturated samples [QcOLr],

[AirqOLd, [ArscO,M The oxygen content, obtained for the "compensated" ( x = z ) series of samples oxygenated at low temperatures, was measured to be close to 7.0 ( y = 1.00_+ 0.05) whatever the previous high-temperature treatments. The M6ssbauer spectra at room temperature of samples saturated in oxygen, referring to the treatments [O~¢OLT], [AirqOLT], [Ars~OLx] for x = z = 0 . 0 9 and x = z = 0 . 2 4 (fig. 1), were computer-fitted to four ( x = 0 . 0 9 ) or three ( x = 0.24) doublets corresponding to electric quadrupole splitting AEQ. They are labelled here as A, B, C and D as AEQ increases, i.e. close to the values 0.6, 1.0, 1.7 and 2.0 ram/s, respectively. The same four subspectra observed for a long time for YBazCu3_xFexO6+ywere identified to originate from Fe on the pyramidal Cu(2) site (doublet A) and from different coordinations of Fe on the Cu ( 1 ) site (doublets B, C, D) [7,8]. The evidence for A-subspectrum assignment to the Cu (2) site follows from the antiferromagnetism of Cu (2) planes in desoxygenated samples; it was observed simultaneously in neutron diffraction by Tranquada et al. [9 ] and in M6ssbauer spectroscopy by Pavlukhin et al. [7,8] and Nowik et al. [10]. No generally accepted assignments of B, C and D subspectra to defined oxygen coordinations of Fe on Cu ( 1 ) have yet been found; this will be attempted in the present analysis. The comparison of the spectra of oxygen-saturated

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samples shows that the relative intensity of the doublet A progressively increase while that of doublet D decreases in the series [Os¢OLx]:~[AirqOLv]~" [Ars¢OLT] (table 1). Thus, the occupancy of the Cu(2) site by Fe atoms increases at the expense of the population of Cu ( 1 ) sites. This tendency of iron to migrate from Cu ( 1 ) sites into Cu (2) sites as the oxygen fugacity decreases during the intermediate thermal treatment is easily explained by the inability of iron to adopt the two-fold coordination contrary to univalent copper. At the same time one observes (table 1 ) an increasing orthorhombicity of the lattice for the same series. The macroscopic lattice symmetry changes from the tetragonal one for [Os¢OLT] to the orthorhombic one for [Ars¢OLx] for both x = 0.09 and x = 0.24, whereas the intermediate members of the series [AirqOLT] exhibit an orthorhombic symmetry for x=0.09 and a tetragonal symmetry for x=0.24. As the short Cu( 1 ) - O chains are known to survive in the tetragonal phase [ 11 ], the changes in the symmetry for the series [ O~OLx ] =~ [AirqOLx ] ~ [ ArscOLx] can be attributed to a lengthening of the chains which themselves lead to an expansion of the orthorhombic domains. If one assumes that the linear C u ( 1 ) - O chains are disrupted by Fe atoms in tetrahedral coordination, then the increase of the tetrahedral iron content on the Cu ( 1 ) sites should be at the origin of the transition of the structure from orthorhombic to tetragonal. Therefore, one can consider the decrease of intensity of the D-subspectrum in the series [OscOLT] =:~ [AirqOL-r]~ [AFscOLT] to be due to the departure of Fe atoms from the Cu ( 1 ) site which are characterized by FeO4 tetrahedra (as inferred by lattice symmetry). The subspectrum B does not exhibit significant changes in the series. However, when compared with the same doublet of YBa2Cus_xFexO6+y [2,7], it shows a remarkable effect of Ca in the case of joint Ca-Fe substitution. In particular, for the conventional preparation [O~¢OLT], the introduction of a Ca content equal to that of Fe ( x = z = 0 . 0 9 - 0 . 2 4 ) , with respect to YBa/Cu3_xFexO7, results in a decrease of the intensity of the B doublet by about 10%, and an increase of the intensity of the D doublet by the same value. Since the phases without Ca are generally oxygen richer, i.e. "O7+6", the B doublet should be associated with an Fe coordination number ex-

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