Room temperature Mössbauer spectra of 57Fe-doped La2SrCu2O6, La2CaCu2O6 and La1.6Sr0.4CaCu2O6

PHYSICA

PhysicaC 181 (1991) 1-10 North-Holland

Room temperature M6ssbauer spectra of 57Fe-doped La2SrCu206, La2CaCu206 and Lal.6Sro.4CaCu206 Structuralimplications C. M e y e r a, F. H a r t m a n n - B o u t r o n

a, y . G r o s a a n d P. S t r o b e l b

a LaboratoiredeSpectrombtriePhysique, UniversitbJoseph-Fourier/Grenoble 1, B.P. 87, 38402Saint-Martind'HbresCbdex, France b Laboratoire de Cristallographie du C.N.R.S., B.P. 166X, 38042 Grenoble Cbdex, France

Received 24 June 199 l Revised manuscript received 30 July 1991

In order to elucidate the properties of iron in the pyramidal Cu (2) sites of YBa2Cu3Ov_y, 1% 57Fewas substituted to copper in the simple bilayer compounds La2SrCu2Or, La2CaCu206 and La~.6Sro.4CaCu206 (in their insulating forms). It appears that iron enters these systems as high-spin Fe3+. The room temperature Mrssbauer spectra consist of paramagnetic quadrupole doublets, and a magnetic hyperfine structure appears below about 20 K. At RT, two quadrupolar doublets with 131 ~ 1 mm/s and 131 ~ 0.3 mm/s are observed in variable proportions in all three compounds. They have been interpreted by structural considerations involving, first a small displacement of the iron with respect to the copper position inside the oxygen pyramid, and second the possibility of additional oxygen insertion into the 2b vacancy site, leading in some cases to an octahedral oxygen neighbourhood.

1. Introduction

A lot o f M r s s b a u e r studies have been carried out on 57Fe-doped YBa2Cu307_6 [1]. However the M r s s b a u e r spectra o f this c o m p o u n d are intricate and their interpretation is difficult because: - there are two c o p p e r sites Cu ( 1 ) and Cu ( 2 ) which can be occupied by iron; - when iron is substituted to Cu ( 1 ) (preferred site), it m a y have several oxygen neighbourhoods (from 2 to 6 oxygens) with p r o b a b l y various possible valence states. W h e n iron is not in the high-spin ( H S ) Fe 3+ state, the electric field gradient ( E F G ) is the sum o f a lattice contribution which can be c o m p u t e d by lattice s u m m a t i o n and o f a valence contribution which is difficult to estimate except for high spin states [lh]. On the other hand, superconductivity in YBa2Cu307_~ and several other c o m p o u n d s (e.g., 2212 Bi and T1 c o m p o u n d s ) seems to be mainly controlled by the presence o f a bilayer o f p y r a m i d a l copper sites, similar to the Cu ( 2 ) bilayer o f YBCO. F o r this reason, after looking at Y2BaCuO5 (green phase)

in which p y r a m i d s are isolated [2], and at YBaCuFeO5 in which the bilayer is asymmetrical [ 3 ], it seems interesting to study simple c o m p o u n d s with a symmetrical copper bilayer, such as La2SrCu20 6 [ 4 7] a n d Lal.9Cal.lCU206 [4,8] which belong to the structural family AEBCu20 6 (or " 2 1 2 6 " ) . After this work was started, it was found by C a v a et al. [ 9,10 ] that LaE_xSrxCaCu206 p r e p a r e d under m o d e r a t e l y high oxygen pressure (up to 20 atm. ) becomes a superconductor with T¢= 60 K for x - - 0 . 4 . This was c o n f i r m e d by Kinoshita et al. [ 1 l ] who found To~ 18 K for Lal.85(Cao.s6Sro.14)l.15Cu206.o3 p r e p a r e d under 8 atm. pressure. Subsequently superconductivity with Tc ~ 60 K was achieved again in La2_xSrxCaCu206+y by Liu et al. for x ~ 0 . 2 - 0 . 3 [12] and by Sakurai et al. for x = 0 . 2 [13,14]. Superconductivity with T c ~ 4 5 K was also observed by Fuertes et al. [15] in La2CaCu206.o37s (superconducting fraction ~ 1%) and by Kinoshita et al. [ 16 ] in Lal.75Cal.25Cu206+y (superconducting fraction ~ 12%), while Okai [ 17 ] obtains T~ ~ 70 K in Lal.7Cal.3Cu206+y (superconducting fraction ~0.5%).

0921-4534/91/$03.50 © 1991 Elsevier Science Publishers B.V. All fights reserved.

2

C Mo,er et al. / R T MOssbauer spectra qf "~kk'-doped .4 :B( 'u:O,

On the contrary, superconductivity has not yet been achieved in La2_ ~Ba,-CaCu20~, [18 ].

2. Structural characteristics

Our initial aim was to compare the quadrupole splittings J-= 7 e -,q O x ~ l l + jir I 2 of 57Fe substituted to copper in the pyramidal sites of various compounds. Indeed in YBa2Cu30,., the experimental quadrupole splitting of S7Fe in the M6ssbauer subspectrum attributed to HS Fe 3+ in the Cu(2) sites is much smaller than the computed value [ 1h ]. Possible reasons for this could be that iron is displaced from the copper position towards the apex of the pyramid [ l h ] , or that, if there is room enough, iron attracts an additional oxygen in the intermediate plane of the CuO2 bilayer, thus achieving an octahedral neighbourhood (in that case one could also think of an iron pair constituted by two iron atoms in two sheets on each side of the interstitial oxygen). However, what will emerge from the present work, is that the various compounds with pyramidal hilayers are not really comparable, because interatomic distances, which are controlled by the ionic radii of the components, may vary appreciably; this strongly influences the position of the iron in pyramidal sites, as well as the possibility of oxygen insertion leading to an octahedral neighbourhood. Let us recall the tetragonal crystallographic structure 14/mmm n°123 of La2SrCu206 [4-7] and Laj ~Ca I.l Cu206 [ 4,8 ], which is represented in fig. I. with its successive shifted bilayers. In this structure, site 2b midway between the copper atoms of the two CuO2 sheets is in principle empty as in Sr3Fe206 (while it is filled by oxygen in Sr3Fe207 and Sr3Ti2OT). As regards site 2a (A1), also in the intermediate plane of the bilayer, and site 4e (A2) in the apex plane, their fillings by lanthanum and by the alkaline earths are not the same in the two compounds, this being apparently related to the ionic radii of La, Sr and Ca, for which Pauling values are [19]: rc

0.99 A,

rv3+ =0.93 A ,

r L. . . .

1.15,~,

rsr2+ = 1.13 ,~ "

ru. . . .

1.35 A .

°,@ ?. ~ ~

• cu O tQ.A ©o

Fig. 1. C~stal structure of La~SrCu20~and ( La~ ,Sr~):CaCu2(I~.

These fillings are reported in table I, to which wc have added the results of recent studies of the superconducting compounds La2 ,Sr,CaCu20,,. According to the most recent data in this table, in La2SrCu206 site 2a ( A I ) is mostly filled with La 3+ ions, while site 4e (A2) contains about 50% La 3+ and 50% Sr 2+. On the contrary in all the other compounds, site 2a (A1) is mostly occupied by Ca ~+ ions and site 4e (A2) is mostly occupied by La 3+ ions. The strontium compound should therefore exhibit the largest amount of disorder (related to the 4e site filling) and the largest intersheet distance (because rLa3+ >> rca2+ ). Table 2 reports a number of interatomic distances in A2BCu206 compounds, as well as in YBa2Cu3Ov .,~. It appears that the bilayer (intersheet) C u - C u distance in La2SrCu206 (3.664 A) is appreciably larger than that in L a 2 C a C u 2 0 6 (3.30 A) which is comparable to those in YBa2Cu307 (3.38 A) and in YBazCu306 (3.27 A) as expected ( rca2+ ~ fv3+ ). The same is true for the Cu-2b distance which is half the Cu-Cu distance. On the other hand the bilayer (in-

C. Meyer et al. / R T M6ssbauer spectra of 5ZFe-doped A 2BCu206

Table l Site occupancies in La2SrCu206, La2CaCu206, (La,Sr) 2CaCu206

La2SICu206 NGuyen (1980) [4] Caignaert (1990) [6] Lightfoot (1990) [7]

La LgCaL~C u 2 0 6 NGuyen (1980) [4] lzumi (1990) [8] La2CaCu206 Fuertes (1990) [ 15 ] La L6Sro.4CaCu206 Cava (1990) [ 10] (condition: no Sr in 2a) LaLaSro.2CaCu206 Sakurai ( 1991 ) [ 13 ]

2a (Al) bilayer intermediate plane

4e (A2) apex plane

50% La, 50% Sr 84% La, 16% Sr 78% La, 22% Sr

75% La, 25% Sr 58% La, 42% Sr 53.5% La, 46.5% Sr

2a (AI)

4e (A2)

10% La, 90% Ca 12% La, 88% Ca

90% La, 10% Ca 89% La, 11% Ca

23% La, 77% Ca

88.5% La, 11.5% Ca

20% La, 80% Ca

70% La, 20% Sr, 10% Ca

14% La, 0% Sr, 86% Ca

83% La, 10% Sr, 7% Ca

Table 2 Interatomic distances in the compounds of interest and in YBa2Cu3OT_6 in/k (see also table 2 of the paper by Sakurai et al. [ 13 ] )

Cu-Cu (bilayer) O(1)-O(1) (bilayer) Cu-O( l ) plane Cu- (2b) vacancy (2b in middle of bilayer) Cu-O ( 1 ) O( 1 )-2b Cu-apical oxygen Apex oxy-apex oxy

La2SrCu206

Lal.6Sro.4CaCu206 +6

3.664 a~

3.393 ¢~

3.370 a)

3.2186 ¢)

0.147 ~)

0.0872 ~)

1.832 ")

1.6965 ¢)

1.938 a) 2.568 a~ 2.210 "~ 2.174 b~ 8.084 a)

1.913 ¢~ 2.500 ¢~ 2.32 insul c) 2.293 SC c)

Lal.9Cal.lCu206 f 3.306 d~ ~[ 3.304 c) f 3.191 a) ~ 3.198 ~) f 0.0575 a) 1 0.053 ¢) f 1.653 a~ 1.652 c) 1.913 a) 2.492 a~ I 2.306 a~ ~ 2.323 c~ [ 7.918 a~ l 7.952 o

YBa2Cu307

YBa2Cu306

3.381 ~

3.275 f)

2.853 ~) ( 0 2 - 0 3 plane) 0.264 "~ ( 0 2 - 0 3 plane) 1.6905 ~)

2.831 f~ ( 0 2 - 0 3 plane) 0.222 f~ ( 0 2 - 0 3 plane) 1.6375 f)

1.929-1.960 ") 2.4116 ~) 2.293 ~

1.948 r) 2.4079 f) 2.428 r~

7.966 e)

8.215 fJ

a) Ref. [6]. b~ Ref. [7]. c~ Ref. [ 10]. Note than for our own sample Lal.6Sro.4CaCu206 IIl, precise measurements lead to: a=3.837(2) A and c= 19.662(4)/k (in agreement with M.J. Rey's thesis, 1991 ) to be compared with: a = 3.8208 A and c= 19.5993 A obtained by Cava et al. d) Ref. [8]. e) Ref. [27]. f) Ref. [28]. t e r s h e e t ) o x y g e n - o x y g e n d i s t a n c e is l a r g e r i n A2BCu20 6 compounds (3.37-,3.19 A) than in YBaECU3OT_6 ( 2 . 8 5 3 - - . 2 . 8 3 1 / i ) , t h i s c o r r e s p o n d i n g t o t h e fact t h e C u O 2 s h e e t s a r e f l a t t e r (less c o r r u gated). Accordingly the distance u of the copper to t h e b a s i s o f t h e p y r a m i d s is s m a l l e r ( 0 . 1 4 7 ~ 0 . 0 5 7

A, i n s t e a d o f 0 . 2 6 4 ~ 0 . 2 2 2 A ) . F i n a l l y t h e C u - a p i c a l - o x y g e n d i s t a n c e is 2.2 A in L a 2 S r C u 2 0 6 , o f t h e o r d e r o f 2 . 3 0 A i n Lal.6Sro.4CaCu206, L a 2 C a C u 2 0 6 a n d Y B a 2 C u 3 0 7 , a n d 2.43 A i n Y B a 2 C u 3 0 6 ; t h e fact t h a t t h i s d i s t a n c e is a b o u t t h e s a m e in all s u p e r c o n d u c t i n g p y r a m i d s h a s b e e n n o t i c e d b y C a v a et al.

4

('. M e y e r el al. / R T Mdsshauer spectra e l ~~k~'-doped .t:B( 'u,O<,

Fable 3 Oxygen occupancies of sites 2b, 8g, 4e in %

La2SrCu20. Lal usSr, t 5('u2(). 25 La2CaCu20 4CaCu20~, .~ La~ 8Sr0.eCaCu20~

2b 0 ( 3 )

8g()( I )

(;aignacrt ( 1990 ) [6 ] Lightfoot ( 1990 ) [ 7 ] Fuertes ( 1990 ) [ 15 ] ('ava ( 1990 ) [ 10 ] Sakurai ( 1991 ) [ 13 ]

5.7% 28% 3.7% 3%

[ 10 ]. Note also that the total thickness of the bilayer is of the o r d e r o f 8 A, in all cases. Recent neutron studies of La2Sr('u20~. ka~ ssSr~ , 5 C u e 0 6 . 2 s . La2CaCu20¢,.o37 and La2 ,Sr,CaCu20~ a have shown some degree o f occupancy o f the 2b ( 0 3 ) site in the intermediate plane o f the bilayer. In strontium c o m p o u n d s , this is acc o m p a n i e d by vacancy f o r m a t i o n in the 8g (O( I ) ) sites o f the CuO2 sheets, which gives rise to oblique distorted pyramids (see ref. [6], fig. 3 ). On the other hand, in La2 ,Sr~CaCu20~ a, some oxygen deficiency has been observed in the 4e ( 0 ( 2 ) ) apices o f the pyramids. All these results are collected in table 3. In strontium c o m p o u n d s , the partial occupation of the 2b site (which increases with (5) is assumed to be favoured by the large ( C u - 2 b vacancy) distance (1.83 A, to be c o m p a r e d with 1.94 ei, for the CuO b o n d in the CuO2 plane) and to be probably responsible for the absence o f superconductivity in this c o m p o u n d . On the contrary in calcium c o m p o u n d s the small C u - 2 b distance (1.65 - 1 . 7 a,) is not favourable to oxygen insertion; if such insertion occurs, oxygen should preferably be located near the large La 3+ ions [15]. In all cases however, there is a problem with the distance 2 b - O ( 1 ) between the vacancy and the oxygens o f the CuOe plane: it is a r o u n d 2.4-2.5 ,& which is lower than twice the ionic radius o f O 2 : 2.70-2.80 A; when the vacancy is filled, this might induce some distortion o f its neighb o u r h o o d or favour the f o r m a t i o n o f oblique distorted p y r a m i d s as observed in strontium compounds.

3. P r o b l e m s a n d related to iron s u b s t i t u t i o n

What happens when iron is substituted to copper?

4e+.)(2 )

09.3% 95.7% 100% 100% I ()O0!l)

100% 100% 100%

96% 07%

M6ssbauer spectra to be described below shov, that iron enters as high-spin Fe ~+ (r):~,,~ = 0 . 6 4 ~ ) to replace Cu 2+ (r~ ,~,+ = 0 . 6 9 A). The length of the Fe ++02 bond is usually of the order 1.95-2.1 4. ()n the other hand, in concentrated iron c o m p o u n d s where iron is surrounded by an oxygen pyramid, the dislance between the iron and the apex oxygen is appreciably shorter ( ~ 1.85-1.95 ,4,) than the copper apex distance in copper c o m p o u n d s (2.2 .A in La2SrCu206, 2.3 A in LaeCaCueO~, and YBa2Cu~O? ): in FeTiRO> [20] the surrounding is 1 > 1 . 8 4 .-~. 2 × 1.92 +~,, 2 × 1.97 A with a distance u = 0 . 2 3 .A. to the basis plane: similarly in Pb4Fe308CI [21] the surrounding is 1 × 1.93 +,~,4 × 2 . 0 1 ]~ with u = 0 . 4 9 A. It may therefore be assumed that when iron is substituted to copper inside a pyramid, it will deviate from the copper site and move by ~ 0 . 1 - 0 . 3 5 ,-k tow a r d s the apex in order to reduce its bond length to 1.9-2.1 .&. In calcium compounds, where the displacement should be o f the order 0.35 .A, this will simultaneously increase the iron distance to the 2b vacancy from 1.65 to 1.95-2 .'k and perhaps facilitate oxygen insertion into the 2b vacancy even in c o m p o u n d s such as La2CaCu20,, ( a n d also YBa2Cu307 ,). In conclusion, one could expect to observe: if the 2b vacancy is empty, a p y r a m i d a l site with displaced iron: - if the 2b vacancy is filled with an 0 ( 3 ) oxygen. and octahedral site with displaced iron. The case o f an oblique distorted pyramid, with a filled 2b 0 ( 3 ) vacancy and a missing 8g O ( 1 ) oxygen, does not seem very likely since Fe 3+ has a large electrical charge than Cu 2+ and should therefore hold back O ( 1 ) oxygen. The case o f a missing O ( 2 ) apex seems i m p r o b a b l e for the same reason. As regards the problem of electrical neutrality. Fe ~ + ions associated with a standard or oblique p y r a m i d

C. Meyer et al. / R T M6ssbauer spectra of 57Fe_dopedA2BCu2O6

or with an octahedron lead to a positive or negative excess electrical charge with respect to the matrix. The same is true for a pair of Fe 3+ ions inside two connected oblique pyramids. In all these cases, a more or less distant compensating charge is necessary. On the contrary, two opposite Fe 3÷ ions in two CuO2 sheets with an O ( 3 ) (2b) oxygen between them (i.e., two connected octahedra around two opposite irons) will be electrically neutral with respect to the matrix.

4. Preparation and X-ray diffraction study

A2BCu206 compounds, with a doping concentration of I at.% 57Fe/Cu, have been prepared by solid state reaction. For La2SrCu206+ , and Lal.9CatACu206_,~ the required quantities of La203, SrCO3, (CaCO3), CuO and 57Fe203 were first fired at 900°C for 12 h. The Sr compound was then ground and heated at 1100 ° C for 12 h, yielding a single-phase product, according to X-ray diagrams. On the contrary, successive grindings and refirings of the Ca compound in air during 12 h at 1050, 1000 and 950°C did not succeed in eliminating some La2CuO4 impurity whose lines were still present on the final X-ray diagram. Lal.6Sro.aCaCu206_ ~ has been prepared by firing in air at 1050 °C the required amounts of powder oxides. The product was then ground and heated four times at 1000°C in 1 arm. Oz. The final X-ray diagram mainly exhibits the expected phase, with minor impurity lines probably due to SrCuO2. This sample will be called I. It was attempted to make it superconducting by heat treatment at 700°C under 165 arm. 02 (sample I ~ s a m p l e II) and then by heat treatment at 950°C under 150 atm. O2 for 12 h (sample II--.sample III). But these treatments did not result in any weight increase, indicating the absence of oxygen uptake, and no superconductivity was observed in samples II and III. However, these attempts had an interesting effect on the RT Mrssbauer spectra as shown below. Obviously, it cannot be excluded that the 1% substituted iron has some detrimental effect on superconductivity in LaL6Sro.4CaCu206. In Lat.sSro.2CuxFe~_xO4 superconductivity disappears when x>1.5% [22]. On the other hand, in YBa2(Cut_xFe~)307_6, it persists until x ~ 1 3 %

5

[23,1d]; up to 30% of the iron can go into the pyramidal Cu (2) sites, corresponding to a partial concentration ~5%, without destroying superconductivity [ 12]. Lat.6Sro.aCaCu206, with its pyramidal Cu sites, is more similar to YBCO than to (La,Sr)zCuO4 (which has octahedral sites) and should not be much affected by 1% iron substitution. All the samples were studied by susceptibility measurements and by Mrssbauer spectroscopy at various temperatures (to be described in part II). These techniques show that all compounds order around 15-20 K. The isomer shifts at room temperature, and the saturation values of the hyperfine fields, correspond to high-spin Fe 3÷ which is an "S state" ion. Observed quadrupole effects are therefore entirely due to the lattice, and at low temperature iron should be a good local magnetic probe.

5. Impurity phases As regard possible impurity phases not seen on Xray diagrams, we observe in the Ca compounds additional magnetic hyperfine structure which does not have the same thermal behaviour as the main spectrum. In La2CaCu206 this impurity spectrum is already present at room temperature. An extrapolation of the hyperfine field variation yields TN~380 K. The spectrum area corresponds to 15-20% of the total area. This could be some Fe-rich compound not detected by X-rays, for instance LaCaFeO4 (TN = 373 K [24] ). In ( Lal.6Sro. 4) CaCu206 I, parasitic magnetic lines are observed below 77 K. They represent only 2-3% of the total spectrum and might exist at room temperature without being detectable. According to the hyperfine parameters, one could think of LaSrFeO4 [25].

6. Room temperature Mi~ssbauer spectra The room temperature spectra of the various samples are represented in fig. 2 and the values of the parameters deduced from their analysis are collected in table 4. La2SrCu206 exhibits an asymmetrical quadrupole

6

C. Meyer et al. / R T MOssbauer spectra ~)[ eZFe-doped A_,B('u:O,,

Anyhow the quadrupole splitting is small: 131 ~ 0.3 mm/s. Lat.9Cal.iCu206 has a spectrum which is clearly the superposition o f a main doublet with a large q u a d r u p o l e splitting IAI = 1.27 m m / s and o f a minor one with a small q u a d r u p o l e splitting I~1 = 0.27 m m / s . The small q u a d r u p o l e splitting is quite comparable to that o f the previous c o m p o u n d , corresponding probably to the same kind o f site. Note that none o f these I~[s does correspond to La2CuO4where substituted Fe 3+ has 131 ~ 1.6 m m / s . La~.6Sro.4CaCu206_,~: in this case the three samples I, II, III exhibit again two kinds o f q u a d r u p o l c doublets: [~Jl -~ 1 m m / s and 131 "--0.3 r a m / s , but their relative weights d e p e n d s on the oxygen treatment. They are, respectively, equal to 51 and 49% in sampie I, to 34 and 66% in sample II, to 19 and 81% in sample Ill; i.e. under oxygen t r e a t m e n t the doublet with smallest 3 becomes p r e d o m i n a n t . A possible interpretation o f this fact will be given below.

100 99

t00 7 98 03

~100 98

100

7. Comparison of room temperature quadrupole splittings with computed quadrupole splittings

(e)

-3

-2

-1

0

1

2

3

VELOCITY (MM/S} Fig. 2. Room temperature Mfssbauer spectra of (a) La2CaCu206, (b-d) (Lal.6Sro.4)CaCu206 (1), (II) and (!11), and (e) La2SrCu2Oo. doublet with a small splitting 131-1½e2qQI ~ 0 . 3 m m / s . This spectrum can be adjusted, either with a single doublet in the presence o f coupled isomer shift and q u a d r u p o l e splitting distribution (fig. 1 and table 4), either with two q u a d r u p o l e doublets representing two hypothetic sites; in this last case the fit is not unique, and in the absence o f physical data it is not possible to choose between the different solutions, an example o f which is: 6=0.273mm/s

and

3=0.324mm/s(48%),

6=0.173mm/s

and

3=0.296mm/s(52%).

Let us first recall that for "S state" high-spin Fe 3 ~ the quadrupole splitting is only due to the lattice EFG. In order to get some insight in the characteristics of the Fe site in each c o m p o u n d q u a d r u p o l e splitting calculations have been p e r f o r m e d by lattice summation, with the help o f the available structural data. Because o f the uncertainties on the values o f the Sternheimer factor: ( 1 - y~) = 11 and o f the 57Fe electric q u a d r u p o l e m o m e n t : Q = 0.2 barn, these calculations are only indicative and are mainly useful for c o m p a r i s o n between different neighbourhoods a n d / o r different compounds. 7. 1. L a 2 S r C u z O c ,

It has been m e n t i o n e d previously that in this comp o u n d the 2b vacancy site can be occupied by a surn u m e r a r y oxygen O ( 3 ) , giving rise, for the copper site, either to an octahedral neighbourhood, or to an oblique distorted p y r a m i d a l neighbourhood. Electrical field gradient calculations have been per-

C. Meyer et al. / R T M6ssbauer spectra o f 57Fe_doped A 2BCu206

Table 4 Parameters extracted from the room temperature M6ssbauer spectra F/2

La2SrCu206 Lat.gCamCu206 Lal.6Sro.4CaCu206 1 Lal.6Sro.4CaCu20611 La16Sro4CaCu206111

0.235 S 0.183 1 0.195 0.28 0.155 0.253 0.209 0.22 S 0.203 1 0.218 0.225 J" 0.190 0.212

3

%

6

F/2

3

%

~

F/2

2~

H

8H

%

0.312

100

0.268 0.36 0.308

l0 49 66

0.257 0.243 0.238

0.228 0.168 0.149

1.266 1.012 1.036

74 51 34

0.286

0.2*

0*

343

40

16

0.304

81

0.228

0.155

1.006

19

= isomer shift with respect to ot iron at room temperature. 1"/2 = half width at mid height (when there are two values, they correspond to the two lines of the doublet ).

A= quadrupole splitting. 2~= d ( 3 cos20 - l ) / 2 (where O is the angle between the h.f. field and the main axis of the EFG ). H = hyperfine field. 5H= half width of h.f. field distribution. % = relative percentage of the subspectrum.

f o r m e d for several cases: regular p y r a m i d ( 2 b unoccupied), oblique distorted pyramid (2b o c c u p i e d + a n 8g v a c a n c y ) , o c t a h e d r o n , two dist o r t e d p y r a m i d s c o n n e c t e d by an O ( 3 ) a t o m w i t h their Cu 2+ r e p l a c e d by two Fe 3+, two o c t a h e d r a c o n n e c t e d by an 0 ( 3 ) w i t h two F e 3+. T a b l e 5 reports the results o b t a i n e d for A by s u m m a t i o n inside a s p h e r e w i t h 100 A radius, w i t h the d a t a o f C a i g n a e r t et al. [ 6 ] a n d o f L i g h t f o o t et al.

[ 7 ] ; for this calculation, the 2a a n d 4e sites o c c u p i e d b o t h by La 3+ a n d Sr 2+ h a v e b e e n e n d o w e d w i t h average charges. It a p p e a r s t h a t only single or d o u b l e o c t a h e d r a p r o v i d e small q u a d r u p o l e splittings c o m patible w i t h the small e x p e r i m e n t a l v a l u e IAI ~ 0 . 3 m m / s . T h e a s s u m p t i o n o f an o c t a h e d r a l s u r r o u n d ing for the iron s e e m s r a t h e r plausible, since s o m e 2b sites are o c c u p i e d by o x y g e n e v e n in the u n d o p e d c o m p o u n d , T h e 2 b - C u d i s t a n c e b e i n g r a t h e r large

Table 5 Computed quadrupole splitting 3 (ram/s ) and main axis orientation of the EFG, at the copper site, obtained by summation inside a 100 A radius sphere Compound

Coordination

La2SrCu206

Standard pyramid Oblique pyramid

La2CaCu206

Quadrupole splitting and main axis orientation

Quadrupole splitting and main axis orientation

Caignaert's structure

Lightfoot's structure

2.042 c 2.046; (c, z) = 76 ° in at-plane

Octahedron Two connected oblique pyramids with two irons Two connected octahedra with two irons

- 0.106

Standard pyramid

Izumi's structure

c

1.985; (c, z) = 75 ° in at-plane 0.037 c

2.4c Cava" s structure

La 1,6Sro.4CaCu206

Standard pyramid

2.5c

2,150 e 1.897; (c, z) = 77 ° in ac-plane 0.156

c

1.851; (c, z) = 76 ° in ac-plane 0.281 c

( ". Meyer et al. / R T Mdssba uer spectra (?1 ~:k~'-doped A 2B('u:O~,

( 1.83 A) and the Cu-apex distance being rather small (2.2 A), the iron displacement with respect to the copper site (if any) should be rather small _<0.1 A. towards the apex. 7.2. L a i . ~ C a 1 1 C u : O ~ ,

The calculations have been performed with the structural data of Izumi et al. [8] (very similar data have been obtained by Cava et al. [ 10] ). The computed value of A is reported in table 5. In this compound, the Ca 2+ atom being smaller than La 3+ and Sr 2+, there is in principle not enough room for oxygen to fill the 2b vacancy, and effcclively there is no [8] or little [ 15] detectable 2b occupation by neutron diffraction. However, according to the M6ssbauer spectra, 10% of the spectrum corresponds to a quadrupole doublet with small splitting (IAI ~ 0 . 2 7 m m / s ) . There are two possible explanations: - when a Cu 2+ ion is replaced by a n F e 3+ ion, in a few cases this could favour the filling of 2b by an additional oxygen (for example in the presence of a neighbouring L a 3+ ion in a 2a site) leading to an octahedral oxygen neighbourhood, whence a small quadrupole sitting. - this doublet is an impurity; it could even be a superparamagnetic fraction of the impurity phase which gives rise to the RT magnetically ordered spectrum (but then it should decrease with temperature in favor of the magnetic part, which is not the case). Let us now turn to the main spectrum with IAl,~x~= 1.27 m m / s . The computed value for iron substituted to copper in the pyramidal site (table 5) is about twice larger: A,h~o = + 2.4 m m / s . As already mentioned in ref. [ 1h ] a possible explanation for this reduction could be that iron substituted to copper moves away from the copper site towards the apex o f the pyramid (in order to reduce the Fe-apex bond length, see the discussion of section 3). In La2CaCu206 the distance of copper to the pyramidal basis is uc~=0.06 A. Figure 2 displays the effect o f increasing u upon the A value (computation in a 100 sphere): in order to get IAI = 1.27 m m / s , it is necessary that the distance uF~ of iron to the basis o f the pyramid be u = 0 . 3 5 A, corresponding to a displacement with respect to the Cu site by 0.3 ~,. In this position, the distance of iron to all its oxygen neigh-

bours are more equal (1 ×2.01, 4 × 1.94). In the same way, A was computed as a function of u for an Fe octahedron (fig. 2). For example, a distance u = 0 . 2 /~ correspond to A = - 0 . 3 m m / s to be compared with the experimental value observed for the small doublet: IAI = 0 . 2 7 m m / s . For an F e - F c pair in two opposite octahedra (cross curve), A= - 0 . 2 7 m m / s requires a distance u = 0 . 2 8 A. The F e - O ( 3 ) distances corresponding to u = 0 . 2 ,h and u = 0 . 2 8 A are, respectively, 1.85 and 1.93 A, which looks reasonable. 7.3. L a l

ogro.4CaCu2Oe,_,+1,

II, 1H

The only available structural refinement is that of Cava et al. [10] established for superconducting (La~ 6Sr..4)CaCu205 L~4by neutron diffraction. Note that the stoichiometric compound La~+6Sro.4CaCu205 ~4 is insulating and that our compound is also insulating. Fortunately, according to Cava et al. [10], there is no major difference between superconducting (La~ .6Sr0. 4 ) C a C u 2 0 6 and insulating La2CaCu20+. We can therefore also use the data ref. [8] in our calculations on the insulating compound. The result is reported in table 5. As already mentioned the M6ssbauer spectrum consists of two doublets with IAI _~1 m m / s and IAI-~0.3 m m / s . In sample I their intensities are comparable, but the double IAI -~ 0.3 m m / s becomes dominant under oxygen treatment when I -,11 .Ill. In addition, double IAI ~-0.3 m m / s cannot be due to an impurity phase, since at low temperature the magnetic behaviours of the subspectra corresponding to the two doublets are the same (see part II). These facts, and the comparison with La2SrCu20<, and La~+,+Caj+~Cu206, strongly suggest that tAI ±1 m m / s is due to (displaced) iron in a pyramidal neighbourhood, while IAI ~-0.3 m m / s is due to (displaced) iron in an octahedral neighbourhood. The relative increase of the second doublet under oxygen treatment could be due to the fact that isolated interstitial oxygen atoms 0 3 move towards iron atoms, therefore increasing the proportion of octahedral neighbourhoods. It is likely that both in pyramidal and octahedral neighbourhoods, iron moves towards the apex. The calculations o f fig. 3 for La2CaCu20+ can also be used

C Meyer et al. /RTM6ssbauer spectra of 57Fe-dopedA2BCu206 A

mm/~

'

' La2CaCu206

2.5

1.5 1

0

.

-0.5

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

-\ .

.

.

~

-1

II

0

i

,

i

i

0.1

0.2

0.3

0.4

,

~

0.5 (A)

Fig. 3. Calculated quadrupole splitting A as a function of the distance u between the the Fe atom and the pyramid basis. In the octahedron case, the cross curve corresponds to a Fe pair inside two connected octahedra.

for L a l . 6 S r 0 . 4 C a C u 2 0 6 ; in the pyramid a distance u-~0.4 A would lead to A= 1 m m / s , while in the octahedron a distance u-~ 0.3 A would lead to d = - 0 . 3 m m / s . Note that for such distances A,heo should be positive for the pyramid and negative for the octahedron. The interpretation of the low-temperature magnetic spectra [26] will effectively suggest that dexp is negative for the small A subspectrum.

7.4. Application to YBa2Cu3Oz_y The results obtained in this paper seem to corroborate the assumption according to which in Y B a 2 C u 3 0 7 _ v the pyramidal Cu (2) site is occupied by HS Fe 3+. As a matter of fact, in the superconducting phase YBa2Cu3OT, several authors [ 1 ] have assigned the Fe 3+ small quadrupole doublet to the Cu (2) site. This quadrupole splitting at room temperature is A=0.4-0.6 ram/s, depending on the preparation. The amount of this subspectrum is always less then 15% relative to the main spectra which represent iron in Cu ( 1 ) site and do not correspond to HS Fe 3+. Note that the ratio F e ( 2 ) / F e ( 1 ) can be increased up to 40% by heat treatments, as described for instance in refs. [ lc] and [ le]. In the oxygen deficient antiferromagnetic compound Y B a 2 C u 3 0 6 , the

9

iron substituted for Cu (2) is assumed to correspond to the Fe 3+ magnetic hyperfine structure detected on the spectrum at room temperature, reflecting the magnetic ordering of the Cu (2) moments. However, the value of d cannot be measured directly because of the angular part of the quadrupole splitting, but has to be determined on the paramagnetic quadrupole doublet above the N6el temperature. This explains why not so many data are available, due to the fact that this experiment has to be carried out at high temperature ( T > TN-~400 K), without any oxygen contamination. Anyway, a value of 0.67 m m / s is reported in ref. [ 1d ], for instance. Note that we have performed our own experiments (to be published separately) which give rise to similar values. In conclusion the quadrupole splittings are small, both in YBa2Cu307 and in YBa2Cu306. This might also be due either to an important displacement, by ~ 0 . 5 0.6 A, of the iron towards the apex of the pyramid (but then the iron-apex distance would be a little too small ~ 1.7-1.9 A) or to an octahedral neighbourhood due to oxygen insertion in the mid-plane of the bilayer.

8. Conclusion

In conclusion, it seems that the room temperature M6ssbauer spectra of the five compounds La2SrCu206, LaECaCu206 and Lal.6Sro.4CaCu206 I, II, III can be interpreted in a coherent way, by assuming that the subspectra with quadrupole splittings: I ~ l - 1 m m / s and [all ~ 1 m m / s and Idl ~0.3 m m / s , correspond, respectively, to iron in a pyramidal oxygen neighbourhood, and to iron in an octahedral oxygen neighbourhood (i.e., a p y r a m i d + a n additional oxygen in a 2b site). The respective proportions of the two subspectra depend on the distance between the two CuO2 layers and also on the oxygen treatment. In calcium compounds, it is necessary to assume that both in the pyramid and in the octahedron, iron is slightly displaced (by ~0.3 A) from the regular copper position towards the apex oxygen 0 2 .

c'. M e y e r et al. / R 7"MOssbauer spectra o f ~7Fe-doped A 2B('u20o

1o

Acknowledgement We want to thank prepared

J . P . L e v y a n d J. D e c k e r

who

some of the samples.

References [ 1 ] (a) M.E. Lines and M. Eibschutz, Physica C 166 (1990) 235; (b) E. Baggio-Saitovitch, R.B. Scorzelli et al., J. Appl. Phys. 67 (1990) 4509; (c) H.J. Bornemann, G. Czjzek and K. Kmiec, J. Appl. Phys. 67 (1990) 5082. (d) D. Hechel, I. Nowik, E.R. Bauminger and I. Felner, Phys. Rev. B 42 (1990) 2166; (e) Z.Q. Qiu, Y.W. Du, H. Tang and J.C. Walker, J. Appl. Phys. 67 (1990) 5458. ( f ) S. Katsuyama, Y. Ueda and K. Kosuge, Physica C 165 ( 1990 ) 404. (g) R.A. Brand, C. Sauer, H. Lutgemeier, P.M. Meuffels and W. Zinn, Hyperfine Interactions 55 (1990) 1229. (h) F. Hartmann-Boutron, C. Meyer, Y. Gros, P. Strobel and J.L. Tholence, Hyperfine Interactions 55 (1990) 1293, and references therein. See also: Proc. Int. Conf. on the Applications of the M6ssbuaer Effect, ICAME 1989, Budapest (Hyperfine Interactions 55), and: the Proc. Int. Conf. on Hyperfine Interactions HFI 1989, Prague (Hyperfine Interactions 59 ). [2] C. Meyer, F. Hartmann-Boutron, Y. Gros, P. Strobel, J.L. Tholence and M. Pernet, Solid State C o m m u n . 74 (1990) 1339. [3] C. Meyer, F. Hartmann-Boutron, Y. Gros and P. Strobel, Solid State C o m m u n . 76 (1990) 163. [4] N. Nguyen, L. Er-Rakho, C. Michel, J. Choisnet and B. Raveau, Mates.Res. Bull. 15 (1980) 897. [ 5] J.B. Torrance, Y. Tokura, A. Nazzal and S.S.P. Parkin, Phys. Rev. Lett. 60 (1988) 542. [6] V. Caignaert, N. Nugyen and B. Raveau, Mater. Res. Bull. 25 (1990) 199.

[7] P. Lightfoot, S. Pei, J.D. Jorgensen et al., Physica (" 169 ( 1990 ) 464. [ 8 ] F. lzumi, E. Takayama-Muromachi, Y. Nakai and H. Asano, Physica C 157 (1989) 89. [9] R.J. Cava, B. Batlogg, R.B. van Dover and J.J. Krajewski, Nature 345 (1990) 602. [ 10] R.J. Cava, A. Santoro, J.J. Krajewski, R.M. Fleming, J.V. Waszczak, W.F. Peck Jr. and P. Marsh, Physica C 172 (1990) 138. [ 11 ] K. Kinoshita, H. Shibata and T. Yamada, Jpn. J. Appl. Phys. 29 (1990) L1632. [ 12] H.B. Liu, D.E. Morris, A.P.B. Sinha and X.X. Tang, Physica (? 174 ( 1991 ) 28. [ 13 ] T. Sakurai, T. Yamashita, J.O. Willis, H. Yamauchi and S. Tanaka, Physica C 174 ( 1991 ) 187. [ 14] T. Sakurai, T. Yamashita, H. Yamauchi and S. Tanaka, J. Appl. Phys. 69 (1991) 3190. [ 15 ] A. Fuertes, X. Obradors, J.M. Navarro, P. Gomez-Romero, N. Casan-Pastor, F. Perez, J. Fontcuberta, C. Miravitlles, J. Rodriguez-Carvajal and B. Martinez, Physica C 170 ( 1990 ) 153. [ 16 ] K. Kinoshita, H. Shibata and T. Yamada, Physica ( 171 (1990) 523. [17] B. Okai, Jpn. J. Appl. Phys. 30 (1991) L179. [ 18] M. Hiratani, Shin-Ichiro Saito, M. Suga and T. Sowa, Solid State C o m m u n . 75 (1990) 425. [ 19 ] Coordination dependent alues of the ionic radii can be tound in: R.D. Shannon, Acta Crystallogr. A32 ( 1976 ) 75 I. [20] P.J. Schurer and A.H. Morrish, Phys. Rev. B 16 (1977) 951. [21 ] J. Pannetier and P. Batail, J. Solid State Chem. 39 ( 1981 ) 15. [ 22 ] P. lmbert, G. Jehanno and J. Hodges, Hyperfine Interactions 50 (1989) 599. [23] T.J. Kistenmacher, Phys. Rev. B 38 (1988) 8862. [24] G. le Flem, J.L. Soubeyroux and C. Delmas, J. Magn. Magn. Mater. 15-18 (1980) 1315. [25] M. Shimada, M. Koizumi, M. Takano, T. Shinjo and T. Takada, J. de Phys. Coll. 40C2 (1979) 272. [26] F. Hartmann-Boutron, Y. Gros, C. Meyer, P. Strobel and J.L. Tholence, to be published in J. Magn. Magn. Mater. ( ICM'91 proceedings). [ 27 ] M. Francois et al., Solid State C o m m u n . 66 ( 1988 ) 1117. [28] P. Bordet etal., Nature 327 (1987) 687.