Neutron diffraction and Mössbauer spectroscopy studies of the mixed valent T1-1212 ferrite (Tl0.5Pb0.5)Sr3Fe2O8

Physica B 228 (1996) 251-260

ELSEVIER

Neutron diffraction and M6ssbauer spectroscopy studies of the mixed valent T1-1212 ferrite (Tlo.5Pbo.5)Sr3Fe2Oa N. Nguyen, D. Groult*, V. Caignaert, A. Ducouret, B. Raveau Laboratoire CRISMAT, URA CNRS 1318, ISMRa, Bd du Mar~chal Juin, 14050 Caen Cedex, France

Received 2 April 1996

Abstract The mixed valent 1212-ferrite (Tlo.5Pbo.5)Sr3Fe20 s has been characterized using powder neutron diffraction data collected at 1.5 and 293K. The structure is confirmed to be close to that of the T1-1212 superconductor (Tlo.sPbo.5)Sr2CaCu207 but with no deviation from a full occupancy of (TI, Pb) and oxygen sites. 57Fe-M6ssbauer spectroscopy data depend upon the temperature between 4.2 and 293 K. They have been explained on the basis of a mixture of trivalent and tetravalent iron states at 4.2 and 150 K. At temperature higher than 150 K the existence of intermediate iron states Fe"+ (3 < n < 4) due to electron transfer between Fe 3+ and Fe 4+ sites has to be taken into account to fit the data. EFG calculations based upon neutron diffraction results at 293 K confirm that local displacements of the apical oxygen O(2) can be considered to explain the appearance of three M6ssbauer iron sites instead of the structure expected one. Keywords: Neutron diffraction; M6ssbauer spectroscopy; Ferrite; Mixed valent iron

1. Introduction After the discovery of the high Tc superconductors Bi2SrzCan-1CunO2.+4 and T1Sr2Ca, 1Cu. 02.+ 3 (for a review see Ref. [1]), the investigation of the systems B i - S r - M - O and T1 S ~ M - O where M = Fe, Co has allowed a new series of closely related layer structures to be generated. This is the case for the oxides BizSr2CoO6, T1SrzCoO5 and T1Sr4Fe209 [2 5] which are isostructural with the 2201, 1201 and 1201-0201 parent cuprates, respectively, and also for the oxide Bi2Sr3Fe209 [6 8] whose structure differs from that of its Bi-2212 counterpart Bi2Sr2CaCu20 s by the introduction of additional oxygen leading to double distorted octahedral iron layers instead of double pyramidal * Corresponding author.

copper layers. The detailed knowledge of the structure of these oxides is of great interest in order to understand the relationships between superconductivity, magnetism and chemical bonding in these materials. Recently, new members (Tll-xPbx) Sr3Fe208 (0 ~< x ~< 0.5) have been isolated [9] whose structure has been assumed to derive from that of the T1-1212 cuprates (Tll-xPbx)Sr2CaCu207 (0 ~< X ~< 0.5) [10]. One important issue, that has to be answered for these phases, deals with the actual valence states of iron for which the mean oxidation degree can vary from 3.5 (x = 0) to 3.25 (x = 0.5) if one admits that T1 and Pb are trivalent and tetravalent, respectively, and that the oxygen stoichiometry is exactly 8 oxygen atoms per formula unit. In that aim, we report herein on the neutron diffraction and M6ssbauer spectroscopy studies of the limit phase (Tlo.sPbo.5)Sr3Fe2Os showing that

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N. Nguyen et al. /Physica B 228 (1996) 251 260

true trivalent iron coexists with intermediate valent states between Fe a + and Fe 4 + whose proportions depend upon the temperature.

2. Experimental Polycrystalline samples of (Tlo.sPbo.s)Sr3Fe208 have been prepared in suitable amount for neutron diffraction measurements from mixture of the starting materials T1203 (Aldrich 99%) PbO (Prolabo 98%), FeeO3 (Merck 99%) and SrCO3 (Prolabo 99%) using a two-step procedure as described elsewhere [9]. Powder neutron diffraction data were collected at 1.5 and 293 K by means of the highresolution diffractometer D1A of ILL (Grenoble, France) for the wavelength 1.9060A. Intensities were measured between 0.05 and 150 ° (20) with increments of 0.05 °. The structure was refined with the Rietveld method using the F U L L P R O F program (version 3.1) [11]. M6ssbauer absorption spectra were recorded between 4.2 and 293K by means of a conventional constant acceleration spectrometer and a 5VCo:Rh source. Least-squares refinements of the spectra were performed using the M O S F I T program [12].

Fig. 1 shows a schematic drawing of the structure with the atom nomenclature. The final atomic parameters are given in Table 1. The corresponding calculated and observed neutron diffraction patterns (Fig. 2) show the validity of the calculations. Examination of Table 1 allows the following comments to be made: (i) No departure from the full occupancy of the cationic sites as well as the anionic sites was detected so that the "O8" formula is definitively established. All attempts to vary the occupancy factors yielded indeed values which were within less than one standard deviation from their stoichiometric ones. (ii) (T1, Pb) cations and O(1) oxygen atoms of the rock-salt-type layers are shifted from their ideal positions (la) (000) and (lc) (1/2 1/20) towards more general positions (41)(x 0 0) and (4n) (x 1/2 0), respectively. Such a shifting has been observed previously for other thallium cuprates. Note that the location of (T1, Pb) on (41) sites led to a better Bragg-R factor (Ri = 4.26%) than (4j) (xxO) sites (Ri = 4.90%). However, taking into consideration

FeO 2

3. Results and discussion

SrO

3.1. Powder neutron diffraction study Preliminary X-ray diffraction (XRD) analysis of the specimens showed a single phase which can be indexed in a tetragonal ocell with parameters a ~ 3.82,A and c _-12.83A. Refinements of the structure were carried out in the space group P4/mmm from neutron diffraction data collected at room temperature and at 1.5 K. 89 and 95 reflections were, respectively, considered for these refinements. Peak shapes were treated assuming a pseudo-Voigt profile function and intensities were calculated over an angular range of four full-widths at half maximum. Preliminary structure calculations were performed using atom parameters close to that of the parent cuprate (T10.sPbo.5)SrECaCuzOv [10] as previously shown from XRD data [9].

TIPbo

@

Q

SrO FeO 2

SrO FeO 2

SrO

Fig. 1. Schematic drawing of the structure of 1212-Fe compound (Tlo.sPb0.s)Sr3FezOs with the atom nomenclature.

253

N. Nguyen et al. /Physica B 228 (1996) 251-260 Table 1 Atomic parameters refined for (Tlo.sPbo.5)Sr3FezO8 at room temperature (esd's are given in parentheses) Atom

Site

x

y

Z

Biso (/~ 2 )

Occupancy

TI, Pb Sr(l) Sr(2) Fe O(1) 0(2) 0(3) 0(4)

41 ld 2h 2g 4n 2g lb 4i

0.0763 (18) 0.5 0.5 0 0.3860 (22) 0 0 0

0 0.5 0.5 0 0.5 0 0 0.5

0 0.5 0.2004 0.3500 0 0.1559 0.5 0.3356

0.51 (13) 0.54 (8) 0.48 (5) 0.31 (3) 2.38 (28) 1.46 (7}* 0.62 (9) 0.66 (4)

1 1 2 2 1 2 1 4

Rp = 14.4% Rwp = 13.1%, Rexp =

(2) tl) (3) (2)

8.10%, Z 2 = 2.62, Bragg R-factor = 4.26%, Rf fac-

Note: Conventional Rietveld R-factors: tor = 3.30%. Cell parameters a = b = 3.81474 (6),~, c = 12.8293 (3)A. *Anisotropic thermal parameters: vl~ = v22 = 0.016,~2, D33 = 0.024~2. Beq = 8n2Veq.

600

400

200

,I

20

40

60

80

100

120

140

2-THETA Fig. 2. Powder neutron diffraction patterns of (Tlo.sPbo.5) Sr3Fe208 and final Rietveld difference plots at room temperature.

254

N. Nguyen et al. / Physica B 228 (1996) 251 260

the ability of Pb(IV) to exhibit an octahedral coordination rather than a tetrahedral one, like in Pb-based 1212 cuprates [13, 14], the existence of two kinds of sites (la) (000) for lead and (41) (x00) for thallium has to be considered. Unfortunately, the too small number of neutron diffraction reflections did not allow Fourier difference maps to be used. Nevertheless, structure calculations carried out with lead sitting in (la) (000) and thallium sitting in (41) (x 0 0) did not change significantly the Bragg-R factor (Ri = 4.40%). Consequently, the (x 0 0) positions reported in Table 1 for (T1, Pb) can be replaced by these two kinds of positions for lead and thallium, respectively, which better agree with the usual coordinations of TI(III) and Pb(IV). This latter point seems to be supported by bond valence calculations as will be discussed further. (iii) isotropic thermal parameters of all the atoms are rather low except for those of O(1) and 0(2) atoms that exceed 1 ~2. For O(1) the high B value of 2.4 ~2 can be explained on the basis of a static disorder arising in the [(T1, Pb)O]~ layers as already shown for other parent ferrites and cuprates. This behavior is certainly due to local ordering of T1, Pb and O(1) atoms as will be discussed below. For the oxygen atoms 0(2) the appearance of o 2 a thermal parameter larger than normal (B = 1.5 A ) may suggest the presence of a static or dynamic atom disorder, perhaps correlated to that of the (T1, Pb)O plane. As it is well known, at least three probes can be considered for testing the presence of such a disorder: (1) the values of the root-meansquare (rms) displacements, (2) the density Fourier map, and (3) the structural refinements including split sites. As can be seen in Table 1, the v33 parameter of 0(2) which corresponds to a r m s o displacement of 0.15 A indicates that 0(2) is slightly disordered along the c-axis. Unfortunately, the neutron diffraction data collected here were not enough to carry out precise nuclear map calculations. In order to check the third probe, additional structure calculations were carried out using neutron diffraction data collected at 1.5K. Use of the data showed that there is no phase transition down to this temperature. More particularly, the magnetic ordering of iron atoms observed by

M6ssbauer spectroscopy at 4.2 K was not detected likely because of a spin relaxation time lower than that of neutron diffraction measurements. The thermal factors of O(1) and 0(2) obtained from the refinements were still abnormally high (1.41 and 1.05A° 2 , respectively) compared to the others (Table 2). As a result, the presence of a disorder due to sites only partially occupied by oxygen atoms has been considered. No deviation from full occupancies of the sites was observed. Therefore, the existence of two apical 0(2) sites resulting from a double-well potential has been assumed, accounting for previous C u - K edge-polarized EXAFS analysis of T1Ba2Ca3Cu4011 specimens 1-15]. In this latter case it has indeed been shown that the best fits of the absorption spectra recorded at 10, 127, 135 and 156K were obtained when the apical oxygen atoms (identical to 0(2)) were diso tributed over two sites separated by 0.17 A. Interestingly, refinements carried out with two 0(2) sites were positive. Positional parameters and occupancies of the two sites are given in Table 2. They correspond to the agreement factors Rex p = 5.56, g 2 = 3.75 and Ri = 4.84%. The distance between the two sites is ~0.15A, close to that found for T1-1234 from EXAFS data [15]. The interatomic distances reported in Table 3, confirm the ability of iron to exhibit a strongly distorted octahedral coordination so that the structure can be described as built up from almost regular FeO5 pyramids. Supposing TI(III) and Pb(IV) species located in (x00) positions would lead to a tetrahedral coordination of these ions with (T1, Pb)-O distances ranging between 2.02 and 2.24A. Bond valence calculations based on these results gave for thallium a valence of +2.95 close to the expected value, whereas a valence of + 3.27 was obtained for lead, which is rather far from the expected value of +4. On the opposite, leaving thallium in the (x00) positions and introducing lead on the (1 a) (0 0 0) site i.e. in a flattened octahedron with four equatorial Pb-O(1) distances of 2.40A and two apical Pb 0(2) distances of 2.00A yielded to a valence of + 3.76 close to the expected value. The valences were calculated from the formula e x p [ ( R - d)/0.37)] where d is the cation-oxygen

N. Nguyen et al. / Physica B 228 (1996) 251-260

255

Table 2 Atomic parameters refined at 1.5 K for (Tlo.sPb0.5)SraFe208 Atom

x

y

2

Biso(A2)

Occupancy

T1, Pb Sr{ 1) Sr(2) Fe O(1) 0(2) 0(3) 0(4)

0.0780 (14) 0.5 0.5 0 0.3806 (15) 0 0 0

0 0.5 0.5 0 0.5 0 0 0.5

0 0.5 0.2009 0.3500 0 0.1566 0.5 0.3359

0.10 (10) 0.30 (6) 0.23 (4) 0.21 (3) 1.41 (20) 1.05 (5)" 0.3l (7) 0.48 (3)

1 1 2 2 1 2 1 4

(1) (1) (1) (1)

Note: Cell parameters: a = b = 3.81002 (5)A, c = 12.7964 (2),~. Rietveld-R factor Rp = 13.6%, Rwv = 11.4%, R,xp = 5.75%,/2 = 3.94, Bragg-R factor = 4.70%, Rf-factor = 3.50%. "Refinement assuming a double site for 0(2): z

Bi~o(~,.)

Occupancy

0(2) O'(2)

0.1629 (9) 0.1507 (9)

0.91 (6) 0.91 (6)

0.96 (3) 1.04 (3)

Rexp = 5.86%,

Z2 = 3.94,

Ri = 4.84%,

Rf = 3.56%.

Table 3 Selected interatomic bond distances (A) for (Tlo.sPbo.5)Sr3Fe208 (T1, Pb)-O(1) × 2 (T1, Pb)-O(2) × 2 Sr(l)-O(3) x 4 -0(4) x 8 Sr(2)-O(l) x 1 0(2) x 4 -0(4) x 4 Fe 0(2) × 1 0(3) x 1 -0(4) × 4

2.244 2.021 2.697 2.844 2.607 2.757 2.578 2.490 1.925 1.916

q

m

'X

(6) (3) (1) (2) (2) (1) (2) (4) (2) (3)

distance (,~) and R is the bond-valenceoParameter tabulated in Ref.o[-16]: namely R = 2.003 A for TI(III) and R = 2.042 A for Pb(IV). As a result of the calculations, the distribution of Pb(IV) in octahedral coordination on the (la) sites and that of TI(III) in tetrahedral coordination on the (41) sites appears as most probable. This may also explain the static disorder of O(1) that involves the occurrence of [(T1, Pb)O]~ chains with two kinds of coordinations: tetrahedral for TI(III) and octahedral for Pb(IV) (Fig. 3).

¥V Fig. 3. Possible local ordering of TI(III) and Pb(IV) species in [(TI, Pb)O] ~ rock-salt type layers.

3.2. M6ssbauer spectroscopy study F i g . 4(d)

shows

the

57Fe-room-temperature

M6ssbauer spectrum of (Tlo.sPbo.5)Sr3Fe208 powder sample. The spectrum was first fitted with

N. Nguyen et al. / Physica B 228 (1996) 251 260

256

a '

,

-2

r

-1

0

1

2

/

180K

b -2

-I

0

l

C

d -2

-I

0

1

2

Velocity, mm/s Fig. 4. M6ssbauerresonancespectra of(T10.sPbo.5)Sr3FezOsas a function of the temperature between 150 and 293 K.

three different quadrupole-splitting paramagnetic doublets A', B' and C'. However, one observes that a better fit is obtained if one introduces a fourth paramagnetic singlet whose relative intensity is about 10% of the full intensity. The isomer shift of

this singlet IS = 0.29 mm/s is typical of Fe 3 + and could be assigned either to the presence of a paramagnetic impurity phase not visible by neutron diffraction measurements or to the existence of small superparamagnetic particles of the principal phase. In order to decide between these two possibilities a supplementary electron diffraction investigation of the 1212-Fe powder was carried out. About one-hundred microcrystals have thus been investigated and analyzed by energy dispersive spectroscopy (EDS). Beside microcrystals of the tetragonal 1212-Fe phase whose averaged composition was in good agreement with the expected one, less than 10% of the crystals exhibited unexpected ED patterns corresponding to an impurity phase of orthorhombic symmetry with unit cell parameters a ~ 3.8,~,, b ~ 5.4A, c ~ 21.0,~ strongly related to the perovskite one. EDS analysis of these unknown crystals showed that they contain Pb, Sr and Fe as the main elements. In Table 4 we have listed the M6ssbauer parameters (isomer shift (IS), quadrupole splitting (QS), full-width at half maximum (F) and relative intensity (%)) obtained at room temperature for the 1212-Fe compound. The first site A' which exhibits an isomer shift of 0.27 mm/s is surely that of high spin (S = 5) Fe 3 + ions located in a distorted oxygen octahedron (QS = 1.01 ram/s). The two other sites B' and C' exhibit isomer shifts of 0.18 and 0.09mm/s, respectively; they are typical of mixed valence states between Fe 3+ and Fe 4+ as previously shown for other perovskite-like oxides SrFeO3_x [17]. In order to get more information for fitting these data, M6ssbauer spectroscopy measurements were carried out at 4.2 K. The spectrum, which has been reported in Fig. 5, can be fitted quite satisfactorily by using three sets of magnetic hyperfine splitting noted A, B, C in Table 5 and one single line D whose relative intensity is identical to that found at RT and assigned to a paramagnetic impurity phase. The first two sets A and B with isomer shifts of 0.39 and 0.44mm/s and hyperfine fields Hf = 47.2 and 44.0T likely characterize high spin Fe 3+ in distorted ( 5 + 1) oxygen environments. Their relative proportion being ~(81 _+ 5)% of A + B + C sites, this involves that holes ([hi = 0.25at-1) should be distributed over the remaining (19 _+ 5)% sites. Because of the

N. Nguven et al. / Physica B 228 (1996) 251-260

257

Table 4 Hyperfine M6ssbauer parameters for (T10.sPbo.5)Sr3Fe208 Site

Type

IS(+_0.02) (mm/s)

QS(_0.02) (mm/s)

F[+_0.02) (mm/s)

If(_+5) (%)

1.20 0.52 0.0

0.36 0.31 0.28

72 23 5

1.18 0.78 0.49 0.0

0.32 0.33 0.28 0.50

67 8 20 5

1.12 0.93 0.61 0.0

0.28 0.32 0.34 0.50

54 22 19 5

1.01 0.92 0.76 0.0

0.28 0.25 0.25 0.32

53 20 18 9

Temperature = 150 K A B Da

Fe 3 ÷ Fe 4+ Fe 3 +

0.38 0.03 0.36

Temperature = 180 K A' B' C' Da

Fe 3÷ Fe "+ Fe "+ Fe 3+

0.35 0.21 0.06 0.31

-12

-10

-8

-6

-4

-2

0

2

4

6

8

IO

12

Velocity mm/s Fig. 5. M6ssbauer at 4.2 K.

resonance spectrum o f ( T l o . s P b o 5 ) S r 3 F e 2 0 8

Temperature = 240 K A' B' C' Da

Fe 3+ Fe "+ Fe "+ Fe 3+

0.32 0.21 0.07 0.31

Temperature = 293 K A' B' C' Da

Fe 3÷ Fe" + Fe" + Fe 3 +

0.27 0.18 0.09 0.29

Note: IS = Isomer shift relative to ~-Fe; Q S = quadrupole splitting; F = half-height width of the doublet; If = fitted relative intensity. Impurity phase. a

value of the hyperfine field He = 22.9 _+ 0.2 T, close to that reported for the tetravalent iron oxide SrLaMgo.sFeo.504 (Hf = 18.1 _ 0 . 1 T ) [18], the third Zeeman site C can be assigned to the valence state Fe 4 + in good accordance with both the hole concentration and its relative intensity. At room temperature the M6ssbauer spectroscopy does not identify the two valence states probably because of rapid electron transfers between Fe 3+ and Fe 4+ over big orbitals. As a result, the doublets B' and C' (Table 4) should be assigned to oxidation states which are neither Fe 3 + nor Fe 4 + but intermediate states according to ~ F e 3+ + f i F e 4+ ~ ' F e

. + + f i F e "'+

with 3 < n < n' < 4 . The electrical conductivity has been shown to be a semiconducting behaviour with an activation

Table 5 Hyperfine M6ssbauer parameters for (Tlo.~Pbo s)Sr3Fe208 at 4.2K

Site

Type

IS(+0.02) tmm/s)

2e(_0.02) (mm/s)

Hf(_+0.02t T

I f ( + 5) %

A B C D"

Fe 3+ Fe 3+ Fe 4+ F e 3+

0.39 0.44 0.14 0.41

-0.46 0.36 -0.09 QS = 0

47.2 44.0 22.9 -

63 9 16 12

Note: IS = Isomer shift relative to ~-Fe; 2e = quadrupole shift; H f = hyperfine field; If = fitted relative intensity. Impurity phase.

energy of 0.18eV. The conduction mechanism would be fast electron hopping from Fe 3 + to the orbital hole big of Fe 4+ through the equatorial bonds Fe O(4)-Fe. At this stage of the M6ssbauer study, it appears very interesting to determine the beginning temperature of the electron delocalization. In that goal, the temperature dependence of the M6ssbauer spectrum has been measured. The results obtained at 150, 180 and 240 K have been gathered in Fig. 4. Note that at 150 K (Fig. 4(a)) (Tlo.sPbo.5)SraFe2Os is paramagnetic but that iron still exhibits localized Fe 3 + and Fe *+ oxidation states whose proportions are in good agreement with the formulation (Table 4). On the other hand, a change of the M6ssbauer spectrum is clearly visible at 180K (Fig. 4(b)). It corresponds to the appearance of electron transfer between iron sites as stated here above, which requires to fit the spectra recorded at 180, 240 and

258

N. Nguyen et al. / Physica B 228 (1996) 251 260

293K with three quadrupole paramagnetic doublets instead of two. It should be pointed out that in this range of temperature, the proportion of true Fe 3+ decreases linearly with the temperature (Table 4). Similar results have been obtained for the parent Tl1201-0201 type ferrite (Tll_xPb~)Sr3Fe209 (0 ~< x ~< 1). Relationships between structure, magnetism and electron transport properties will be more detailed in a forthcoming paper [19].

3.3. Electric field gradient calculations at 293 K Although only one Fe site has to be taken into account for neutron diffraction refinements, three main Fe sites have to be considered to fit the M6ssbauer spectrum of the 1212-Fe compound. At a local scale, besides the possibility for iron to exhibit three different charge states, the existence of three different oxygen octahedral configurations corresponding to three different QS values in the paramagnetic RT phase should also be assumed. In order to check the origin of such local octahedron modifications, EFG calculations have been carried out by taking into account either the whole crystal or the atomic shell limited to the first six oxygen neighbours of Fe. Because of the lack of data about the crystal field parameters of the Fe 4+ ion, the valence contribution to the quadruple splitting QS (sum of lattice and valence contributions) cannot be calculated, thereby limiting the E F G calculations to the M6ssbauer sites A' and B' which correspond to Fe 3 + (A') for which only the lattice term needs to be evaluated and to the intermediate state Fe" + (B') in fact surely close to Fe 3+ and for which the valence contribution to QS can be neglected. Using the mean atomic parameters deduced from neutron diffraction refinements at room temperature (Table 1), monopolar lattice EFG calculations have been first performed by taking into account the whole crystal. One obtains a main component V~z of the E F G tensor which is positive and parallel to the c-axis (Vzz = 0.336,~ 3). The corresponding quadrupolar splitting, which can be calculated from the formula QS=(1-~)

-2-eQv= ( l + ~ - j")Jr]2x~ 1/2 ,

depends upon the Sternheimer antishielding factor (1 7~)- This factor varies indeed with the valence state of Fe: (1 - 7 ~ ) = 11.97 for Fe z+, (1 - 7 ~ ) = 10.14 for Fe 3+ [20]. It is unknown for Fe "+ with n > 3 . Therefore, chosing here ( 1 - ~ , ~ ) = 10.14 while the mean oxidation state of iron is +3.25 leads to some uncertainties on the calculated value of QS. Taking also into account the fact that the valence contribution to QS has been neglected, one can estimate that the averaged value of QS for the whole crystal is 1.02 _+0.04 mm/s. Similarly, we have then calculated the monopolar EFG lattice and the quadrupole splitting QS for a distorted oxygen FeO6 octahedron whose Fe-O distances have been listed in Table 3. The main component of the EFG tensor is aligned along c (V~ = 0.285 A 3) and corresponds to the averaged QS (octa) value 0.87_+ 0.04mm/s. The difference between the two values of QS (AQS = 0.15 _+ 0.04mm/s) is representative of the monopolar contribution to QS of the more distant atoms of Fe. Moreover, it should be noted that the multipolar lattice contribution to QS seems to be very weak since the averaged value of QS = 1.02 _+0.04 mm/s calculated for the whole crystal in the monopolar approach is equal to the experimental QS value (QS = 1.01 + 0.02mm/s) observed for the M6ssbauer site A' (Table 4) whose relative intensity is (58 + 5)% of the full intensity. Now let us turn to the origin of the presence of three Fe sites with different local oxygen environments. Based on previous EFG calculations carried out for parent ferrites with 1201-0201 (PbSr4Fe209) and 2212 (Bi2SraFezOg)-types layered structures [21, 8], local displacements of the apical oxygen 0(2) along the c-axis have been considered. We have thus performed QS calculations for different zO(2) positional levels between zO(2)= 0.140 and zO(2)= 0.180, taking account of the value zO(2) = 0.1559(1) obtained from neutron diffraction refinements (Table 1). However, because of the reasons developed here above for (1 - 7 ~ ) , the calculations have been limited to the M6ssbauer sites A' and B'. The best fit of the experimental QS values was obtained for the positional zO(2) values reported in Table 6. Observe that the main site A' is quite well fitted for a zO(2) parameter equal to that obtained from neutron diffraction data. It -

N. Nguyen et al. / Physica B 228 (1996) 251 260 Table 6 Results of monopolar EFG calculations with QS values corrected for the effects of the more distant atoms of Fe than oxygen neighbours Site

z(Oz) ( _+0.004)

QS calc ( + 0.04) (mm/s)

QSobs ( __+.0.02) (mm/s)

dFe-O(2) ( _+0.05) (,A)

A' B'

0.156 0.165

1.02 0.90

1.01 0.92

2.49 2.37

corresponds to a more elongated FeO6 octahedron (Fe-O(2) = 2.49 A) than the one corresponding to the M6ssbauer site B' for which a local displacement of the apical oxygen 0(2) towards the iron site (Fe-O(2)= 2.37A) should be assumed. Note that this local displacement of 0(2) ,~ 0.12A appears in good agreement with the rms displacement ,~0.15A found from neutron diffraction refinements (Table 2). It supports also recent X-ray structural data [7] obtained for the 2212-Fe (BizSr3Fe209) counterpart which exhibits indeed a complex variation of the same apical Fe-O bond throughout the crystal.

4. Concluding remarks Neutron diffraction data collected at 1.5 and 293K for the 1212-Fe compound (Tlo.sPbo.5)Sr3 Fe208 confirm the structural model suggested from XRD patterns and remove definitively the ambiguity about the actual oxygen stoichiometry. Further they allow to bring new insight on the peculiar behavior of the apical oxygen 0(2) in agreement with previous works on high T¢ superconducting oxides, including EXAFS measurements, ion channelling experiments, infrared reflectivity and Raman spectroscopy. SVFe M6ssbauer spectroscopy data agree well with the composition determined by neutron diffraction refinements and with the existence of local displacements parallel to the c-axis of the apical oxygen 0(2). This leads, for iron, to different octahedral oxygen environments whose quadrupole splittings have been well fitted by EFG calculations. A part of iron cations is in the trivalent

259

state but which decreases as the temperature increases from 4.2 to 293 K. The remaining part is in mixed valence states Fe "+ intermediate between true Fe 3+ and Fe 4 + oxidation states, which correspond to fast electron transfer from site to site.

Acknowledgements The authors would like to thank Dr. E. Suard for her help as local contact during the collection of the neutron diffraction data at I.L.L. (Grenoble). They are also indebted to Professor M. Hervieu for making the electron diffraction investigation and to Dr. J.M. Greneche (Le Mans) for fruitful discussions.

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