Mössbauer investigation of 57Fe-doped TlSr2CoO5

C. R. Acad.

Sci. Paris,

t. 2, Sbrie

Miissbauer TISr,CoO,

II c,

p. 637-643,

1999

investigation

of 57Fe-doped

Jean-Pierre DOUMERC*, Martine COUTANCEAU, Leopold FOURNkS, Jean-Claude GRENIER, Michel POUCHARD, Alain WATTIAUX ICMCB-CNRS, Chateau Brivazac, 87, av. du Docteur-A.-Schweiaer, E-mail: [email protected]

33608 Pessac cedex, France

Abstract - The thallium

cobaltite TlSr,CoO, undergoes a metal-insulator transition near room temperature. Mossbauer investigation of a 57Fe-doped sample shows that iron actually enters into the lattice and that a long range antiferromagnetic order sets in below 150 K explaining the origin of a peak previously observed in the temperature dependence of the magnetic susceptibility. An unexpected complex behaviour is found for the high temperature phase for which previous electron diffraction and X-ray diffraction studies revealed a single site for Co atoms. The results are interpreted in terms of a dynamical formation of two types of structural domains characterized by a fluctuation of Co-O bond lengths inducing a fluctuation of Co spin state. 0 1999 AcadCmie des sciences I Editions scientifiques et medicales Elsevier SAS. Miksbauer

I metal-insulator

transition

I spin state ordering

I spin state fluctuation

I magnetic

ordering

Version fraqaise abrCgCe - hude MSssbauer of 57Fe-doped TISr,CoO,. Le cobaltite de thallium TlSr,CoOs subit une transition metal-isolant (MIT) au voisinage de la temperature ambiante. La transition est du premier ordre, comme le montre, par exemple, la coexistence d’une phase de haute temperature (HT) metallique et d’une phase de basse temperature (BT), isolante sur plusieurs dizaines de degres, ainsi que l’existence d’une hysterese. Alors qu’il n’existe qu’un seul site pour le cobalt dans la phase HT, il y en a deux dans la phase BT. La phase HT est caracterisee par de fortes interactions ferromagnetiques (F), mais, lorsque la temperature diminue, la MIT intervient avant I’apparition d’un eventuel ordre B grande distance (LRO). Aucun LRO ferromagnetique n’est observe a BT. Si un ordre s’etablit a BT, comme le suggtre l’existence d’un accident a 150 K sur les courbes donnant la variation thermique de la susceptibilitt magnetique, il ne peut &tre qu’antiferromagnttique (AF). Une telle hypothttse pourrait 8tre confirmee par diffraction de neutrons, mais nous ne disposons pas d’echantillon en quantids sufftsantes. Comme on le sait, la RPE ne permet pas d’etudier les ions Co3+, m8me s’ils sont dans une configuration de spin fort (HS) ou de spin intermediaire (IS) [ 11. Dans ces conditions, il ttait interessant d’entreprendre l’ttude Mossbauer d’un echantillon dope au 57Fe. Trois types differents de spectres ont ete obtenus en fonction de la temperature (&WC 5). 11scorrespondent a des domaines de temperature pour lesquels les autres proprietes du materiau, telles que les proprietes magnetiques et electriques, subissent des changements importants [8], ce qui demontre que le fer est bien entre dans le reseau du cobaltite. A 4,2 K, on observe la coexistence d’un sextuplet et d’un doublet, dont la contribution a l’absorption est d’environ 6 % (figure 6). Lorsque la temperature augmente, cette contribution augmente et le sextuplet disparait completement a 150 K, temperature qui peut &tre considtree comme la temperature de NCel de l’oxyde (J&n 6). Le champ hyperfin moyen (H) (qui vaut 43,1 T B 4,2 K) ne diminue pas sensiblement avec la temperature. En revanche, la proportion des atomes de fer soumis a H diminue avec 7: Ceci peut s’expliquer par l’existence de domaines de tailles variees et done de G temperatures de gel ), variees. Le doublet subsiste done seul jusqu’a 315 K, temperature pour laquelle apparaZt un deuxitme doublet ayant un eclatement quadrupolaire beaucoup plus faible (tableau), ainsi qu’un singulet, representant B peine 10 % de l’absorption totale (Jgure 8). Les spectres Mlissbauer subissent done un changement tres important lors de la MIT. Leur tvolution n’est toutefois pas facile a expliquer et semble m?me a premiere vue en contradiction avec les donnees structurales. En effet, dans la phase BT, nous avons deux sites pour le cobalt et nous observons un seul doublet ! En revanche, pour la phase HT, les rayons X r&lent un seul site pour le cobalt et un spectre complexe est obtenu ! Pour rendre compte de l’existence de deux doublets a

*Correspondence and reprints 1387-1609/99/00020637

0 1999

Acadkmie

des sciences

I l&ions

scientifiques

et mCdicaks

Else&r

SAS.

Tous

droits

rhservb

637

J.-P.

Doumerc

et al.

HT, nous partons de la description de la phase HT mttallique presentte dans la reference [2], qui considere la configuration de spin des atomes de cobalt comme un melange des configuration HS et IS et place le niveau de des orbitales Fermt au sein de la bande Y*cO-O p rovenant du recouvrement des orbitales x2-y” par l’intermediaire 0:2p. Un tel modele rend bien compte du caractere m&allique de la phase HT, mais laisse subsister une difficult6 concernant les autres electrons. Alors que pour les electrons .x2-y2 nous avons affaire B un modele classique d’electrons itinerants, pour les electrons dxzgz, nous devons faire appel B un mtcanisme de saut, qui respecte l’observation d’un seul site par diffraction X (XRD). Cette situation n’est pas sans rappeler celle que l’on rencontre dans les oxydes a valences mixtes, tels que certains manganites, ou, pour une densitt tlectronique donnee, le systeme n’est plus stable par rapport B une dtmixion, avec formation de domaines a l’echelle nanometrique [9]. Comme un seul site est observe par XRD, les domaines ne sont pas ordonnts mais fluctuent, tandis qu’un electron dx+ saute d’un site B l’autre, ce qui constitue une autre maniere de dtcrire un tquilibre de spin dynamique. On peut penser que, pour minimiser la perte d’energie coulombienne, le gaz d’electrons x”-y2 presente B son tour une fluctuation dynamique de concentration. De tels transferts electroniques, qu’ils soient intra- ou interatomiques, sont bien stir correlts a des fluctuations des distances Co-O. Par consequent, pourvu que la frequence du phtnomene soit inferieure a la frequence caracteristique de la spectroscopic Mossbauer, une partie des atomes de fer voit un type de region et l’autre partie un autre type de region. En refroidissant au-dessous de la temperature ambiante, la phase HT n’est plus stable ; il y a alors nucleation et croissance de domaines de la phase BT, dans laquelle il existe deux sites differents pour le cobalt. Pourquoi observe-t-on alors un seul doublet ? On peut penser que le fer, afin de creer un site qui lui convient mieux, favorise la formation de defauts qui se situent B la frontieres des domaines. De tels domaines, correspondant a deux orientations possibles des axes a et b de la maille orthorhombique, ont et6 observes par diffraction electronique. Au contraire, on fait disparaitre en chauffant ces domaines statiques ; le fer doit subir I’un ou l’autre des environnements fluctuants decrits plus haut et deux doublets sont observes. Au refroidissement, la MIT correspond done au passage d’une phase HT, dont le caracttre metallique resulte de la delocalisation d’electrons dans une bande dc,mo favorisee par le caractere dynamique de I’tquilibre de spin, a une phase BT ou les configurations HS et IS s’ordonnent parfaitement, formant un ordre d’ktats de spin (SSO) qui, d’une certaine maniere, joue un role analogue a celui de l’ordre de charges (CO) invoque pour expliquer certaines des transitions Ctudiees dans des systemes tels que les manganites a CMR. 0 1999 Acadtmie des sciences / Editions scientifiques et medicales Elsevier SAS. Mhsbauer

I transition

m&al-isolant

I ordre

d’Ctats

1. Introduction The thallium cobaltite TlSr,CoO, undergoes a metal-insulator transition (MIT) near room temperature. The transition is first order asevidenced e.g. from the coexistence of both high temperature (HT) and low temperature (LT) phasesover a large temperature range and from the observation of a hysteresis loop [2]. Whereas a single site wasfound for Co atoms in the HT-phase, 2 different sites were observed in the LT-phase using X-ray diffraction (XRD) [2, 31. The HT ph aseis characterized by strong ferromagnetic interactions, but as the temperature is lowered the MIT occurs before any ferromagnetic long range order (LRO) sets in. Ferromagnetic LRO does not still occur in the low temperature phase as the magnetization remains proportional to the magnetic field at least in the investigated temperature range. The conclusion is that if any LRO occurs at low temperature it should be antiferromagnetic and

638

de spin

/ fluctuation

d’hts

de spin

I ordre

magnhique

the signature of the onset of such an LRO could be a peak observed in the temperature dependence of the magnetic susceptibility at 150 K. This normally could be confirmed using neutron diffraction, but up to now quantities of the compound that can be prepared are too small for carrying out such experiments. Study of ESR line broadening is no longer applicable to our material since, as it is well known, Co3+ ions even in HS or intermediate spin (IS) [l] state do not provide any significant ESR signal. Therefore, it was worthwhile investigating a sample doped with 57Fe in order to study whether any local field could be probed at the iron nucleus at low temperature and studying how the MIT affects the Mossbauer spectra.

2. Experimental The investigated sample was prepared according to the method previously described

MGssbauer

[4] for undoped materials, but instead of Sr2C0205, Sr,Co,~9s57Fe,,,,05 was used as the source of cobalt and iron. The sample purity was checked using XRD. The magnetization of the sample was measured from 5 to 350 K in applied magnetic fields up to 2 T using a SQUID magnetometer (MPMS-5S, Quantum Design). Miissbauer measurements were performed using a constant acceleration HALe spectrometer with a room tempeDER-ty rature 5PCo source (Rh matrix) in a transmission geometry. The spectra in the temperature range 4.2 5 T < 293 K were recorded in a variable temperature cryostat. Between 293 and 413 K, an investigation of the temperature dependence was carried out in a furnace filled with dried argon. The velocity was calibrated by using pure iron metal as standard. The refinement of the Mossbauer spectra showed an important and abnormal widening of the peaks, so that the spectra have been fitted assuming a distribution either of quadrupolar splittings (above TN) or of hyperfine fields at low temperature.

investigation

The HT phase of TlSr$ZoO, is isostructural with the tetragonal form of the I 201 member of the thallium cuprate series . A drawing of the structure is shown inJigure 1. The unit cell is tetragonal and can be described with a P4/ mmm space group already reported for the cuprate analogues [6]. The Co3+ ions occupy

Tl Sr CO

Figure

1. Structure

of the HT-phase

of TlSr,Co05.

TISr,CoO,

elongated octahedra with 4 equatorial short distances and two axial long distances @gun 2).

HT-phase

LT-phase

Figure 2. Surroundings of the Co-atoms in the tetragonal HT-phase and in the 2 sites of the orthorhombic LTphase.

The LT phase was first described with a unit cell similar to that of the HT phase, but with a significantly larger cell volume. An electron diffraction (ED) study showed that the actual unit cell is rather orthorhombic with lattice parameters related to that of the tetragonal cell by the following equation [3] :

r1 130

(~,~*CJ (~,&)Co)

3. Crystal structure

of 57Fe-doped

=

-10 02 3 0

A detailed structural determination will be published elsewhere [3]. It includes a Rietveld refinement of high resolution X-ray synchrotron data using the Fullprof program [7], the conventional R-factors being: RP = 14.7 %; KP = 18.1 %; x2 = 8.3 %; RBragg= 8.4 % and R,= 11.2 %. One of the main structural features is the existence of two different sites for the Co atoms (figure 2). Site I (occupied by one third of the cobalt atoms) is an elongated octahedron characterized by four very short equatorial Co-O distances of 1.79(2) A. Site II no longer has a D,, point symmetry, one of the becoming very long Co-O distances (2.47(3) A) and thus leading rather to a 5-fold coordination ($igure 2). The average Co-O distance is much shorter for site I (I .91 A) than for site II (2.04 A), the latter being very close to that in the t-phase (2.03 A). A drawing of the Co-O layers is given in jgure 3. The ED study also reveals twins connecting domains undergoing a rotation of 62 so that the ti axis of a given domain is parallel with the 6 axis of its neighbours. We could estimate the domain width at about 0.5 pm [3]. Such a feature is often observed in orthorhombic LT phases originating from a tetragonal HT phase through a slight distortion of the crystal network.

639

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et al. VELOCITY I

-10 I

0 I

-5

(mm::)

5 I

I

0

0.2

a 0.4

f

I----+b I

Figure 3. Drawing of a COO, layer in the orthorhombic LT-phase. Site I is shown in grey.

4. Results 4. I. Magnetic

‘;: az 0

0 ?

ii

o.4

$ $

0.8

I

In order to compare the magnetic behaviour of the 57Fe doped sample with that of the undoped material the susceptibility was measured between liquid helium temperature and 350 K (jgure 4). The temperature dependence of the susceptibility is very close to that found for the undoped sample [8]. One of the main features is a peak at I50 K. We then observe a huge increase of the susceptibility near room temperature as the amount of HT phase (that is characterized by strong ferromagnetic interactions) increases. 4.2. Miissbauer

measurements

The Mossbauer spectra recorded at various temperatures ranging from 4.2 to 413 K are given in jgure 5.

I

0

100

I

200 T(K)

Figure 4. Temperature dependence ceptibility of 57Fe doped TlSr,CoO,.

640

I

300

of the magnetic

sus-

I

I

I

-1 I

0 I

1 I

2 I

spectra

recorded

-3

measurements

I

-2 I

Figure 5. Mkbauer 413 K.

at 4.2,

3 c

171 and

Three kinds of spectra can be identified as temperature increases. They correspond to temperature domains where changes of other properties such as magnetic and transport properties were already observed [8], which gives evidence that iron ions have entered the material network. At 4.2 K, spectra can be described by the coexistence of a sextuplet (6= 0.38 mms-‘, E = -0.35 mms-‘, H = 43.1 T) and ofa doublet (6 = 0.38 mms-‘, A = 0.6 1 mms-‘). The amount of Fe giving rise to the doublet can be evaluated to about 6 % with respect to the total Fe amount. Then, at rising temperature, the relative absorption of this doublet increases (jgunr 6) and the sextuplet disappears near 150 K, which can be considered as a Ntel temperature and corresponds to the already mentioned peak observed in the temperature dependence of the magnetic susceptibility, confirming again that iron actually has entered the cobaltite network. The hype&e mean field does not vary much with temperature. Instead, the part of the sample where LRO is probed by iron nuclei strongly drops as Tincreases. Such a behaviour could result either from the variation of the domain sizes with T or from a distribution of

MSssbauer

-5 VELfCITY

investigation

(mms;b,

-10

of 57Fe-doped

VELOCITY

5

-3 I

-2 I

-1 I

0

1

TISr,CoO,

(mm&) 1

2

I

I

3

I

0 0.1 0.2

0.8

0

0.18 0 0.1 0.2

0.8

0

';: s zoo.2 z a

0.8

0.4

I

8 20 a 0.3 0.6

Figure 7. Temperature from 150 to 290 K. Figure 6. Temperature from 4.2 to 150 K.

dependence

of M&batter

Between 150 and 315 K, a doublet is observed of which the parameters are given in Mijssbauer

Temperature

(K)

parameters

for 150

I TI

Spectrum

413 K. component

150

Doublet

290

Doublet Doublet Doublet Singlet

413

Chemical

0.37 0.26

Dl (67 %) D2 (23 %) (10 %)

Doublet

Dl

(67 %)

Doublet

D2

(23 %)

Singlet

spectra

the table. Its temperature dependence is given in figure 7. Above 3 15 K, the spectra can be decomposed into three components: two doublets referred to as Dl and D2 in the table and a singlet. Their relative absorptions do not change much

(mm&‘)

335

of Mijssbauer

spectra

domain freezing-temperatures resulting from the T-dependence of the coherence length.

Table.

dependence

(10 %)

shift

6

Quadrupolar splitting (mm.s-‘)

A

1.48

0.20

1.30 1.25

0.19

0.36

0.27 0.14 0.14

1.25 0.28

0.21

641

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et al.

VELOCITY -3

-2

-1

0

(mms-‘) 1

2

3

0 0.1 0.2

0 0.06 0.12

Figure 8. Temperature from 293 to 413 K.

dependence

of Miissbauer

spectra

with temperature as shown injgure 8. LT type spectra are recovered upon cooling.

5. Discussion All the &values found in the various temperature ranges correspond to trivalent iron (table), which was expected from the overall composition of the sample and the conditions of preparation. The Mossbauer study also clearly shows the onset of magnetic LRO below 150 K. The MIT found for undoped samples is strikingly reflected in the large change of spectra around room temperature: to our knowledge this is the first time that a MIT is probed so clearly using Mossbauer resonance. However, the details of the spectra corresponding to the LT and HT phases are not straightforward to explain. At first sight, they are in apparent contradiction with the structural change occurring at the transition. In the LT phase structure refinement using XRD reveals the existence of two sites for Co-atoms and a single doublet is found. In the HT phase, a single site is available for the Co-atoms and a rather complex spectrum is obtained. For discussing the present results we must keep in mind that iron atoms may not only choose the most convenient between available cobalt environments but also

642

even create their own. The latter possibility should be invoked at least for explaining the small singlet contribution (table) observed for the HT phase as the cobalt site undergoes a strong axial distortion. In order to account for two doublets with different A-values in the HT phase, we have to recall briefly the picture given in ref. [2], which in turn finds in the present Mossbauer data an additional support. In ref. [2] we described the spin state of Co-atoms in the HT metallic phase as a mixing of HS and IS states, which sets the Fermi level in a tic -() band originating from the overlap of g-? orbitals via 0:2~ orbitals. Such a model accounts very well for the metallic character of the HT phase, but raises a difficulty concerning the status of the tzg (d,, .) electrons. Whereas for X2-y2 electrons we dea9 with a classical picture of itinerant electrons, for d,, z electrons we have to consider the possibility o fyhopping, in which case XRD only reveals a single site for Co-atoms. The situation is not far from the one described for various mixed valent transition metal oxides such as manganites, where for given electronic density the system is no longer stable with respect to a phase separation [9]. However, phase separation leading to large electrically charged regions would lead to a large increase of energy and instead small domains of nanometer scale are formed in order “to spread the charge more uniformly” [9]. As a single site is found from XRD, the domains are not ordered, but move as a dxzyz electron jumps from site to site, which is” another way for describing the dynamical spin equilibrium. We may think that in order to decrease the loss of coulombic energy, in turn the electron gas of 2-g e1ectrons tends to have a more or less inhomogeneous ‘dynamical’ distribution. Now we have to remember that any charge transfer either intra-atomic or inter-atomic as well as fluctuations of charge-carrier density (such as electron or hole bags) lead to fluctuations of metal-oxygen bond lengths. Therefore, provided that the frequency of the phenomenon is smaller than the characteristic time of Mossbauer spectroscopy, one part of the Fe-ions sees one type of region and the other part of Fe-ions sees the other type region. Upon cooling below room temperature the HT phase as described above is no longer stable and the LT phase, where 2 different sites for Co-ions are observed, nucleates and forms larger and larger domains. Why is a single dou-

MSssbauer

blet observed? Iron favours the formation of defects, which are probably boundaries of domains, in order to create its own site. Such domains correspond to the 2 possible respective orientations of the a and b axis of the orthorhombic cell of the LT phase and were observed by electron diffraction. On the contrary, upon heating above room temperature, as the phase transition occurs, such ‘static’ domains no longer exist and, as iron cannot diffuse at these temperatures, Fe-ions must undergo either one or the other of the two types of surrounding described above and two doublets are observed.

6. Conclusions Use of Mossbauer spectroscopy, at a local probe introduced as doping 57Fe ions into the network of T1Sr,Co05, allowed us to show that the LT phase is antiferromagnetically ordered below I50 K. It also supplied new features for the description of the structural properties of the metallic phase. Whereas a picture of a

References [l]

For an introduction to the relationships between the electronic configuration of Co” and the crystal field effects see e.g.: Buffat B., Demazeau G., Pouchard M., Hagenmuller P., Proc. Indian Acad. Sci. 93 (1984) 313.

investigation

of 57Fe-doped

TISr,CoO,

homogeneous metal could have been proposed up to now, the present study suggests that the HT phase is better described as containing domains differing by the electronic configuration of Co-ions. In one type of domain, o*cO-O energy levels are empty corresponding to an IS state for trivalent cobalt. This IS state is stabilized by a dynamical compression of Co-O equatorial bonds in agreement with crystal field theory. In the second type of domains (J*~~-~ levels are partly occupied by itinerant electrons and, in addition, the concentration of these domains must be large enough to reach the percolation threshold in order to account for the metallic behaviour of the HT phase. Here Co-O equatorial distances are dynamically extended. Upon cooling, this HT phase transforms into an LT phase where the two different electronic configurations of Co-ions are well ordered and an insulating state results from this ‘spin state ordering’ (SSO). In some sense, the SSO plays a role similar to that of charge ordering (CO) in mixed valent systems such as CMR manganites.

[5] Raveau B., Michel C., Hervieu M., Groult D., Crystal Chemistry of High-T, Superconducting Oxides, Springer Series in Materials Science 15, Springer-Verlag, Berlin, 1991. [6] Kim J.S., Swinnea J.S., Steinfink Met. 156 (1989) 347.

H., J. Less Common

[2] Doumerc J.-I?, Grenier J.-C., Hagenmuller I?, Pouchard M., Viliesuzanne A., J. Solid State Chem., 147 (1999) 211, in press and references therein.

[7] Rodriguez-Carvajal 1995.

[3] Coutanceau M., Demourgues mere J.-I?, to be published.

[8] Coutanceau M., Doumerc J.-P, Grenier J.-C., Pouchard M., Sedmidubsky D., Solid State Commun. 96 (1995) 569.

A., Grenier

J.-C.,

Dou-

[4] Coutanceau M., Doumerc J.-P, Grenier J.-C., Maestro I?, Pouchard M., Seguelong T., C. R. Acad. Sci. Paris, s&ie II 320 (1995) 675.

J., FullProf

Program,

[9] See e.g.: Moreo A., Yunoki S., Dagetto (1999) 2034 and references therein.

ILL, Grenoble,

E., Science 283

643