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JOURNAL OF SOLID STATE CHEMISTRY 130, 117128 (1997)ARTICLE NO. SC977287

LaMnO3 Revisited

J. Topfer and J. B. GoodenoughCenter for Materials Science and Engineering, ETC II 9.104, The Uni versity of Texas at Austin, Austin, Texas, 78712

Received September 26, 1996; in revised form January 15, 1997; accepted January 22, 1997

Samples of LaMnO3 with 044440.18 have been preparedvia a precipitation method followed by heating at different tem-peratures and oxygen partial pressures. Oxidation is accommod-ated by the formation of cation vacancies to give La1Mn1O3

with /(3). Room temperature X-ray diffraction revealsa two-phase region separating an O-orthorhombic phase stableover 044440.06 and a rhombohedral phase, stable in the range0.1044440.18, that transforms below room temperature to anO-orthorhombic phase. Transport and magnetic studies indicatethe following evolution of electronic properties with increasing .Oxidation creates small-polaron holes that become increasingly

trapped at cation vacancies with decreasing temperature in theparamagnetic domain. Some of these trapped holes are releasedon cooling through the onset of long-range magnetic order. In theO-orthorhombic structure, the trapped holes form super-paramagnetic clusters below room temperature that becomemagnetically coupled with the onset of antiferromagnetic order

in the hole-poor matrix to form a magnetic glass. The O-orthorhombic structure sustains the cooperative JahnTeller de-formation. The rhombohedral phase suppresses the cooperativeJahnTeller deformation, and the hole-poor matrix becomesferromagnetic. With increasing , the perovskite tolerance factorincreases, and at a critical value tc+0.97, a transition fromsmall-polaron to a peculiar nondispersive delocalized state of theconduction electrons occurs below Tc . At highest , these conduc-tion electrons introduce a double-exchange spinspin coupling inthe matrix that varies as cos (ij/2); this ferromagnetic couplingcompetes with the antiferromagnetic Mn: t3O: 2pMn: t

3

superexchange, which varies as cos ij. Consequently an equilib-rium ij increases with to give a cant angle 04444180,which introduces metamagnetic behavior of the matrix between

clusters and a ferromagnetic magnetization that decreases rela-tively sharply with increasing in the range 0.134440.18. 1997 Academic Press

INTRODUCTION

Early attempts to prepare the perovskite LaMnO

showed that it is oxidized with firing in air; stoichiometry

To whom correspondence should be addressed at the present address:Hermsdorfer Institut fur Technische Keramik, Naumburger Strasse, 07629Hermsdorf, Germany.

was approached by firing at lower oxygen activities (1).Although characterized at the time as LaMnO

>B , it wasrealized that the perovskite structure cannot accept excessoxygen in an interstitial site. However, the nonstoichio-metry is accommodated as cation vacancies, as has been

shown by several studies on the defect chemistry of thatmaterial either by TGA measurements (24) or diffractionexperiments (3, 5, 6). Accordingly, the composition ofLaMnO

>B is better expressed as La\CMn\CO with"/(3#) or

LaMn>\BMn>B O>B

"3#

3La

>B)B>BMn>\B>BMn>B>B)B>BO.

[1]

Since we determine analytically the ratio La/O"Mn/O"3#, we refer in this paper to the system LaMnO

>B ;the conversion to La

\CMn\CO is straightforward.Most studies of the manganese oxides with the perovskite

structure have concentrated on the properties of theLa

\VA

VMnO

systems containing a larger rare-earth atom

such as n"La, Pr, or Nd and a larger alkaline-earth atomA"Ca, Sr, or Ba. Recent interest in this family of materialshas been stimulated by the observation of a colossalnegative magnetoresistance (CMR) in alkaline-earth dopedmanganites (7) and by the charge-ordering transitionnear x"0.5 (8) that was first recognized (9) in the

La\VCaVMnO system in an interpretation of the magneticorder found by neutron diffraction (10). The occurrence ofCMR has also been reported for LaMnO

>B (11).In the parent compound nMnO

, the mismatch be-

tween equilibrium nO and MnO bond lengths is givenby a tolerance factor

t"(nO)

(2(MnO)[2]

that decreases with decreasing temperature. The cubic per-ovskite adjusts to a t(1 by a cooperative rotation of the

1170022-4596/97 $25.00

Copyright 1997 by Academic PressAll rights of reproduction in any form reserved.


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MnO

octahedra that buckles the MnOMn bond anglefrom 180 to (180! ), where increases with decreasingt to lower the space group symmetry from cubic to tetrag-onal, rhombohedral, or orthorhombic. In the parent perov-skite LaMnO

, the symmetry is lowered to orthorhombic

by a cooperative rotation of the MnO

octahedra around

the cubic [11 0] axis as in GdFeO (12). Superimposed onthis distortion is a cooperative JahnTeller deformationsetting in well above room temperature that removes theorbital degeneracy of the localized, high spin Eg electronicconfiguration at the octahedral-site Mn> : t

e ions;

a cooperative shifting of each O\ ion of an ab plane awayfrom one of its two near-neighbor Mn atoms toward theother creates long and short OMnO bond lengths per-pendicular to each other at each Mn atom, the long bondsrotating 90 within an ab plane on going from one Mnatom to another (9). The JahnTeller deformation lowersthe c/(2a ratio to less than unity without any furtherlowering of the space group symmetry from orthorhombic

(Pbnm).The JahnTeller electronic ordering couples the Mn>

spins S"2 ferromagnetically within the ab planes of theparent LaMnO

compound via a dominant eO : 2pNe

superexchange interaction; these planes are coupled anti-parallel to one another via t

O : 2pLt superexchange

interactions, which are dominant along the c axis. ThisA-type antiferromagnetic order is modified by thecooperative rotation of the MnO

octahedra about the

orthorhombic b axis, which introduces an antisymmet-ric Dzialoshinski exchange coupling D

GH S

G;S

Hthat cants

the antiparallel coupling to give a weak canted-spin

ferromagnetism.The end member CaMnO

is an antiferromagnetic insu-lator; each localized electronic configuration A

gat a

Mn> : t

e ion has its spin S"3/2 coupled antiparallel toits six neighbors (Type-G antiferromagnetic order) viat

O : 2pLt superexchange interactions. The t configura-tions of a Mn>/Mn> couple remain localized over theentire compositional range 04x41 of a La

\VA

VMnO

system. The -antibonding e electrons, on the other hand,undergo a transition from localized to itinerant characterwith increasing x; the CMR effect is a maximum at thecrossover from localized !e to itinerant !* electronicbehavior below

(7). Moreover, the itinerant * electrons

introduce a global ferromagnetic coupling, and by x"0.3the systems La

\VA

VMnO

are ferromagnetic metals below

the Curie temperature .

In this paper we explore the transition from polaronic toitinerant behavior of the Mn e electrons in the systemLaMnO

>B where the introduction of cation vacanciesin the MnO

array strongly perturbs any antibonding

itinerant-electron * band states of e-orbital parentage.Earlier studies showed that LaMnO

>B undergoesa transition from polaronic behavior and antiferromagnetic

order at low values of to a ferromagnetic metal at +0.12(13, 14).

EXPERIMENTAL

LaMnO>B samples were prepared by a wet chemical

precipitation route. LaO (dried before use at 950C) andMnCO

(Mn-content gravimetrically determined) were sub-sequently dissolved in acetic acid. To obtain a clear solu-tion, a few crystals of hydroxyl aminochloride were added toreduce higher valent manganese ions. A solution of citricacid was added to the hot solution of the metal ions, anda white precipitate was obtained. The solution was slowlyevaporated on a hot plate, and the obtained solid wascalcined at 800C. The precursor powders were pressed intopellets and fired at 1300C for 24 h in air. Then the pelletswere crushed in an agate mortar, pressed again into pellets,and sintered again at 1300C for 24 h in air. After that, finalequilibration runs under a given set of conditions (Table 1)

were performed in either argon, air, or oxygen. The oxygenpartial pressure in the argon used was independently deter-mined with the help of an EMF cell based on doped zirco-nia; the measured cell voltage indicated an oxygen partialpressure of about 10\ atm. Samples heated in argon oroxygen were slowly cooled down in the furnace in the gasatmosphere, and those heated in air were quenched. Toobtain higher concentrations of Mn>, samples were pre-pared by direct heating of pellets of the precursor powder at800C in oxygen. Another set of samples was synthesized bysintering pellets of the starting precursor material at 1000Cfor 24 h in air before a final equilibration run at 800C in

oxygen was carried out.The products were checked by powder X-ray diffractionwith CuK radiation and Si as internal standard. The non-stoichiometry parameter was determined by chemicaltitration: the samples were dissolved in an excess of VO>in sulfuric acid and potentiometrically titrated withKMnO

solution. This procedure was checked with

MnO

, and an accuracy of O

$, referred to as

AMO

, was achieved.Magnetic properties of the products were measured with

a Quantum Design MPMS SQUID magnetometer. Suscep-tibility measurements were performed in a zero-field-cooled(ZFC) and in a field-cooled (FC) mode in a field of 100 Oe.The field dependence of the magnetization and hysteresisloops M"M(H) were recorded at 5 K on field-cooled sam-ples.

The electrical resistance of the sintered pellets was meas-ured between 30 and 300 K in a four-probe device. Seebeckcoefficients were obtained from measurements in home-built devices between 30 300 K and 3001000 K; the high-temperature measurements were performed in air. Thermalanalysis of the materials was carried out with a PerkinElmer thermal analysis system. TGA runs were performed

118 TO PFER AND GOODENOUGH


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TABLE 1Preparation Conditions, Values of as Determined by Titration, Lattice Parameters, Effective Magnetic Moment Extracted fromthe CurieWeiss Part of the 1/ vs TCurves (300600 K) and Calculated Magnetic Moment (Spin-Only) for LaMnO3 Compounds

and gas atmosphere a (As ) b (As ) c (As ) ()

A. Initial firing 1300C24 h, 1200C Ar 0.00 5.532(4) 5.738(1) 7.693(5) 5.55 4.9024 h, 1300C air 0.04 5.542(2) 5.658(2) 7.711(2)24 h, 1200C air 0.05 5.537(2) 5.656(2) 7.715(3) 6.19 4.8124 h, 1100C air 0.06 5.543(4) 5.598(4) 7.765(6)24 h, 1000C air 0.07 5.92 4.7224 h, 900C air 0.09 5.503(2) 60.40(1)24 h, 850C air 0.10 5.480(1) 60.72(1)48 h, 800C O

0.11 5.472(1) 60.68(1) 5.49 4.61

48 h, 750C O

0.12 5.476(1) 60.69(1)100 h, 700C O

0.13 5.471(1) 60.63(1)

B. Initial firing 1000C48 h, 800C O

0.14 5.469(1) 60.58(1)

C. No initial firing50 h, 800C O

0.16

100 h, 800C O

0.18 5.471(2) 60.54(1) 4.86 4.43

in various gas atmospheres with a heating/cooling rate of1 K/min.

RESULTS

1. Sample Preparation and X-Ray Characterization

The nonstoichiometry of the samples of LaMnO>B asdetermined by redox titration is summarized in Table 1 to-gether with the firing conditions. Heating at 1200C at logP-+!5 gives the stoichiometric perovskite with "0.00,

while heating in air at 1200C results in "0.05. Loweringthe temperature in air or oxygen leads to samples withhigher concentrations of cation vacancies and Mn>. Thevalues of obtained are in good agreement with results ofthermogravimetric studies on the deviation from stoichio-metry as a function of temperature and oxygen partialpressure (2, 4). Dense samples, which were first sintered at1300C and then heated at lower temperatures, could not beoxidized higher than "0.13; even prolonged firing at

700C in oxygen did not increase the Mn> concentration.Therefore samples were either heated at only 1000C beforea final anneal at 800C in oxygen or annealed at 800Cwithout any prefiring of the precursor material. As a resultof this procedure, higher values were accessible; however,the densities of these samples were considerably lower thanthose of samples prefired at 1300C.

The room temperature X-ray patterns of some of thesamples are shown in Fig. 1. LaMnO

>with 0440.06

crystallizes in the orthorhombic GdFeO

structure (Pbnm)at room temperature; all reflections could be indexed on thebasis of the orthorhombic cell. This is in agreement with

a recent neutron-diffraction study on a sample with

"0.005 (15). A monoclinic structure, as proposed forsmall in the system La

\VSr

VMnO

(16), has not been

observed. The orthorhombicity decreases with increasing ,i.e., with increasing tolerance factor t. It is noteworthy thatat room temperature all the orthorhombic samples fulfill the

criterion c/a((2 characteristic of a cooperative JahnTel-

ler deformation superimposed on the orthorhombic struc-ture resulting from t(1. For '0.09, the rhombohedralstructure (R3c) is observed with the rhombohedral angle decreasing with increasing or t. This is in good agree-ment with other studies (1, 14). Two representative X-raypatterns of samples with rhombohedral room temperaturestructure are displayed in Fig. 1. The reflections of thesample with "0.18 are broadened due to a small particlesize; this sample was prepared at 800C in oxygen withoutan intermediate high-temperature firing. The evolution ofthe lattice parameters is shown in Fig. 2. Between the or-thorhombic and rhombohedral phase field, a two-phaseregion was observed; the sample with "0.07 exhibitsreflections of both phases. Deviations from the regularevolution (Vegards law) of the cell parameters were foundon both sides of this composition, indicating an onset of thetwo-phase field. The exact width of this region is not known.However, a similar two-phase field between orthorhombicand rhombohedral compounds of the system La

\VSr

VMnO

has been reported (16).

2. Magnetic Properties

The temperature dependence of the inverse magnetic sus-

ceptibility of samples measured in a field of 100 Oe with

LaMnO>B REVISITED 119


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FIG. 1. Examples of room temperature X-ray diffraction powderpatterns (CuK ) for orthorhombic perovskites ("0.00, 0.05) and rhom-bohedral perovskites ("0.10, 0.18).

heating after cooling in zero field (ZFC) is shown in Fig. 3.All samples exhibit a ferromagnetic Weiss constant

that

increases with . The onset of magnetic long-range orderbelow a critical temperature ranges from

,"135 K for

"0.00 to "240 K for "0.18. Each sample showsa ferromagnetic component below the ordering temper-ature. The Curie temperatures of the samples with 0.05440.11 are at about 170 180 K. In Table 1, the effectivemagnetic moments

extracted from the paramagnetic

range are compared to the magnetic moments based on

FIG. 2. Variation of room temperature cell parameters with forLaMnO

>B .

FIG. 3. Temperature dependence of the inverse magnetic susceptibilityof LaMnO

>B samples measured in a field of 100 Oe.

spin-only contributions; the experimental values are con-siderably larger than expected. In addition, the curves1/( ) are not linear above

, and a change of the slope

occurs near 300K.In Fig. 4 the molar magnetizations measured between

5 and 300 K in a field of 100 Oe for samples with "0.05and "0.18 are compared; both zero-field-cooled (ZFC)and field-cooled (FC) runs are shown. Whereas the ZFCmeasurements exhibit a broad maximum in M( ) below

,

no such maximum is found for the FC runs in which M ( )decreases with temperature and is almost temperature-inde-pendent at low temperatures. The difference between theFC and ZFC magnetization at low temperature becomes

smaller in the rhombohedral phase.Figure 5 shows the field dependence at 5 K of several

samples in a field up to 40 kOe. The following observationsare noted: (i) at higher the samples become saturated at themaximum applied field, and at lower they do not become

FIG. 4. Zero-field-cooled (ZFC) and field-cooled (FC) magnetizationfor "0.05 and "0.18.



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FIG. 5. Field dependence of the molar magnetization at 5 K for sam-ples with various values of (sample with "0.18 only measured to30 kOe).

totally saturated; and (ii) the saturation magnetization(taken as magnetization at maximum field) increases with to a maximum value at around "0.12. The evolutionwith of the number of Bohr magnetons, calculated fromthe magnetization data at maximum field, is plotted inFig. 6. At +0.12, the magnetization approaches the theor-etical value expected for full aligment of the spins (straightline at the top of the graph), but for '0.12 it decreaseswith increasing . The inset ofFig. 6 shows the M"M(H)curves of the samples with "0.05 and 0.18 at 5K. Thesample with "0.18 shows saturated moments at a field of30 kOe, the sample with "0.05 is not yet saturated underthat field. The area of the hysteresis loops and the re-manence are small. In Fig. 7 the temperature variation of

the magnetization in a field of 40 kOe is plotted. Comparedto the low-field magnetization data of Fig. 4, the absolutevalues of M are much larger and the decrease in M with

FIG. 6. Variation with of the magnetic moments per Mn ion at 5 Kobtained from saturation magnetizations taken at 40 kOe (30 kOe for"0.18). (Inset) Plot of the magnetization vs field at 5 K for "0.05 and"0.18.

FIG. 7. Temperature variation of the magnetization in a field of40 kOe for samples with various values of.

increasing temperature is smoother. Whereas samples withsmaller values of show a regular shape of the M( ) curve,the "0.16 sample is anomalous below 90 K.

3. Transport Properties

Measurements of the thermoelectric power as a functionof temperature, ( ), between 30 and 1000 K are shown inFig. 8 for selected samples. From these plots we note severalfeatures: (i) all samples show a maximum in the ( ) curvesbetween 140220K and the absolute values of at thismaximum temperature, ? , increase with decreasing ;(ii) the thermopower of samples with smaller could not bemeasured at very low temperatures because of a high sampleimpedance whereas () drops to values close to 0 V/Kbelow 100 K in samples with 50.13; and (iii) at temper-atures '

?

the Seebeck coefficient decreases to small

positive or negative values at high temperatures, becoming

FIG. 8. Thermopower for selected samples of LaMnO>B as a func-

tion of temperature. (Inset) Thermopower as a function of in thetemperature range 7001000 K.



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temperature-independent above 800 K. The evolution of( ) in the temperature range 700 1000K is shown in theinset of Fig. 8; it is consistent with a small-polaron modeldescribed by

"!k

e

ln

1!c

c

, [3]

where c"n/N is the fractional occupancy ofN Mn sites bymobile charge carriers and is a spin-degeneracy factor.From the values of at high temperatures (e.g., "5 V/Kfor "0.00, or "!16 V/K for "0.11) and "2, onemay estimate the composition of LaMnO

>at that tem-

perature; this estimate gives

(e.g., "0.16 for "0.00

as starting material or "0.18 for "0.11 as starting

material). A ' suggests that oxidation of the samples

takes place during the measurements, which were made inair. The evolution of the values of ( ) reached at hightemperatures for different values of the starting materialsconfirms that the kinetics of this oxidation reaction is reflec-ted in the measured values of the Seebeck coefficient: thehigher the nonstoichiometry in the starting material, themore negative the value of reached at high temperaturesand therefore the larger the nonstoichiometry

in the final

composition. The maximum value of "0.18 demon-

strates that this composition represents the maximum at-tainable extent of oxidation or nonstoichiometry, , in thisperovskite at 1 atm pressure.

Further experimental evidence for oxidation in air athigher temperatures is shown in Fig. 9, where the evolution

FIG. 9. Thermopower for "0.00 between 100 and 1000 K duringheating and cooling. (Inset) TGA curves for "0.00 for the temperaturerange 300973 K (heating and cooling rate 1 K/min, holding at 973 Kfor 2 h) in different gas atmospheres: (a) Ar, logp

-:!5; (b) N

,

log p-:!3.5; (c) air; values of represent the final compositions cal-

culated from the mass gain.

of () with temperature in air for "0.00 is shown incombination with results of thermogravimetric experiments.Above about 600 K, the Seebeck coefficient drops to smallpositive values and a mass gain in the TGA signals the onsetof oxidation. On cooling, ( ) does not return to its originalvalue because the sample remains oxidized. This conclusion

is also confirmed by thermogravimetry in different atmo-spheres: (i) the higher the oxygen partial pressure in thesurrounding gas atmosphere, the higher the extent of oxida-tion (no oxidation in argon, "0.04 after heating in oxy-gen-contaminated nitrogen, and "0.15 after heating inair); (ii) after oxidation, the sample mass stays constantduring cooling, i.e., the sample does not give up oxygen onlowering the temperature.

Figure 10 shows the thermopower ( ) between 30 and320 K for samples with values of ranging from 0.05 to 0.18.Similar results had been reported for some values of (11,14). The trend mentioned above is clearly visible: the smallerthe , the higher the Seebeck coefficient at

?

. The

temperature of this maximum coincides with (except for"0.18, where

is somewhat higher than ? ). As the

temperature decreases to , the factor ofEq. [3] changes

from 2 in the paramagnetic phase at high temperatures to1 in the ferromagnetic phase; the onset of magnetic long-range order removes the spin degeneracy of the chargecarriers. This change in would lead to an enhancement of( ) of up to 60 V/K during cooling to

. For 40.10,

the observed change in () is larger than that, whichindicates that, in addition to a vanishing of the spin degener-acy, holes are progressively trapped out as the temperatureapproaches

. The drop of() with decreasing temper-

ature below is remarkable; it indicates a release of thecharge carriers from the traps. In the rhombohedral samples

FIG. 10. Variation of thermopower with, temperature () for selectedsamples between 30 and 320 K.



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FIG. 11. Temperature dependence of the electrical resistance normalized to the resistance at room temperature between 30 and 310 K for the systemLaMnO

>B : (a) "0.12; (b) "0.13; (c) "0.14; (d) "0.18.

there appears to be also a change in the character of themobile charge carriers from small polarons.

The temperature dependence of the resistance R()for several samples is shown in Fig. 11. LaMnO

>Bwith 40.13 is semiconducting while for "0.14 and"0.18 a resistance maximum at 130 K is found. Below130K, metallic-like behavior with positive R/ is ob-served down to 50 K; below 50 K the resistance increasesagain with decreasing temperature. The low density of therhombohedral samples introduces an important grain-boundary resistance, and the resistance maximum occursat about 8090 K below ?+ , whereas it appearsto be closer to

in dense samples prepared electrochemi-

cally (11).A tentative phase diagram is proposed in Fig. 12; it in-

cludes the orthorhombicrhombohedral transition temper-ature

as determined by Wold and Arnott (1). At values of

40.10,

marks the onset of the cooperative JahnTellerdeformation that is superimposed on an orthorhombic dis-tortion, e.g., at about 600C for "0.00 (

for small not

included in Fig. 12); the linear fall off of

with increasing

at higher oxidation represents the orthorhombicrhom-bohedral transition in the absence of a cooperative JahnTeller deformation. At room temperature, our data suggesta two-phase region (Fig. 2) in the interval 0.06440.09.The onset of long-range magnetic order is denoted as

; it

is somewhat higher in the rhombohedral phase than in theO-orthorhombic phase. Below

, the transition with in-

creasing from a canted-spin antiferromagnet at "0.00 toa nearly full ferromagnetic magnetization at "0.13 ap-pears to be via a heterogeneous spin configuration as ina spin glass (SG), see Fig. 4. At higher values a metamag-netic canted-spin configuration (MCS) appears. All samplesare insulators (I) in the paramagnetic phase (P) at '

;

below

there is a change at +0.14 from an insulatingstate to a peculiar electronic state (17).

DISCUSSION

1. Electronic States

Figure 13a presents a schematic electron-energy diagramfor the MnO

array of the parent compound LaMnO

.



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FIG. 12. Magnetic phase diagram of the system LaMnO>B . Temper-ature of thermopower maximum, ? (full circles); Curie temperature,

, or, Neel temperature for small ,

,(full rhombi); temperature of the

orthorhombic to rhombohedral phase transition from Ref. (1),

(fulltriangles). The full lines connecting the points are a guide for the eye. PI,paramagnetic insulator; CAFI, canted-spin antiferromagnetic insulator;SGI, magnetically heterogeneous, spin-glass-like insulator; FI, ferromag-netic insulator with, nearly complete parallel alignment of spins; MCS,metamagnetic, canted-spin configuration.

The empty 5d and 6s bands of the La> ions overlap the Mn4s band. The fact that LaMnO

and CaMnO

are both

antiferromagnetic insulators places both the Mn>/Mn>

and Mn>

/Mn>

redox couples within the relatively largeenergy gap between the filled O : 2p band and the emptyMn: 4s and La : 5d, 6s bands. The existence of a cooperativeJahnTeller deformation in LaMnO

indicates that in the

stoichiometric compound the octahedral site, high-spinMn> : d configuration is localized and separated dis-cretely from an empty Mn> : d configuration by theCoulomb energyN required to induce the disproportiona-tion reaction 2Mn>PMn>#Mn>. In octahedral co-ordination, the symmetry of the cubic crystalline field splitsthe d-fold manifold into a more stable, orbitally threefold-degenerate set oft orbitals and a less stable, orbitally two-fold-degenerate set ofe orbitals to give the high-spin localiz-ed configurations Mn> : te, A

g; Mn> : te, Eg; and

Mn> : te, Ag

.TheMn>/Mn> redox couple lies high enough in energy

above the filled O : 2p orbitals that hybridization of the filledoxygen wave functions and the empty d wave functions ata Mn> ion may be described by second-order perturbationtheory to give ligand-field orbitals (18)

R"NL ( fR!LL ) and C"NN( fC!!NN),

[4]

FIG. 13. Schematic band diagrams for (a) LaMnO

, (b) semicon-ducting mixed-valence manganites with intermediate values of , and(c) LaMnO

>B with '0.13 with metallic-like resistivity below .

where fR

and fC

are, respectively, the - and -bondingatomic orbitals, the

Gare symmetrized O : 2pN , 2s, and 2pN

orbitals, and the covalent mixing parameters G

(G"

bG

/EG) increase with the cationanion resonance integral

bG

(i.e., with the degree of overlap of the cation and anionwave functions) and with decreasing energy E

Gto lift

the respective oxygen electron to the corresponding emptyMn 3d wave function (18). In the discussion to follow,Eq. [4] is assumed to remain valid for the Mn> ionsalso even though the energy EN to lift an O 2pN electroninto an empty e orbital of the Mn>/Mn> couple is rela-tively small. The covalent mixing parameters stand in therelation

;L(N [5]

and the electronelectron electrostatic energiesL andN toadd an electron to a t or e manifold are in the relation

L'N . [6]

A measure of the strength of a (180!) MnOMninteraction is the corresponding spin-independent cationcation resonance integral b

"(

G, H

H) between the

nearest-neighbor Mn atoms:

bL+LL and bN+NNcos [7]



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In Eq. [7], the relatively small term ;NNcos is

omitted from bN to simplify the discussion. The tight-bind-ing bandwidth L+2zb for the antibonding * d-likebands with z"6 nearest neighbor MnO Mn interactionsb

is

L+12LL(L [8]

so that the t configurations with spin S"3/2 remain local-ized as the (MnO

)\ array is oxidized to (MnO

)\ in

CaMnO

. Therefore it is necessary to replace bN and bL bythe spin-dependent resonance integrals tL"bLsin(GH/2) andtN"bNcos(GH/2), where GH is the angle between spins onneighboring Mn atoms at sites R

Gand R

H. Since the spin

angular momentum is conserved in an electron transfer, thePauli exclusion principle constrains the half-filled t config-urations at neighboring atoms to couple antiferromagneti-cally, which introduces a sin(

GH/2) into tL; however, transfer

of an e electron to an empty e orbital is energeticallyfavored, so ferromagnetic intraatomic exchange introducesa cos(

GH/2) into tN . Since the t configurations remain un-

changed on oxidation, the antiferromagnetic Mn : tO :2pLMn : t spinspin interactions are described by superex-change second-order perturbation theory. The ferromag-netic Mn: teO : 2pNMn : te e-electron interactions aredescribed by superexchange third-order perturbation the-ory where the charge transfer is virtual because it costs anenergy, e.g., N in stoichiometric LaMnO , and by first-order double-exchange theory where a real Mn>#Mn>PMn>#Mn> charge transfer is on a time scalethat is fast relative to the relaxation of the S"3/2 spins ofthe t configurations. Replacemment of bN by tN givesa tight-binding bandwidth for the e electrons as

N+12NN cos cos(GH/2). [9]

From the uncertainty principle, the time for a real electrontransfer in a mixed-valent system is

F+h/2N+h/24tN [10]

and a transition from polaronic to itinerant electronic be-

havior occurs as decreases to below the period 1/0 of theoptical-mode lattice vibration that would trap a hole ata distiguishable Mn> ion. Increasing tN by increasing N(with hydrostatic pressure to increase bN ) or decreasing (by increasing the tolerance factor t ofEq. [2] by choosinga larger n> and/or A> ion) or decreasing

GH(by intro-

ducing long-range ferromagnetic order) shortens

and, atcrossover from small-polaron to itinerant-electron behav-ior, can induce a release of holes trapped in localized states(19). It is this modulation that is responsible for the colossal

magnetoresistance (CMR) found in the Ln\V

AVMnO

systems.

2. Stoichiometric LaMnO3

In LaMnO

, a cooperative JahnTeller distortion to

remove the orbital degeneracy leads to an anisotropic mag-netic order with ferromagnetic Mn : teO : 2pNMn : teinteractions within the ab plane of the orthorhombic cell(Pbnm) and antiferromagnetic Mn : tO : 2pNMn : t coup-ling between the planes in the c direction (9) (Type A mag-netic order), as confirmed by neutron diffraction (10). Asshown in Fig. 3, the sample with "0.00 exhibits a weakspin-canting ferromagnetism with an antiferromagneticNeel temperature

,+135K. The appearance of a small

canting of the antiferromagnetically ordered spins arisesfrom Dzialoshinskys antisymmetric exchange (20). The factthat stoichiometric LaMnO

exhibits weak ferromagnetism

was first noted by Matsumoto (21). The magnitude of that

ferromagnetic moment is about 0.4 in LaMnO asreported recently (22). It has been shown that, in the lightlydoped material La

Sr

MnO

(Type-A magnetic or-

der), the ferromagnetic component and the antiferromag-netic long-range order set in at the same temperature of136 K (23). Above room temperature, LaMnO

shows

CurieWeiss behavior; however, the plot 1/ vs is nota straight line (Fig. 3) and the effective magnetic moment is5.5

per Mn ion, i.e., significantly larger than expected for

a spin-only contribution of Mn>. Either local, short-rangeferromagnetic interactions stabilize short-range ferromag-netic order (or superparamagnetic clusters) in this material

up to temperatures of several hundred degrees above , orthe Weiss molecular field increases with temperature. It hasbeen reported that in the temperature range 400 470C,a transition in the CurieWeiss plot occurs (2426) with aneffective magnetic moment above that transition of 4.9

per Mn ion as expected for Mn> (26). Unfortunately, thistemperature is above the accessible temperature range ofthe SQUID device used in this study. Structural transi-tions have been reported for nominally stoichiometricLaMnO

: from orthorhombic to rhombohedral at 600C

(1) and from orthorhombic to cubic at 300C followed bycubic to rhombohedral at 530C (15). However, no provis-ion was made to preserve the oxygen stoichiometry in these

studies, and oxidation sets in above about 500C in air(Figs. 8 and 9). Therefore the origin of the change in slopeof the inverse-susceptibility vs temperature plot remainsunresolved.

Nominally stoichiometric LaMnO

is an insulator.However, a positive thermopower indicates some oxidation((0.01) in our samples, and the increase to a large positivevalue at a ?+, indicates a trapping out at , ofmost of the holes that are present and a release of someholes below

,.



10/12

3. Paramagnetic LaMnO>B

Oxidation of the MnO

array lowers the Fermi energy$

ofFig. 13a into the Mn>/Mn> redox couple as illustratedin Fig. 13b; it also creates cation vacancies: LaMnO

>B"La

\CMn\CO. Oxidation of a Mn> : te configura-tion to a Mn> : te configuration leaves empty e states at

the top of the Mn> : te manifold of energies. Contractionof the equilibrium MnO bond length at a smaller Mn>ion would lift the empty e states of the Mn> ion above thetop of the Mn> : te manifold. Therefore, if the time+h/2N for a Mn>#Mn>"Mn>#Mn> charge

transfer is long compared to the time to trap the hole bya breathing-mode contraction of the equilibrium MnObond lengths, the holes become polaronic with energiesdiscretely above the Mn> : te energies as shown inFig. 13b. Cation vacancies tend to trap holes, and polaronicholes become trapped at a higher energy than mobile polar-ons. The distribution of holes between trapped and free

polaronic states is temperature-dependent.In the paramagnetic temperature domain, the holes re-main polaronic for all compositions , but the concentrationof mobile holes decreases with decreasing temperature. Asa result, ( ) is described by Eq. [3], and it increases withdecreasing temperature not only because the spin-degener-acy parameter changes from "2 to "1, which can givean increase of 59 V/K, but also because trapping out of thepolaronic holes decreases the effective occupancy factor c.The motional enthalpy of the diffusive motion of the mobileholes as well as the activation energy for release of thetrapped holes introduce an activation energy into the con-ductivity, so the conductivity has a semiconductive temper-ature dependence. Therefore the paramagnetic domain iseverywhere designated PI in Fig. 12.

The cooperative JahnTeller distortion that is superim-posed on the O-orthorhombic deformation due to a toler-ance factor t(1, Eq. [2], gives an O-orthorhombic phase,

i.e., c/a((2. Within the paramagnetic domain, the O-orthorhombic to rhombohedral transition temperature

was found by Wold and Arnott (1) to decrease sharplywith increased for 0440.10; the cooperative JahnTeller deformation disappears for '0.10, and

for the

O-orthorhombic to rhombohedral transition decreases lin-early with . Recently, a

:200K has been found for

"0.15 (22). The long-range magnetic-ordering temper-ature

increases somewhat on crossing from the O-orthor-

hombic to the rhombohedral phase, which has a largerMnOMn bond angle and therefore a larger tN. Quenchingfrom higher temperatures may give a single crystallographicphase; but a two-phase range 0.05((0.10 was found(Fig. 2) in our samples, which might reflect a differentquenching rate.

The evolution with of the paramagnetic CurieWeissplots (Fig. 3) shows a continuous increase of the Weiss

constant

with . With an increasing tolerance factor t, theangle decreases and the ferromagnetic MnOMn inter-actions introduced by the e electrons increase relative tothe antiferromagnetic Mn : tO : 2pLMn : t interactions.,'

'0 in LaMnO

shows the coexistence of antifer-

romagnetic and ferromagnetic interactions. The ferromag-

netic component becomes more dominant in the intermedi-ate compositions where (

is found, e.g., see the curves

for "0.07 and 0.11 in Fig. 3. However, for 50.14,

'

is remarkable, and a drop in the magnetization

relative to ferromagnetic alignment of the Mn spins (Fig. 6)indicates the reappearance of a competitive antiferromag-netic coupling. Moreover,

significantly larger than the

theoretical value, except for the "0.18 sample, indicatesthe possible presence of superparamagnetic clusters toabout room temperature; trapping of holes at cation va-cancies introduces a high Mn>/Mn ratio locally withina cluster where ferromagnetic Curie temperatures of up to350 K may be supported. At higher temperatures, a Weiss

molecular field that increases linearly with temperature cangive an effective Curie constant, and hence

, that is larger

than theoretical.

4. Magnetically Ordered LaMnO>B

Below , three compositional domains can be distin-

guished in Fig. 12: 0((0.07, 0.10((0.14, and0.14440.18. The first appears to contain ferromagneticclusters embedded in an antiferromagnetic matrix givingrise to spin-glass (SG) magnetic behavior, see Fig. 4. Withthe collapse of the cooperative, static JahnTeller deforma-

tion associated with the O-orthorhombic structure, thematrix becomes ferromagnetic, but some clusters may be-come antiferromagnetic at the higher values of . Onpassing from +0.13 to +0.14 there is a transition ofthe charge carriers below

from polaronic behavior to

fast, real charge transfer between at least pairs of Mn atoms,and the magnetization exhibits a reduced magnetizationwith metamagnetic behavior characteristic of a spiral-spinconfiguration at low fields and a canted-spin configurationat high fields (MCS), see Fig. 4. The transition froma canted-spin antiferromagnetic insulator (CAFI) at "0to a spin-glass insulator (SGI) at "0.05 is not defined byour data.

For 40.06, we postulate that the holes trapped atcation vacancies form hole-rich clusters within which thecooperative JahnTeller deformations become dynamicrather than static to give three-dimensionalas againsttwo-dimensionalferromagnetic coupling within a clus-ter. Moreover, ferromagnetic order within a cluster in-creases cos(

GH/2) to near unity, so

4h/2N may be

shortened to where real charge transfer takes place, thusincreasing the onset temperature for the formation of super-paramagnetic clusters to near room temperature. Below

,



11/12

the O-orthorhombic matrix orders antiferromagnetically,but with weakly canted spins as in the parent LaMnO

compound. Magnetic coupling through the antiferromag-netic matrix between superparamagnetic clusters randomlydistributed in space gives rise to the spin-glass behavior. Thepeak in the ZFC curve of Fig. 4 for "0.05 is associated

with the temperature of freezing of the superparamagneticspins into different directions as a result of the competingferromagnetic and antiferromagnetic intercluster interac-tions. Magnetic order within the matrix lowers the activa-tion energy for escape of holes from the clusters to thematrix, thus introducing a sharp drop in ( ) on loweringthe temperature through

.

Segregation of the SGI phase at +0.06 from the FIphase at +0.10 is consistent with a continuous increase inthe magnetization through the two-phase region (Fig. 6).Comparable values of the magnetic moment (e.g., 2.5

for

"0.08) have been reported (22).In the range 0.10440.18, the matrix no longer sus-

tains a cooperative, static JahnTeller deformation, andferromagnetic Mn>OMn> superexchange interactionsare stabilized in three dimensions by strong e-electron coup-ling to the oxygen vibrations along a MnOMn bond axisbetween neighboring Mn atoms as has been demonstratedin the LaMn

\VGa

VO

system (27). In addition the Mn>OMn> interactions are ferromagnetic either by superex-change or double exchange; evidence for fast electron trans-fer in compositions with 50.13 suggest a double-ex-change mechanism below

in this compositional range.

A ferromagnetic matrix couples any ferromagnetic clustersferromagnetically. However, at these higher values of, the

cation-vacancy concentration apparently becomes highenough that some clusters within vacancy-rich regions maycontain enough Mn> ions to form antiferromagnetic do-mains within the ferromagnetic matrix, so the full ferromag-netic magnetization is not quite realized for 0.10(40.13.

In the range 0.13(40.18, the volume of the anti-ferromagnetic Mn>-rich clusters increases with . Moreimportant, the increasing value of the tolerance factordecreases the angle of bending of the MnOMn bondsfrom 180, which increases N in accordance with Eq. [9].Below

, cos(

GH/2) increases in the ferromagnetic matrix,

and the ( ) curves of Fig. 10 and the R/R

curves ofFig. 11 indicate that the conduction electrons are no longer

small polarons; fast, real charge transfer between at leastpairs of Mn> and Mn> ions occurs to give ferromagneticdouble-exchange interactions. The ferromagnetic double-exchange interactions vary as cos(

GH/2) whereas the antifer-

romagnetic superexchange interactions vary as cos GH

. Inthis situation, optimization of the angle

GHbetween neigh-

boring spins can stabilize a cant angle (0(GH(180) in the

matrix that increases as the relative strength of the fer-romagnetic double exchange decreases with increasing ,i.e., with decreasing concentration of mobile e electrons.

Canted spins in high fields give a net ferromagnetic moment;at low fields, this type of canting may transform into a heli-cal spiral. We believe the metamagnetic type of increase inthe magnetization with applied field in the range 0.13(40.18 is due to transformations of ferromagnetic spiralspins in the matrix to a canted-spin ferromagnetism.

Two additional observations should be noted. Althoughthe low density of the samples in the range 0.13440.18renders unreliable interpretation of the R/R

curves in

Fig. 11, the Seebeck data of Fig. 10 should be reliable. Themaximum in () at a

+

with a drop to a constant,

essentially zero value below 100 K is similar to the behaviorfound in the system (La

\VNd

V)

Ca

MnO

at the criticaltolerance factor t

for a transition below

from small-

polaron to double-exchange behavior (17). In that case, theconduction electrons do not become metallic in a conven-tional sense; the mobile holes become condensed into vib-ronic states ( ) that show no energy dispersion as illustratedin Fig. 13c. The mobile electrons are extended over at least

Mn>OMn> pairs, and these pairs apparently movewithout any activation energy through the MnO

array.

Our data indicate that a similar phenomenon takes place inthe canted-spin matrix in the range 0.13(40.18.

Finally, the peculiar temperature dependence of the mag-netization of the "0.16 sample (Fig. 7) may reflect themagnetic inhomogeneity. An escape of trapped holes to thematrix with increasing temperature, which is suggested bythe R/R

curves ofFig. 11, would increase the ferromag-

netic component of the coupling in the matrix, therebyreducing the equilibrium cant angle

GHand increasing some-

what the magnetization with increasing temperature.

CONCLUSIONS

The system LaMnO>B contains cation vacancies, not

oxygen interstitials; it corresponds to La\CMn\CO. An-

nealing in air above 1000C yields an O-orthorhombicroom temperature phase stable for 40.06. For 50.10,the high-temperature rhombohedral phase undergoesa transition to an O-orthorhombic structure below roomtemperature as a result of a decrease with temperature of thetolerance factor t(1. The O-orthorhombic structure re-flects a cooperative JahnTeller deformation superposed onthe O-orthorhombic structure. The room temperature toler-ance factor t, calculated from tables of ionic radii, increaseswith through the critical value where, with increasing ,a transition from localized to itinerant electronic behavioroccurs below a ferromagnetic Curie temperature

. In the

cation-stoichiometric perovskites n

Ca

MnO

, wheren may be a combination of rare-earth atoms, the criticaltolerance factor is t

+0.96. The introduction of cation

vacancies into the LaMnO>B system perturbs the periodic

potential of the Mn\CO array so as to shift the critical

tolerance factor from 0.96 to t+0.97.



12/12

The magnetic and transport data for the systemLaMnO

>B reveal the presence of heterogeneous magneticbehavior for '0. The parent compound LaMnO

is

a Type-A antiferromagnet, the cooperative JahnTeller de-formation of the O-orthorhombic structure stabilizinganisotropic MnOMn superexchange interactions: ferro-

magnetic (001) planes coupled antiparallel to one another,but with a weak ferromagnetic moment superimposed dueto spin canting by an antisymmetric superexchange com-ponent. In the range 0(40.06, where X-ray diffractionindicates a single O-orthorhombic phase at room temper-ature, the magnetic data reveal stabilization of super-paramagnetic clusters, presumably associated with holestrapped at cation vacancies, below room temperature;the static cooperative JahnTeller deformation wouldbe suppressed within the hole-rich clusters, where strongelectron coupling to dynamic JahnTeller deformationswould give ferromagnetic superexchange interactions. Theclusters are embedded in an O-orthorhombic matrix that

orders with the Type-A antiferromagnetic order belowa Neel temperature

,. Magnetic coupling between the

spatially disordered ferromagnetic clusters through the anti-ferromagnetic matrix is frustrated and leads to spin-glassbehavior.

In the O-orthorhombic and rhombohedral phases, thereis no cooperative JahnTeller deformation, and ferromag-netic Mn>OMn> superexchange interactions becomeisotropic, i.e., three-dimensional. The Mn>OMn> in-teractions are also ferromagnetic, either via superexchangewhere charge transfer is slow or by double-exchange whereit is fast relative to the period of an oxygen-atom vibration

along a MnOMn bond axis. Consequently the matrixbecomes ferromagnetic. However, in vacancy-rich domainsthere are clusters with high concentrations of Mn> ions,and charge ordering within a cluster (Mn> : teO : 2pLMn> : te interactions) could introduce antiferromagneticclusters embedded in a ferromagnetic matrix in the domain'0.11. For 50.14, the transport data indicate the onsetof double-exchange coupling below

, which may intro-

duce a spin canting of the ferromagnetic matrix that in-creases with ; at larger cant angles spiral-spin configura-tions are stabilized. The result is a relatively sharp drop inthe magnetization with increasing and a metamagnetictype of behavior.

The transport properties show polaronic behavior in theparamagnetic phases with increased trapping out of mobilepolarons with decreased temperature. However, a max-imum thermopower near

signals that below

an in-

creasing bandwidth N associated with magnetic ordercauses a continuous release of holes from the traps onlowering the temperature through

. At lowest temper-

atures, some of the holes released from the traps tend tobecome retrapped; but at higher '

, most of them ap-

pear to become condensed into a vibronic state in which

mobile holes are coupled strongly to the oxygen vibrationsalong a MnOMn bond axis and move diffusively throughthe crystal without any activation energy. This peculiarelectronic state gives a zero thermopower, which signalsthat the electron energies have no dispersion even thoughthe conducting e electrons are delocalized over at least two

Mn atoms.

ACKNOWLEDGMENTS

J. S. Zhou is thanked for making the transport measurements available.One of the authors (JT) thanks the DAAD for a scholarship. The NSF isalso thanked for financial support.

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