Structural and magnetic properties of

LaFe0.5Cr0.5O3 studied by neutron diffraction, electron

diffraction and magnetometry

A.K. Azad a,*, A. Mellergard a, S.-G. Eriksson a, S.A. Ivanov b,S.M. Yunus c, F. Lindberg d, G. Svensson d, R. Mathieu e

a Studsvik Neutron Research Laboratory, Uppsala University, SE-611 82, Nykoping, SwedenbKarpov Institute of Physical Chemistry, 10 105064 Moscow, Russia

c Institute of Nuclear Science & Technology, Bangladesh Atomic Energy Commission,

G.P.O Box 3787, Dhaka 1000, BangladeshdDepartment of Structural Chemistry, Stockholm University, SE-10691 Stockholm, Sweden

eDepartment of Materials Science, Uppsala University, Box 534, SE-75121 Uppsala, Sweden

Received 14 January 2005; received in revised form 28 June 2005; accepted 5 July 2005

Abstract

The structural and magnetic properties of the perovskite type compound LaFe0.5Cr0.5O3 have been studied by

temperature dependent neutron powder diffraction and magnetization measurements. Rietveld refinement of the

neutron diffraction data shows that the compound crystallizes in an orthorhombic perovskite structure with a

random positioning of the Fe and Cr cations at the B sublattice. The magnetic structure at 10 K is a collinear

antiferromagnetic one with the magnetic moment per site being equal to 2.79(4) mB. Magnetisation measurements

confirm the overall antiferromagnetic behaviour. Moreover, it indicates a weak uncompensated magnetic moment

close to the transition temperature TN � 265 K. This moment can be described by a magnetic cluster state, which

remains up to 550 K. Electron diffraction patterns along with high-resolution transmission electron microscopy

images reveal that the crystallites are composed by domains of different orientation, which share the same cubic

perovskite sub-cell reflections.

# 2005 Elsevier Ltd. All rights reserved.

Keywords: A. Oxides; C. Neutron scattering; C. Electron diffraction; D. Crystal structure; B. Magnetic properties

www.elsevier.com/locate/matresbu

Materials Research Bulletin 40 (2005) 1633–1644

* Corresponding author. Tel.: +46 155 221871; fax: +46 155 263001.

E-mail address: azad@studsvik.uu.se (A.K. Azad).

0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2005.07.007

1. Introduction

Structural and magnetic properties of mixed metal oxides are very complex, especially when the oxide

contains more than one magnetic species, or when the crystal structure permits some degree of atomic

disorder [1,2]. The combination of 3d and 4d or 5d elements in a perovskite structured oxide like

AB0xB

001�xO3 or A2B

0B00O6, where A is an alkaline-earth or rare-earth element, and B0/B00 are transition-metal elements, are easy to synthesise as ordered compounds because of the large difference in the atomic

radii. Synthesis of 3d–3d ordered compounds is however difficult because of the very similar ionic sizes.

The undiluted perovskites LaCrO3 and LaFeO3 show antiferromagnetic G-type ordering with Neel

temperatures of 280 and 750 K, respectively. A non-collinear antiferromagnetic ordering of Fe3+ and

Cr3+ and a weak ferromagnetic (FM) component below the Neel temperature was found by the study of

the magnetic characteristics of the LaFexCr1�xO3 compound (0.17 � x � 0.84) [3]. The solid solution of

LaFexCr1�xO3 was described as a B-site disordered system, since no spontaneous magnetic moment was

observed down to liquid helium temperature [4,5]. It was expected that an ordering of the cations on the B

sites into alternate (1 1 1) planes would result in an overlap of the oxygen 2p orbitals with empty

chromium eg orbitals on one side and half filled iron eg orbitals on the other side, yielding a FM Fe–O–Cr

interaction. However, since no spontaneous magnetic moment was observed, it was concluded that the

system was B site disordered.

Band-structure calculations based on the local-spin density approximation (LSDA) predicted that

the ordered LaFe0.5Cr0.5O3 compound, in which Fe and Cr form a NaCl-type lattice, is a half-metallic

ferrimagnetic (FiM) material [6]. Recently, Ueda et al. [7] succeeded in synthesising ordered

La2FeCrO6 as an artificial superlattice of LaFeO3/LaCrO3 (1 1 1) layers using a laser-ablation

technique. Based on the measured value for the saturation magnetic moment and referring to the

Kanamori–Goodenough (KG) rule [8,9], it was argued that the Fe and Cr moments were ferromag-

netically coupled, being inconsistent with the theoretical prediction from band-structure calculations.

A possible explanation for this discrepancy is that LSDA calculations may not be reliable for

transition-metal oxides. The band-structure calculation of Miura and Terakura [10] based on the

generalized gradient approximation (GGA), and also the local density approximation (LDA) + U

method, shows that the ground state magnetic order is FiM in GGA but is sensitive to the local

Coulomb repulsion U. Even a small value of U converts the ground state from FiM to FM. It was

therefore argued that the true ground state of ordered La2FeCrO6 should be FM being consistent

with the KG rule. From the diversified results and explanations it is thus obvious that a clear

understanding of the structural and magnetic properties, and the cation ordering are yet to be

achieved. Neutron diffraction is considered to be a very suitable tool for probing these properties in

such a perovskite system with varying degrees of frustration and dilution. The technique is capable of

providing a microscopic evidence of the existence of magnetic long-range order and short-range

order and a direct measure of the magnitude and orientation of the magnetic spins, which are essential

for characterising such chromium, substituted lanthanum orthoferrites. In this variable temperature

study neutron diffraction, electron diffraction and magnetisation measurements have been carried

out in order to determine the structural and magnetic properties of the compound LaFe0.5Cr0.5O3.

A combination of neutron and electron diffraction is useful to determine the correct space-group,

type of ordering and accurate site occupancies. Low temperature neutron diffraction and magnetisa-

tion measurements can confirm the magnetic structure, spin arrangements and magnetic behaviour.

To our knowledge no neutron diffraction work has been done to determine the nuclear and magnetic

A.K. Azad et al. /Materials Research Bulletin 40 (2005) 1633–16441634

structure of this material. Our study clearly shows the nuclear and magnetic structural properties,

antiferromagnetic transition temperature and the existence of some uncompensated magnetic

moment close to the Neel temperature (TN). We also showed that a magnetic cluster state exists

above TN and holds up to 550 K, which can be described by a distribution of a weak ferromagnetic

component.

2. Experimental

A polycrystalline sample of LaFe0.5Cr0.5O3 was prepared by the solid state sintering method.

Stoichiometric amounts of high purity powders of La2O3, Fe2O3 and Cr2O3, were mixed in an agate

mortar using ethanol. The La2O3 powder was dried at 950 8C for 15 h in a continuous flow of oxygen

before measuring the weight. The mixed powder was placed in an alumina crucible and calcined at

1100 8C for 15 h. The sample was pressed into a pellet and reacted at 1300 8C for 48 h and finally at

1350 8C for 48 h in N2 environment. The sample was reground in each step, and grinding and pelleting

cycles were carried out to ensure the homogeneity of the sample. Part of the material was heated in a

closed quartz tube for 19 days in vacuum at 900 8C and examined by electron and X-ray diffraction,

which shows no change in structure.

X-ray powder diffraction (XPD) patterns were obtained from Guinier film data (Cu

Ka1 = 1.540598A). These data were used to index the pattern. Indexing and refinement of the lattice

parameters were made with the programs TREOR90 [11] and Chekcell [12], respectively. Neutron

diffraction measurements were performed at a number of temperatures in the range 600 K � T � 10 K

using the neutron powder diffractometer (NPD) at the 50 MW R2 Research Reactor at Studsvik,

Sweden. A detector system consisting of a bank of 35 3He detectors was used for recording the neutron

patterns. The scanning of data was done covering a 2u range 4–139.928 with a step size of 0.088. Aneutron wavelength of 1.470(1) A was used in the experiment from a monochromator system

consisting of two parallel copper crystals in (2 2 0) mode. The NPD data sets were refined by the

Rietveld method using the FullProf software [13]. A magnetic phase was included in the refinement as

a second phase for which only the Fe/Cr cations were defined. The diffraction peak shapes were

considered as pseudo-Voigt and background intensities were described by a Chebyshev polynomial

with six coefficients. Corrections for absorption effects were subsequently carried out in the Rietveld

refinements, utilising the empirical value mR = 0.0942 cm�1. Peak asymmetry corrections were made

during refinements.

For the transmission electron microscopy studies the sample was crushed and dispersed in butanol and

a drop of this dispersion was put on a copper-grid covered with an amorphous carbon film. The

microscopes used in this study were a JEOL 3010UHR operated at 300 kVand a JEOL2000FX operated

at 200 kV both with double tilt holders. Simulated electron diffraction (ED) patterns and high-resolution

transmission electron microscopy (HRTEM) images were calculated with the program suite NCEMSS

[14].

Hysteresis, zero-field-cooled (ZFC) and field-cooled (FC) magnetisation measurements were per-

formed using a SQUIDmagnetometer. ZFC and FCmeasurements were done over the temperature range

5–300 K at 0.1 T magnetic field. Hysteresis measurements were done in the field range �4 T at a

temperature 10 K.

A.K. Azad et al. /Materials Research Bulletin 40 (2005) 1633–1644 1635

3. Results and discussion

3.1. Nuclear and magnetic structure

The room temperature XPD patterns could be indexed as a pseudo-cubic orthorhombic phase. NPDdata

were collected at 10 different temperatures 600, 500, 400, 295, 280, 270, 260, 250, 240 and 10 K.At first the

refinement was undertaken using the 600 K data to avoid any magnetic contribution. The Rietveld

refinement confirms that the material crystallises in orthorhombic lattice symmetry. The unit cell

parameters are related to the ideal cubic perovskite as a � H2ap, b � H2ap and c � 2ap (ap � 3.9 A)

where ap is the lattice parameter of the ideal cubic perovskite. The pattern can be indexed in the

orthorhombic space-group Pbnm (cab non-standard setting of Pnma, no. 62). In the Glazer’s notation

[15] this orthorhombic distortion is connected with the tilting (a+b�b�). This data can be refined in the

A.K. Azad et al. /Materials Research Bulletin 40 (2005) 1633–16441636

Table 1

Main crystallographic and magnetic information for LaFe0.5Cr0.5O3 from NPD data at 600 K, 295 K and 10 K

600 K 295 K 10 K

Nuclear phase Nuclear phase Nuclear phase Magnetic phase

Space-group Pbnm Pbnm Pbnm Pmmm

a (A) 5.5556(7) 5.5371(8) 5.5281(6) 5.5281(6)

b (A) 5.5314(5) 5.5189(6) 5.5132(5) 5.5132(5)

c (A) 7.831(1) 7.808(1) 7.7957(8) 7.7957(8)

V (A3) 240.64(6) 238.60(5) 237.59(5) 237.59(5)

La in 4c (x y 0.25)

x �0.0034(1) �0.0073(3) �0.0065(9) –

y 0.0184(5) 0.0239(7) 0.0264(4) –

B (A3) 0.93(3) 0.43(7) 0.14(3) –

Fe/Cr in 4b (0.5 0 0) Fe/Cr in

(0.5 0 0; 0 0.5 0;

0 0.5 0.5; 0.5 0 0.5)

B (A3) 0.56(7) 0.24(4) 0.10(3) 0.10(3)

Magnetic moment (mB) – – – 2.79(4)

O(1) in 4c (x y 0.25)

x 0.067(1) 0.069(3) 0.071(1) –

y 0.489(9) 0.489(8) 0.0489(7) –

B (A3) 0.85(1) 0.40(5) 0.23(7) –

O(2) in 8d (x y z)

x 0.7247(6) 0.7205(6) 0.07198(5) –

y 0.2761(5) 0.2783(5) 0.2804(6) –

z 0.0368(4) 0.0374(8) 0.0369(4) –

B (A3) 0.91(7) 0.67(5) 0.40(5) –

Reliability factors

Rp (%) 3.92 3.94 4.09 –

Rwp (%) 4.99 5.03 5.32 –

RB (%) 2.97 2.62 1.99 –

x2 1.41 1.48 1.86 –

Rmag (%) – – – 2.45

monoclinic symmetry space-groupP21/nwithout any ordering at the B-site. According toWoodward [16],

theP21/n space-group can be used to describe the same tilting system (a+b�b�) if they have ordering among

the B-site cations. Since we have not observed any B-site ordering and, moreover, the monoclinic space-

group does not give better refinement results, the orthorhombic space-groupwas selected as the correct one.

TheFe3+ andCr3+ ions are randomly distributed at theB-cation sublattice. From refinement ofNPDdatawe

observed that the mixing of Fe/Cr is about 0.48(5)/0.52(5) at 295 K. It is also important to state that the

neutron coherent scattering length of Fe andCr is 9.45(2) and 3.635(7) fm, respectively, although their ionic

radii is very similar (0.645 and 0.615 A). Main crystallographic and magnetic information are listed in

Table 1. The observed and calculated patterns, differences and the peak positions after Rietveld refinement

are shown in Fig. 1. The crystal structure was distorted due to the small size of the La3+ cation (tolerance

factor, t = 0.916), which force the (Fe/Cr)O6 octahedra to tilt in order to optimise the La–O bond distances.

Using the ionic radii of Fe3+ and Cr3+, RB0 = 0.645 A and RB00 = 0.615 A, respectively [17], (RB0-RB00)/

RB0 = 0.0465 for this compound, implying that it will be difficult to order the Fe3+ and Cr3+ cations on the

octahedral sites of the structure [18]. The (Fe/Cr)O6 octahedral volume is calculated to be 10.491(2) A3.

The average B–O bond lengths at 295 K compare well with the expected values as calculated from ionic

radii sums; hFe/Cr–Oi 1.9894(2) A (calculated 2.045 A). A bond valence sum calculation [19] using the

initial valence of Fe and Cr as 3+, and bond lengths fromNPD data refined at room temperature shows that

Fe and Cr valences are 3.22+ (R0 = 1.759, B = 0.37) and 2.93+ (R0 = 1.724, B = 0.327), respectively.

A.K. Azad et al. /Materials Research Bulletin 40 (2005) 1633–1644 1637

Fig. 1. Observed (circles) and calculated (continuous line) NPD intensity profiles for LaFe0.5Cr0.5O3 at room temperature

(295 K). The short vertical lines indicate the angular position of the allowed Bragg reflections. At the bottom in each figure the

difference plot, Iobs � Icalc, is shown.

The NPD data below 260 K reveal that some extra reflections are clearly appearing in the diffraction

pattern, and the intensity of these reflections were increased with decreasing temperature due to the

magnetic ordering of the 3d cations. Above 260 K the intensity of the strongest magnetic reflection

(2u = 18.768) becameweaker and broader and finally vanished at 550 K. Belayachi et al. observed this type

A.K. Azad et al. /Materials Research Bulletin 40 (2005) 1633–16441638

Fig. 2. Variation of (a) unit-cell parameters and unit-cell volumewith temperature from NPD data refinements. (b) A plot of the

integrated intensity vs. temperature of the magnetic (1 0 1) peak at 2u = 18.768.

of feature bymagnetisation measurements and the Neel temperaturewas determined to 550 K [3]. In order

to confirm themagnetic origin of the peaks we have collectedNPD data at 600 K.X-ray powder diffraction

datawas also collected at 300, 573, 773 and 973 K.The intensity of the strongestmagnetic reflection (1 0 1)

at 2u = 18.768 fromNPD datawas zero at 600 K. The NPD data show that themagnetic transition occurs in

the temperature range 260–270 K, being consistent with the transition temperature determined from

magnetisationmeasurements TN � 265 K. TheXPD data in the temperature range 300–993 K do not show

any intensity at the (1 0 1) reflection position (2u = 19.678), which clearly indicate the magnetic origin of

the (1 0 1) reflection. Temperature dependentX-ray (300–993 K) aswell as neutron (10–500 K) diffraction

measurements does not show any structural phase transition within the measured temperature range due to

the thermal vibration of the atoms. Thevariation of lattice parameters and unit cell volumewith temperature

obtained from refinement of NPD data is shown in Fig. 2(a). The unit-cell parameters as well as the cell

volume increasewith the increasing temperature. The small anomaly in the cell parameters and cell volume

between 250 and 270 K is related to the paramagnetic to antiferromagnetic phase transition, which was

determined at 265 K. Fig. 2(b) shows the integrated intensity versus temperature plot of themagnetic Bragg

peak (1 0 1) at this angle. We tried to calculate the possibility of spin-glass transition, if any. The computer

programPEAKFIT [20]was used for this purpose.This programwasdeveloped to analyse diffuse signals in

magnetically disordered spinel systems assuming several possible functions for the various physical

parameters contributing to the intensity of the specific reflection. The intensity of the (1 0 1) peak has been

expressed as the sum of three different contributions. The first contribution is for the normal nuclear and

magnetic intensity for the long-range order (LRO). The contribution is for the diffuse peak, which describes

the short-rangemagnetic correlation, and the third contribution is for the sample background, which comes

from incoherent scattering and quite local ordering.

A.K. Azad et al. /Materials Research Bulletin 40 (2005) 1633–1644 1639

Fig. 3. Observed (circles), and calculated (continuous line) plots of NPD Rietveld profiles at 10 K. The short vertical lines

indicate the angular position of the allowed Bragg reflections. At the bottom in each figure the difference plot, Iobs � Icalc, is

shown.

However, the difference between intensity and background was not sufficient to calculate the spin-

glass contribution of the diffuse features of the magnetic Bragg reflections. The magnetic structure can be

modelled as antiferromagnetic with the lattice parameters of the magnetic and nuclear unit cells being

equal. For analysis of the low temperature NPD data a multiphase Rietveld refinement (nuclear + mag-

netic) using the Fullprof software was performed. The magnetic structure was orthorhombic and refined

in the space-group Pmmm as an independent phase for which only the Fe/Cr cations were defined

and only magnetic scattering was calculated. The magnetic unit cell was identical to the nuclear unit cell.

A.K. Azad et al. /Materials Research Bulletin 40 (2005) 1633–16441640

Fig. 4. The orientation of the magnetic moments in LaFe0.5Cr0.5O3 corresponds to a G-type magnetic structure. The arrows

indicate the direction of the magnetic moments.

Fig. 5. ED patterns recorded along [1 0 0] (a) and [0 1 0] (b). Space-group Pbnm is confirmed by zonal absences 0 k l: k = 2n

and h 0 l: h + l = 2n along with row absences h 0 0: h = 2n, 0 k 0: l = 2n and 0 0 l: l = 2n.

The final result of the refinement is shown in Fig. 3. After the full refinement of the profile, including the

magnitude of the magnetic moment and its orientation, a best discrepancy factor Rmag of 2.45% was

reached. The total magnetic moment was found to be 2.79(4) mB. Fig. 4 shows the orientation of the

magnetic moments in the unit cell. The magnetic unit cell parameters were a = 5.5281(6) A,

b = 5.5132(5) A and c = 7.7956(8) A, which was related to the chemical unit cell by the propagation

vector k = (0 0 0).

3.2. Electron diffraction

ED studies and HRTEM images agreed with the Rietveld refined model. As can be seen in Fig. 5 the

Pbnm space-group is confirmed by the reflection conditions 0 k l: k = 2n and h 0 l: h + l = 2n in the

[1 0 0] and [0 1 0] zone-axes patterns. These reflection conditions are not consistent with space-group

P21/n wherein a [1 0 0] zone-axis pattern would have the systematic absences 0 k 0: k = 2n and 0 0 l:

l = 2n. However, it should be mentioned that frequently ED patterns like that shown in Fig. 6(a) is found

when examining the crystal. At a first glance the reflections in this pattern seem to violate to the reflection

conditions of the Pbnm space-group. However it is revealed by HRTEMmicrographs that this ED pattern

is the product of intergrowths of domains oriented along the Pbnm zone-axes [1 0 0] and [0 1 0]. Such a

micrograph is shown in Fig. 7 along with fast Fourier transforms of the different regions. The later agrees

with the [1 0 0] and [0 1 0] zone-axis patterns in Fig. 5 (a) and (b). This is not surprising since these two

orientations share the same sub-cell reflections namely those found in the cubic perovskite zone-axis

pattern h1 1 0i, shown in Fig. 6(b).

3.3. Magnetisation measurements

Fig. 8(a) shows the field-cooled (FC) and zero-field-cooled magnetisation vs. temperature for

LaFe0.5Cr0.5O3 measured with an applied field of m0H = 0.1 T. The temperature at which the FC and

A.K. Azad et al. /Materials Research Bulletin 40 (2005) 1633–1644 1641

Fig. 6. (a) Zone-axis pattern which seems to break the orthorhombic symmetry in favour of monoclinic symmetry. Reflections

0 0 l: l = 2n + 1 is due to double diffraction for example 0 0 1 is the product of 0 2 2 acting as primary beam. (b) Computer

simulated cubic perovskite zone-axis pattern h1 1 0i. The reflections of this pattern are present as sub-cell reflections in both

Pbnm zone-axis patterns [1 0 0] and [0 1 0].

ZFC curves part can be used as an indicator of the magnetic ordering temperature TN � 265 K. Judging

from the weak field induced magnetisation, the overall behavior is AFM, but the magnetic irrever-

sibility and in particular the negative FC magnetisation observed at low temperature indicate a FiM

like behaviour. Using the hysteresis curve shown in Fig. 8(b), it is possible to estimate the magnitude of

the uncompensated moment per formula unit to be �0.04 mB, which proves that the dominating

magnetic behavior is AFM. The ordered AFM magnetic moment of 2.79(4) mB, as found from NPD

data at 10 K, is lower than the expected value, since the average magnetic moment calculated from

the high spin state of Fe (5 mB) and the Cr moment (3 mB) is 4 mB. This discrepancy can be attributed to

the effect of disorder in the magnetic interactions. The antiferromagnetic exchange interactions are of

Fe/Cr–O–Fe/Cr superexchange type, where the Fe/Cr–O–Fe/Cr bond angle is 1808. Magnetic disorder

can arise from antisite disorder of the B-site octahedral sublattice and/or the presence of oxygen

vacancies [20].

A.K. Azad et al. /Materials Research Bulletin 40 (2005) 1633–16441642

Fig. 7. A HRTEMmicrograph that reveal domains of orientations [0 1 0] (area a) and [0 1 0] (area b) intergrowth giving rise to

ED patterns shown in Fig. 6(a). FFT’s of the encircled areas are shown in the lower part of the figure.

4. Conclusions

The nuclear (or chemical) cell is related to the ideal cubic perovskite unit as a � H2a0, b � H2a0 and

c � 2a0 (a0 � 3.89 A). The magnetic ordering is found to be AFM and yields a magnetic moment per B-

site of 2.79(4) mB. The magnetization measurements indicate that the magnetic ordering takes place at

TN � 265 K. Moreover, these results evidence a weak uncompensated magnetic moment at low

temperature. An electron microscopy examination of LaFe0.5Cr0.5O3 support the Rietveld refined model

in the orthorhombic space-group Pbnm. ED patterns along with HRTEM images reveal that the

crystallites are composed by domains of different orientation which share the same cubic perovskite

sub-cell reflections.

Acknowledgements

We are grateful to the Royal Swedish Academy, the Swedish Research Council (VR), the Swedish

Foundation of Strategic Research (SSF) and Bangladesh Atomic Energy Commission (BAEC) for

A.K. Azad et al. /Materials Research Bulletin 40 (2005) 1633–1644 1643

Fig. 8. (a) FC and ZFC magnetisation vs. temperature at m0H = 0.1 T; (b) Magnetisation vs. field T = 10 K for LaFe0.5Cr0.5O3.

financial support. We also gratefully acknowledge the support from J. Eriksen and H. Rundlof during

sample preparation and neutron data collection, respectively.

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