Available at: http://publications.ictp.it IC/2009/061

United Nations Educational, Scientific and Cultural Organization and

International Atomic Energy Agency

THE ABDUS SALAM INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS

SYNTHESIS, XRD, 151Eu MÖSSBAUER AND XPS STUDIES OF

NANOCRYSTALLINE EuCrO3 FORMED BY MECHANICAL ALLOYING AND SUBSEQUENT SINTERING

H.M. Widatallah1

Department of Physics, Sultan Qaboos University, P.O. Box 36, 123, Muscat, Oman and

The Abdus Salam International Centre for Theoretical Physics, Trieste, Italy,

S.H. Al-Harthi Department of Physics, Sultan Qaboos University, P.O. Box 36, 123, Muscat, Oman,

C. Johnson

Chemistry Department, The Open University, Walton Hall, Milton Keynes, MK7 6AA, U.K.,

T.H. Al-Shahumi, I.A. Al-Omari, A.D. Al-Rawas, A.M. Gismelseed Department of Physics, Sultan Qaboos University, P.O. Box 36, 123, Muscat, Oman,

D.E. Brown

Physics Department, North Illinois University DeKalb, IL 6011 5, USA

and

C.I. Wynter Nassau Community College, Garden City, NY 11530-6793, USA.

MIRAMARE – TRIESTE

August 2009

1 Regular Associate of ICTP. Corresponding author: hisham@ictp.it

1

Abstract

The influence of mechanical alloying and subsequent sintering of a 1: 1 molar mixture of

Eu2O3 and Cr2O3 on the formation of EuCrO3 perovskite-related nanocrystalline particles is

investigated. Pre-milling the mixture for (75 h) has led to the formation of nanocrystalline EuCrO3

particles (~25 nm) at 900oC (12 h). This temperature is ~300ºC- 400ºC lower than those at which the

material is prepared via the conventional solid state routes. The core and surface structures of the

resulting EuCrO3 nanoparticles were systematically investigated using a host of techniques. The 295 K 151Eu Mössbauer spectrum of nanoparticles revealed only an electric quadrupole hyperfine splitting

reflecting the distorted orthorhombic perovskite structure of the material. Rietveld analysis of the x-

ray diffraction pattern of the nanoparticles favors a structural model with a partial degree of cationic

inversion where ~11% of the Eu3+ and Cr3+ ions substitute each other at the octahedral B- and

dodecahedral A- sites respectively. This cationic distribution has been contrasted with the one taken

for granted for bulk EuCrO3 where the Eu3+ and Cr3+ ions exclusively occupy the A- and B- sites

respectively. A complex surface structure emerged from x-ray photoelectron spectroscopy data where

extremely thin layers of un-reacted Eu2O3 were found to cover most of the nanoparticles’ surfaces with

some traces of monodisperse un-reacted Cr2O3 nanoparticles and elemental Cr. The small bare surface

portions of the nanoparticles enabled the estimation of the binding energies associated with Eu3+ 3d5/2,

Eu3+ 4d3/2, Cr3+ 2p3/2 and O 1s core levels for EuCrO3.

2

1. Introduction

The perovskite-related lanthanide orthochromites, with the general formula LnCrO3 (Ln is a

lanthanide ion) have been widely studied as refractory conducting ceramics by virtues of their

electrical conductivity, oxidation resistance and high melting points (>2400°C). These

properties have led to applications in which the materials were used, e.g., as high-temperature

furnace-heating elements and interconnects in solid state oxide fuel cells1-4. One interesting

material in this family is the orthorhombically distorted europium-orthochromite, EuCrO3 1.

While EuCrO3 has not yet been extensively harnessed relative to other lanthanide

orthochromites, the observation that high power optical pumping of the material triggers a

magnetic phase transition has led to the alluring prospects of using the material as an active

medium in high-density optical storage and optical processing devices5. The unit cells of

EuCrO3 and related materials such as EuFeO3 (orthoferrite) are assumed to be iso-structural

with that of GdFeO3 1,6. Hence material crystallizes in a distorted orthorhombic perovskite

structure (space group D2h16-Pbnm). The distortion from the ideal perovskite is mainly in the

twelve O2--coordinated position of the larger Eu3+ ion, so-called A12 dodecahedral site. The

Cr3+ ion reside in the less distorted six O2- -coordinated position referred to as B6 octahedral

site1,6 However, while accurate atomic coordinates are available for EuFeO3, to our

knowledge, they are not available for EuCrO3. It is believed that the magnetic structures of

both EuFeO3 and EuCrO3 are similar7 Consequently in EuCrO3 the nearest-neighbor Cr3+

ions, as those Fe3+, are assumed to be antiferromagnetically aligned, but a slight canting of the

Cr3+ spins creates a weak ferromagnetic moment below a Néel temperature of 181 K7,8.

Conventionally lanthanide orthochromites, including EuCrO3, are synthesized using

the ceramic technique by combining appropriate metal oxides and/or carbonates and heating

them in air for prolonged times at ≥1200°C 7-10. The process does often involve multiple

heating treatments and regrinding steps to help overcome the solid state diffusion barrier.

Alternative synthesis routes have been developed such as nitrate decomposition, hydrothermal

reaction, co-precipitation, the hydrazine method and decomposition of molecular precursors9.

These methods often involve multiple stages and the precursor always requires heating in a

furnace to ca. 1200°C. Hence it is only natural to assume that the experimental data reported

on EuCrO3 was drawn from bulk or large polycrystalline particles that were prepared at

elevated sintering temperatures7,8. There exists, to our knowledge, no reported work o the

preparation and crystal structure of EuCrO3 nanoparticles. Motivated by the well-established

experimental evidence that magnetic nanoparticles may depict novel properties relative to

their bulk counterparts11 we have, for some time, been trying to identify processing schemes

3

whereby magnetic nanoparticles can be formed so as to subsequently compare their properties

with those of the corresponding bulk12-14. In this respect we recently have reported on using

the simple route of mechanical alloying (mechanosynthesis) followed by sintering to produce

nanocrystalline particles of the perovskite-related compounds SrFeO3-δ (0.5 ≥ δ ≥0.0), at a

significantly modest temperature relative to those at which the materials are conventionally

prepared12. However, while this synthesis route proved useful in significantly lowering the

formation temperature of perovskite12, spinel13 and other oxide nanoparticles14, it can be

susceptible to some inherent disadvantages associated with both mechanical alloying and

ceramic techniques such as the inhomogeneity of the final product due to the presence of

small amounts of un-reacted precursors or contaminants from the milling tools. It is also

possible that the prepared nanoparticles can have a distorted crystallographic structure and/or

a different surface composition relative the conventionally synthesized bulk12, which is often

an advantage as this may lead to the enhancement or suppression of particular structural-

dependant properties to suit particular applications. In this paper we show that the same route

can be used to form EuCrO3 nanocrystalline particles at the relatively low temperature of

900oC. We use x-ray diffraction to explore the structural phase evolution during the synthesis

process of the EuCrO3 nanoparticles. The core and surface structures of the produced

nanoparticles are systematically investigated with a host of techniques. Of particular

importance to us is using Rietveld analysis of x-ray powder diffraction to determine the

accurate atomic coordinates within the unit cell of the produced nanocrystalline EuFeO3. In

the absence of any x-ray photoelectron data regarding the binding energies of the core electrons

of Eu, Cr and O in EuCrO3, the present study will hopefully provide such a data.

2. Experimental

A 1: 1 molar mixture of high purity Eu2O3 and Cr2O3 was prepared by weighing appropriate

amounts of both components and mixing them in an agate mortar. The mixture was then

milled in air for various times of up to 75 h using a planetary ball mill (Fritsch Pulverisette 6)

with tungsten carbide vial (250 ml) and balls (20 mm) at a milling of 300 rpm and a powder-

to-balls mass ratio of 1: 15. The 75 h pre-milled mixture was sintered in air in the range 400

°C-700 °C for periods of 12 h followed by quenching in air. Powder x-ray diffraction (XRD)

patterns were collected with a Philips PW1710 diffractometer using CuKα radiation (λ

=1.5406 Å). The program WinMProf 15 was used for Rietveld structural refinement of the

XRD data. Transmission electron microscopy (TEM) was performed using a JEOL (1200EX)

system equipped with an AMT digital camera operating at an accelerating high voltage of 120

4

kV. Atomic force microscopy (AFM) and magnetic force microscopy (MFM) experiments

were performed using a Nanoscope V with a Multimode SPM unit (Digital Instruments). An

AS-12V E-head piezoelectric scanner and Sb-doped Si magnetic tips with resonant frequency

of 60-100 kHz and spring constants: 1-5 N/m were used in all scans. The thermal variation of

the saturation magnetization was recorded using a DMS-1660 vibrating sample magnetometer

(VSM) in a magnetic field of 1.35 T. The temperature was stabilized within 0.5K. 151Eu

Mössbauer measurement at 300 K was performed using a 151Sm2O3 source in the transmission

mode. Isomer shift calibrations are given relative to iron foil. X-ray Photoelectron

Spectroscopy (XPS) was recorded from a EuCrO3 sample attached on a double sided adhesive

conductive carbon tape. All XPS spectra were collected using a monochromatic Al Kα

radiation (hν= 1486.6 eV) of source voltage 15 kV and emission current of 20 mA at base

pressure of 2 × 10-10 mbar and constant analyzer transmission energy of 20 eV. As charging

effects are unavoidable in the XPS study of EuCrO3 sample, charge compensation was

performed by electron gun flooding and the accuracy of the measured binding energy values

is estimated to be equal to ± 0.2 eV.

3. Results and Discussion

3.1 Synthesis

The XRD patterns recorded form the 1: 1 mixture of Eu2O3 and Cr2O3 following milling at

different times are shown in Figure 1. The progression of the reaction process can be followed

via the relative intensity of the X-ray diffraction lines in Figure 1. For the non-milled (0 h)

mixture, the XRD pattern shows reflection peaks characteristic of the initial precursors, viz,

the cubic polymorph of Eu2O3 and Cr2O3. Examining the XRD profile obtained in the first

hour of the milling process, it is obvious that the reflections peaks of the cubic Eu2O3 phase

dramatically broaden and decrease in intensity. Concomitantly very broad peaks indexable to

the monoclinic modification of Eu2O3 develop. This is implicative that milling quickly

induces a cubic-to-monoclinic phase transformation in Eu2O3. A similar result on milling-

induced phase transformation in Eu2O3 has been reported by Seifu et al and was interpreted in

terms of milling-induced transformation of the complicated cubic structure Eu2O3 (16 formula

units per unit cell) to its simple monoclinic structure (6 formula units per unit cell) 16,17.

However, the reflection peaks of the Cr2O3 precursor remain almost unchanged during the first

1 h of milling. As milling proceeds for 10 h, the peaks of the Cr2O3 component start to

broaden and decrease in intensity suggesting the onset of a fragmentation process for the

5

Cr2O3 particles. Simultaneously, the reflection peaks of both of Eu2O3 polymorphs

progressively broaden and overlap suggesting that both phases co-exist as nanoparticles.

Given the nature of mechanical milling, these results suggest that high surface area

nanoparticles of Eu2O3 and Cr2O3 are formed by milling in the first 10 hours and it would be

reasonable to expect these nanoparticles to be tightly pressed together with a large interface

between the reactant particles. This has resulted in a milling-induced reaction where the

reflection peaks of sought perovskite-related EuCrO3 phase dominated the XRD pattern of the

20 h pre-milled mixture. The reflection peaks of resulting EuCrO3 continued to intensify with

milling time as is seen from the XRD patterns recorded from the milled mixture after 30 h and

40 h that show the precursors’ peaks to continuously vanish. Further milling to 60 h and 75h

shows that the reaction leading to the formation of the required EuCrO3 phase is almost

completed except for a small amount of Cr2O3 as is clear from the presence of its most

dominant peaks at the angles 2θ ~24.4º and ~36.2º.

20 30 40 50 60 70 80

10 h

30 h

40 h

60 h

*******

EuCrO3

cubic Eu2O

3Cr

2O

3

75 h

20 h

1 h

0 h

Inte

nsity

(arb

it. u

nits

)

2θ / (degree)

Figure 1: The x-ray powder diffraction patterns recorded form the 1: 1 molar mixture of Eu2O3 and Cr2O3 milled for the times indicated.

6

20 30 40 50 60 70

****

**

*

**

**

**

*

EuCrO3

900Co

850Co

400Co

800Co

700Co

600Co

500Co

as milled

Inte

nsity

(arb

it. u

nits

)

2θ

Figure 2: The x-ray powder diffraction patterns recorded from the 75 h pre-milled 1: 1 molar mixture of Eu2O3 and Cr2O3 following sintering (12 h) at the temperatures indicated.

Figure 2 shows the XRD patterns recorded from the 75 h pre-milled mixture of Eu2O3

and Cr2O3 following heating at different temperatures (12 h). As the sintering temperature

increases from 400°C, in steps of 100°C, the reflection peaks of the EuCrO3 phase start to

sharpen indicating an increase in the crystallite size, whereas those of the un-reacted Cr2O3

start to gradually disappear with increasing sintering temperature. It was only after sintering

the 75 h pre-milled mixture Eu2O3 and Cr2O3 at 900°C (12 h) that the XRD pattern refined,

within experimental errors, to a single perovskite-related EuCrO3 phase. One concludes that

the present preparation route of high enrgy ball milling followed sintering leads to the

formation of a single perovskite-related EuCrO3 phase at 900°C. This temperature is ~300°C-

400°C lower than those at which EuCrO3 forms by the conventional high-temperature 7,8.

The morphology and size of the EuCrO3 particles produced was inferred from both

TEM and AFM. The TEM image shown in Figures 3(a) reveals the EuCrO3 nanoparticles

obtained at 900°C to be generally spherical or oblate in shape with an average diameter of ~25

nm. The tendency of a few particles to agglomerate can be realized. The analysis of the

morphology of the nanoparticles also involved observing their surfaces. Figure 3(b) shows the

AFM a 3μm × 3μm topography tapping mode image of the EuCrO3 nanoparticles. The cross-

sectional height data along the dotted line of Figure 3(b) is shown in Figure 3(c). Figure 3(d)

shows the 3D topography image of same image. All AFM images were taken simultaneously.

7

The mean surface roughness, which can be taken as a measure of the nanoparticle size in the

sample, found from the AFM topography image is 73 nm. The agglomeration of the

nanoparticles is evident from the height data of the AFM topography image of Figure 3(c)

where clusters of different sizes such as those surrounded by closed loops can be observed.

This pronounced agglomeration and stacking of the nanoparticles in the AFM relative to the

TEM one is, of course, is partly related to the ways in which the samples were prepared.

Discrepancies in the lateral sizes obtained from TEM and AFM techniques are expected due

to the effect of the AFM tip convolution which always results in larger values of the lateral

sizes relative to the real ones 18.

Figure 3: (a) TEM image of the EuCrO3 nanoparticles prepared at 900oC (see text) (b)

Topography (AFM) tapping mode image of the EuCrO3 nanoparticles. (c) The height data along the dotted line in (b). (d) The 3D topography image of (b). All AFM images were taken simultaneously.

(c) (d)

(a) 300 nm

(b)

8

3.2 Mössbauer and XRD structural analysis of the EuCrO3 nanoparticles

Figure 4: The 151Eu Mössbauer spectrum recorded at 295 K from the EuCrO3 nanoparticles prepared by sintering the 75 h pre-milled 1: 1 molar mixture of Eu2O3 and Cr2O3 at 900oC (12 h).

Figure 4 shows the 151Eu Mössbauer spectrum recorded at 295 K from the EuCrO3

nanoparticles to be a single broad line. The value of the isomer shift of 0.640 mm/s relative to

EuF3 confirms that in these nanoparticles the Eu ions are in the trivalent oxidation state

(Eu3+)19. This in turn shows that the pre-milling of the initial reactant mixture Eu2O3 and

Cr2O3 in air and its subsequent sintering have not led to reducing Eu3+ to Eu2+. The linewidth

value of 3.562 mm/s is consistent with the reported values for EuMO3 perovskites (M=Cr, Co

and Fe)7,20. This value, being considerably large compared to the theoretical 151Eu Mössbauer

natural linewidth of 1.31 mm/s, is suggestive of the presence of unresolved hyperfine

interactions. To find out whether these hyperfine interactions are of magnetic and/or electric

origins, we can investigate the magnetic behavior of our EuCrO3 nanoparticles at 295KoC, the

temperature at which the Mössbauer spectrum was recorded. For this sake we show in Figure

5 the thermal variation of the magnetization of the EuCrO3 nanoparticles in the temperature

range 100K-830K. While the temperature step of ~20 K used in the measurements is not

convenient for the exact determination of the Curie temperature of the nanoparticles, it is clear

that they exhibit a weak ferromagnetic character at temperatures ≤ 180.3 K and a

paramagnetic behavior at temperatures ≥ 200K. Thus while Figure 5 cannot help us compare

-15 -10 -5 0 5 10 15

-6

-5

-4

-3

-2

-1

0

1

Tran

smis

sion

(%

)

Velocity (mm/sec)

9

between the actual Curie temperature of the EuCrO3 nanoparticles and the corresponding bulk

(181 K) 7,8, we can safely infer from it that nanoparticles are paramagnetic at 295K.

Consequently, one concludes that the observed 151Eu Mössbauer line broadening (Figure 4) is

not of a magnetic origin, but is rather implicative of that the perovskite structure here is a

distorted one as expected. This is because the low symmetry at the Eu3+ sites in the EuCrO3

distorted perovskite structure results in mixing the 151Eu nuclear ground state with some low-

lying excited states7 leading, to an inevitable electric quadrupole interaction. Hence the

Mössbauer observed broadening could solely be attributable to the electric quadrupole

splitting of the 151Eu nuclear ground state.

0 100 200 300 400 500 600 700 800 9000.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

200 K

180.3 K

Mag

netiz

atio

n (e

mu/

g)

Temperature (K)

Figure 5: Thermal variation of magnetization of the EuCrO3 nanoparticles prepared by sintering the 75 h pre-milled 1: 1 molar mixture of Eu2O3 and Cr2O3 at 900oC (12 h).

Having established the Eu oxidation state and distortion of the crystal structure from

Mössbauer spectroscopy we intend in this part to investigate the crystal structure of the

material obtained at 900°C. As we have shown in Figure 2, the XRD pattern only exhibits

reflection peaks attributable to the perovskite-related structure of EuCrO3 (space group:

Pnma)1,6. The apparent peak-broadening reflects an increasing structural disorder as well as

small crystallite size12-14. We have performed Rietveld analysis for the XRD data of the

material obtained at 900°C to rigorously examine (i) whether the EuCrO3 nanocrystalline

particles truly single-phased and (ii) whether the cationic distribution in these nanocrystalline

10

particles is similar to that of bulk EuCrO3 where the Eu3+ and Cr3+ exclusively occupy the

dodecahedral A- and octahedral B- sites respectively1,6. Bearing in mind the relatively low

signal-to noise nature of the experimental data, it was found that the observed XRD pattern

was amenable to a reasonable fitting using a structural model corresponding to a single

EuCrO3 phase but with the same space group but different cationic distribution from that of

GdFeO3 6. The lattice parameters obtained for the nanoparticles are in good agreement with those

expected for the corresponding bulk. It is interesting to note in Table I how the cationic disorder

on Eu3+ and Cr3+ crystallographic A- and B- (or respectively the 4c and 4a in Wyckoff

notation) sites has been partly interchanged relative to that of bulk, namely about 11% of the

Eu atoms occupying the Cr sites and vice versa. This finding could be a consequence of the

milling process that has induced a disorder in that cationic positions relative to that of the

expected GdFeO3- configuration (see Table I). In this context, we have recently reported a

milling-induced structural distortion for SrFeO2.875 perovskite-related nanocrystalline particles

prepared using an identical synthesis route to the one used in this work12 Incidentally,

according to the magnetic structure discussed earlier the refined structure should bring about a

change in the Cr-Cr, Cr-Eu and Eu-Eu magnetic exchange interactions relative to those

expected for an ideal GdFeO3-configuration of the bulk material. This in turn should bring

about some change in the magnetic properties of the EuCrO3 nanoparticles relative to their

bulk material. It is possible from Figure 5 that the Curie temperature of the nanoparticles is

slightly higher than that reported for bulk EuCrO3 (181 K). However, as correlating the

crystal structure and magnetic behavior of the present nanoparticles is beyond the scope of

this article, we hope to investigate such possibilities in a forthcoming article. However, the

best Rietveld refinement of the XRD data (Figure 6) was obtained on the basis of a two-phase

model with (~ 96.8%) of the profile refining to a nanocrystalline EuCrO3 phase in which the

fractional coordinates are similar to those shown in Table I, The rest of the diffraction profile

(~ 3.24%) was indexable to, a presumably un-reacted, nanocrystalline Cr2O3 phase. A final

comment in this context goes for the implications of the very small nanocrystalline Cr2O3

phase suggested by the XRD Rietveld refinement as being impurity phases in the prepared

nanocrystalline EuCrO3. The 1: 1 molar ratio of the initial reactants’ mixture of Eu2O3 and

Cr2O3 clearly presupposes the presence of Eu2O3 with the same number of molecules as that of

the un-reacted Cr2O3 in the final material obtained at 900°C. This small amount of Eu2O3 was

undetectable by XRD due to the nature of the planetary milling process and the very different

mechanical properties of “hard” Cr2O3 and “soft” Eu2O3, manifested in the first stages of the

milling (Figure 1). It is possible that this soft Eu2O3 has formed very thin layers that could not

11

be detected by XRD, around the nanocrystalline EuCrO3 particles as milling proceeded. Such

a model could be proved or refuted in the subsequent part when we discuss the surface

structure of the nanocrystalline EuCrO3 particles.

20 30 40 50 60 70 80

Experimental data Fitted data Difference Cr2O3 EuCrO

3

Inte

nsity

/ (a

rb. u

nits

)

2θ degrees

Figure 6. Observed (dots), fitted (solid line) and difference x-ray diffraction patterns of the EuCrO3 nanoparticles prepared at 900oC (12 h). The bars refer to the positions of the Bragg’s reflection peaks.

Table I. Refined parameters from powder XRD for the nanocrystalline EuCrO3 material obtained at 900oC. Space group: Pnma. Cell parameters: a = 5.5024(4) Å, b = 7.6338(7) Å, c = 5.3535(4) Å; crystallite size: ~25 nm. Fitting parameters: Rwp: 3.6, Rp: 2.9.

Ion Wyckoff’s position x/a y/b z/c occupancy Eu1 4c 0.9923 0.0534 0.2500 0.893 Cr1 4c 0.9923 0.0534 0.2500 0.107 Eu2 4a 0.5000 0.0000 0.0000 0.107 Cr2 4a 0.5000 0.0000 0.0000 0.893 O1 0.0940 0.4730 0.2500 1.000 O2 0.2960 0.6950 0.0480 1.000

For bulk EuCrO3 with GdFeO3-like structure (see text): Eu is located at ±(x, y, ¼; ½-x, y+½, ¼) with x=-0.018 and y=0.060; Cr is located at (½, 0, 0; 0,½,0; ½, 0, ½; 0, ½,½); 4O are located at the same fractional positions listed for Eu atoms but with x=-0.05 and y=0.47); 8O are located at ± (x, y, z; ½-x, y+½, ½-z; -x,-y,z+½; x+½, ½-y, z) with x= -0.29, y= 0.275 and z= 0.05 11.

12

3.3 Surface analysis of the EuCrO3 nanoparticles

XPS data were recorded so as to determine the valence state of the cations on the surface of

the EuCrO3 nanoparticles and the chemical compositions of the surfaces. The Eu 3d core level

XPS spectrum (Figure 7) recorded from the EuCrO3 nanocrystalline particles depicts a simple

spin-orbit doublet with a spin orbit splitting of 29.9 eV. To appreciate the origin of this

spectrum we note that generally for compounds with Eu2+ and Eu3+ mixed valence states, the

Eu 3d XPS spectrum is composed of two doublet namely Eu2+(3d5/2,d3/2) and the

Eu3+(3d5/2,d3/2) doublet21,22 The binding energy values of 1133.7 eV and 1163.6 eV as well as

the spin orbit splitting of the Eu 3d core level XPS spectrum (Figure 7) are in good agreement

with earlier reports for Eu3+(3d5/2,d3/2) doublet21-23. The Eu 3d spectrum reveals no traces of

Eu2+ ions, whose binding energies are expected to be ~1124 eV and ~1153.6 eV21-24. The Eu

4d core-level spectrum recorded from the surface layers of the EuCrO3 nanoparticles, shown

in the inset of Figure 7, is typical of trivalent europium compounds showing a typical

Eu3+(4d5/2,4d3/2) spin orbit splitting between~ 135.0 and 142.5 eV 25,26 Taken together, these

results reveal that only trivalent Eu3+ ions exist on the surface of the EuCrO3 nanoparticles.

115 120 125 130 135 140 145 150

Eu 4d3/2

1163.6

Shake-down satellite (128.5 eV)

Eu 4d5/2

binding energy (eV)

1120 1140 1160 1180 120012k

14k

16k

18k

20k

22k

24k

26k

(1153.6)

(1124)

1133.7

Δ = 29.9 eV

Eu3+(3d5/2) Eu3+(3d3/2)

Inte

nsity

Binding energy (eV)

Figure 7. The XPS Eu 3d core level spectrum recorded from of the EuCrO3 nanoparticles prepared at 900oC (12 h). The corresponding Eu 4d core-level spectrum is shown in the inset.

Having established the valence state of the Eu ions on the surface of the nanoparticles,

we now turn to the compositional assignment of the Eu 3d and Eu 4d binding energies. The

13

lack of published XPS data on EuCrO3 makes this not an easy task. However, the low binding

energy values of the Eu3+ 3d5/2 core level (1133.7 eV) and Eu3+ 4d5/2 core level (134.9 eV) are

in excellent agreement with those reported by Mercier et al 27 for Eu2O3. These results are

consistent with our earlier inference, based on the Rietveld refinement of the XRD data, that

un-reacted Eu2O3 can form thin, though not necessarily thickness-uniform, surface layers

surrounding most of the EuCrO3 nanoparticles. The two high binding energy peaks, namely in

both Eu3+ 3d3/2 and Eu3+ 4d3/2 XPS spectra, are however not quite symmetrical as the low

binding energy ones. This is suggestive of a contribution from a small amount of another Eu-

containing compound, which we naturally assume to be EuCrO3, that while having low

binding energy Eu3+ 3d5/2 and Eu3+ 4d5/2 XPS peaks that are coincident with those of Eu2O3, its high energy peaks, Eu3+ 3d3/2 and Eu3+ 4d3/2, may slightly shifted towards higher binding

energy values relative to those of Eu2O3.

575 580 585 5902k

3k

4k

5k

6k

7k

8k

9k

524 526 528 530 532 534

527.2 eV

3

1

Cr3+2p 1/2

Cr3+2p 3/2

Inte

nsity

/ (a

rb. u

nits

)

binding energy (eV)

1 3

2

529.4 eV

530.6 eV

2

O 1S

Bindind energy (eV)

Figure 8. The XPS Cr 2p core level spectrum recorded from of the EuCrO3 nanoparticles prepared at 900oC (12 h). The corresponding O 1s core-level spectrum is shown in the inset.

Figure 8 shows the XPS Cr 2p spectrum recorded from the EuCrO3 nanocrystalline

particles. Two dominant features in the spectrum, peaking at binding energies of 576.4 eV and

586.3 eV are observed. These features are readily associated with Cr3+ 2p3/2 and Cr3+ 2p1/2 in

Cr(III) compounds such as Cr2O3 28-31. While both features are clearly composed of multiple

peaks, we only analyzed the intense and straightforward Cr3+ 2p3/2 peak which was resolvable

14

into three components. The central main component (2) at the binding energy of 576.4 eV,

which amounts to ~ 86% of the whole peak, is exactly similar to that reported for Cr2O3 29.

The component (1) at 573.8 eV with a spectrum weight of not more ~ 4 % relative to entire

Cr3+ 2p3/2 XPS peak, detected due to the surface sensitivity of the photoemission technique, is

similar to that reported for metallic chromium (Cr0) 32,33. Such a presence of the metallic

chromium could be thought of to stem from the weak Cr-O bonds on the surfaces of the

extremely small nanoparticles of the un-reacted Cr2O3 which can be easily broken thermally

as the temperature increased up to 900oC. This in turn could lead to the formation of

monodisperse metallic Cr nanoparticles. As the third component (3), that amounts to ~ 10 %

of the Cr3+ 2p3/2 XPS peak, at the high binding energy of 578.8 eV does not match reported

both Cr2O3 or metallic Cr, we attribute it to EuCrO3; though, unexpectedly, it is similar to the

reported value of the Cr6+ 2p3/2 peak in CrO3 34.

We now turn to the O 1s core level spectrum recorded from the EuCrO3 nanoparticles

shown in the inset of Figure 8. The spectrum could be resolved into three components at the

binding energies 530.6 eV, 529.2 eV and 527.2 eV. The first component is typical of O 1s

core level in Cr2O3 29. The central and main O 1s peak is exactly at the binding energy

reported for Eu2O3 31. This result confirms that most of surface of our EuCrO3 nanoparticles is

covered with a small layer of Eu2O3. Despite lacking information about the O 1s binding

energy value for EuCrO3, the above discussion infers that the third O 1s, at the binding energy

of 527.2 eV is for EuCrO3. Finally we have to stress that the data deduced from our XPS

analysis, which is summarized in Table II, requires confirmation by using more polished

“clean” EuCrO3 surfaces.

Table II: Summary of the measured or estimated binding energy values for the Eu3+3d, Eu3+4d, Cr3+2p, Cr0 and O 1s core levels for the components detected on the surface of the EuCrO3 nanoparticles. Corresponding reported values are shown for comparison. Level/material This study Reported values Reference Eu3+ 3d5/2 in Eu2O3 1133.7 eV 1133.7 eV [27] Eu3+ 4d3/2 in Eu2O3 134.9 eV 134.9 eV [27] Eu3+ 3d5/2 in EuCrO3 1133.7 eV ----------- Eu3+ 4d3/2 in EuCrO3 134.9 eV ----------- Cr3+ 2p3/2 in EuCrO3 578.8 eV ----------- Cr3+ 2p3/2 in Cr2O3 576.4 eV 576.7 eV [29] Cr3+ 2p1/2 in Cr2O3 586.3 eV 586.1 eV [29] Cr L3M23V in Cro 573.8 eV 574 eV [32], [33] O 1s in Eu2O3 529.2 eV 529.2 eV [31] O 1s in Cr2O3 530.6 eV 530.2 eV[8,9] [29] O 1s in EuCrO3 527.2 eV ----------

15

4. Conclusion

Sintering a 1: 1 molar mixture of Eu2O3 and Cr2O3 pre-milled in air for (75 h) led to the

formation of EuCrO3 perovskite-related nanocrystalline particles (~ 25 nm ) at 900 oC (12 h)

which is ~ 300ºC- 400ºC lower than those temperatures at which the material is prepared via

the conventional solid state routes. 151Eu Mössbauer and x-ray photoelectron spectroscopic

techniques have shown only Eu3+ to exist with no traces of Eu2+ in the produced the EuCrO3

nanoparticles. The presence of only hyperfine electric quadrupole splitting in the 151Eu

Mössbauer spectrum of the EuCrO3 nanoparticles was associated with their distorted

orthorhombic perovskite structure of the . We have deduced from the XRD pattern of the

EuCrO3 nanoparticles a structural model having a partial degree of cationic inversion with

11% of the Eu3+ and Cr3+ ions substitute each other at the octahedral B- and dodecahedral A-

sites respectively. This structural model is different from that assumed for bulk EuCrO3 where

the Eu3+ and Cr3+ ions exclusively occupy the A- and B- sites respectively. A complex surface

structure was revealed from XPS data where about 3% unreacted monoclinic Eu2O3 forms

extremely thin layers around the EuCrO3 nanoparticles. Also some traces of monodisperse

Cr2O3 nanoparticles and elemental Cr were found to exist at the nanoparticles’ surfaces. A

small portion of a “clean” EuCrO3 surface component enabled us, for the first time, to

estimate the binding energies associated with Eu3+ 3d5/2, Eu3+ 4d3/2, Cr3+ 2p3/2 and O 1s

electron core levels for the compound.

Acknowledgments

This research was partly supported by Sultan Qaboos University (Research Grant:

SQU/Sci/Phys/06/04). H.M. Widatallah is thankful to the Abdus Salam International Centre

for Theoretical Physics (ICTP), Trieste, Italy, where parts of this paper have been written and

to the very supportive Intisar Sirrour. This work was done within the framework of the

Associateship Scheme of ICTP.

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