A Raman study of cation-disorder transition temperatureof natural MgAl2O4 spinel

Nguyen Van Minha,b, In-Sang Yanga,*

aDivision of Nano-sciences, Department of Physics, Ewha Womans University, Seoul 120-750, South KoreabDepartment of Physics, Hanoi University of Education, Hanoi, Viet Nam

Received 1 October 2003; received in revised form 28 November 2003; accepted 1 December 2003

Available online 12 February 2004

Abstract

We have measured Raman spectra of samples of natural MgAl2O4 spinel before and after annealing at different temperatures and

quenching. The appearance of 727 cm�1 mode in the samples annealed at 800 8C or higher is believed to be due to the cation-disorder of

spinel. The full-width-at-the-half-maximum (FWHM) values of the 410 cm�1 mode as a function of the annealing temperature clearly shows

an abrupt increase at the temperature between 700 and 750 8C as the annealing temperature increases. Thus, we find the annealing temperature

at which the cation-disorder occurred in the natural MgAl2O4 spinel to be between 700 and 750 8C.

# 2004 Elsevier B.V. All rights reserved.

Keywords: Cation-disorder; Raman study; MgAl2O4

1. Introduction

Spinel and spinel-like are subjects of continuous scientific

interest and have been deeply investigated in both earth

science and materials sciences, because of their physico-

chemical properties. Spinel oxides also constitute an impor-

tant class of materials with a variety of interesting electrical,

magnetic, and optical properties. In particular, the magne-

sium spinel, MgAl2O4, has the properties of high melting

point, high strength, resistance to chemical attack, and low

electrical loss [1]. The general formula of magnesium spinel

may be written as (Mg)[Al2]O4, where ( ) and [ ] denote

tetrahedral and octahedral sites, respectively. Two types of

spinel exist: normal and inverse spinel. In the normal spinel,

there are two sub-lattice structures of AlO6 and MgO4

coordinations. The inverse spinel can be described by the

formula (Al)[MgAl]O4. The Mg2þ and Al3þ ions occupy the

octahedral sites in equal proportions. Between these two

extremes, there exist intermediate phases with random

cation distributions.

Analysis of the vibrational spectra of the (Mg)[Al2]O4

spinel, and many other spinels, is complicated by dis-

ordering of the cations and history of the samples. Cynn

et al. [2] have shown that for the natural spinel, the Raman

spectra changed when the sample was annealed at high

temperatures, where a new mode around 727 cm�1 and

the asymmetry of the intense mode near 410 cm�1 were

detected. These features are retained after quenching to

room temperature; thus, the spectrum of quenched natural

spinel differs from that before annealing by the appearance

of a mode at 727 cm�1 and asymmetric broadening of the

mode near 410 cm�1. The origin of 727 cm�1 mode is

claimed to be the Al–O stretching vibration of AlO4 groups

created by redistribution of Al-ions from octahedral to

tetrahedral sites [2]. A similar order-disorder and structural

phase transitions were observed by Raman spectroscopy on

zinc thiourea sulphate and lead lanthanum zirconate titanate

single crystals [3]. In previous Raman studies, however, it

was not shown that at what annealing temperature the

cation-disorder appeared. In this study, we have investigated

the annealing temperature at which the cation-disorder

occurred in the natural MgAl2O4 spinel, and found it be

between 700 and 750 8C.

2. Experiments

The natural MgAl2O4 spinel samples were collected from

YenPhu mine (North Vietnam). The samples were fired in

air at different temperatures (100, 200, 300, 400, 500, 600,

700, 750, 800, 850, 920, and 1000 8C) for 2 h. For investiga-

Vibrational Spectroscopy 35 (2004) 93–96

* Corresponding author. Fax: þ82-2-3277-2372.

E-mail address: yang@ewha.ac.kr (I.-S. Yang).

0924-2031/$ – see front matter # 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.vibspec.2003.12.013

tion of the effect of cooling speed, the annealed samples

were cooled in several different ways: quenched in liquid

nitrogen, quenched in air, furnace cooled, and furnace

cooled with a cooling speed of 5 8C min�1 from 1000 8C.

The Raman spectra of samples fired at 1000 8C and then

cooled in various ways were essentially the same each other,

even though the widths of peaks of quenched (either in liquid

nitrogen or in air) samples seems to be slightly larger than

those of slowly cooled samples. In the present study, all the

annealed samples were quenched in the air.

Raman measurements were performed in a back scatter-

ing geometry using Jobin Yvon T64000 triple spectrometer

equipped with a cryogenic charge-coupled device (CCD)

array detector, using 514.5 nm line of an argon ion laser. For

the temperature dependent measurements, samples were

mounted in a variable temperature closed cycle refrigerator

capable of reaching temperature as low as 10 K.

3. Results and discussions

The natural MgAl2O4 spinel is assumed to have a closed-

packed FCC structure described by the Fd3m space group

with eight molecular units per cubic cell, thus, two chemical

formulas in Bravais cell. A correlation of the site symmetries

with the crystal symmetry enables one to identify the

irreducible representations that describe the normal modes

of vibration associated with each atomic species at the center

of the Brillouin zone:

D3d½Mg� : A2u þ Eu þ 2F1u þ F2u

Td½Al� : F1u þ F2g

C3v½O� : A1g þ A2u þ 2F2g þ 2F1u þ Eg þ F1g þ Eu þ F2u

There are five Raman (A1g þ Eg þ 3F2g) and four infrared

(4F1u) active modes that might appear in the spectra.

The Raman spectra of natural MgAl2O4 spinel measured

at various temperature before and after annealing at 1000 8Cthen quenching to room temperature are shown in Figs. 1 and

2, respectively. In both figures, there is no abrupt change in

the spectra as the temperature is lowered, meaning no

structural phase transition. All the modes of sample before

annealing are sharp and symmetric, and there is no peak near

727 cm�1. In contrast, in the spectra of sample after anneal-

ing and quenching, the peaks are broad and asymmetric,

with an additional broad peak at 727 cm�1, as observed in

earlier work by Cynn et al. [2] The shape and the broadness

of the peaks, especially the 727 cm�1 peak, do not change as

the sample temperature is lowered. This indicates that the

additional peak is rather a fundamental phonon mode, not a

two-phonon mode of 311 cm�1 (F2g) and 410 cm�1 (Eg)

modes [4]. This interpretation is consistent with the fact that

the 727 cm�1 mode has A1g character in the previous

polarized Raman measurements [5].

The appearance of the 727 cm�1 mode and the shoulder

peak of the 410 cm�1 mode in the annealed samples does not

mean a new structure due to annealing. We could not detect

any difference in the X-ray diffraction patterns of samples

before and after annealing at high temperature, using the

single crystal X-ray diffractometer (NONIUS-FR-590). It

would not be possible to identify the slight difference in

the Al3þ–Mg2þ configurations of the natural and annealed

spinel.

The frequency of the 727 cm�1 mode is similar to those of

Raman modes assigned as symmetric stretching vibration of

AlO4 in Ca3Al2O8 [6]. Therefore, it seems to be natural to

800 700 600 500 400 3000

100

200

300

400

500

before annealing

300 K

200 K

100 K

10 K

Inte

nsi

ty (

a.u

.)

Raman shift (cm-1)

Fig. 1. Raman spectra, measured at various temperatures, of natural

MgAl2O4 before annealing. The spectra are shifted in y-axis for clarity.

800 700 600 500 400 3000

200

400

600 after annealing

300 K

200 K

100 K

10 K

Inte

nsi

ty (a

.u)

Raman shift (cm-1)

Fig. 2. Raman spectra, measured at various temperatures, of natural

MgAl2O4 after annealing at 1000 8C for 2 h then quenching to room

temperature. The spectra are shifted in y-axis for clarity.

94 N.V. Minh, I.-S. Yang / Vibrational Spectroscopy 35 (2004) 93–96

assign the 727 cm�1 mode as the symmetric Al–O stretching

vibration of AlO4 groups created by redistribution of some

aluminum ions from octahedral to tetrahedral sites as a result

of the cation-disorder which occurred during the annealing

at high temperature. In a similar token, the shoulder of the

410 cm�1 mode can be attributed to the bending mode for Al

ions in tetrahedral sites.

In order to find the annealing temperature at which the

cation-disorder occurs, we have annealed the natural

MgAl2O4 spinel samples at several different temperatures

and quenched to room temperatures, as mentioned in the

experiment section. Fig. 3 shows the Raman spectra mea-

sured at room temperature of the samples annealed at

different temperatures as noted. The Raman spectra of the

samples annealed at 800 8C and above show the additional

peak at 727 cm�1 clearly. The spectrum of the sample

annealed at 750 8C shows a trace of the 727 cm�1 mode

and the shoulder peak of the 410 cm�1 mode starts to grow.

The asymmetry of the 410 cm�1 mode is evident for the

samples annealed above 750 8C due to appearance of a new

additional bending mode at �400 cm�1 for Al ions in

tetrahedral sites. Fig. 4 shows the full-width-at-the-half-

maximum (FWHM) values of the 410 cm�1 mode measured

at room temperature as a function of the annealing tem-

perature. We present the FWHM values of the 410 cm�1

mode as if it were a single mode just to indicate the effect of

the shoulder peak due to cation-disorder. It clearly shows

that the FWHM values abruptly increases at the temperature

between 700 and 750 8C as the annealing temperature

increases. It is at this annealing temperature that the

cation-disorder in the natural MgAl2O4 spinel occurs.

Decomposing the 410 cm�1 mode in to two components

and comparing the relative intensity of the shoulder peak to

that of the main peak would be a good indicator for the

extent of the cation-disoder caused by the annealing. How-

ever, additional peaks at lower wavenumber masked quan-

titative analysis and the resulting information was as good as

the FWHM analysis above.

4. Conclusions

In conclusion, we have measured Raman spectra of

samples of natural MgAl2O4 spinel before and after anneal-

ing at different temperatures and quenching. Before anneal-

ing, the Raman spectra of natural MgAl2O4 spinel show

sharp and symmetric modes, with no peak near 727 cm�1. In

contrast, the spectra of samples after annealing at 800 8C or

higher and subsequent quenching, show broad and asym-

metric peaks, with an additional broad peak at 727 cm�1

possibly due to cation-disorder. The FWHM values of the

410 cm�1 mode measured at room temperature as a function

of the annealing temperature clearly shows an abrupt

increase at the temperature between 700 and 750 8C as

the annealing temperature increases. We find the annealing

temperature at which the cation-disorder occurred in the

natural MgAl2O4 spinel to be between 700 and 750 8C.

Acknowledgements

We wish to thank Luc Huy Hoang and Nguyen The Khoi

for their X-ray diffraction measurements. We thank Korea

Research Foundation Grant KRF-2000-015-DS0014 for

financial support.

800 700 600 500 400 3000

100

200

300

400

500

TA = 850 oC

TA = 800 oC

TA = 750 oC

TA = 700 oC

Inte

nsit

y (a

.u.)

Raman shift (cm-1)

Fig. 3. Raman spectra measured at room temperature of the samples

annealed at different temperatures as noted. The spectra are shifted in

y-axis for clarity.

0 200 400 600 800 1000

10

20

30

40

FWH

M (

cm-1)

Annealing Temperature (oC)

Fig. 4. The full-width-at-the-half-maximum (FWHM) values of the

410 cm�1 mode as a function of the annealing temperature.

N.V. Minh, I.-S. Yang / Vibrational Spectroscopy 35 (2004) 93–96 95

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