Cation coordination and Fe3+ luminescence in LiAlO2 polymorphs prepared by a hydrothermal method

Article abstract of DOI:10.1016/S0025-5408(99)00014-8

The polymorphs α-, β-, and γ-LiAlO₂ were synthesized by a hydrothermal method. The as-prepared product obtained at 240 °C was β-LiAlO₂, which converted completely to γ-LiAlO₂ above 1000 °C. α-LiAlO₂ was obtained by the decomposition of LiAl(OH)₄·H₂O prepared by imbibition of LiOH into LiAl₂(OH)₇·2H₂O hydrothermally at 140 °C. Solid state MAS NMR (magic-angle spinning nuclear magnetic resonance) studies indicate that Li⁺ uniquely occupies octahedral sites in all the polymorphs. This observation indicates that the results of XRD and crystal structure studies on β- and γ-LiAlO₂ reported in the literature that indicate the presence of Li^- in the tetrahedral site are apparently in error with respect to Li⁺ coordination. The other cation, Al³⁺, occupied octahedral sites in α-LiAlO₂ and tetrahedral sites in the β- and γ-LiAlO₂. The Fe³⁺ doped in the various polymorphic forms of this compound was found to uniquely occupy the octahedral Li⁺ site. EPR spectrum of the Fe³⁺ doped in these polymorphs indicates that during the transformation this site is distorted. The Fe³⁺ photoluminescent emission maximum was different for each polymorph. The difference in the luminescence characteristics among the polymorphic forms is due to the change in the site symmetry because of the distortion of the octahedra occupied by Fe³⁺ across the phase transition. The infrared spectrum indicates that site symmetry is lowered during the phase transition.

Full text of DOI:10.1016/S0025-5408(99)00014-8

Materials Research Bulletin, Vol. 34, No. 2, pp. 249–262, 1999
Copyright © 1999 Elsevier Science Ltd
Printed in the USA. All rights reserved
0025-5408/99/$–see front matter
PII S0025-5408(99)00014-8
CATION COORDINATION AND Fe3؉ LUMINESCENCE IN LiAlO2
POLYMORPHS PREPARED BY A HYDROTHERMAL METHOD
T.R.N. Kutty* and M. Nayak
Materials Research Centre, Indian Institute of Science, Bangalore 560 012, India
(Communicated by M. Greenblatt)
(Received May 22, 1998; Accepted May 22, 1998)
ABSTRACT
The polymorphs ␣-, ␤-, and ␥- LiAlO2 were synthesized by a hydrothermal
method. The as-prepared product obtained at 240°C was ␤-LiAlO2, which
converted completely to ␥-LiAlO2 above 1000°C. ␣-LiAlO2 was obtained by
the decomposition of LiAl(OH)4⅐H2O prepared by imbibition of LiOH into
LiAl2(OH)7⅐2H2O hydrothermally at 140°C. Solid state MAS NMR (magic-
angle spinning nuclear magnetic resonance) studies indicate that Liϩ uniquely
occupies octahedral sites in all the polymorphs. This observation indicates that
the results of XRD and crystal structure studies on ␤- and ␥-LiAlO2 reported
in the literature that indicate the presence of Liϩ in the tetrahedral site are
apparently in error with respect to Liϩ coordination. The other cation, Al3ϩ
,
occupied octahedral sites in ␣-LiAlO2 and tetrahedral sites in the ␤- and
␥-LiAlO2. The Fe3ϩ doped in the various polymorphic forms of this com-
pound was found to uniquely occupy the octahedral Liϩ site. EPR spectrum
of the Fe3ϩ doped in these polymorphs indicates that during the transforma-
tion this site is distorted. The Fe3ϩ photoluminescent emission maximum was
different for each polymorph. The difference in the luminescence character-
istics among the polymorphic forms is due to the change in the site symmetry
because of the distortion of the octahedra occupied by Fe3ϩ across the phase
transition. The infrared spectrum indicates that site symmetry is lowered
during the phase transition. © 1999 Elsevier Science Ltd
KEYWORDS: A. inorganic compounds, C. nuclear magnetic resonance
(NMR), C. infrared spectroscopy, D. luminescence
*To whom correspondence should be addressed.
249

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T.R.N. KUTTY and M. NAYAK
Vol. 34, No. 2
INTRODUCTION
The compound LiAlO2 exists in three different allotropes, namely, ␣-LiAlO2, the low-
temperature (Ͻ400°C) form; ␤-LiAlO2, the intermediate temperature (400–800°C) form;
and ␥-LiAlO2, the most stable high temperature (Ͼ1000°C) form. The latter has received
considerable attention because of its possible industrial applications, which include tritium-
breeding material in fusion reactors [1,2] and electrolyte matrix for the molten carbonate fuel
cells [3,4] wherein ␥-LiAlO2 is used as the nonconducting inert matrix to contain carbonate
electrolyte between the anode and the cathode. Apart from this, when doped with Fe3ϩ
,
␥-LiAlO2 emits in the deep red region, rendering it suitable for artificial plant illumination
phosphor blends. It has been proven experimentally [5–7] that light peaking at about 730 nm
is essential for optimum plant growth and flowering. The low- temperature form, ␣-LiAlO2,
is used as a catalyst support [8] and for the preparation of delithiated alumina compounds [9].
Synthesis and structure of ␣-LiAlO2 has been reported by Poeppelmeier et al. [8]. They
proposed that the structure is based upon a NaCl-like structure wherein all the metal cations
are in the octahedral sites with sheets of alternating LiO6 and AlO6 octahedra elongated on
the c axis, forming a hexagonal lattice. ␤-LiAlO2 is isostructural with ␤-LiGaO2 and has an
orthorhombic cell in which all the atoms are said to be tetrahedrally coordinated by oxygen
[10].
Bertaut et al. [11] determined the crystal structure of ␥-LiAlO2 by neutron and X-ray
diffraction on powder samples and indicated that the structure is based on a tetragonal unit
cell. Marezio [12], based on X-ray diffraction results, confirmed that ␥-LiAlO2 is tetragonal,
with the space group P41212 or its enantiomorph P432121. He concluded that all the cations
are tetrahedrally coordinated by oxygen.
The present investigation aims to determine the coordination of the metal cations in the
allotropes through solid-state MAS NMR. The use of isotropic chemical shifts obtained from
MAS NMR spectra to characterize the coordination sphere of Liϩ and Al3ϩ is well known
from the literature [13–17]. Mu¨ller et al. [13], based on their studies on aluminates and
aluminum oxides, showed that the chemical shifts (␦) of 27Al resonance vary with the local
7
coordination of aluminum. The Li resonance absorption in LiO6 group is observed at ␦ ϭ
0–1 ppm (with respect to LiClO4) [15,17].
The present paper also describes the site preference of Fe3ϩ when doped in these
polymorphs, as determined by electron paramagnetic resonance (EPR) technique, which is
highly sensitive to change in site symmetry. The tetrahedral Fe3ϩ always shows a resonance
signal of g-value of about 4 and the octahedral Fe3ϩ shows an EPR signal of g-value of about
2 [18,19]. The resonance absorption lines may show multiplicities depending upon the
distortion of the sites, e.g., a rhombic distortion will yield three absorption peaks (gx, gy, and
gz) and an axial distortion will show two components (g and gЌ). The luminescent property
࿣
of Fe3ϩ in LiAlO2 polymorphs is also presented in this article.
EXPERIMENTAL
Hydrothermal preparation was carried out in a Teflon (PTFE)-lined Morey-type autoclave
made of stainless steel, type SS318 (the major constituents were Fe 68.5%, Cr 21%, Mn 2%,
Ni 4.5%, and Mo 2.5%). The autoclave was charged with the reactants, viz., hydrated
alumina gel mixed with LiOH in the desired molar ratio. Deionized water was added to the

Vol. 34, No. 2
LITHIUM ALUMINATE LUMINESCENCE
TABLE 1
251
The Products Formed for Two Different Compositions of the Reaction Mixture
Temperature (°C)
Li2O/Al2O3 ϭ 1
Li2O/Al2O3 ϭ 1.5
Ͻ140
160
180
200
240
LDHa
LDH
LDH ϩ ␤-LiAlO2
␤-LiAlO2 ϩ ␥-AlOOH
␤-LiAlO2 ϩ ␥-AlOOH
␤-LiAlO2
LDH ϩ ␤-LiAlO2
␤-LiAlO2 ϩ ␥-AlOOH
␤-LiAlO2 ϩ ␥-AlOOH
␤-LiAlO2
aLDH ϭ LiAl2(OH)7⅐2H2O.
required percentage fill so that the autogenerated pressure was in the range of 5 to 40 MPa
when heated in the temperature range 140–250°C for 8–12 h. The Fe3ϩ (0.1 mol%) doped
samples were prepared by coprecipitating ferric hydroxide along with alumina gel from
mixed ferric chloride and aluminum sulfate solution, using 30% NH4OH. The preparation
was carried out as explained above. The lithium content in the washings was monitored to
measure the extent of the reaction and to compare the composition of the solid obtained,
through the wet-chemical route. Imbibition of LiOH into LiAl2(OH)7⅐2H2O was done by
charging the autoclave with the previously prepared LiAl2(OH)7⅐2H2O and LiOH in the
required molar ratio and heating below 140°C for 12 h.
Phase identification of the powders was carried out by powder X-ray diffraction (XRD),
using a Scintag diffractometer. Infrared absorption spectra were recorded on a Biorad FT/IR
spectrometer in the range 4000–400 cmϪ1. Solid state 27Al and 7Li MAS NMR spectra were
obtained at 78.2 and 116.6 MHz, respectively, using a high resolution NMR spectrometer
(Bruker 300 MHz) at room temperature, fitted with a magic angle spinning probe for rotating
the sample at a frequency of 7 kHz. 27Al MAS NMR chemical shifts (␦) were referenced to
7
1 M Al(H2O)6Cl3 solution having the assigned ␦ ϭ 0 and that of Li chemical shifts were
referenced to 1 M LiClO4. Photoluminescence spectra were obtained at room temperature,
using a Shimadzu RF-540 spectrofluorophotometer fitted with a Hamamatsu R952 red
sensitive photomultiplier. The electron paramagnetic resonance spectra were recorded with a
Varian E109 X-band spectrometer with a TE011 cavity. The g-values were calibrated using
diphenylpicryl-hydrazil (DPPH) having g ϭ 2.0036 Ϯ 0.0003.
RESULTS AND DISCUSSION
The products formed by the hydrothermal synthesis were highly dependent on pressure–
temperature (P-T) conditions as well as composition. The products formed from two different
initial (charge) compositions are shown in Table 1. Temperature of the hydrothermal
reactions was varied, to study the conditions of preparation at which the phase is stable. At
temperatures Ͻ140°C, the product formed was always a layered double hydroxide
LiAl2(OH)7⅐2H2O (LDH). As the temperature increased to 180°C, LDH decomposed to form
a biphasic mixture of ␤-LiAlO2 and ␥-AlOOH. At 240°C, the phase formed was a monopha-
sic ␤-LiAlO2. An increase in Li2O/Al2O3 above the stoichiometric ratio resulted in better
crystallinity, and the excess Li could be washed out. Our results show that the low-
temperature form, ␣-LiAlO2, cannot be formed under these conditions. At low temperatures,

252
T.R.N. KUTTY and M. NAYAK
Vol. 34, No. 2
the stability of LDH is very high; as the temperature is increased, the conditions favor the
formation of ␤ form. The reaction sequence of the formation of ␤ and ␥ can be represented
as follows:
240°C
Al(OH)3⅐xH2O ϩ LiOH
¡ ␤-LiAlO2 ϩ (x ϩ 2) H2O (70 Ͻ x Ͻ 105)
ϳ40MPa
(hydrated alumina gel )
Ͼ 1000°C
¡ ␥LiAlO2
800°C
␤-LiAlO2 ϩ ␥-LiAlO2
(␤ ϩ ␥ phases)
␤-LiAlO2
(as-prepared)
¡
The ␣ form was synthesized by decomposing the precursor having Li2O/Al2O3 ϭ 1. The
precursor is a LDH with the composition LiAl(OH)4⅐H2O prepared by imbibing LiOH into
LiAl2(OH)7⅐2H2O. Figure 1 shows the XRD pattern of the imbibed sample in relation to
LiAl2(OH)7⅐2H2O. The major difference is that some of the basal reflections, (004) and (006),
are split. The splitting of the XRD peaks is due to the insertion of LiOH or [Li(OH)2]Ϫ ions
into the intermediate layer. On further increase of LiOH, the phases stabilized are
LiAl2(OH)7⅐2H2O and ␤-LiAlO2.
Infrared absorption spectra (Fig. 2) of the LiOH imbibed sample (LiAl(OH)4⅐H2O) show
differences from those of the starting composition (LiAl2(OH)7⅐2H2O) by way of intensity
and sharpness of the absorption bands. However, the pattern is the same, indicating that the
basic structure remained undisturbed. The imbibed sample shows absorption due to hydrogen
bonded or bridged hydroxyl groups similar to that of the starting composition. The LiOH-
imbibed sample shows a very strong and sharp peak at 1380 cmϪ1 and the shoulder peaks at
1618 and 1491 cmϪ1 are almost the same as those of the starting composition. Gain in
intensity of the peak at 1380 cmϪ1 is due to the intercalation of Li(OH)2Ϫ in the intermediate
layer. The increase in sharpness as well as the intensity of absorption band below 1000 cmϪ1
(characteristics of the AlO6 group) indicates cation ordering in the octahedral sites.
Figure 3 shows the XRD patterns of different polymorphs obtained from decomposing the
precursor LiAl(OH)4⅐H2O. The XRD patterns were indexed using the reported powder X-ray
diffraction data on these polymorphs [8,10,12]. The low-temperature form, ␣-LiAlO2, was
stable only up to 400°C, above which the compound converted into the intermediate ␤ form.
Both the phases coexisted up to 500°C. The ␤ form was stable up to 800°C. The intermediate
phase, ␤-LiAlO2, coexists with ␥-LiAlO2 up to 1000°C and was completely converted into
␥-LiAlO2 at higher temperatures. ␣-LiAlO2 is highly sensitive to moisture and converts into
a hydrate (LiAlO2⅐H2O) on long time exposure to humid atmosphere free of carbon dioxide.
Solid-State MAS NMR. Knowledge of Al3ϩ and Liϩ coordination is useful in those cases
where complete X-ray crystal structure analyses have been not carried out because of the
7
nonavailability of single crystals. The Li MAS NMR spectra of different polymorphs of
LiAlO2 are shown in Figure 4. All the allotropes showed a single resonance signal with the
chemical shift value of 0.6 ppm with respect to LiClO4 wherein Liϩ are in the octahedral
sites. The periodic repetitions of the satellite peaks on both sides of the 0.6 ppm peak are
spinning side bands. This was verified by recording the spectrum at two different spinning
speeds, whereby the peaks showed shift in the maxima. The nominal chemical shift value of
7Li with respect to LiClO4 indicates that the chemical environment of 7Li in both compounds
is the same. This shows that Liϩ ions uniquely occupy octahedral sites in all the polymorphs.
These results indicate that the conclusion drawn on the basis of X-ray diffraction on powder

Vol. 34, No. 2
LITHIUM ALUMINATE LUMINESCENCE
253
FIG. 1
XRD tracings of (a) LiAl2(OH)7⅐2H2O, (b) LiAl(OH)4⅐H2O, and (c) the multiphase obtained
on imbibing with excess LiOH.
samples by Poeppelmeir et al. [8] regarding the Liϩ coordination in ␣-LiAlO2 is correct,
whereas Marezio’s [10,12] conclusion on the Liϩ coordination in ␤- and ␥-LiAlO2 is
doubtful. Accurate deduction of the coordination number of a lighter element such as Liϩ is
difficult by powder X-ray diffraction techniques. Nuclear magnetic resonance is a versatile
technique for determining the local environment of such ions by referencing to a standard
substance wherein the coordination is known.
Figure 5 shows the 27Al MAS NMR spectra obtained by referencing [Al(H2O)6]Cl3 (␦ ϭ
0). The chemical shift values differed among the polymorphs. The low-temperature
(Ͻ400°C) form, ␣-LiAlO2, showed a chemical shift of 9.5 ppm corresponding to the
resonance behavior of 27Al in the octahedral site. The NMR signal of ␤-LiAlO2 is observed
at ␦ ϭ 78.6, indicating the resonance behavior of 27Al in tetrahedral sites [14]. The ␥-form

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T.R.N. KUTTY and M. NAYAK
Vol. 34, No. 2
FIG. 2
Infrared absorption spectra of (a) LiAl2(OH)7⅐2H2O and (b) LiAl(OH)4⅐H2O.
showed a splitting in the resonance absorption peak with a chemical shift of 70 and 66 ppm,
respectively. The appearance of two resonance peaks is due to the presence of two types of
tetrahedral Al, perhaps differing from one another by their distortion accompanied by a
variation in the nearest as well as the next-nearest environment around the aluminum nuclei.
The line broadening indicates that ␥-LiAlO2 is more disordered than the lower temperature
polymorphs.
The above results indicate that the ␣-to-␤ transformation involves a change in coordination
of Al3ϩ accompanying the structural conversion from hexagonal to an orthorhombic unit cell;
whereas, the ␤-to-␥ transformation does not involve any change in the coordination of Al3ϩ
.
The difference in the chemical shift and the observed splitting in the resonance peaks indicate
multiplicity in the local site symmetry for Al3ϩ, accompanying the orthorhombic-to-tetrago-
nal transformation.
Infrared Spectra. Figure 6 shows the infrared spectra of ␣-, ␤-, and ␥-LiAlO2 in the range
400–1200 cmϪ1. All the polymorphs show multiple absorption bands in the region below
1200 cmϪ1. ␣-LiAlO2 shows a spectrum different from that of either ␤- or ␥-LiAlO2. The
absorption peaks observed at 790, 737, 650, 636, and 477 cmϪ1 are due to the presence of
two types of structural groups, viz., AlO6 and LiO6 group. The two absorption peaks at 790
and 737 cmϪ1 are due to the Al–O antisymmetric stretching (␯3) vibrations, and the shoulder

Vol. 34, No. 2
LITHIUM ALUMINATE LUMINESCENCE
255
FIG. 3
XRD pattern of (a) ␣-LiAlO2, (b) ␤-LiAlO2, and (c) ␥-LiAlO2.
at 650 cmϪ1 and peak at 636 cmϪ1 are due to the antisymmetric bending (␯4). The absorption
peak at about 477 cmϪ1is due to antisymmetric stretching vibrations of LiO6 group.
The IR absorption spectra of ␤- and ␥-LiAlO2 are distinctly different from the spectrum
of ␣-LiAlO2. The spectrum of ␤-LiAlO2 shows a broad absorption band centered at 844
cmϪ1; a triplet at 670, 640, and 610 cmϪ1; and three sharp peaks at 513, 457, and 422 cmϪ1
.
By comparing the present spectrum to that of a compound wherein only AlO4 polyhedra are
present, e.g., BaAl2O4 [20], the absorptions due to AlO4 groups can be identified. Accord-
ingly, the broad absorption band centered at 844 cmϪ1 can be assigned to Al–O antisym-
metric stretching (␯3), the triplet thereafter is due to the antisymmetric bending (␯4), and the
422 cmϪ1 absorption peak is due to symmetric bending (␯2). The sharp peaks at 513 and 457
cmϪ1 are due to the LiO6 antisymmetric stretching and antisymmetric bending, respectively.
␥-LiAlO2 showed a similar spectrum, hence the assignment of the bands remained the same.

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Vol. 34, No. 2
FIG. 4
7Li MAS NMR spectra of (a) ␣-LiAlO2, (b) ␤-LiAlO2, and (c) ␥-LiAlO2.
However, the absorption bands show modifications: The 844 cmϪ1 peak corresponding to the
antisymmetric stretching of AlO4 is broader. The band corresponding to antisymmetric
bending of AlO4 shows a doublet instead of triplet as observed in ␤-LiAlO2. The antisym-
metric stretching band of LiO6 has broadened, which may be due to the merging of the third
absorption band of AlO4 antisymmetric bending. The antisymmetric bending of LiO6 shows
a doublet. The intensity of the absorption band due to symmetric bending of AlO4 has been
reduced. The assignment of absorption bands to lithium polyhedra in ␥-LiAlO2 to LiO4
7
groups by Tarte [21] appears to be incorrect, since solid-state Li MAS NMR shows that
lithium is in 6-coordination in all the polymorphs.

Vol. 34, No. 2
LITHIUM ALUMINATE LUMINESCENCE
257
FIG. 5
27Al MAS NMR spectra of (a) ␣-LiAlO2, (b) ␤-LiAlO2, and (c) ␥-LiAlO2.
The modification of the IR spectrum from one polymorph to another indicates the change
in site symmetry. The presence of multiple IR absorption bands indicates the lowering of the
site symmetry. The LiO6 absorption in the ␣-LiAlO2 shows a broad single band that has split
into two sharp bands in ␤-LiAlO2 and into a triplet in ␥- LiAlO2, which indicates the Liϩ site
is more symmetric in the ␣-LiAlO2, is lowered in the ␤-LiAlO2, and is more asymmetric in
␥-LiAlO2. The site symmetry of Liϩ in ␣-LiAlO2 is D3d, which is lowered to C2v in
␤-LiAlO2 and is further reduced to Cs in ␥-LiAlO2. The asymmetry in this site may be

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T.R.N. KUTTY and M. NAYAK
Vol. 34, No. 2
FIG. 6
IR spectra of (a) ␣-LiAlO2, (b) ␤-LiAlO2, and (c) ␥-LiAlO2.
affected by the distortion of octahedra in different directions during the phase transition. The
difference in the Al–O stretching frequency region between the polymorphs, ␣-, ␤-, and
␥-LiAlO2, is due to the change in the coordination viz., Al3ϩis in octahedral coordination in
␣-LiAlO2 and in tetrahedral coordination in the other two polymorphs. Furthermore, mod-
ifications in the IR absorption bands of ␥-LiAlO2 are due to the distortion of the AlO4
tetrahedra. Also, broadening of the infrared bands usually indicates the distortion in the
coordination polyhedra. The effect of change in the site symmetry is reflected in the optical
properties, such as luminescence. The change in local environment around the optically
active ion due to the polyhedral distortion results in altering of the crystal field, which causes
the difference in the splitting of energy states of the optically active ion from one polymorph
to another.

Vol. 34, No. 2
LITHIUM ALUMINATE LUMINESCENCE
259
FIG. 7
EPR spectra of Fe3ϩ(0.1 mol%) doped (a) ␣-LiAlO2, (b) ␤-LiAlO2, and (c) ␥-LiAlO2.
EPR Spectra of Fe3؉ Ions. Figure 7 shows the EPR spectrum of Fe3ϩ (0.1 mol%) doped
in different polymorphs. All the polymorphs show a single resonance absorption peak
corresponding to the g-value of ϳ 2.00. Ferric ion with a d5 configuration, with S ϭ 5/2,
should show a 5-line spectrum corresponding to the five S-state transitions. Because of large
initial splitting, only the signal corresponding to a ϩ1/2 7 Ϫ1/2 transition is observed in the
spectrum recorded using an X-band spectrometer. It is well known from the literature [18,19]
that the octahedral Fe3ϩ shows an EPR signal at g ϳ 2.00 and the tetrahedral Fe3ϩ shows an
EPR signal at g ϳ 4.2. Thus, the Fe3ϩ ions preferentially occupies the octahedral Liϩ sites,
rather than Al3ϩ sites, in LiAlO2, irrespective of the polymorphic form. Table 2 shows the
linewidth of the EPR signal in the different polymorphs. The linewidth of EPR signal in
TABLE 2
Linewidth of the Fe3ϩ EPR Signal in
Three Polymorphs at 25°C
Polymorphic form
Linewidth (Gauss)
␣-LiAlO2
␤-LiAlO2
␥-LiAlO2
350
400
700

260
T.R.N. KUTTY and M. NAYAK
Vol. 34, No. 2
FIG. 8
Photoluminescence excitation and emission spectra of Fe3ϩ (0.1 mol%) doped in different
polymorphs of LiAlO2 at 300 K. (a), (b), and (c) are the excitation spectra of ␣-, ␤-, and
␥-LiAlO2 for ␭emi ϭ 712, 724, and 732 nm, respectively. (aЈ), (bЈ), and (cЈ) are the emission
spectra of ␣-, ␤-, and ␥-LiAlO2 when excited at 294 nm in the case of ␣-LiAlO2 and 264 nm
in the case of ␤- and ␥-LiAlO2. (d), (e), and (f) are low temperature (200 K) emission spectra
of ␣-, ␤-, and ␥-LiAlO2, respectively. ⌽ is the spectral radiant output power and qr is the
quantum output per unit wavelength interval.
␥-LiAlO2 is two times that of ␣-LiAlO2. The EPR of Fe3ϩ shows line broadening because
of the increased disorder in the lattice.
Luminescence Spectrum. Figure 8 shows photoluminescence–excitation and emission–spec-
tra of Fe3ϩ doped (0.1 mol%) LiAlO2 in the various polymorphic forms. The excitation spectra
of ␣-LiAlO2 have a strong band at 294 nm followed by a weak band at 404 nm and a broad band
of medium intensity at 612 nm. Other polymorphs of LiAlO2:Fe3ϩ exhibit similar spectrum with
the difference being that the excitation bands, except for the peak at 404 nm, have shifted to
shorter wavelengths. The 404 nm peak remained at the same position. The excitation spectrum of
␤-LiAlO2 shows a strong band at 264 nm followed by a weak band at 404 nm and a medium
strong band at 512 nm. The half bandwidth of the 264 nm band has reduced considerably from
that of the 294 nm band in ␣-LiAlO2. The excitation spectrum of ␥-LiAlO2 is similar to that of
␤-LiAlO2 except that the intensities of all the bands have increased. Emission spectrum of all the
polymorphs show a single broad band with different emission maximum. The emission of
␣-LiAlO2:Fe3ϩ peaks around 712 nm and that of ␤- and ␥-LiAlO2 at 724 and 732 nm,
respectively. The emission spectra (Fig. 8d–f) recorded at low-temperature (200 K) show a sharp
band followed by a broad band with superimposed fine structures. For ␣-LiAlO2, the sharp band
is observed at 670 nm; whereas, in the case of ␤- and ␥-LiAlO2, it is observed at 710 nm.

Vol. 34, No. 2
LITHIUM ALUMINATE LUMINESCENCE
261
Ferric ion (Fe3ϩ) has d5 electronic configuration and involves the intraconfigurational d-d
transitions. The electric dipole transitions are both spin and parity forbidden. However, mixing of
the opposite parity states by odd components of the crystal field violates the selection rules. The
ion situated at a site that lacks inversion symmetry exhibits stronger optical transitions than those
at centrosymmetric sites. In all the polymorphs, Fe3ϩ is in octahedral sites, as indicated by the
EPR results. Even though, in ␣-LiAlO2 both Al3ϩ and Liϩ are in octahedral sites, Fe3ϩ occupies
the Liϩ site (octahedral) as in the other polymorphs, as indicated by the similarity of excitation
and emission spectra. For the reasons given above, a strong excitation band is not expected from
this type of transition. Hence the strong band in the short wavelength region of the excitation
spectra (264 nm in the case of ␤- and ␥-LiAlO2; 294 nm for ␣-LiAlO2) can be assigned to the
Fe3ϩ4O2Ϫ charge transfer band. The shift in the charge transfer band to shorter wavelengths in
␤ and ␥ forms and the increase in intensity are due to the change in site symmetry, as indicated
by the IR results. Prevalence of this charge transfer band is essential for the observation of
luminescence as the energy absorbed is transferred on to the emitting levels. This is further
substantiated by recording the emission spectrum with 404 nm fixed as the excitation maximum,
which shows a very weak emission spectrum. Irrespective of the emission maximum, the charge
transfer band is the strongest among all the excitation bands, indicating that the energy is derived
from the charge transfer band. This type of band is also observed in LiAl5O8:Fe3ϩ[17] at 294 nm.
The weaker bands thereafter are due to the internal transitions of Fe3ϩ center, which are indicated
in Figure 6 by spectroscopic notations. The 404 nm peak position did not change in the different
polymorphs. This can be explained using the Tanabe-Sugano diagram [22], which shows the
4
4
crystal field dependence of the energy levels of the Fe3ϩ. The excited Eg, A1g(4G) state is
parallel to the ground state, i.e., no shift is observed with change in the crystal field; hence, in all
the three polymorphs, the position of this band remained the same. Assignment of the other bands
was done by fixing the 404 nm peak as the highest energy transition.
The sharp band observed in the emission spectrum recorded at low-temperature (200 K) is the
4
zero phonon line (ZPL) and is assigned to T1g(4G)36A1g(6S) transition. The assignment is
further supported by the overlap of this band in the excitation and emission spectrum at the same
position. The associated broad band is a vibronic band, and the superimposed fine structure is due
to the vibronic transitions. The shift in the emission band can be explained based on the change
in site symmetry. IR results indicate a lowering of site symmetry across the phase transition. The
effect of this reduced symmetry is to modify the crystal field, which, in turn, modifies the splitting
of energy states of the optically active ion, which is reflected in the luminescence spectrum.
Lowering of the site symmetry is due to the distortion in the octahedral sites as a result of the
rearrangement of coordination polyhedra during the transformation from ␣ to ␤ or ␥. The
distortion is also indicated by the broadening of the EPR signal of Fe3ϩ, as shown in Figure 6.
As the electric dipole transition is forbidden for Fe3ϩ ion occupying a totally symmetric site, the
intensity of emission is very low as this type of transition lacks strong excitation bands. As the
site symmetry deviates, the probability of transition increases and hence the intensity of the
emission increases.
CONCLUSIONS
The hydrothermal route is a convenient synthetic method for the preparation of LiAlO2 in
different polymorphic forms. The ␥-LiAlO2 prepared through this route is phase-pure and
highly stable. The structural transitions do not involve any change in the Liϩ coordination

262
T.R.N. KUTTY and M. NAYAK
Vol. 34, No. 2
that occupies the octahedral sites in the three polymorphs. Whereas across the ␣-to-␤
transformation, Al3ϩ changes its coordination from octahedral to tetrahedral. The ␤-to-␥
transformation does not involve the change in Al3ϩ coordination, although the distortion due
to the rearrangement of polyhedra is obvious. The MAS NMR results clearly rule out the
presence of Liϩ in the tetrahedral sites in ␤- and ␥-LiAlO2, in contrast to the conclusions of
Marezio [10,12] from XRD studies. The ferric ion occupies the octahedral site in the lattice
and can be an excellent structural probe since its spectroscopic properties vary with the site
symmetry.
REFERENCES
1. B. Rasneuer, Fusion Technol. 8, 1909 (1985).
2. J. Jimenez-Beceril, P. Bosch, and S. Bulbulian, J. Nucl. Mater. 185, 304 (1991).
3. A.J. Appleby and F.R. Foulkes, in Fuel Cell Handbook, p. 297, Van Nostrand Reinhold, New
York (1989).
4. K. Kinoshita, J.W. Sim, and J.P. Ackerman, Mater. Res. Bull. 13, 445 (1978).
5. M.W. Parker and H.A. Bothwick, Plant Physiol. 24, 345 (1949).
6. J. Rabatin, Electrochemical Society Spring Meeting Extended Abstracts, Philadelphia, PA, USA,
8-13 May 1977, p. 1027, abst. no. 333.
7. J.G. Rabatin, J. Electrochem. Soc. 125, 920 (1978).
8. K.R. Poeppelmeier, C.K. Chiang, and D.O. Kipp, Inorg. Chem. 27, 4523 (1988).
9. K.R. Poeppelmeier and D.O. Kipp, Inorg. Chem. 27, 766 (1988).
10. M. Marezio, Acta Crystallogr. 18, 481 (1965).
11. E.F. Bertaut, A. Delapalme, G. Bassi, A.D. Burif-Varambon, and J.C. Joubert, Bull. Soc. Fr.
Mine´r. Cristallogr. 88, 103 (1965).
12. M. Marezio, Acta Crystallogr. 19, 396 (1965).
13. D. Mu¨ller, G. Gessner, H.J. Behrens, and G. Scheler, Chem. Phys. Lett. 79, 59 (1981).
14. D. Mu¨ller, G. Gessner, and G. Scheler, Polyhedron 2, 1195 (1983).
15. M. Carewska, S. Scaccia, F. Croce, S. Arumugam, Y. Wang, and S. Greenbaum, Solid State Ionics
93, 227 (1997).
16. M. Nayak, T.R.N. Kutty, V. Jayaraman, and G. Periaswamy, J. Mater. Chem. 7, 2131 (1997).
17. T.R.N. Kutty and M. Nayak, J. Alloys Compd. 269, 75 (1998).
18. E. Boesman and D. Schoemaker, C. R. Acad. Sci. Paris, 1931 (1961).
19. R.W. Kedzie, D.H. Lyons, and M. Kestigian, Phys. Rev. 138, A918 (1965).
20. M. Nayak and T.R.N. Kutty, Mater. Res. Bull. 31, 227 (1996).
21. P. Tarte, Spectrochim. Acta 23A, 2127 (1967).
22. Y. Tanabe and S. Sugano, J. Phys. Soc. Jpn. 9, 753 (1954).