IR and Raman spectroscopy study of YAG nanoceramics

Article abstract of DOI:10.1016/j.cplett.2010.06.033

Y₃Al₅O₁₂ (YAG) nanoceramics fabricated by low-temperature high-pressure sintering were investigated using Raman and FT-IR techniques. It was found that the Raman scattering intensity decreased significantly with increasing sintering pressure. The spectra showed also broadening of the Raman and IR bands. The XRD and TEM studies showed that during fabrication of nanoceramics the average grain size decreased and a partial amorphization took place. It was concluded that these effects are responsible for the changes in the Raman and FT-IR spectra.

Full text of DOI:10.1016/j.cplett.2010.06.033

Chemical Physics Letters 494 (2010) 279–283
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Chemical Physics Letters
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IR and Raman spectroscopy study of YAG nanoceramics
*
A. Lukowiak , R.J. Wiglusz, M. Maczka, P. Gluchowski, W. Strek
Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wroclaw 2, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Y Al O (YAG) nanoceramics fabricated by low-temperature high-pressure sintering were investigated
3
5 12
Received 15 December 2009
In final form 9 June 2010
Available online 12 June 2010
using Raman and FT-IR techniques. It was found that the Raman scattering intensity decreased signifi-
cantly with increasing sintering pressure. The spectra showed also broadening of the Raman and IR
bands. The XRD and TEM studies showed that during fabrication of nanoceramics the average grain size
decreased and a partial amorphization took place. It was concluded that these effects are responsible for
the changes in the Raman and FT-IR spectra.
Ó 2010 Elsevier B.V. All rights reserved.
1
. Introduction
Fabrication of transparent yttrium aluminum garnet (YAG)
sure [7]. In this Letter, we report the infrared and Raman spectra of
YAG nanoceramics fabricated at different pressures. Changes in
vibrational spectra observed for these nanoceramics such as signif-
icant decrease in Raman scattering intensity are discussed in terms
of deformation of the crystallite grains.
ceramics is a promising substitute for single crystal YAG prepara-
tion. As compared with single crystalline YAG, ceramics have sev-
eral advantages such as: larger dimensions of the prepared
samples, shorter time required for fabrication, lower production
cost, and higher concentration of the active ions introduced with
uniform distribution. At present, the densification methods of
nanostructured ceramics are developed to achieve transparent
ceramics with high optical properties. The YAG ceramic fabricated
by a solid-state reaction method requires long sintering at high
temperature (above 1700 °C) under vacuum [1]. The undesirable
grain growth is sometimes observed during this process, resulting
in opaque samples. Various sintering reagents (e.g. tetraethoxysi-
lane) could act as grain growth inhibitor allowing to obtain trans-
parent ceramics [2]. Moreover, application of pulsed direct current
2
. Experimental
The YAG nanopowder was prepared by modified Pechini meth-
od. High-purity compounds have been used for the synthesis.
Yttrium oxide (99.99%) has been digested in hot water solution
of nitric acid. Yttrium nitrate and aluminum chloride (99.999%)
were dissolved in deionized water in stoichiometric amounts (with
3
+
3+
a Y :Al molar ratio of 3:5). The citric acid and ethylene glycol
were added to the salts solution, which were further hold at the
temperature of 80 °C until a brown resin was formed. The resin
was calcined in air at 900 °C for 16 h.
(
DC) and pressure in spark plasma sintering (SPS) enhanced the
Fabrication of nanoceramics was performed using low-temper-
ature high-pressure sintering method described elsewhere [5].
Briefly, before sintering, pellets (4 mm in diameter and approxi-
mately 2 mm in thickness) were formed from the synthesized
powder by cold pressing under low pressure. The sintering process
was carried out in solid-state high-pressure cell of toroid-type. The
samples of nanoceramics were prepared by hot-pressing at the
temperature of 450 °C during 1 min and the applied pressure was
in the range of 2–8 GPa.
The X-ray diffraction (XRD) patterns were collected in 2h range
of 5–120° with X’Pert PRO X-ray diffractometer (PANalytical).
Transmission and scanning electron microscopy (TEM and SEM)
were performed with a Philips CM-20 Super Twin microscope
operating at 200 kV and a FEI Nova NanoSEM 230, respectively.
The Raman spectra of the ceramics were recorded at room temper-
ature using a confocal micro-Raman apparatus (LabRam HR800,
Horiba Jobin Yvon) with a CCD detector. During measurements
the system was equipped with a Â100 objective and a 1800
densification rate of ceramic powders at lower temperatures (heat-
ing for 3 min at 100 MPa and 1400 °C) [3]. To prepare nanostruc-
tured ceramics, also other sintering methods can be applied
where high pressure is used. Fast sintering process at a very high
pressure (8 GPa), with the high-temperature step not exceeding a
few minutes, was found to permit the densification while avoiding
grain growth [4].
Many interesting studies have concerned structural and optical
properties of garnet nanocrystals but only a few authors have used
the vibrational spectra to characterize nanoceramics. We have re-
cently reported the transparent Nd:YAG nanoceramics composed
of nanosized grains fabricated by low-temperature high-pressure
(
LTHP) sintering technique [5,6]. It was demonstrated that a partial
amorphization has occurred in these samples with increasing pres-
*
Corresponding author. Fax: +48 71 3441029.
E-mail address: A.Lukowiak@int.pan.wroc.pl (A. Lukowiak).
0
009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2010.06.033

2
80
A. Lukowiak et al. / Chemical Physics Letters 494 (2010) 279–283
grooves/mm grating. The 632.8 nm line of He–Ne laser was used as
an excitation source. The scattered Raman signal was integrated
for 30–120 s depending on the sample and measured over a spec-
During the densification of nanostructured ceramics, significant
grain growth is usually observed. In this short-term sintering
method, very high pressure and relatively low temperature during
the preparation prevents the grains growth. Rietveld analysis indi-
cates that at 2 GPa, only a slight increase of grain size is observed
(from 35.5 nm at ambient pressure to 36.8 nm at 2 GPa with a
standard deviation of 0.2 nm). The increase of pressure to 4 GPa
leads to decrease of the crystallite size to 26.0 ± 0.18 nm. At higher
pressures, the calculated grain sizes are similar, i.e., they are about
À1
tral range 100–900 cm . The FT-IR spectra were measured with a
Biorad 575C FT-IR spectrometer as KBr pellets in the 4000–
À1
4
00 cm region. The Origin program (OriginLab Co.) was used
for the deconvolution of the FT-IR and Raman spectra in order to
identify bands’ widths and positions.
2
6 nm. These values are obtained from widths of the XRD peaks.
3
. Results and discussion
However, it is worth noting that the peak broadening depends
not only on the crystallite size but also on the microscopic stress
at grains, defects, and dispersion of crystallite sizes.
The grain sizes were also determined using TEM. There was no
significant difference between the samples obtained at 2 and 8
GPa. In both cases, the grains had dimensions in the range of 10–
55 nm. However, it seems that for the ceramic prepared at 2 GPa
a higher number of large particles (with diameter of 35–55 nm)
is observed.
To obtain Y
was pressed into pellets of 4 mm in diameter and less than
.5 mm in thickness. The most transparent ceramic was obtained
3 5
Al O12 nanoceramics, the prepared YAG powder
1
by sintering at 8 GPa pressure (Fig. 1). The samples prepared at 2
and 4 GPa were opaque whereas the nanoceramics sintered under
higher pressures became transparent as a result of decreasing the
size of pores below the limit where they scatter light.
The YAG powder subjected to pressures up to 8 GPa at 450 °C
for short period of time showed no decomposition since only
reflections resulting from the cubic Y Al O12 structure (space
3 5
group I a-3 d, No. 230, 170157-ICSD [8]) were observed in the
XRD patterns measured for the ceramic samples (Fig. 2).
The experimental XRD patterns were used to calculate density,
lattice parameters and crystallite size in the nanoceramics by the
Rietveld analysis [9], using the isotropic approach [10] in the pro-
gram MAUD 2.0 [11]. The final structure factor calculation gave a
Vovk et al. [12] have explained the decrease of the grain size as
a high pressure-induced deformation of nanocrystals that were re-
laxed by forming twins. We suggested that a partial amorphization
of the crystallites has occurred [7]. In the HRTEM micrographs, the
amorphous part of the ceramic samples is well seen (the light gray
areas in Fig. 3). A partial amorphization has occurred already in the
sample prepared at 2 GPa (Fig. 3a) but the amount of amorphous
part increases with increasing pressure. The thickness of the non-
crystalline part between the crystalline particles (and at the edge
of the grains) was from about 1 nm to 15 nm. No evidence of twin-
ning was observed in the TEM pictures. Although our results indi-
cate presence of an amorphous phase, the amount of this phase is
much smaller than the crystalline phase, even for the sample syn-
thesized at 8 GPa. As a result the XRD patterns presented in Fig. 2
show peaks characteristic for the crystalline YAG but the diffrac-
tion peaks from the amorphous phase cannot be clearly seen due
to their low intensity and large width.
w
weighted factor R in the range of 5.28% and 10.36%. All the sam-
3
ples have density above 4.54 g/cm , that is close to the theoretical
3
density of YAG single crystal (4.56 g/cm ). The lattice parameter a
slightly increases with increasing pressure with the lowest and the
highest values equal to 12.0425 and 12.0545 Å for the samples pre-
pared at 2 and 6 GPa, respectively. A standard deviation was less
than 0.001 Å.
The SEM picture of the fracture surface of the 8 GPa ceramic is
presented in Fig. 4. The size of grains is in the 50–90 nm range,
which is higher than that observed with TEM. This discrepancy
can be most likely attributed to the fact that the particles observed
with SEM consist of a number of smaller crystallites. The TEM and
SEM studies of other YAG ceramics obtained by the LTHP sintering
method that were reported in the literature (YAG [13], 1%Nd:YAG
[
5], 5%Yb:YAG [14]) gave similar results.
YAG crystallizes in the cubic structure with eight formula
Fig. 1. YAG nanoceramic obtained at 8 GPa.
units in the unit cell (four formula units in the primitive cell).
For the O structure of YAG group theory predicts 3A1g + 5A2g
+ 14T1g + 14T2g + 5A1u + 5A2u + 10E + 18T1u + 16T2u Brillouin
zone center modes where only the A1g, E , and T2g modes are Ra-
h
+
8
E
g
u
g
man-active and T1u modes are IR-active [15]. IR and Raman studies
were performed to gain more insight into the structures of the YAG
samples.
À1
The FT-IR spectra (Fig. 5) in the 400–1000 cm range show
well defined absorption bands that can be assigned to characteris-
tic vibrations of crystalline YAG [16]. The bands at about 430, 455,
À1
4
8
75, 513, 567, 690, 729, and 789 cm , as well as the shoulder at
20 cm
À1
represent the characteristic metal–oxygen (Y–O and
Al–O) vibrations. The data are in agreement with previous reports
concerning both single crystals [15,16] and nanocrystals of YAG
[
17,18]. With decreasing particle size, shift and broadening of
absorption bands as well as their intensity decrease were usually
observed for YAG [18–21]. For the investigated ceramic samples,
only a slight frequency shift can be observed for some bands. For
À1
instance, with increasing pressure a 3 cm shift towards lower
À1
À1
À1
frequency is observed for the 430 cm band and 3–5 cm shift
Fig. 2. XRD pattern of YAG powder and nanoceramics obtained by sintering at
different pressures.
towards higher frequency for the 455, 475, and 786 cm bands.

A. Lukowiak et al. / Chemical Physics Letters 494 (2010) 279–283
281
Fig. 3. TEM picture of YAG nanoceramic obtained at (a) 2 GPa and (b) 8 GPa.
Fig. 4. SEM picture of fracture surface of YAG nanoceramic obtained at 8 GPa.
Fig. 6. Raman spectra of YAG samples. The spectra for the samples prepared at 4, 6,
and 8 GPa are presented in a similar intensity scale as those recorded for the
powder sample in order to better show the Raman peaks.
À1
and 789 cm exhibit broadening for the ceramics sintered at 2
and 4 GPa but the bandwidth remains unchanged upon further in-
À1
crease of pressure. The changes were in the range of 3–10 cm
.
Fig. 6 shows that the Raman spectra of YAG nanocrystalline
powder and ceramics are similar to the spectrum of a single crys-
tal. For the prepared samples, Raman bands characteristic for YAG
can be seen [15,22–25]. These well resolved and intense bands at
À1
1
60, 218, 259, 340, 371, 400, 729, and 781 cm are marked with
asterisks. However, some weaker bands are also observed in some
À1
spectra at 142, 294, 545, 689, and 856 cm . In contrast to the sin-
gle crystal, peaks appearing in the spectra of the investigated sam-
ples are much broader due to the crystallite size dependence of the
electron–phonon coupling as well as phonon confinement effects
[
26,27]. However, the Raman band positions are almost invariable
as compared to the single crystal.
Intensity of the Raman spectra decreases gradually with
increasing pressure from 4 to 8 GPa. For instance, in order to reg-
ister a well-resolved spectrum, four times longer integration time
was required for the samples obtained at 6 and 8 GPa than for
the powder or 2 GPa ceramic. Therefore, the spectra presented in
Fig. 6 were normalized and for the all ceramic samples are pre-
sented in a similar intensity scale as those recorded for the powder
sample in order to better show the Raman peaks. Raman spectra
Fig. 5. FT-IR spectra of YAG powder and nanoceramics sintered at different
pressures.
The bandwidths also exhibit significant changes. Some bands be-
À1
come broader (bands at 455 and 513 cm ) and other narrower
À1
(
bands at 475 and 820 cm ). The bands at 430, 567, 690, 729,

2
82
A. Lukowiak et al. / Chemical Physics Letters 494 (2010) 279–283
show that with increasing pressure many bands shift towards low-
er frequencies (the difference between the powder and 8 GPa cera-
mic is about 2 cm ). The widths of the Raman bands of the
at 6 and 8 GPa, which further supports are assumption on partial
amorphization of the studied YAG ceramics. The shift and broaden-
ing of the Raman and IR modes may also be a result of the ion sub-
stitution in YAG crystals [23,33]. We have rejected this possibility
because the spectra were recorded on fractures and furthermore,
the energy dispersive X-ray analysis showed no surface or fracture
contamination.
The difficulties in recording Raman spectra (longer integration
time for the samples obtained at 4–8 GPa) may be partially caused
by the high transparency of the ceramics (as for the 6 and 8 GPa
ceramics). However, we suggest that the observed decrease of
the Raman scattering intensity is, at least partially, related to the
gradual amorphization of the samples during the ceramic fabrica-
tion. The former studies of YAG bulk sample showed that at room
temperature it retains cubic phase up to 101 GPa [34]. However,
amorphization was observed at room temperature for some other
À1
nanocrystalline powder are almost two times larger than the cor-
responding widths of the single crystal. The bands at 340, 371,
À1
À1
4
00, 729, and 781 cm exhibit 2–3 cm broadening for the cera-
À1
mic sintered at 4 GPa followed by gradual narrowing (of 2–4 cm
for the ceramics obtained at higher pressures.
)
Arvanitidis et al. [28] presented the effect of high hydrostatic
pressure (up to 15 GPa) on Raman spectra of YAG single crystals.
With increasing pressure a considerable shift towards higher fre-
quencies was observed for the Raman bands. However, these re-
sults were obtained in other conditions, i.e., upon hydrostatic
pressure at room temperature, than in the present work, i.e., after
the application of nonhydrostatic pressure at 450 °C. The authors
did not consider in detail the changes in the spectrum after gradual
pressure release. In their research the phase transition was not ob-
served. The spectra before and after the application of pressure
were nearly the same. However, the information about the width
and intensity of the bands were not given [28] and could not be
compared with our results.
garnet crystals such as Gd
(at 84 GPa) [34]. This amorphization was irreversible and it was ex-
plained as a result of chemical decomposition into GdGaO perov-
skite and Sc (Ga ) oxide [34]. Although at room temperature
3 2 3 3 5 12
Sc Ga O12 (at 58 GPa) and Gd Ga O
3
2
O
3
2 3
O
no amorphization of YAG could be observed, the structural changes
appeared when the sample was subjected to both high tempera-
ture and high pressure [13,35]. This behaviour was also attributed
Fig. 7 shows the ratio of the intensities of selected Raman peaks
À1
to intensity of the 160 cm peak as a function of the applied pres-
À1
sure. The relative intensity of the peaks at 400 and 781 cm re-
3
to the chemical decomposition into perovskite (YAlO ) and corun-
mains nearly the same for all ceramics. In contrast to this
behaviour, the relative intensity of the 259 or 371 cm peaks in-
creases with increasing pressure. The changes of relative intensi-
ties can be most likely attributed to some slight changes in the
local structure of the studied ceramics and presence of defects in-
duced by applied pressure.
dum structure (Al ), which were observed when the sample was
2 3
O
À1
subjected to 44 kbar and 1000 °C for 16 h [35]. For ceramics ob-
tained by the LTHP sintering, similar decomposition and the grain
size increase was observed when the sintering temperature ex-
ceeded 500 °C at 7.7 GPa [12,13].
Let us now discuss briefly the possible origin of the observed
amorphization of the nanocrystalline YAG samples. First of all,
amorphization of garnet structure was not observed up to 101
GPa at room temperature but it was observed at moderately
high-pressure and high-temperature [34,35]. This result indicates
that temperature increase leads to decrease of pressure at which
the amorphization of YAG occurs. Since the studied samples were
pressed not at room temperature but at 450 °C, the temperature
can be regarded as one of the factor responsible for significant de-
crease of amorphization pressure of the studied samples. Second,
numerous studies of materials under hydrostatic and nonhydro-
static conditions showed that pressure at which amorphization oc-
curs can be greatly reduced in the later case [36,37]. This factor
may also contribute to the observed lowering of the amorphization
pressure since no transmitting medium was used in our experi-
ment. Third, high pressure studies of nanocrystalline materials
showed that the grain size of crystallites also influences the behav-
iour at high pressures. It was shown that surface vs. bulk free en-
ergy contributions within nanocrystalline materials result in
reversals of normal stability relations among bulk phases, and
may stabilize the amorphous form at high pressures [38–40]. For
instance, no amorphization was observed at high pressure for bulk
The observed frequency shift and broadening of bands for the
studied YAG ceramics may be induced by stress and defects [27].
The change of Raman bandwidth accompanied by decreasing Ra-
man scattering intensity may also be attributed to resizing of the
nanocrystals [20,29,30]. However, our TEM studies show that the
crystallite sizes of the studied ceramic samples are similar. We
suppose, therefore, that the observed intensity decrease may be re-
lated to partial amorphization of the samples. Amorphization of
materials under pressure was a subject of numerous studies. It
has been shown that partial amorphization leads not only to signif-
icant decrease of Raman scattering intensity and broadening of Ra-
man bands but also to appearance of additional very weak bands
due to changes in the local structure [31,32]. Our results also show
appearance of additional weak bands for the samples synthesized
TiO
2
whereas the grains smaller than 10 nm amorphize at about 25
. Single crystal of this mate-
GPa [40]. Another example is Ca(OH)
2
rial undergoes amorphization around 20 GPa whereas grains smal-
ler than 50 nm amorphize at around 11 GPa [41]. Therefore, it is
plausible to assume that the observed amorphization of YAG sam-
ples at relatively low pressure (below 2 GPa) can be attributed in
large extent to the small size of the grains.
4
. Conclusions
In summary, the short-term sintering process was found to per-
mit the densification of the Y Al O12 nanocrystalline ceramics
3 5
while avoiding grain growth and compound decomposition. The
XRD analysis, FT-IR, and Raman spectra confirm the well crystal-
Fig. 7. The effect of sintering pressure on relative intensities of Raman bands of YAG
nanoceramics.

A. Lukowiak et al. / Chemical Physics Letters 494 (2010) 279–283
283
lized YAG structure. It was found that with increasing the sintering
pressure, the Raman scattering intensity decreased significantly.
We conclude that high-pressure (2–8 GPa) and moderate temper-
ature (450 °C) during the ceramic fabrication cause partial amor-
phization. These effects lead to a decrease in the intensity of the
characteristic YAG Raman bands. The temperature is probably
one of the factor responsible for amorphization occurred at rela-
tively low pressure. Moreover, it can also be a result of small size
of the grains in nanoceramics. In the infrared and Raman spectra
slight changes are observed with increase in pressure during cera-
mic preparation (frequency shift and broadening) but they cannot
be easily assigned to the crystallite size decrease or stress-induced
changes.
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