Variation of fundamental and higher-order Raman spectra of ZrO2 nanograins with annealing temperature

Article abstract of DOI:10.1103/PhysRevB.59.3173

ZrO₂ nanograins annealed at different temperatures are systematically investigated using x-ray diffraction (XRD) and Raman spectroscopy. Our results show that bulk properties of nano-ZrO₂ diminish but the defect-, surface-, and size-related features display as grain size decreases. A critical size that divides respective predominance is determined to be about 15 nm. Both XRD and Raman spectra of nano-ZrO₂ of size above 15 nm are similar to bulk ones. On the contrary, those of nano-ZrO₂ of size below 15 nm are deteriorated bulk spectra with spectroscopic line broadening and merging, line intensity reducing, and position shifting. Deteriorated XRD spectra are essentially associated with increasing defects in small nanograins. General mode softening, a surface mode around 1040 cm^-1 and 14 weak second-order (overtone and combination) modes are characteristics of Raman spectra of nano-ZrO₂ below 15 nm. They are associated with microstructure change of nanograins, i.e., they reflect the effects of grain size, surface, and the interaction between nanograins. 1999 The American Physical Society.

Full text of DOI:10.1103/PhysRevB.59.3173

PHYSICAL REVIEW B
VOLUME 59, NUMBER 4
15 JANUARY 1999-II
Variation of fundamental and higher-order Raman spectra of ZrO2 nanograins
with annealing temperature
G. G. Siu and M. J. Stokes
Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China
Yulong Liu
Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, People’s Republic of China
͑Received 7 April 1998; revised manuscript received 24 June 1998͒
ZrO2 nanograins annealed at different temperatures are systematically investigated using x-ray diffraction
͑XRD͒ and Raman spectroscopy. Our results show that bulk properties of nano-ZrO2 diminish but the defect-,
surface-, and size-related features display as grain size decreases. A critical size that divides respective pre-
dominance is determined to be about 15 nm. Both XRD and Raman spectra of nano-ZrO2 of size above 15 nm
are similar to bulk ones. On the contrary, those of nano-ZrO2 of size below 15 nm are deteriorated bulk spectra
with spectroscopic line broadening and merging, line intensity reducing, and position shifting. Deteriorated
XRD spectra are essentially associated with increasing defects in small nanograins. General mode softening, a
surface mode around 1040 cmϪ1 and 14 weak second-order ͑overtone and combination͒ modes are character-
istics of Raman spectra of nano-ZrO2 below 15 nm. They are associated with microstructure change of
nanograins, i.e., they reflect the effects of grain size, surface, and the interaction between nanograins.
͓S0163-1829͑99͒02803-9͔
ZrO2 can exist in several polymorphs.8 There is a lot of work
I. INTRODUCTION
performed using IR,9,10 and Raman8,11 techniques to charac-
Nanomaterials have aroused great interest because of their
unusual properties resulting from peculiar microstructure of
nanograins, their basic construction units.1,2 When the aver-
age grain size is controlled in the range of 1–100 nm, their
physical and chemical properties can be significantly differ-
ent from those of bulk materials owing to space confinement
and surface effects.3,4 It is important to understand the man-
ner in which the structures and properties of a single nan-
ograin vary with grain size, from atomic behavior to bulk
solid-state behavior. Nanomaterials synthesized by different
methods could have different microstructures such as nanoc-
rystallite, nanoamorphous grain, or nanoclusters assembles
with some crystal feature, which underlie their physical and
chemical properties. It is essential to study the relationship of
internal microstructure of nanograins with grain size for un-
derstanding physical mechanisms involved in nanomaterials
and cooperative atomic phenomena such as lattice vibration.
The size of nanograins can be controlled by annealing
nanomaterials. Their structures and the approximate mean
size are conventionally determined by x-ray diffraction
͑XRD͒. A few papers have recently reported detailed XRD
studies of the lattice distortion in nanomaterials and the mi-
crostructure changes with annealing temperature.5,6 The vi-
brational properties of nanomaterials, which are strongly de-
pendent on the nanograin size, are usually studied by Raman
spectroscopy, which is a powerful characterization technique
concerning molecular vibrations and dynamical properties of
materials. We intend to combine these techniques and corre-
late their results.
terize the structures and properties of ZrO2. In this paper,
x-ray spectroscopy in range 2␪ϭ10°–80° and Raman spec-
tra over a large frequency range 80–1250 cmϪ1 are measured
systematically for ZrO2 nanograins of the size ranging from
5 nm to 1 ␮m at room temperature. Compared with previous
papers,12,13 our results are improved and complete in the
sense that additional vibrational modes of nano-ZrO2 can be
resolved together with a characteristic mode around 1050
cmϪ1 ͑a surface vibrational mode͒. These results provide
useful information for studying the relationship between the
grain size, the microstructure of nanograins and their vibra-
tions.
II. EXPERIMENT
ZrO2 nanograins of average diameter ϳ5 nm were pre-
pared using the hydrothermal method from a solution of 1N
zirconium nitrate in 2N nitric acids ͑N for normality concen-
tration͒ after heating to 150 °C for 12 h in a sealed hyper-
baric vessel cushioned with polytetrafluoroethylene, and then
cooled to room temperature. White ZrO2 powder was ob-
tained after washing the product alternately and repeatedly
with water and absolute acetone, which was dried in a
vacuum desiccate for 14 h. ZrO2 nanograins of different size
were obtained by annealing the powder at different tempera-
tures for 8 h. A series of annealing temperatures were cho-
sen: 200 °C, 300 °C, 450 °C, 500 °C, 800 °C, and 1100 °C.
The powders were compressed under 10 GPa into circular
pieces of diameter ϳ10 mm and thickness ϳ1 mm.
An x-ray diffractometer ͑Rigaku D/Max-r͒ was used for
structure identification with Cu K␣ radiation. The operating
voltage and current were 50 kV and 200 mA, respectively. A
transmission electron microscope ͑TEM, Hitachi 900͒ was
Zirconium oxide (ZrO2) is a useful functional material in
high-temperature optical and electronic technologies,7 and is
also a typical material in structural-phase transitions because
0163-1829/99/59͑4͒/3173͑7͒/$15.00
PRB 59
3173
©1999 The American Physical Society

3174
G. G. SIU, M. J. STOKES, AND YULONG LIU
PRB 59
TABLE I. The average grain sizes of nano-ZrO2 annealed under
different temperature.
Sample
n1
n2
n3
n4
n5
n6
n7
Ta ͑°C͒
D ͑mm͒
as-grown 200
300 450 500 800 1100
12 15 20 75 120
5.0 8.0
terface stress and effects the interface-structure relaxation.
After 500 °C the grain size increases rapidly when annealing
mainly causes grain growth.
From n1 to n7 the grain size increases and correspond-
ingly their XRD peaks are sharpening and resolving into fine
structure. The as-grown sample n1 has eight broad diffrac-
tion peaks situated in the similar ranges of the monoclinic
polycrystalline ZrO2 spectrum. However, the bandwidths are
large ͑so that the resolution is low͒, the positions shift, and
the relative intensities are different from the literature data.15
It is a deteriorated bulk spectrum, which is closely related to
the fact that the presence of large numbers of lattice vacan-
cies and local lattice disorders may result in considerable
reduction in intensity and even lead to near disappearance of
XRD peaks of crystal planes. The as-grown nanograins pos-
sess maximum structure disorders: oxygen vacancies, local
lattice disorder, cluster pores, and etc. The smallest grain size
also means the largest grain surface. There are huge numbers
of interfaces between and inside nanograins so that many
atoms or molecules of nanograins are in the surface or near-
surface states. Therefore the n1 spectrum is the least bulk-
like. There is no drastic change in the positions and intensi-
ties of XRD peaks from n2 to n5, although a tendency
towards stronger peaks is apparent. It is consistent to the
explanation that annealing under Taр500 °C is able to re-
FIG. 1. XRD spectra of nano-ZrO2 with different annealing tem-
perature Ta :n1, as-grown sample; Taϭ200, 300, 450, 500, 800,
and 1000 °C, respectively, from n2 to n7.
used to take feature micrographs. The average size of ZrO2
nanograins was determined by XRD and TEM. Two Raman
spectrometers were used for investigating the vibrational
modes of ZrO2 nanograins at room temperature. One was a
SPEX-1403 Ramalog system together with an argon ion
laser ͑Coherent Innova-100͒, which provided 514.5- and
488.0-nm lines for excitation. Another was a Bruker 100
system, a near infrared ͑NIR͒ Fourier transform ͑FT͒ Raman
spectrometer with a cw diode-pumped Nd:YIG ͑yttrium
indium-garnet͒ laser for 1.06-␮m excitation and with a
liquid-nitrogen-cooled germanium detector. Backscattering
geometry is adopted for all Raman measurements. In gen-
eral, Raman spectra were collected under the conditions of
laser power 50 mw and resolution 4 cmϪ1. Typically 2500
scans at 4 cmϪ1 interval were averaged to provide a spectrum
of sufficient quality. Results from two spectrometers are
similar and only the NIR-FT Raman results are shown.
III. RESULTS AND DISCUSSION
Figure 1 is the XRD spectra of nano-ZrO2 with different
annealing temperatures, which shows that the samples are
single-phase ZrO2 with monoclinic structure. The average
grain sizes are obtained from the Scherrer equation14
DϭK␭/ ␤ cos ␪͒,
͑1͒
͑
where D is the crystallite size, K a proportional constant, ␭
the wavelength of the x rays, ␤ is the full width at half
maximum ͑FWHM͒ of the diffraction peak in unit of radian,
and ␪ is the Bragg angle. The results are given in Table I,
which agree well with those from TEM studies.5 The in-
crease in grain size as Taр500 °C is moderate, which mani-
fests that low-temperature annealing mainly removes the in-
FIG. 2. Raman spectra of nano-ZrO2 with different annealing
temperatures (n1–n7).

PRB 59
VARIATION OF FUNDAMENTAL AND HIGHER-ORDER . . .
3175
TABLE II. Wave numbers ͑cmϪ1͒ of the C group Raman modes of nano ZrO2. ͑vs, very strong; s, strong;
m, medium; w, weak; sh, shoulder; , observed ambiguity; T, tetragonal phase. The modes with Raman shift
*
larger than 700 cmϪ1 belong to the H group.͒
n1
n2
n3
n4
n5
n6
n7
Ref. 8
Ref. 11
*
92
101
148T
177
189
222
266T
306
315
335
347
355
382
476
502
537
559
616
637
114 m
178 s
112 m
178 s
110 m
107 m
105 m
104 m
101 (Ag) m
178 s
189 sh
220 sh
178 s
190 s
220 sh
178 s
191 s
220 w
178 s
191 s
224 m
178 (Ag) s
191 (Ag) s
224 (Bg) m
179 (Ag)
190 (Ag)
222 (Bg)
270T
220 sh
220 sh
308 sh
305 m
305 m
306 (Ag) m
305 (Ag)
*
*
328 m
338 sh
330 m
340 sh
332 m
342 sh
334 m
345 sh
334 m
346 m
334 s
349 m
334 (Bg) s
350 (Ag) s
334 (Bg)
348 (Ag)
380 m
479 m
380 m
479 m
380 m
479 m
381 m
478 s
501 sh
534 sh
558 m
616 s
382 m
476 vs
501 w
536 w
558 m
616 s
382 s
475 vs
504 m
539 m
558 m
618 s
382 (Bg) s
475 (Ag) vs
504 (Bg) m
539 (Bg) m
560 (Ag) m
618 (2Bg) s
638 (2Ag) s
381 (Bg)
476 (Ag)
500 (Bg)
534 (Bg)
557 (Ag)
615 (2Bg)
637 (2Ag)
534 sh
554 w
612 m
534 sh
556 w
612 m
534 sh
558 w
616 m
633 sh
633 s
637 s
704 w
714 w
725 w
760 w
758 w
758 w
756 w
756 w
758 w
758 w
780
764
move stress and lead to relaxation in the interface structure,
but is difficult to eliminate local lattice disorders or to
change the internal structure of nanograins owing to their
requirement for higher energy. The spectra of n6 and n7 are
much bulklike, qualitatively different from the previous
ones. The diffraction peaks sharpen, strengthen, and can be
indexed according to the standard monoclinic structure. It
shows that the grains have improved greatly. The rapid in-
crease in grain size as TaϾ500 °C is accompanied with a
rapid decrease in the density of vacancies, local lattice dis-
orders, and clusters pores.
spectrum. There is no vibrational mode at 148 cmϪ1 or 266
cmϪ1 in any sample, which are attributed to the tetragonal
phase.8,11 The n6 spectrum is similar to n7 but the spectra
from n1 to n5 are deteriorated bulk spectra with new fea-
tures. These results indicate that the samples are basically
single-phase ZrO2 of monoclinic structure, in good agree-
ment with XRD.
In Fig. 2, the Raman modes display usual anharmonic
effects as the grain size decreases. Broadening, peak merg-
ing, and frequency shifting are observed. There are also
some new features in the spectra. For example, an additional
Raman mode, denoted as S, at 1038 cmϪ1 appears in the
spectra of samples n1–n5 as the grain size is under 20 nm,
which has asymmetric broadening on the low-frequency side
of the peak. S strengthens and shifts toward higher fre-
quency ͑1050 cmϪ1͒ as the grain size decreases. Further-
more, 15 weak modes are observed in the sample n1 of grain
size 5 nm.
It is noteworthy that the Raman spectra of nano-ZrO2 vary
systematically with grain size in the mode position, line-
width, and intensity. Their general behavior divides into
three categories: Group C includes lines in the region 100–
700 cmϪ1; line S varies in the range 1030–1050 cmϪ1; and
Group H concerns the weak features in the region 700–1250
cmϪ1. The spectral lines in Group C strengthen and resolve
into sharp peaks with increase of grain size, following the
evolution tendency of the XRD spectra. On the contrary, the
line S and Group H behave oppositely and the Group H is
very weak.
Bulk ZrO2 is monoclinic, belonging to the space group
C52h (P2l /c). A unit cell contains four formulas. Its vibra-
tion properties have been investigated.11 According to group
theory, there are 36 lattice vibrational modes:
G
monoϭ9Agϩ9Auϩ9Bgϩ9Bu
͑2͒
among which 9Ag and 9Bg modes are Raman active, while
8Au an 7Bu modes are infrared active and others are acoustic
modes.
The NIR-FT Raman spectra of nano-ZrO2 with grain size
ranging from 5 to 120 nm (n1–n7) are shown in Fig. 2,
which are similar using 514.5 and 488.0 nm beam for exci-
tation. To compare with published data, Table II lists the
Raman frequencies of nano-ZrO2 together with the frequen-
cies of the ZrO2 modes given in other sources.8,11 For the
sample n7 annealed at 1100 °C ͑the grain size is larger than
120 nm͒, our results are similar to Refs. 8 and 11 and can be
interpreted based on the single-crystal or polycrystalline

3176
G. G. SIU, M. J. STOKES, AND YULONG LIU
PRB 59
FIG. 4. IS /IC versus grain size.
1050
475
FIG. 3. Raman-mode wave numbers as functions of annealing
temperature.
and some that do appear are displaced in frequency and
changed in intensity. The powder ones shows a continuous
distribution over the range of the single-crystal polar-mode
frequencies. Systematic change in grain size makes the ten-
dency in spectral change clear.
The sample n7 has a bulk vibrational spectrum of mono-
clinic ZrO2 crystal. The spectrum deteriorates continuously
from n6 to n1, in the region of Group C. Each mode is
denoted by its wave number as subscript. Their Raman shifts
are listed in Table II where each row shows the variation of
a mode frequency with grain size. The tendency of frequency
shifting is clearly shown by Fig. 3, a graph of mode fre-
quency versus annealing temperatures where each row of
Table II becomes a line. The frequency variation is usually
several cmϪ1 and red shift with grain-size reduction is com-
mon but blue shift also occurs. As grains contract, lines
broaden and some merge with each other. In the meantime,
the relative intensities of Raman modes also change. For
example, the Ag modes C178 and C191 of n7 attenuate about
twofold towards n1 and merge into a broad peak C178 of
n1. C178 is stationary but C191 shifts to 178 cmϪ1 of n1,
with a redshift as large as 13 cmϪ1. Its FWHM almost
doubles. Another example is the Ag mode C475 of n7, the
strongest in the spectrum, which weakens to being similar to
others towards n1. We conclude that Group C concerns bulk
phonons and manifests the persistence of the basic polar-
mode properties of the single crystal in nanograin.
The mode S1050 is the characteristic of n1. Single-crystal
or polycrystalline ZrO2 does not have any S mode, which
appears only in ZrO2 nanograins of small grain size ͑р15
nm͒. Figure 2 shows that the intensity of S drops as nan-
ograins grow. S disappears completely when grain size
reaches 50 nm. The grain-size dependence of the S intensity
relative to that of C475 of n1 is shown in Fig. 4. The ratio of
the number of superficial atoms NS to the total number of
atoms NT versus grain size following Ref. 19 is plotted in the
inset. Both increase rapidly when particle size decreases be-
low 15 nm, suggesting some inner link. However, they have
different coordinates scale ͑relative intensity and percentage͒
and IS1050 /IC475 has a dramatic drop for a diameter larger
than 15 nm but the ratio NS /NT is a smoothly and slowly
varying function, which make them dissimilar. It would not
be possible to draw causal relation from comparison between
two. On the other hand, this behavior is similar to the Raman
spectra of small particles of silicon.20 S mode behaves
similarly to the Raman-scattering cross section of surface
phonon in very small GaP spherical crystals calculated by
Ruppin.21 As the bulk-mode transverse and longitudinal
cross sections of GaP sphere keep almost the same when
sphere size changes, the surface-mode cross section exceeds
them for sphere diameter smaller than about 20 nm, becomes
comparable with the bulk-mode scattering for sphere diam-
eter ϳ100 nm and drops dramatically to being negligible for
diameter Ͼ200 nm. The predominance of the surface-mode
scattering at extremely small size in the order of magnitude
of sphere diameter ϳ20 nm is the most striking modification
of the spectral results. The basic reason why the powdered
specimen show new spectral features is that the wave-vector
The behavior of Group C shows clearly the dependence of
Raman scattering by polar modes in a collection of randomly
oriented crystallites on the size of the individual crystallites,
as summarized in the Chapter 4 of Ref. 16. Similar behavior
¨
appears in LiIO3 powder. Gualberto and Arguello obtained a
good agreement between the experimental results and theo-
retical calculation of the spectral differential cross section of
LiIO3 powder.17 Otaguro et al. studied the orientation depen-
dence of the single-crystal polar-mode frequencies of LiIO3
in the similar frequency range.18 The details of the powder
spectra are quite different from the single-crystal spectrum:
not all the single-crystal peaks appear in the powder spectra

PRB 59
VARIATION OF FUNDAMENTAL AND HIGHER-ORDER . . .
3177
2
ប2␲
1
1
e2
*
Ϫ0.248ERy , ͑3͒
EϭEgϩ
ϩ
Ϫ1.786
ͩ
ͪ
2R2 me mh
␧R
where R is the radius, ␧ is the dielectric constant, me and mh
are the effective masses of the electron and the hole, respec-
tively, and ERy is the effective Rydberg energy. On the right-
hand side, the first term is the energy gap, the second is the
quantum-size energy of localization, the third is the Coulomb
interaction energy, and fourth results from the spatial corre-
lation effect. The first term is constant and the fourth is
independent of the size and usually small. The second
(ϰ1/R2) and third (ϰ1/R), respectively, represent an in-
crease and a decrease in energy and are mutually canceling.
The quantum size energy of localization will prevail as grain
size decreases. When it dominates over the Coulomb energy,
the excitation energy increases and could be higher than that
of the eigenstate, which results in a blueshift of the peak.
However, why should the energy of surface vibrations be
related to the energy of an exciton? More relevant interpre-
tation should relate the blue shift of surface vibration to sur-
face itself. It results likely from surface relaxation as the
particle size decreases. A surface optic phonon was observed
on the TaC͑100͒ surface by inelastic electron scattering24 and
for TaC with simple rocksalt crystal structure the dispersion
curve of the surface mode on ͑100͒ can be calculated apply-
ing the diatomic linear-chain model.25 The theoretical disper-
sion values based on this nearest-neighbor central-interaction
model are smaller than the experimental ones because of
modified force constants ͑surface relaxation͒ near the surface
or owing to the effect of second- and third-neighbor forces.
Although there is no simple theoretical model for the surface
mode of monoclinic ZrO2, similar change in force constants
might occur owing to surface relaxation, which strengthens
as the particle size decreases. Corresponding increase in
force constants causes the blueshift.
Group H includes weak features at 704, 714, 725, 807,
835, 859, 874, 911, 928, 938, 955, 1145, 1167, and 1224
cmϪ1, clearly observable in Fig. 5. They are sharp with
Lorentzian line shape and linewidth in the region 10–20
cmϪ1. Their frequencies are too high to be related to the
first-order silent modes. They are likely the second-order ac-
tive Raman modes. A Raman or infrared spectrum of a crys-
tal ͑molecule͒ may contain numerous combination and over-
tone bands arising from anharmonicity. An ideal crystal has
infinite spatial correlation length of phonon, and hence, the
phonon eigenstates are plane wave. It leads to the usual se-
lection rule, phonon momentum qХ0 for the first-order ͑the
upper limit of q is three orders of magnitude smaller than the
FIG. 5. Fundamental and higher-order Raman spectra of as
grown nano-ZrO2 over the range 80–1250 cmϪ1
.
conservation condition no longer limits the modes that con-
tribute to light scattering when the particle size D is so small
that Dqр1, where បq is the momentum transferred.16,18 For
visible incident light of a typical wavelength ϳ514 nm ͑the
green light of Arϩ laser͒ into a crystal of refractive index
ϳ1.5, its wave vector ϳ1.8ϫ107 mϪ1 so that the maximum
qϳ3.6ϫ107 mϪ1 ͑Ref. 16͒ and D would be smaller than
ϳ30 nm. Hayes and Loudon,16 however, pointed out that the
scattering is nevertheless difficult to observe, and the crys-
tallite in real powders are not usually spherical or even ellip-
soidal, but have irregularities that broaden the surface-mode
peak. Systematic reduction in size of nano-ZrO2 provides an
opportunity to observe the emerging and development of the
surface-mode peak. We attribute S mode to surface-mode
scattering, which is another observed surface phonon in ad-
dition to one from scattering by CdS crystallites.22
Brillouin-zone boundary͒ and qϩq ϭ0 for the second-order
Ј
Figure 2 also shows that the mode S blueshifts from 1040
cmϪ1 (n4) to 1050 cmϪ1 (n1). It seems to be tempting to
attribute this blue shift to quantum-size-effect related mecha-
nism such as the surface electron-phonon interaction. A
nanocrystal exciton model has been used to explain the blue
shift of the absorption edge in photoacoustic spectrum and
the absorption peaks of first-order differential PAS of nano-
ZrO2,5 which was used in previous Raman-scattering work,
too.12 However, this interpretation of the blueshift of mode S
is not convincing. By the spheroid model of nanoclusters, the
energy of the exciton ground state of nano ZrO2 can be ex-
pressed as23
two-phonon ͑now phonon wave vectors q and q are typi-
Ј
cally three orders of magnitude larger than the light wave
vectors over the major part of the Brillouin zone͒ Raman
scattering. The order of a real crystal, however, is not perfect
owing to defects, impurities, disorder, etc. It means that the
spatial correlation function of the phonon becomes finite in
extent. It relaxes the selection rule, which, in turn, results in
observation of symmetry-forbidden modes, together with
line broadening and asymmetry of line shape, as well as
redshifting of phonon mode. A combination band belongs to
the direct product of representations to which the internal
fundamental vibrations belong and its observed frequency is

3178
G. G. SIU, M. J. STOKES, AND YULONG LIU
PRB 59
TABLE III. Wave numbers ͑cmϪ1͒ of the H group, Raman com-
binations and overtones, in nϪ1.
their fundamental modes. Figure 2 gives the ratio of intensi-
ties I478 /I956ϭ22. The higher-order Raman modes in n1 are
listed in Table III, in good agreement with quantum theory.
The appearance of high-order modes in Raman spectra of
nano-ZrO2 brings new possibility for studying the micro-
structures of nanograins because the selection rules of high-
order modes are different from those of fundamental modes.
In summary, sample n5 distinguishes between the
samples that show noteable ‘‘sizeϩinterface’’ effects and the
samples in which bulk properties predominate. Its surface
mode S and Group H almost disappear and its bulk phonon
modes start to intensify, showing reduction of the lattice va-
cancies and local lattice disorder. Correspondingly, its lattice
distortion weakens and all XRD peaks start to strengthen.
Nanograin samples show bulklike spectra as DϾ15 nm,
which manifests grain growth improves crystallite quality.
Samples of Dр15 nm, on the contrary, show deteriorated
bulk spectra with clear ‘‘finger print’’ of size and interface
effects.
Mode
Position
Combination
Error
H1
H2
H3
H4
H5
H6
H7
H8
704
714
725
807
835
859
874
913
938
328ϩ380
534ϩ178
612ϩ114
328ϩ479
612ϩ220
380ϩ479
534ϩ343
534ϩ380
328ϩ612
478ϩ478
534ϩ612
554ϩ612
612ϩ612
4
Ϫ2
1
0
Ϫ3
0
3
Ϫ1
2
1
1
H9
H10
H11
H12
H13
955
1145
1167
1224
Ϫ1
0
the sum of their frequencies.26 For overtone bands, the char-
acters of vibration representations are given by27,28
IV. CONCLUSIONS
2
We have systematically measured XRD and Raman spec-
tra of ZrO2 nanograins of size in the region 5–120 nm at
room temperature. Both the structure and vibration of nan-
ograins of size under 15 nm show characteristics different
from bulk ones. Their essentially monoclinic bulk properties
diminish. Their spectral-line weakens, broadens, and merges.
In Raman spectra shifts of modes are common, an extra sur-
face vibrational mode S around 1040 cmϪ1 and weak second-
order Raman modes ͑Group H͒ appear. The results show the
effects of defects, size and interfaces in typical ZrO2 nan-
ograins. On the other hand, nanograins of size above 20 nm
are very similar to bulk solids. The bulk characteristics re-
cover in both spectra, showing the predominance of interior
order. It suggests that the modes S and Group H are closely
related to the change of nanograin microstructure.
␹
R͒ϭ ␹ R͒
,
for fϭ1,
͑
͓
͑
͔
f2
f
␹
R͒ϭ 1/2͒ ␹ R͒ 2ϩ␹ R2͒ , for fу2, ͑4͒
͑ ͓ ͑ ͔ ͑
͕ ͖
f f
͑
f2
where ␹f(R) is the character under the operation R for the
f-fold degenerate irreducible representation ir of the point
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