Page 303 - Book Hosokawa Nanoparticle Technology Handbook

FUNDAMENTALS CH. 5 CHARACTERIZATION METHODS FOR NANOSTRUCTURE OF MATERIALS
crystalline phases from Raman spectrum. Comparing
with the X-ray and neutron diffraction, the Raman
scattering technique has the following features [1–5]:
1. Raman scattering is useful for the identification
of crystalline phases with less long-range order,
less long-range periodicity and intermediate
periodicity. Thus, this technique is also effective
for characterizing the nanoparticle samples with
small crystallite sizes. For example, in the initial
stage of crystallization, the Raman spectroscopy
enables to investigate how ordering exists in the
sample. From the Raman peak width, the crys-
tallite size can be estimated.
2. In substitutional solid solutions such as (Zr,
Yb)O , the Zr and Yb ions coexist at the cation
2–
site, and the oxygen ions and vacancies coexist at
the anion site. Thus, strictly speaking, the perio-
dicity is broken. In the system where the periodic-
ity is broken, various vibration modes appear due
to the breaking of the selection rule [4, 5]. For
example, in the cubic fluorite-type structured
(Zr,Yb)O 2
solid solution, the Zr and Yb ions
coexist at the cation site, and the oxygen ions and
vacancies coexist at the anion site. According to
the factor group analysis, only one Raman band is
expected for a defect-free fluorite-type structure,
but the cubic fluorite-type structured (Zr,Yb)O 2–
solid solution exhibits many bands and continuous
Figure 5.2.8
Raman spectra of monoclinic, tetragonal, and cubic spectrum due to the disordering and coexistence
zirconia phases. of some ions and vacancies (Fig. 5.2.8). In this
way, the Raman scattering technique is useful for
Peak positions of the optical Raman bands are investigating the disordering.
governed by the force constants of lattice vibrations, 3. Raman scattering is sensitive to the atoms with
so to speak, by the spring constant. Vibrations of a large polarizability. Since the anions such as
simple molecule can be classified into stretching oxygen and fluorine have large polarizability,
vibrations of bonds and bending vibrations. The inter- the Raman scattering is often effective for
pretation of phonon vibrations of a little complicated
molecules and of crystals is not very easy. A phonon studying the displacements and defects of
vibration does not always contribute to the Raman oxygen atoms in oxides [4, 5].
scattering. A vibration is classified into Raman active 4. The micro-Raman technique enables the phase
mode and infrared (IR) active mode, which are deter- identification from a small area with the spatial
mined by the selection rule governed by the symme- resolution of about 1 m. Recently, near-field
try of crystal and molecule. Therefore, the Raman and
IR (Section 5.3.3) techniques are alternative charac- Raman scattering instruments have been devel-
terization methods. The selection rule is strongly oped and their spatial resolution can be several
dependent on the crystal structure. For example, the tens of nanometers. Measurements of the Raman
fluorite-type structure has one Raman-active mode, spectra with these instruments enable the phase
while the tetragonal and monoclinic zirconia phases identification and the characterization of stresses
have six and eighteen Raman-active modes, respec-
tively. Thus, the Raman spectrum is strongly depend- in a very small amount of sample (less than
ent on the material and crystalline phase, which several nanograms). Utilizing the high spatial
enables the phase identification of materials and resolutions, the Raman scattering technique has

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