Polycrystalline Li2Ni(WO4)2 samples were prepared using a conventional solid-state reaction method. The high-purity (> 99.5%) powder mixtures of NiO, Li2CO3, and WO3
were heated to 550 and 650 ℃ for 24 h and ground intermittently to pelletize. The samples were then annealed at 700 ℃ for 160 h, followed by furnace cooling to room temperature at a rate of approximately 150 ℃/h [39,40].
Muthuselvam et al. [39] described the Rietveld refinement of synchrotron X-ray powder diffraction pattern of Li2Ni(WO4)2 (Fig. 2-25), suggesting the triclinic symmetry with space group P1̅. The lattice parameters of Li2Ni(WO4)2 at room temperature and low temperatures are shown in Table 2-5.
Table 2-5: Structural parameters of Li2Ni(WO4)2 obtained by refinement of synchrotron X-ray diffraction data (λ = 0.619 Å) for T = 300 K (room temperature, RT) and high-resolution neutron powder diffraction data (λ = 1.6215 Å) for T = 2 K (low temperature, LT) phases.
The space group is P1̅ [39].
Fig. 2-26 displays the densities of states of Ni-3d and O2p for Li2Ni(WO4)2. The theoretical band structure calculations [39] predicted that Li2Ni(WO4)2 has band gap energy of 2.7 eV. Lv et al. [43] examined the ultraviolet–visible diffuse reflectance and Raman scattering spectra of Li2Co(WO4)2 and Li2Ni(WO4)2 (Fig. 2-27). They discovered the primary absorption bands at approximately 2.6 and 2.9 eV, respectively, which can be
attributed to charge-transfer excitations from the 2p orbitals of O ions to the 5d orbitals of W ions. The other absorption bands observed in the visible frequency region can be attributed to the d–d transitions of Ni and Co ions. Fig. 2-28 demonstrates the temperature-dependent dielectric constant of Li2Ni(WO4)2. The anomalies near TN1 = 18 K and TN2 = 13 K connect the long range 3D antiferromagnetism ordering [37]. Fig. 2-29 exhibits the magnetic susceptibility of Li2Co(WO4)2, with two magnetic phase transitions at approximately 9 K and 7 K [37]. Fig. 2-30 illustrates the magnetic susceptibility of Li2Ni(WO4)2, with two magnetic phase transitions at approximately 18 K and 13 K.
Li2Ni(WO4)2 exhibited strong Raman-active phonon modes (Fig. 2-31) at approximately 899 and 902 cm−1, respectively, due to the symmetric stretching vibrations of WO6 octahedra. Maczka et al. [55] studied the infrared and Raman scattering spectra of Li2M(WO4)2 (M = Co, Ni, and Cu). The observed phonon modes were appropriately assigned, as shown in Table 2-6. The vibrational properties of Li2Co(WO4)2 and Li2Ni(WO4)2 were well correlated with their crystal structures.
Table 2-6: Infrared and Raman phonon modes for Li2Ni(WO4)2, Li2Co(WO4)2, and Li2Cu(WO4)2 with assignment based on comparison with data reported for ZnWO4 [56].
Fig. 2-1: Pseudobinary phase diagram showing the variation of composition with temperature at 1- and 350-atm oxygen pressure. The approximate lower oxygen limit of the perovskite phase is indicated with the dashed line [24].
Fig. 2-2: Room-temperature Mössbauer spectra of ground SrFeO3−δ crystals. Subspectra:
blue—Fe4+ (single line), red—Fe3.5+ (doublet with smaller splitting), and green—Fe3+
(doublet with larger splitting) [21].
Fig. 2-3: Temperature dependence of the lattice parameters of Sr8Fe8O23 [50].
Fig. 2-4: Linear scans along the longitudinal direction through the Bragg reflection (004) for the cooling and heating processes of Sr8Fe8O23. (a) The evolution of the lattice parameter of the c-axis (unit, Å ) as a function of temperature. (b) The temperature dependence of the peak width (full width at half maximum, FWHM). Both graphs display the hysteresis behavior around the transition temperature of magnetic ordering [22].
Fig. 2-5: Neutron diffraction data from the single crystals of SrFeO3.00 and SrFeO2.87
measured at various temperatures. The scans were performed along the [1,1,1] direction around the structural Bragg reflection (0, 0, 1) cub [50].
Fig. 2-6: Temperature dependence of the intensity, incommensurability, and magnetic correlation length of the helical magnetic Bragg reflections of the magnetic phase I in the single-crystal SrFeO3.00 (left) and the magnetic phases I and II in SrFeO2.87 (right) [50].
Fig. 2-7: Magnetic susceptibility of the single crystals of SrFeO3.00, SrFeO2.95, SrFeO2.85, SrFeO2.81, and SrFeO2.77 measured in field cooling and subsequent field heating runs at B
= 1 T. Curves for the latter four samples were shifted by amounts indicated in the legend [21].
Fig. 2-8: Magnetic ordering of iron moments in the system of SrFeO3−δ [50].
Fig. 2-9: (a) SrFeO3.00, (b) SrFeO2.95, (c) SrFeO2.85, (d) SrFeO2.81, and (e) SrFeO2.77 display the resistivity in zero-field cooling and heating runs (black) compared with field cooling and heating runs with a 9-T field (red) [21].
Fig. 2-10: Temperature dependence of the resistivity of a single crystal of SrFeO2.81, measured in the ab plane and along the c-axis. The top inset presents the temperature dependence of magnetic susceptibility (χ) measured along the c-axis in ZFC and FC runs in a magnetic field of 1 T, and the bottom inset presents room-temperature X-ray diffraction profile showing (004) Bragg peak obtained in θ-scan [51].
Fig. 2-11: Susceptibility, resistivity, and magnetoresistance (MR) measurements of SrFeO2.875. (a) Susceptibility curves (MT), field cool (FC), and zero FC (ZFC) at an applied field of 1 T show a transition at T ≃120 K and a sharp antiferromagnetic transition at approximately 70 K. Resistivity curves (RT) for both cooling and heating measurements show two transitions at T ≃120 K and 50 K as indicated by green arrows, respectively. (b) MR data, both for the applied field perpendicular and parallel to c-axis, display a negative MR of approximately 45% at T ≃55 K. The inset shows that the transition temperature is reduced by the applied field [22].
Fig. 2-12: The temperature-dependent Fe K-edge XANES spectra of single-crystal SrFeO2.81 measured at two different angles of incidence θ = 0° (with electric field E parallel to the ab plane) and 70° (with electric field E nearly parallel to the c-axis) on heating and cooling processes. Corresponding spectra were obtained for FeO, Fe3O4, and Fe2O3 powder samples at room temperature, with angle θ = 0° for reference [51].
Fig. 2-13: Temperature dependence of normalized Fe L3,2-edge XANES spectra of single-crystal SrFeO2.81 at two angles of incidence θ = 0° and 70° during (a) heating and (b) cooling. The bottom panels show the corresponding XLD spectra [51].
(a) (b).
Fig. 2-14: (a) Conduction band and valence band positions for P25 and SrFeO (32d) [52].
(b) Band gap energy and redox enthalpy as a function of Fe content (x) in SrTi1−xFexO3−y
solid solutions [53].
Fig. 2-15: (a) Far-infrared ellipsometric and (b) Raman scattering spectra of SrFeO3.00 at 15, 160, and 300 K [21].
Fig. 2-16: Raman scattering and far-infrared ellipsometric spectra were compared for the charge-order composition of SrFeO2.85 at low temperature and room temperature. (a) Raman scattering spectra in parallel (ZZ) and cross (XZ) polarization and (b) far-infrared ellipsometric spectra: optical conductivity and real part of the dielectric permittivity 1
[21].
Fig. 2-17: Temperature dependence of the Raman scattering spectra: (a) polarized Raman scattering spectra of SrFeO2.85 in ZZ direction and (b) unpolarized Raman scattering spectra of SrFeO2.69 [21].
Fig. 2-18: (a) Room-temperature synchrotron X-ray powder diffraction pattern of Ba2CuTeO6. (b) A single-crystal diffraction pattern obtained using an X-ray (Cu-Kα) beam perpendicular to the ab plane. The inset is the optical image of grown single crystal [29].
Fig. 2-19: (a) The temperature dependence of magnetic susceptibility measured in an applied magnetic field of 10 kOe for H║ab and H⊥ab of single-crystal Ba2CuTeO6. (b) The dχ/dT vs T curves measured with a field of 10 kOe reveal an anisotropic cusp of TN
≃15 K [29].
Fig. 2-20: The χ(T) for Ba2CuTeO6 single-crystal arrays with the magnetic field of μ0H = 1 T applied parallel (upper panel) and perpendicular (lower panel) to the ab plane. The dashed and solid lines indicate chain and ladder model fits, respectively. The inset in the upper panel displays the low temperature region for both orientations along with the high-field behavior for H⊥ab. The inset in the lower panel displays M(H) of Ba2CuTeO6 for H⊥ab [35].
Fig. 2-21: Temperature dependence of the magnetic susceptibility χ(T) for Ba2CuTeO6
measured in an external field of µ0H = 1 T applied parallel and perpendicular to the ab plane. The red solid lines are fits using a two-leg ladder model. The inset shows the inverse susceptibility with a fitting to a Curie-Weiss law [57].
Fig. 2-22: (a) Low-energy range of Raman spectra taken at T = 9 K in four different polarizations (aa, ab, bb, and cc). The gray shading denotes a two-magnon continuum. (b) Magnetic excitations in (bb) polarization for different temperatures. The spectra are vertically shifted by a constant amount [36].
Fig. 2-23: Raman spectra measured in (cc) and (bb) polarizations for different temperatures.
The shadings emphasize scattering due to two-magnon excitations [36].
Fig. 2-24: Band structure (top panel) and density of states (bottom panel) of configuration AF1 in Ba2CuTeO6. The top of the valence band has been set to zero [29].
Fig. 2-25: Rietveld refinement of powder synchrotron X-ray diffraction pattern for Li2Ni(WO4)2 at room temperature [39].
Fig. 2-26: Partial Ni-3d and O-2p densities of states in Li2Ni(WO4)2 at 2 K. The Fermi energy is set to zero [39].
Fig. 2-27: UV-Vis diffuse reflectance spectra of (a) Li2Co(WO4)2, (b) Li2Ni(WO4)2, and (c) WO3, and the inset shows the optical absorption coefficient between 1.5 to 3.75 eV [43].
Fig. 2-28: The temperature dependence of the dielectric constant (εr) under an applied frequency 100 kHz in Li2Ni(WO4)2. (a) The inset shows the first order derivative of the εr
as a function of temperature. (b) The inset shows the temperature dependence of pyroelectric current [39].
Fig. 2-29: Temperature dependence of the magnetic susceptibility of Li2Co(WO4)2
measured in a 1-T magnetic field. The left axis of the inset highlights the low-temperature regime, and the right axis highlights the derivative d(χT)/dT . The peaks of d(χT)d/dT reveal a minor reduction of (χ)T near TN1 ≃9 K and TN2 ≃7 K [37].
Fig. 2-30: (a) Magnetic susceptibility (black circles) as a function of temperature for Li2Ni(WO4)2 in a 1-T magnetic field. (b) The peaks of d(χT)d/dT reveal the minor reduction of (χ)T near TN1≃18 K and TN2 ≃13 K [39].
.
Fig. 2-31: Raman scattering spectra of (a) Li2Co(WO4)2, (b) Li2Ni(WO4)2, and (c) Li2Cu(WO4)2 [55].
Chapter 3
Theory background and experimental techniques
3.1 Spectroscopic ellipsometry
Ellipsometry is commonly used to characterize both thin films and bulk materials. Its most common application is the determination of thin film thickness and its optical constants (n, k, or ε1 and ε2). Ellipsometry has also been used to study doping concentration, surface and interfacial roughness, alloy ratio, crystallinity, optical anisotropy, and depth profile of material properties [58].
The measured values are expressed as psi (
) and delta (
). These values are related to the ratio of Fresnel reflection coefficients Rp and Rs for p- and s-polarized light, respectively. The ellipsometric parameter ρ is commonly expressed in terms of the two real-valued ellipsometric parameters psi (
) and delta (
) as follows:tan( )ei
， (3.1.1)where tan( ) = p
s
R
R ， (3.1.2)
p s (3.1.3)
where tan( ) is the magnitude of the ratio of the p- to s-direction complex reflection coefficients for the sample, and
is the phase difference between the p- and s-reflectioncoefficients.
When a beam of light is directed to an interface at an angle a1, and the reflectance beam from the substrate surface has a reflectance index N2, from classical electrodynamics, the Fresnel reflection coefficients Rp and Rsare derived as follows:
Rp
From Snell’s law, we know that
N1sina1=N2sina2 ， (3.1.6)
and substituting it into Eqs. (3.1.4) and (3.1.5)
1 2 We can also obtain a similar result for Rs
R12s 1 2 1 2 Thus, we can obtain the ellipsometric parameter ρ:
tan( )ei
Now, assuming N = 1 (in air)
and substituting Eqs. (3.1.11) and (3.1.13) into Eq. (3.1.10)
and squaring each side of Eq. (3.1.14), we get
Moreover, we can define a complex dielectric function () as
2
2 1 2
N i ， (3.1.16)
where
1and
2 are the real and imaginary parts of the dielectric function. Comparing with Eqs. (3.1.15) and (3.1.16), we obtain2 2 2
where n and k are the refractive index and extinction coefficient, respectively. Combining Eqs. (3.1.17), (3.1.18), and (3.1.19), n and k are as follows:
2 2 spectroscopic ellipsometer with the focusing optics used in this investigation.
Spectroscopic ellipsometry measurements were conducted under angles of incidence between 60° and 75° by using a J. A. Woollam Co. M-2000U ellipsometer over a spectral range of 0.73–6.42 eV. The experimental data were reproducibly observed at three spots on the sample surfaces using a specially designed focusing optics coupled with an ellipsometer for spot (100 × 100 µm2) measurements.
3.2 Raman scattering measurement
Raman scattering was first observed by Raman and Krishnan in 1928 [59], although the Raman effect had been predicted by Smekal in 1923 [60]. Raman spectra are established by analyzing inelastically scattered light. Raman scattering originates from a change in the polarizability of molecules or susceptibility of crystals by quasi-particles [61]. Fig. 3-2 shows the schematic of Rayleigh and Raman scattering processes. The frequency of the incident light is i, and the frequency of the scattered light is s. The
Ei = hi ， (3.2.1) Ei = hs ， (3.2.2) The scattered photon has an energy that is less than the incident photon called Stokes Raman scattering, and the energy that is greater than the incident photon called anti-Stokes Raman scattering. The difference between the inverse of these two wavelengths is known as the Raman shift, a value that is directly related to the energy. The Raman shift depends on the scattering media, demonstrating that the Raman spectrum is strongly related to the properties of the scattering materials. Raman scattering measurements yield information on lattice vibration and the excitations of the charge, spin, and orbital degrees of freedom [61].
Fig. 3-3 shows the sketch map of micro-Raman spectroscopy. The first laser light wavelength is 785 nm with the input power of 0, 1, 10, 25, 50, and 100 mW. The second laser light wavelength is 532 nm with the output power of 0, 0.2, 2, 5, 10, and 20 mW. The third laser light wavelength is 488 nm with the output power of 0, 0.4, 4, 10, 20, and 40 mW. The laser is in conjunction with the grating density of 400 grooves/mm or 1200 grooves/mm. The laser light is focused by the 50× microscope objective with NA = 0.5 and directed perpendicular to the surface of the sample, with the spatial resolution of 4 μm.
Then, the scattering light is collected at 180° and compared with the incident light.
Alternatively, the laser light is focused on the 100× microscope objective with NA = 0.9 and directed perpendicular to the surface of the sample, with the spatial resolution of 1 μm.
The charge-coupled detector with the model DU 420A-OE-152 can reduce lower the temperature to −60 ℃ to −65 ℃ with a 1024 × 256 resolution. The spectral resolution of these instruments is typically less than 0.5 cm−1. Fig. 3-4 shows a mechanical cross-sectional device. The sample was clamped on the gap for cross-cross-sectional measurement. A cryostat is used for temperature-dependent measurements. The continuous liquid helium flow into the cryostat allows measurements to be conducted in the temperature range of
10–300 K. Fig. 3-5 illustrates the setup for temperature-dependent Raman scattering measurements. The sample was mounted inside the cryostat. Coaxial shield flow liquid helium transfer line, flow meter panel for helium gas flow control, silicon diode sensor, and temperature controller were used to monitor the sample’s temperature.
Fig. 3-1: Spectroscopic ellipsometer with the focusing optics.
Fig. 3-2: Schematic of Rayleigh, anti-Stokes, and Stokes scattering.
Focusing optics coupled tube
Sample
Fig. 3-3: Sketch map of the setup of the micro-Raman scattering spectroscopy.
Fig. 3-4: The cross-sectional mechanical setup.
Sample position
Fig. 3-5: Temperature-dependent Raman scattering experimental setup.
LHe transfer line
LHe dewar
Vacuum pump
Temperature controller Flow meter
Cryostat on the stage of optical microscopy
Chapter 4
Results and discussion
4.1 Optical properties of SrFeO
3-single crystals
In this section, we present the spectroscopic ellipsometry and Raman scattering measurements of SrFeO2.86 and SrFeO2.75 single crystals. These samples were acquired from Prof. C. H. Du’s research group at the Department of Physics, Tamkang University.
We used spectroscopic ellipsometry to investigate the dielectric function and optical constants. Fig. 4-1 displays the ellipsometric parameters and of SrFeO2.86 and SrFeO2.75 single crystal at 70°angles of incidence. Optical constants can be derived from these ellipsometric parameters using Eqs. 3.1.17 and 3.1.18. Fig. 4-2 reveals the refractive index n and extinction coefficient k of SrFeO2.86 and SrFeO2.75 single crystals. We observed that the shapes of the refraction index dispersion of SrFeO2.86 and SrFeO2.75 single crystal were similar despite the different oxygen contents. As displayed in Fig. 4-2(a), when the photon energy increased from 2.1 to 3.8 eV and from 4.7 to 5.4 eV, the refractive index decreased, which indicates anomalous dispersion. As illustrated in Fig. 4-2(b), when the photon energy increased from 2.3 to 3.9 eV and from 4.6 to 5.3 eV, the refractive index decreased, which displays anomalous dispersion. Fig. 4-3 illustrates the optical absorption coefficient spectra of SrFeO2.86 and SrFeO2.75 single crystals measured at room temperature.
We fitted these absorption spectra by using a standard Lorentzian model [62]. The background was fitted using the Lorentzian functions [62]. The absorption was resolved into two peaks. An increase in the photon energy resulted in a progressive increase in the absorption. Two absorption bands were observed at approximately 2.98, and 5.05 eV for
the SrFeO2.86 single crystal. Galakhov et al. [63] presented the valence band region of the XPS spectra, showing the similar absorption bands. Similar absorption bands were also observed for the SrFeO2.75 single crystal. The first-principles calculations [31,33,64]
indicated that 2.98 and 5.05 eV peaks were associated with the charge transfer transitions between oxygen 2p states to the iron 3d states.
The absorption coefficient included contributions from the direct and indirect bandgap transitions in the typical solid state and is expressed as follows [65,66]:
(5.1)
where Eg,dir is the magnitude of the direct bandgap, Eg,ind is the magnitude of the indirect bandgap, Eph means the absorbed (emitted) phonon energy, and A and B are constants. The model described above, which supposes a simple band shape, causes the extraction of the direct energy gap when (·E)2 is plotted as a function of photon energy. Fig. 4-4 (a) displays the direct bandgap analysis of SrFeO2.86 and SrFeO2.75 single crystals. Plotting (α．E)0.5 as a function of photon energyled to an indirect band gap shown in Fig. 4-4(b) and (c). The values of band gap of both samples are 2.00 ± 0.01 and 1.98 ± 0.01 eV at 300 K, respectively.
Rothschild et al. [52] performed the four-point probe conductivity measurements and demonstrated the band gap of SrFeO3- at approximately 1.9 ± 0.1 eV. Ghaffari et al. [53]
employed the ultraviolet photoelectron spectroscopy and determined the band gap of SrFeO3- at approximately 1.8 eV. L. Wang et al. [67] measured the spectroscopic ellipsometry spectra of SrFeO2.5, SrFeO2.5+ and SrFeO3 thin films. The decrease in band gap from 2.05 to 0 eV indicated that thin films transitioned from semiconductor to metal.
Our experimental results are comparable to these previous studies [52,53,67].
Fig. 4-5 demonstrates the room-temperature Raman scattering spectra of SrFeO2.86
and SrFeO2.75 single crystals. We fitted these Raman scattering spectra using the standard Lorentzian functions [62]. The factor group analysis indicated that SrFeO2.86 has a
tetragonal structure (space group I4/mmm), with atoms located in the 8f(Fe), 16m(O), 8i(Sr), phonon modes. Our spectrum consists of 15 first-order phonon modes at approximately 83, 94, 107, 117, 123, 135, 175, 226, 267, 320, 332, 414, 428, 478, and 626 cm-1 and four second-order phonon modes at approximately 800, 988, 1234, and 1363 cm−1. The numbers of observed phonon modes are less than the predicted result because of the weak intensity of some phonon modes. The phonon modes at approximately 84, 94, 107, 117, 123, 135, 175, 226, and 267 cm-1 modes were associated with the Sr atoms movements and the rotation of FeO6. The phonon modes at approximately 320, 332, 414, 428, and 478 cm−1 were assigned to the bending vibrations of oxygen ions in the FeO6 octahedra. The phonon mode near 626 cm−1 was related with the stretching vibrations of oxygen ions in the FeO6
octahedra [68-70].
Factor group analysis indicated that SrFeO2.75 has an orthorhombic structure (space group Cmmm) [19] including one formula unit per primitive cell, with atoms located in the 4f(Fe), 4h(O), 2d(Sr), 4g(Sr), 4i(Fe), 16r(O), 2c(Sr), and 2b(O) Wyckoff sites. Sr = Ag + B1g + 3B1u + B2g + 3B2u + 3B3u + B3g represent the motion of Sr. The motion of Fe is represented as Fe = Ag + Au + B1g + 2B1u + 3B3u + B3g. The motion of O is represented as
of the phonon modes of SrFeO2.75 at the center of the Brillouin zone are expressed as total
= 7Ag + 6B1g + 5B2g +4B3g + 9B1u + 10B2u + 10B3u + B1u + B2u + B3u + 4Au. The modes Ag, B1g, B2g, and B3g are Raman active; B1u, B2u, and B3u modes are infrared active; Au mode is neither Raman nor infrared active. The SrFeO2.75 structure with the Cmmm space group contains 22 Raman-active modes. Our spectrum is composed of 12 first-order phonon modes at approximately 84, 92, 107, 116, 130, 138, 175, 321, 333, 416, 479, and 624 cm-1 and two second-order phonon modes at approximately 800 and 1380 cm-1. The phonon modes at approximately 84, 92, 017, 116, 130, 138, and 175 cm-1 were assigned to the displacement of Sr atoms and the rotations of FeO6 octahedra. The phonon modes at approximately 321, 333, 416, and 479 cm−1 were attributed to the bending vibrations of oxygen ions in the FeO6 octahedra. The phonon mode near 624 cm−1 was associated with the stretching vibrations of oxygen ions in the FeO6 octahedra [68-70]. Fig. 4-6 (a) and (b) reveals the room-temperature polarized Raman scattering spectra of SrFeO2.86 single crystal between the different frequency ranges. The inset displays the optical image of the SrFeO2.86 single crystal. Fig. 4-7 displays the room-temperature cross sectional polarized Raman scattering spectra.
Table 4-1 summarizes the symmetry of Raman-active phonon modes observed in different scattering configurations for SrFeO2.86 single crystals. According to the selection rule, the A1g and B1g symmetry phonon modes appear in the parallel configurations of the ab plane. The B2g symmetry phonon mode only appears in the cross configurations of the ab plane. The Eg symmetry phonon mode appears in the cross configurations of the ac or bc plane [71-76].
The modes at approximately 320, 332, and 428 cm−1 displayed a larger intensity in the parallel configuration (YY) than in the perpendicular configuration (YX) and did not appear in the cross sectional parallel configuration (ZZ), thus revealed to be B1g symmetry.
The mode at 626 cm−1 displayed a larger intensity in the parallel configuration (YY) than in the perpendicular configuration (YX) and displayed a larger intensity in the cross-section parallel configuration (XX) and (ZZ) than in the perpendicular configuration (YX), thus revealed to be A1g symmetry.
Table 4-1: Symmetry of Raman-active phonon modes observed in different scattering configurations for SrFeO2.86 single crystals.
Symmetry A1g B1g B2g Eg
X(YY)X̅
○ ○
X(YZ) X̅
○
X(ZZ) X̅
○
Y(XX)Y̅
○ ○
Y(XZ)Y̅
○
Y(ZZ) Y̅
○
Z(XX) Z̅
○ ○
Z(YX) Z̅
○
Z(YY) Z̅
○ ○
Fig. 4-8 illustrates the room-temperature polarized Raman scattering spectra and optical image of SrFeO2.75 single crystals. Table 4-2 summarizes symmetry of Raman-active phonon modes observed in different scattering configurations for SrFeO2.75 single crystals. According to the selection rule, the Ag symmetry phonon mode appears in the
Fig. 4-8 illustrates the room-temperature polarized Raman scattering spectra and optical image of SrFeO2.75 single crystals. Table 4-2 summarizes symmetry of Raman-active phonon modes observed in different scattering configurations for SrFeO2.75 single crystals. According to the selection rule, the Ag symmetry phonon mode appears in the