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[2025.05.19.Draft.A Chemical synthesis study of alpha-Al2O3 Fe3+. Optics and lattice dynamics of alpha-Al2O3 Fe3+.docx](https://mdr.nims.go.jp/filesets/56f70c41-8efb-4fed-951b-513642fa28b8/download)

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Koushi Tawara, Tsubasa Sekiya, [Gaku Okuma](https://orcid.org/0000-0002-2997-9166), Toshihiro Yoshizumi

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[Optics and lattice dynamics of α-Al2O3:Fe3 +](https://mdr.nims.go.jp/datasets/0d4c4dba-5588-478d-8d6f-85c1c2915c66)

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A chemical synthesis study of -Al2O3:Fe3: Optics and lattice dynamics of -Al2O3:Fe3Koushi Tawara a, Tsubasa Sekiya b, Gaku Okuma c,d, Toshihiro Yoshizumi a,ba Department of Electrical Engineering, Electronics and Applied Physics, Faculty of Engineering, Saitama University, Saitama 338-8570, Japanb Mathematics, Electronics and Informatics Division, Graduate School of Science and Engineering, Saitama University, Saitama 338-8570, Japanc Research Center for structural materials, National Institute for Materials Science, Ibaraki 305-0047, Japand Materials and Structures Laboratory, Institute of Integrated Research, Institute of Science Tokyo, Yokohama 226-8503, JapanAbstractThis study investigates the chemical synthesis of trivalent iron cation Fe3 substituted -phase alumina, -Al2O3:Fe3. -Al2O3:Fe3 samples were synthesized from -Al2O3 and -Fe2O3 powders within the composition range of Al2xFexO3 (=0.020.04). X-ray diffraction (XRD) analysis confirmed that the synthesized samples were in -phase crystalline structure. UV–Vis absorption spectroscopy revealed that the predominant oxidation state of iron cations substituting Al3 sites in -Al2O3 is Fe3. Fourier-transform infrared (FTIR) and Raman scattering spectroscopy were employed to examine the lattice dynamics of the -Al2O3:Fe3 samples. Keywords:Chemical synthesis of -Al2O3:Fe3, Infrared and Raman scattering measurements, Lattice dynamics Corresponding authorE-mail address:yoshizumi@mail.saitama-u.ac.jp (T. Yoshizumi)1. Introduction-phase aluminum(Ⅲ) oxide (-Al2O3) is an insulating and visible light transparent material [1] with a wide bandgap of the electronic structure [2]. Substitution of trivalent aluminum cation Al3 sites in -Al2O3 with trivalent iron cation Fe3 affects its optical absorption property. Optical absorption properties of -Al2O3:Fe3 have been studied in mineralogical yellow sapphire samples [3,4]. In this study, a chemical synthesis process of -Al2O3:Fe3, in which Fe³⁺ is the predominant substituent cation, was carried out using -Al2O3 and -Fe2O3 as starting materials. For substitution of Al3 cations in -Al2O3 with Fe3 cations, -Fe2O3 in which iron cations are trivalent Fe3 was used as a starting material for synthesizing Al2O3:Fe3. Additionally, both -Al2O3 [58] and -Fe2O3 [9,10] have crystalline structure with  space group. Optical absorption spectra of the synthesized samples were measured for confirming Fe3 substitution into Al3 sites of -Al2O3. Lattice dynamics of -Al2O3 have been studied by infrared [11,12], Raman scattering [13,14], inelastic neutron scattering [5,6,15,16]. In this study, infrared and Raman scattering spectrometry were performed to investigate the lattice dynamics of Al2O3:Fe3 samples.2. Experimental section-Al2O3 and -Fe2O3 were used as starting materials to chemically synthesize -Al2O3:Fe3 in a composition range of Al2xFexO3 (x=0.020.04). The starting powders were mixed with a solvent in a mortar. After complete evaporation of the solvent, the mixture was calcined at 1100 C for 24 h in air using an electric furnace, with a heating rate of xx °C/min. Then, the calcined sample was ground again with a solvent in a mortar. After drying, a second calcination was performed under the same conditions (1100 C, 24 h, in air). Following the two calcination steps, the sintered sample was ground into powder for subsequent characterization. X-ray diffraction (XRD) measurements were performed using a diffractometer with Cu K source (D2 PHASER, Bruker). UV-Vis absorption spectra were obtained using a spectrometer (V-770, JASCO) with a diffuse reflectance method. Infrared measurements were performed using an Fourier-transform infrared (FT-IR) spectrometer (TENSOR Ⅱ, Bruker) with attenuated total reflectance (ATR) method. In the FT-IR measurements with ATR, 16 scans were recorded with a resolution of 4 cm1. Unpolarized Raman scattering spectra were acquired using a confocal Raman microscope (in Via, Renishaw). Raman spectra were collected with a 532 nm excitation laser source with a laser power of 7.5 mW, using 10 scans and a resolution of 4 cm1. The XRD, UV-Vis, FT-IR and Raman scattering measurements were performed at room temperature.3. Results and discussionsFigure 1 shows the XRD patterns of the chemically synthesized samples, Al2xFexO3 (x=0.020.04). No peaks corresponding to the α-Fe₂O₃ starting material were observed in the XRD patterns, indicating that α-Fe₂O₃ was not present in the final products. Furthermore, the XRD patterns revealed that the samples are in -phase crystalline structure. These results indicate that Al3 cation sites of the -Al2O3 starting material were substituted with iron cations.The UV-Vis absorption spectra of the synthesized samples are shown in Figure 2. Optical absorption of samples in the compositional range of Al2xFexO3 (x=0.020.04) revealed peaks around 260, 330, 376 and 453 nm and broad bands around 530, 700 nm. UV-Vis measurements have been used for studying iron cations substituted -Al2O3. Optical transmittance measurements of -Al2O3:Fe3 thin films showed absorption bands in vicinity of 259 nm which is attributed to Fe3 cations in -Al2O3 [17]. Optical studies of natural mineral samples have shown absorption peaks near 330, 376, and 453 nm, where the latter two are associated with Fe³⁺–Fe³⁺ pairs [3]. Peak around 450 nm attribute to Fe3 substituted in Al3 sites is a main factor in color of yellow sapphires [4]. Additionally, previous literature revealed that increasing absorption coefficient from 500 to 600 nm are attributed to h-Fe3 pair absorption [3,4]. Measured results of Figure 2 also revealed this absorption band from 500 nm to 600 nm around 530 nm peak. Furthermore, a weak broad band around 700 nm was observed in samples of this study. This weak broad band around 700 nm also attribute to high concentration of Fe3 [3]. Therefore, UV-Vis absorption peaks and bands indicate the main valence of iron cations is Fe3 in the samples of this study. The crystalline lattice dynamics of -Al2O3 has been experimentally studied by infrared [11,12], Raman scattering [13,14], inelastic neutron scattering measurements [5,6,15,16]. Furthermore, phonon dispersion of -Al2O3 have been studied by theoretical calculations [7,8,18,19]. In this study, infrared and Raman scattering spectra of -Al2O3:Fe3 synthesized using -Al2O3 and -Fe2O3 starting materials were measured. Figure 3 shows FT-IR absorbance spectra of -Al2O3:Fe3 samples measured with the ATR method. Intensity of FT-IR absorbance of -Al2O3:Fe3 samples decreases as iron cation concentration increases. -Al2O3 has a space group  and the irreducible representations of  in the group theory [13,19]. In the irreducible representation, infrared active modes of -Al2O3 are  and  [7,8,11,12]. Lattice dynamics studies of -Al2O3 showed frequencies of infrared active modes in  and  []. -Al2O3:Fe3 samples in this study also revealed peaks at frequencies of transverse optic modes around 11.9, 17.4 THz corresponding with infrared active modes of  [7,8] and 11.211.4, 11.4, 16.817.1, 19.0 THz corresponding with modes of  [7,8]. Furthermore, slight frequency shifts were observed as Fe3 concentrations increases. Figure 4 shows the Raman shift spectra of the -Al2O3:Fe3 samples measured using the excitation laser of 532 nm wavelength. In the group theory, irreducible representations of Raman active modes of -Al2O3 are  and  [7,8,13]. -Al2O3:Fe3 samples in this study revealed 12.4 and 19.2 THz corresponding to , and 11.3, 12.8, 13.4, 17.2 and 22.4 THz corresponding to [7,8,12], respectively. Compared with the -Al2O3 [13,14], furthermore, additional peaks around 14.5, 20.1, 24.2 THz and bands of 4.89.9 THz and 15.016.9 THz were also observed in -Al2O3:Fe3. Observed Raman shift spectra of the -Al2O3:Fe3 samples are more complicated than that of -Al2O3. The measured Raman shift spectra of -Al2O3:Fe3 indicates that the substitution with Fe3 cation effects the Raman shift spectrum of -Al2O3 starting material. These infrared and Raman scattering spectroscopy results provide a knowledge for studying crystalline lattice dynamics of -Al2O3:Fe3. Further investigation using inelastic neutron scattering and density functional theory (DFT) calculations will be beneficial for understanding the phonon dispersion in -Al2O3:Fe3.4. Summary and conclusionsThis work focused on the chemical synthesis of α-Al₂O₃:Fe³⁺ samples using α-Al₂O₃ and α-Fe₂O₃ as starting materials in the compositional range of Al₂₋ₓFeₓO₃ (x = 0.02–0.04). The -Al2O3:Fe3 samples were synthesized by heating at 1100 C. Results of UV-Vis measurements revealed that the main valence state of iron cations substituted at the Al3 cation sites in -Al2O3 is Fe3. Infrared measurements using the ATR method showed that the absorbance intensity decreases with increasing Fe3 concentration. Raman shift spectra of -Al2O3:Fe3 revealed additional peaks along with the Raman-active modes of -Al2O3. These infrared and Raman scattering measurement results indicate that Fe3 substitution using -Fe2O3 as a starting material affects the phonon dispersions of -Al2O3.AcknowledgementsWe thank Rio Miyazaki for assistance with experiments.References[1] E. Gaudry, A. Kiratisin, P. Sainctavit, C. Brouder, F. Mauri, A. Ramos, A. Rogalev, J. Goulon, Structural and electronic relaxations around substitutional Cr3 and Fe3 ions in corundum, Phys. Rev. B 67 (2003) 094108. [2] R.H. French, Electronic band structure of Al2O3, with comparison to AlON and AlN, J. Am. Ceram. Soc. 73 (1990) 477489.[3] E.V. Dubinsky, J. Stone-Sundberg, J.L. 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Stolz, Lattice dynamics of sapphire (Corundum) Part I: Phonon dispersion by inelastic neutron scattering, Z. Phys. B 21 (1975) 319324.[16] H. Bialas, Contours of constant phonon energies in the bc-plane of sapphire (Al2O3) by means of inelastic neutron scattering, Z. Phys. B 27 (1977) 121123.[17] N. Yu, Q. Wen, D. R. Clarke, P. C. Mclntyre, H. Kung, M. Nastasi, T.W. Simpson, I.V. Mitchell, D. Li, Formation of iron or chromium doped epitaxial sapphire thin films on sapphire substrates, J. Appl. Phys. 78 (1995) 54125421.[18] W. Kappus, Lattice dynamics of sapphire (Corundum) Part II: Calculations of the phonon dispersion, Z. Phys. B 21 (1975) 325331.[19] K. Iishi, Lattice dynamics of corundum, Phys. Chem. Minerals 3 (1978) 110.Figure 1. XRD patterns of powder samples in the composition range of Al2xFexO3 (x=0.020.04).Figure 2. UV-Vis absorption spectra of -Al2O3:Fe3 samples.Figure 3. FT-IR absorbance spectra of -Al2O3:Fe3 powder samples. The ATR method was used for the infrared measurement.Figure 4. Raman shift spectra of Al2O3:Fe3 powder samples. The excitation laser of 532 nm wavelength was used.image1.tifimage2.tifimage3.tifimage4.tif