Optical Properties of Nanocrystalline Monoclinic Y2O3 Stabilized by

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Optical Properties of Nanocrystalline Monoclinic Y2O3 Stabilized by Grain Size and Plastic Strain Effects via High-Pressure Torsion Hadi Razavi-Khosroshahi,*,† Kaveh Edalati,‡,§ Hoda Emami,‡ Etsuo Akiba,‡,∥ Zenji Horita,‡,§ and Masayoshi Fuji† †

Advanced Ceramics Research Center, Nagoya Institute of Technology, Gifu, Japan WPI, International Institute for Carbon-Neutral Energy Research, §Department of Materials Science and Engineering, Faculty of Engineering, and ∥Department of Mechanical Engineering, Faculty of Engineering, Kyushu University, Fukuoka, Japan

‡

ABSTRACT: Yttrium oxide (yttria) with monoclinic structure exhibits unique optical properties; however, the monoclinic phase is thermodynamically stable only at pressures higher than ∼16 GPa. In this study, the effect of grain size and plastic strain on the stability of monoclinic phase is investigated by a high-pressure torsion (HPT) method. A cubic-to-monoclinic phase transition occurs at 6 GPa, which is ∼10 GPa below the theoretical transition pressure. Microstructure analysis shows that monoclinic phase forms in nanograins smaller than ∼22 nm and its fraction increases with plastic strain, while larger grains have a cubic structure. The band gap decreases and the photoluminescence features change from electric dipole to mainly magnetic dipole without significant decrease in the photoluminescence intensity after formation of the monoclinic phase. It is also suggested that monoclinic phase formation is due to the enhancement of effective internal pressure in nanograins.

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INTRODUCTION Yttria (Y2O3) is a sesquioxide ceramic with high melting point, high hardness, and good chemical stability.1 Y2O3 doped with rare earth elements is used in different applications such as phosphor materials, fluorescent lamps, and field emission displays because of its optical properties.2 Recently, nanostructured Y2O3-based materials have found potential applications in lasers3 and bioimaging4 because (1) their color can be finley tuned by doping with lanthanide ions, (2) they are stable, unlike organic dyes, and (3) they are nontoxic when compared to their semiconductor counterparts like CdSe and PbS. As shown in the theoretical phase diagram of Figure 1, Y2O3 has a cubic structure at ambient pressure and transforms to the monoclinic and hexagonal phases with increasing pressure.5 In situ Raman spectroscopy or X-ray diffraction analysis by the diamond anvil cell (DAC) technique also reported that the cubic-to-monoclinic transition in Y2O3 occurs at 11 GPa,6 12 GPa7 or 15 GPa;8 however, a reverse phase transformation is also reported when the pressure is released. Earlier in situ experiments under high pressure showed that the formation of monoclinic phase can result in increasing photoluminescence (PL) intensity6,9 and shifting the PL peaks to lower energies.9 It © XXXX American Chemical Society

Figure 1. Theoretical pressure−temperature phase diagram of Y2O3. Data were taken from ref 3.

was also reported that nanoparticles of Y2O3 in monoclinic phase, stabilized by chemical synthesis methods, show better PL intensities than those in cubic phase.10,11 Despite these reports, stabilization of monoclinic phase at ambient pressure Received: November 14, 2016

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DOI: 10.1021/acs.inorgchem.6b02725 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

formation on the band gap and photoluminescence (PL) features is also examined.

without use of high-temperature chemical reactions is still a challenging task. Size effect is a factor that can affect the stability of highpressure phases in different ceramics such as Al2O3,12 BaTiO3,13 ZrO2,14 and TiO2.15 Metastable polymorphs can be stabilized by reducing the particle size to the nanometer level because of the lower free surface energy of metastable phases at reduced sizes. In Y2O3, it has been reported that the monoclinic phase becomes stable when the particle size is less than 4 nm,16 8 nm,17 10 nm,18 or 22 nm10 because of the surface energy effect.19 Despite these reports, there have been few studies on the effect of grain (crystallite) size on high-pressure phase transitions in Y2O3. In addition to temperature, pressure, and particle/grain size, plastic strain can also influence phase transformations. Earlier studies showed that the formation of metastable high-pressure phases could be accelerated in several materials by means of plastic strain: orthorhombic phase (black phosphorus) in P,20 diamond-like phases in C,21 and ω phase in Ti.22 Application of shock-wave compression23 and high-energy ball milling24 methods to Y2O3 suggested that plastic strain may cause cubic-to-monoclinic phase transformation. Although these methods induce higher plastic strain to the sample when compared to the DAC method, the methods provide uncertain evidence concerning the effect of plastic strain because of simultaneous changes of strain, strain rate, and pressure. The high-pressure torsion (HPT) method, as schematically shown in Figure 2a, is an ideal method to study the effect of

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EXPERIMENTAL PROCEDURES

Starting material was Y2O3 powder with a purity level of 99 mol % with Eu impurities (Y3+:Eu3+ = 99:1). Almost 0.2 g of powder, with an average particle size of ∼2 μm, was placed between two Bridgman anvils20 at pressures of 2, 4, and 6 GPa. Shear strain γ (γ = 2πrN/h, where r is distance from disc center, N is number of turns, and h is sample thickness)26 was introduced by rotating the two anvils with respect to each other for either N = 0 (mere compression), 1/16, 1/8, 1 /4, 1, or 5 turns at room temperature. The average strain introduced to the entire disc sample was calculated by considering r = 3.5 mm. The HPT-processed material, which had a disc shape with 10 mm diameter and ∼0.8 mm thickness, was examined by (i) X-ray diffraction (XRD) with Cu Kα radiation, (ii) PL spectroscopy with an excitation wavelength of 245 nm, (iii) UV−vis diffuse reflectance spectroscopy, and (iv) high-resolution transmission electron microscopy (TEM). These characterizations were conducted on the entire HPT disc after crushing, while the TEM samples were prepared from the edge part of the HPT-processed discs.

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RESULTS AND DISCUSSION Figure 2b shows the XRD profiles of Y2O3 processed by HPT at pressures of 2, 4, and 6 GPa for N = 1. It is evident that no phase transition occurs at 2 and 4 GPa, while a cubic-tomonoclinic phase transition occurs at 6 GPa. The monoclinic phase remained stable for at least 400 days under ambient conditions. The formation of monoclinic phase at 6 GPa, which is far below the critical pressure of monoclinic formation (11− 16 GPa),6−8 should be due to the effect of grain size and plastic strain, as will be discussed. XRD profiles of HPT-processed samples at 6 GPa for various turns are shown in Figure 3a. Examination of Figure 3a indicates several important points. First, peak broadening occurs after HPT processing, implying that lattice strain

Figure 2. (a) Schematic illustration of HPT method. (b) XRD profiles for Y2O3 powder and samples processed by HPT for N = 1 turn at various pressures.

plastic strain and grain size on phase transitions.20,25 In this method, material is compressed between two anvils under high pressure, and plastic strain is induced by rotating the two anvils against each other. Although HPT is mainly used as a severe plastic deformation (SPD) method to achieve nanograins26,27 or phase transitions28,29 in metals, there have been several attempts to employ the method for plastic deformation of hard and brittle ceramics.30−33 In this study, the effect of grain size and plastic strain on the formation of monoclinic Y2O3 phase is investigated for the first time by HPT method. The effect of monoclinic phase

Figure 3. (a) XRD profiles and (b) fractions of cubic and monoclinic phases versus shear strain, obtained by Rietveld analysis, for Y2O3 powder and samples processed by HPT for various turns at 6 GPa. B

DOI: 10.1021/acs.inorgchem.6b02725 Inorg. Chem. XXXX, XXX, XXX−XXX

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The formation of monoclinic phase at 6 GPa, which is appreciably below the critical pressure required for phase transformation under pure compression (11−16 GPa),5−8 clearly proves that the plastic strain and grain size have significant effects on the kinetics and thermodynamics of phase transformations. It is well-known that plastic strain is an effective parameter to overcome the kinetic barrier for nucleation of new phases under high pressure.20−24 In pure compression, new phase nucleates on limited lattice defects that are present in the initial microstructure.33,34 However, when the material is severely deformed by HPT, large fractions of lattice defects as nucleation sites are repeatedly produced.37 Therefore, it is quite reasonable that the plastic strain can enhance the kinetics for phase transformation, as shown in Figure 3b. The pileup of lattice defects, which usually occur at the grain boundaries during plastic deformation, can also influence the thermodynamics for phase transformation by providing a highpressure tensor for atomic rearrangement.38 As the grain sizes are reduced to the nanometer level by HPT processing, the volume fraction of grain boundaries increases and thus the pileup of lattice defects is enhanced, which can subsequently result in increasing the internal pressure.38 The current results on Y2O3, together with earlier studies on TiO2 processed by HPT33 as well as BN and several other materials processed by rotational DAC,34,39 confirm that high-pressure phases become thermodynamically stable in nanograins at pressures lower than those expected for coarse-grained materials under pure compression. The monoclinic phase in Y2O3 remains stable even after the pressure is released because of the energy barrier for the nucleation of cubic phase, which makes the cubic/ monoclinic interfaces energetically unfavorable.16−19 Figure 5a shows UV−vis diffuse reflectance spectra for the starting powder and samples processed by HPT at 6 GPa for N = 1/8, 1, and 5. In all samples, below a certain wavelength, known as absorption edge, an intense absorption occurs. The bimodal shape of absorption edge is similar to that reported earlier for Y2O3.40 It is evident that the absorption edge shifts to

resulting from lattice defects and grain refinement is generated during the process. Although the peak broadening is not appreciable after pure compression (N = 0), the peak broadening becomes more significant when the samples are rotated, confirming that the plastic strain, which has a much higher effect than pure compression on the microstructural evolution, could be successfully introduced to the hard and brittle Y2O3. Second, only cubic phase is present under pure compression, while the monoclinic phase appears after HPT processing. Third, the fraction of monoclinic phase increases with increasing plastic strain and reaches ∼90% at large strains, as shown by Rietveld analysis in Figure 3b. The trend of monoclinic phase fraction versus plastic strain is similar to those reported in other HPT-processed ceramics.30−34 The color of samples also changes from white to pink after HPT processing. Since an earlier publication reported that the color of Y2O3 is independent of its crystal structure,35 the pink color may not be due to the formation of monoclinic phase in this study. Kitamura et al.36 showed that when the particle size of Y2O3 is reduced to the nanometer level, the color changes from white to pink. Therefore, the pink color of the HPT-processed samples should be due to formation of nanograins, as shown in Figure 4a.

Figure 4. (a) High-resolution TEM image and FFT difractograms corresponding to grains A, B, and C. (b) Histogram of grain size distribution for cubic and monoclinic phases in Y2O3 sample processed by HPT for N = 5 turns at 6 GPa.

Figure 4a shows a representative high-resolution TEM micrograph of sample after HPT processing for 5 turns at 6 GPa and corresponding fast Fourier transformation (FFT) for three selected nanograins. Examination of 150 grains, as summarized in a histogram in Figure 4b, indicates three important points. First, all grains are at the nanometer level with an average grain size of 17 ± 10 nm. The formation of nanograins by HPT processing was also reported in other ceramics such as ZrO2,30 BaTiO3,31 and TiO2.32 Second, a large fraction of monoclinic phase is formed after HPT processing, although the fraction could not be determined quantitatively by TEM due to the orientation or superimposition of nanograins. Third, nanograins smaller than ∼22 nm have a monoclinic structure, while larger grains always have a cubic structure, indicating the substantial role of grain size on phase stability.

Figure 5. (a) UV−vis diffuse reflectance spectra and (b) band gap estimation via Kubelka−Munk theory for Y2O3 powder and for samples processed by HPT at 6 GPa for various turns. C

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that materials with better crystallinity and larger particle/grain sizes (smaller surface-to-volume ratio) show more intense PL spectra because quench sites for photoexcited electrons are abundant on the free surfaces and grain boundaries.11 In summary, the HPT method can be considered as a mechanical route to synthesize the monoclinic Y2O3 phase for optical applications. This mechanical route contrasts with the chemical routes, in which the monoclinic phase is stabilized by chemical reactions at high temperature.

higher wavelengths with increasing plastic strain. The band gap of samples can be estimated from the spectra of Figure 5a by use of Kubelka−Munk theory:40,41 α = F(R ) = (1 − RL)2 /2RL

(1)

αhν = B(hν − Eg )n

(2)

Here RL is the percentage of reflected light, α is the absorption coefficient, h is Planck’s constant, ν is the light frequency, B is the absorption constant, Eg is the band gap, and n is 2 for the indirect band gap. The variations of (αhν)2 versus hν are plotted in Figure 5b. Extrapolating the linear parts of curves gives the band gaps 5.82, 5.77, 5.68, and 5.69 eV for the starting powder and for the samples processed by HPT for N = 1/8, 1 and 5 turns, respectively. The estimated band gap for the starting powder is consistent with the reported band gap of cubic Y2O3, 5.8 eV.42 It is evident that as the monoclinic phase fraction increases with increasing plastic strain, the band gap becomes narrower. It should be noted that the band gap changes in this study cannot be attributed to the quantum confinement effect because (i) the quantum confinement effect occurs when the grain radius is smaller than the Bohr radius, where the Bohr radius of Y2O3 is as small as ∼2 nm,43 and (ii) the absorption edge should shift to higher energies by the quantum confinement effect, whereas it shifts to lower energies after HPT processing. Figure 6 shows the PL spectra of starting powder and samples processed by HPT at 6 GPa. In Y2O3 with Eu

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CONCLUSIONS (1) The cubic Y2O3 transforms to a high-pressure monoclinic phase upon introduction of plastic strain at a pressure of 6 GPa (∼10 GPa below the critical pressure), when the grain sizes are reduced below ∼22 nm because of increasing the effective internal pressure. (2) The monoclinic phase fraction increases with increasing plastic strain, which results in band-gap narrowing and changes in PL features without any significant decrease in PL intensity. (3) PL transition features change from electric dipole to mainly magnetic dipole upon formation of monoclinic phase, indicating that the phase transition alters the electric field strength of Y2O3 under ambient pressure.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; tel +81-572-248110. ORCID

Hadi Razavi-Khosroshahi: 0000-0002-4344-9612 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the ALCA Program, Japan; in part by CREST, JST, Japan; and in part by MEXT, Japan (26220909 and 15K14183). K.E. thanks Kyushu University for the Qdai-Jump research grant (28325) and MEXT, Japan, for a Grant-in-Aid for Scientific Research (B) (16H04539). Highpressure torsion was conducted in the IRC-GSAM center at Kyushu University.

Figure 6. PL spectra for powder and for samples processed by HPT at 6 GPa for various turns.

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impurities, four PL transitions are possible: 5D0 → 7FJ (J = 1, 2, 3, 4).44 The 5D0 → 7F1 transition originates from S6 symmetry (centrosymmetric), while the other three transitions originate from C2 symmetry (noncentrosymmetric).45 Figure 6 shows that as the plastic strain increases with increasing number of HPT turns, the intensity of 5D0 → 7F2 decreases, while the 5D0 → 7F3 intensity increases. Since the 5D0 → 7F2 transition has the character of electric dipole and the 5D0 → 7F3 transition has mixed character with mainly magnetic dipole,45 it is suggested that the PL transition feature changes from electric dipole to mainly magnetic dipole by formation of monoclinic phase. Moreover, since 4f electrons are shielded by outer electrons of 5s2 and 5p6, they are hypersensitive to the surrounding environment and crystal structure. Therefore, the cubic-tomonoclinic transition alters the electric field strength of Y2O3 and results in a new peak at 629 nm.6 It is noteworthy that although the grain size and crystallinity of Y2O3 are significantly reduced after HPT processing, PL intensity (the total peak area of PL spectra in Figure 6) does not significantly decrease because of the formation of monoclinic phase. It is well-known

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DOI: 10.1021/acs.inorgchem.6b02725 Inorg. Chem. XXXX, XXX, XXX−XXX