Reversible Photoinduced Interconversion of Color Centers in α-Al2O3

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J. Phys. Chem. C 2009, 113, 20949–20957

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Reversible Photoinduced Interconversion of Color Centers in r-Al2O3 Prepared under Vacuum M. Itou, A. Fujiwara, and T. Uchino Department of Chemistry, Graduate School of Science, Kobe UniVersity, Nada, Kobe 657-8501, Japan ReceiVed: September 1, 2009; ReVised Manuscript ReceiVed: October 27, 2009

Introduction of oxygen vacancies in R-Al2O3 has been quite challenging because of its high thermal stability and high melting temperature. Here, we present a method to introduce oxygen vacancies in R-Al2O3. The method is based on vacuum heating of a polycrystalline R-Al2O3 powder, producing a sphere-shaped solid consisting of R-Al2O3 single crystals. The thus prepared R-Al2O3 contains not only oxygen monovacancies, namely, the F+ and F centers, but also aggregated oxygen vacancies, including the F2 and F2+ centers. We have found that a completely reversible interconversion between the F- and F2-type centers occurs by irradiating different wavelengths of light. For example, ∼90% of the F2 and F2+ centers can be converted to each other by alternate irradiation of ∼300 and ∼220 nm light even at room temperature. The observed interconversion can be interpreted in terms of the photoionization of the neutrally charged centers such as F2 and F centers by irradiation of ∼300 and ∼220 nm light, respectively, and the subsequent electron capture by the positively charged F+ and F2+ centers. In the present R-Al2O3 crystal, the positive charge of the F+ and F2+ centers is likely to be compensated by negatively charged aluminum vacancies, which can be introduced especially by the present vacuum heating process. 1. Introduction Because of its high mechanical strength, high optical transparency, low dielectric loss, and high electrical resistivity, R-alumina (R-Al2O3) has been widely utilized for diverse applications ranging from biological implants to abrasives, insulators, high-temperature optics, and refractories. Because the existence of defects or color centers, such as oxygen vacancies, can strongly affect the electrical, optical, and thermal properties of the crystalline samples of interest, the structure and properties of color centers in R-Al2O3 have been extensively studied during the past decades.1-11 The color centers in R-Al2O3 exist mainly in the form of F+ (an oxygen vacancy occupied by one electron) and F (an oxygen vacancy occupied by two electrons) centers,11 which can be intentionally introduced upon bombardment with particles1,3,5,6,9,10 (neutrons, electrons, energetic ions) or by thermochemical reduction2 under a flowing Ar-gas atmosphere around its melting temperature (2054 °C). The F center has an absorption band at 205 nm (6.1 eV) and a photoluminescence (PL) band at 410 nm (3.0 eV),5,11 whereas the F+ center shows several absorption bands at 232 nm (5.3 eV) and 256 nm (4.8 eV) and a PL band at 330 nm (3.8 eV).3,11 It should also be noted that the F+ and F centers exhibit not only PL but also thermoluminescence (TL) and optically stimulatedluminescence(OSL)afterX-ray,β,andγirradiations.12-15 Thus, the F+ and F centers in R-Al2O3 have attracted renewed interest in view of dosimetry applications,16 including environmental dosimetry, medical dosimetry, and space dosimetry.17,18 In addition to the F+ and F centers, color centers associated with two oxygen vacancies, for example, F2, F2+, and F22+ centers, have been shown to exist in R-Al2O3.19-22 As compared with the case of the F+ and F centers, however, studies on the optical and electronic properties of the F2-type centers are rather few. This is because the introduction of F2-type centers into R-Al2O3 is much more difficult than that of F-type centers. Thermochemical reduction of nominally pure R-Al2O3 cannot

normally create F-center aggregates. Thus far, only a high dose (∼1018 n/cm2) of fast-neutron irradiation enables one to obtain R-Al2O3 with observable amounts of F2-type centers;19-22 however, such a high dose of neutron irradiation often accompanies optical-damage processes and further induces concentration quenching of the relevant luminescence centers.6,11 Note also that the thermal stability of the vacancies created by particle irradiation is generally low as compared with those produced by thermochemical reduction.2,4,11 Previously, Ramı´rez et al. demonstrated that the F2-type centers can be successfully introduced in Mg-doped Al2O3 crystals even by thermochemical reduction at temperatures of ∼1800 °C,23 implying that the addition of aliovalent cations is promising in producing and stabilizing the F-center aggregates in R-Al2O3. However, the electronic structures of the F2-type centers in R-Al2O3:Mg are highly perturbed by the doped Mg ions,23,24 which complicates the absorption and emission characteristics of the resulting F2type centers. As an alternative to the previous methods to introduce oxygen vacancies in nominally pure R-Al2O3 crystals, we here present a method based on a vacuum heating process. We found that when a polycrystalline R-Al2O3 powder is heated under vacuum (∼10-3 Pa) by using a graphite crucible, the sample powder is melted at ∼1900 °C, which is below the melting temperature of R-Al2O3 (2054 °C), and the resulting R-Al2O3 crystal contains substantial amounts of the F- and F2-types centers such as the F, F+, F2, F2+ centers. We also found that reversible photoinduced interconversions among the above color centers occur by alternate irradiation of ∼300 and ∼220 nm light at room temperature; for example, ∼90% of the F2 center can be converted to the F2+ center and vice versa. The mechanism of the photoinduced interconversion is discussed in terms of the photoexcitation and recombination processes of the F- and F2types centers in the thus formed colored R-Al2O3 crystals.

10.1021/jp908417m CCC: $40.75  2009 American Chemical Society Published on Web 11/12/2009

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Figure 1. XRD patterns of the samples prepared by heating a polycrystalline R-Al2O3 powder under different conditions. (a) Ground powder of the sample prepared under Ar. (b) Ground powder of the sample prepared under vacuum. (c) and (d) As-prepared sphere-shaped sample (see the inset for the optical image) prepared under vacuum. These two XRD patterns were obtained by using the same sample only by changing its orientation to the incident X-ray beam. The inset shows an example of the sphere-shaped sample prepared under vacuum.

2. Experimental Section We used high-purity polycrystalline R-Al2O3 powders (purity 99.999%, Kojundo Chemical Laboratory Co., LTD) as starting materials. To achieve highly reducing and vacuum atmospheres, we heated the R-Al2O3 powders (∼0.01 - ∼0.1 g) in a graphite crucible by using a high-frequency induction heating unit, which is rated at 4 kW at a maximum frequency of 420 kHz, under continuous evacuation with a turbo molecular pump down to ∼10-3 Pa. The heating temperature was raised up to ∼1900 °C at a rate of ∼500 °C/min and maintained at the same temperature for 3 min. The temperature of the system was monitored with a radiation thermometer. We also performed a similar heat treatment under flowing Ar atmosphere for comparison. We found that only when the sample was heated in vacuum, the starting R-Al2O3 powders were melted and condensed to form a translucent sphere-shaped solid with a smooth and lustrous surface (see the inset of Figure 1). The size of the sphere can be varied from several tens of micrometers to a few millimeters by changing the amount of starting polycrystalline R-Al2O3 powders mounted in a carbon crucible. When the sample was heated under Ar, however, the resulting sample appears to be a well-sintered ceramic body, showing an opaque and a nonlustrous appearance. Powder X-ray diffraction (XRD) patterns of the ground samples were obtained with a diffractometer (Rigaku, SmartLab) using Cu KR radiation at ambient temperature. We also measured several XRD patterns of the as-prepared solid as it is (without grinding) by changing the orientation of the sample to the incident X-ray beam. The surface morphology of the crushed sample was studied with a scanning electron microscope (JEOL, JSM-5601LVS). Steady state PL spectra and time-resolved PL signals in the millisecond time region were recorded on a spectrofluorometer (JASCO, FP 6600) by using a monochromated xenon lamp (150 W) and a mechanical shutter. Timeresolved PL measurements in the nanosecond time region were also carried out with a 150 lines/mm grating and a gated imageintensified charge coupled device (Princeton Instruments, PIMAX:1024RB) by using the third-harmonic (355 nm) and the fourth-harmonic (266 nm) of a pulsed Nd:yttrium-aluminum-

Itou et al.

Figure 2. Scanning electron microscope image of a crushed surface of the sphere-shaped sample prepared under vacuum.

garnet (YAG) laser (Spectra Physics, INDI 40, pulse width 8 ns, a repetition rate 10 Hz) as an excitation source. All the PL measurements were performed at room temperature. 3. Results 3.1. Structural Investigation. Figure 1 shows the XRD patterns of the samples prepared under vacuum and Ar conditions. It is clear from Figure 1 that all the observed peaks of the ground powders are perfectly indexed to the XRD pattern of R-Al2O3 (JCPDS no. 43-1484). We should note, however, that when the XRD patterns of the sample prepared in vacuum are measured without grinding, the resulting XRD patterns depend strongly on the orientation of the sample to the incident X-ray beam. By carefully choosing the orientation, only one sharp XRD peak that satisfactorily matches with one of the XRD peaks of pure R-Al2O3 was found [see curves (c) and (d) in Figure 1]. These results allow us to assume that the grain size of the crystallites in the sphere-shaped solid is so large that it yields only one peak in the XRD patterns. To get a better knowledge about the crystallites in the sphere-shaped solid, we crushed the sphere and investigated the surface morphology with a scanning electron microscope (see Figure 2). We see from Figure 2 that the crushed surface is quite smooth and sharp in the length scale of J500 µm, showing no grain boundaries originated from the starting micrometer-sized R-Al2O3 polycrystals. Thus, the sphere-shaped solid obtained by vacuum heating can be considered as an aggregate of single crystal R-Al2O3 crystallites with a size at least of ∼500 µm. As for the sample prepared under Ar, however, no definite evidence of the evolution of single-crystal R-Al2O3 was obtained. 3.2. Photoluminescence Characteristics. 3.2.1. The F and F+ Centers. The PL bands attributed to the F and F+ centers were observed in both samples prepared under vacuum and Ar conditions [see Figure 3a,b]. The broad PL band peaking at ∼413 nm, which can be excited with photons of a wavelength ranging from 200 to 230 nm, is assigned to the F-center emission band. The observed decay time of the 413-nm PL band is ∼35 ms (see Figure 4), in good agreement with the decay time (34-36 ms) reported previously for the F center in R-Al2O3.5,9 For the sample prepared in vacuum, however, a much slower component with a decay time of ∼480 ms was also observed. Such a long-lived phosphorescent component has also been reported previously in some of the colored R-Al2O3 samples.5 On the other hand, the PL band at ∼330 nm and the corresponding two PL excitation (PLE) bands at ∼230 and ∼260

Photoinduced Interconversion of Color Centers in R-Al2O3

Figure 3. PL and PLE spectra of the F- and F+-centers in (a) the vacuum-prepared sample and (b) the Ar-prepared sample. Excitation (λex) and emission monitoring (λem) wavelengths to obtain the respective PL and PLE spectra are shown.

Figure 4. PL decay curves of the F-center of (a) the vacuum-prepared sample (λex ) 220 nm, λem ) 413 nm) and (b) the Ar-prepared sample (λex ) 211 nm, λem ) 416 nm). The thick lines shown in (a) and (b) are the results of least-squares fitting obtained by using a doubleexponential function and a single-exponential function, respectively, and the fitted decay time constants are also shown.

nm are attributed to the F+-center emission. For the F+-center emission band, we also carried out time-resolved PL measurements by using the fourth harmonic (266 nm) of a pulsed Nd: YAG laser as an excitation source. We found that the decay time was below our instrumental resolution (∼10 ns), in accordance with the previous estimation of the radiative lifetime of the F+-center emission (j7 ns).3,25

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Figure 5. PL characteristics of the F2-center in the vacuum-prepared sample. (a) PL and PLE spectra. Excitation (λex) and emission monitoring (λem) wavelengths to obtain the respective PL and PLE spectra are shown. (b) PL decay curve monitored at 503 nm by excitation with 303 nm light. The thick line is the result of least-squares fitting obtained by using a single-exponential function, yielding a decaytime constant of 68.2 ms.

Although both the vacuum- and Ar-prepared samples exhibit PL emissions attributed to the F and F+ centers, the wavelengthintegrated PL intensity of the vacuum-prepared sample is almost ten times higher than that of the Ar-prepared sample. This indicates that heating in vacuum can create a much larger amount of oxygen vacancies than heating in Ar. It is also interesting to note that the PL peak intensity of the F+ center in the sample prepared under vacuum is almost three times as high as that of the F center in the same sample [see Figure 3a]. On the other hand, the sample prepared under Ar yields only a very low PL intensity for the F+ center [see Figure 3b]. These results clearly demonstrate that, as will be discussed in more detail in a later section, the concentration and charged state of oxygen vacancies are strongly affected by heating environment. 3.2.2. The F2 Center. According to the previous investigations,19,21 the F2 center, which is an oxygen divacancy with four electrons, in R-Al2O3 is characterized by a PLE band peaking at 303 nm (4.1 eV) and a corresponding PL band peaking at 503 nm (2.4 eV) with a decay time τ of 53 ms. As shown in Figure 5, the corresponding PL characteristics (λPLE ) 303 nm, λPL ) 503 nm, τ ) 68.2 ms) are indeed observed in the sample prepared in vacuum, demonstrating that the F2 center does exist in the sample. However, any PL signals related to the F2 center were not observed in the sample prepared in Ar. Previously, the F2 center was observed from neutron irradiated R-Al2O3 samples with an F center concentration of ∼1017 cm-3.21 Thus, we suggest that the present vacuum-prepared sample also has a

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Figure 7. Photorecovery spectrum [IPR(λ)] of the F2-center emission band of the vacuum-prepared sample. To obtain this spectrum, the sample was first preirradiated with a 303 nm light for 3 min to induce substantial photobleaching of the F2 center. Next, the sample was irradiated with a certain wavelength λ of light (200 < λ < 300 nm) for 3 min, and then, the F2-center emission at 503 nm was monitored [IPR(λ)] at the instant of irradiation of a 303 nm light. The above cycles of measurements were performed every 3-5 nm from 200 to 300 nm.

Figure 6. (a) Temporal emission behavior of the F2-center in the vacuum-prepared sample. (1) The F2-center emission intensity at 503 mn was monitored continuously for 1 h during irradiations with a 303 nm light. (2) Then, the sample was stored in the dark for 1 day without irradiation. (3) The F2-emission intensity was monitored once again for 10 min by using the same condition employed in (1). (b) Changes in the F2-center emission intensity under serial irradiations with a 303 nm light for 10 min [(1), (3), and (5)]. Before monitoring curves (3) and (5), a 225 nm light was irradiated for 3 min [(2) and (4)]. In all these measurements, a monochromated Xe lamp (150 W) was used as an irradiation source.

comparable amount of the F center, resulting in the formation of the aggregate F2 center in an observable amount. It has been reported that the F2 center in R-Al2O3 shows a photobleaching phenomenon.11,21,22,26 That is, the photoexcitation of the F2 center by ∼300 nm light results in a concomitant decrease in the F2-center absorption band at ∼300 nm. The observed bleaching of the ∼300 nm absorption band has been interpreted in terms of the photoionization of the F2 center,11,26 the excited states of which lie very near the conduction band edge, and the subsequent trapping of the photoexcited electrons by certain defect centers. In the present vacuum-prepared sample, we also confirmed the photobleaching of the F2 center by monitoring its PL emission intensity at 503 nm as a function of the exposure time of excitation light at 303 nm (see Figure 6). As shown in Figure 6a, the initial PL intensity at 503 nm suddenly drops down to ∼10% within several tens of seconds of exposure and keeps this decreased value for a longer duration of exposure time, probably reaching an equilibrium state. Figure 6a further demonstrates that the emission intensity still retains this decreased value after stopping the excitation light and keeping the sample in the dark for one day at room temperature. This demonstrates that the optical bleaching of the F2 center is not thermally recovered at room temperature. We, however, found

that the decreased PL intensity of the F2 center recovers by irradiation of a 225-nm light [see Figure 6b] and that the degree of photorecovery depends strongly on the irradiation wavelength of ultraviolet light (see Figure 7). It should also be worth mentioning that the observed bleaching and recovery of the F2center emission are completely reversible, as confirmed by the changes in the emission intensity under alternate irradiations with 303 and 225- nm lights [see also Figure 6b]. Although a recoverable nature of the optical bleaching of the F2 center has been inferred in previous studies,11,26 the results shown in Figure 6 present the definite evidence for reversibility of the photoinduced processes associated with the F2 center. 3.2.3. The F2+ Center. For the sample prepared in vacuum, we further observed the PL emission corresponding to that of the F2+ center, which is characterized by a PLE band peaking at 358 nm (3.46 eV) and a PL band peaking at 383 nm (3.24 eV),11,20 as shown in Figure 8a. In contrast to the case of the F2 center, however, the F2+-center emission hardly shows a photobleaching phenomenon during irradiation with a 358 nm light. This is in agreement with the previous PL measurements on the F2+ center; that is, the excited state of the F2+ center lies well below (. kT) the bottom of the conduction band and, hence, cannot be photoionized by the excitation into the first excited state at room temperature.11,26 The PL decay profile for the F2+-center emission is shown in Figure 8b. To our knowledge, there have been no reports on the PL decay characteristics on the F2+-center emission at 383 nm. The F2+-center emission shows a nonexponential decay on the time scale of nanoseconds. We tentatively fit the decay data I(t) with a triple-exponential function with τ1 ) 4.5 ns, τ2 ) 19 ns, and τ3 ) 71 ns. The observed nonexponential decay presumably results from a feeding of the radiative transitions from some long-lived reservoirs and/or trapping states, although the detailed mechanism of the feeding process is unknown for now. We also found that the F2+-center emission exhibits a reversible increase and decrease in intensity when the sample is subject to irradiations with 303 and 225 nm lights, respectively, before monitoring the F2+-center emission (see Figure 9). As mentioned in the previous subsection, the alternate irradiations with 303 and 225 nm lights induce the photobleach-

Photoinduced Interconversion of Color Centers in R-Al2O3

J. Phys. Chem. C, Vol. 113, No. 49, 2009 20953 hν ) 303 nm: F2 f F+ 2 + e

(1)

hν ) 225 nm: F+ 2 + e f F2

(2)

Note also that the above two reactions are completely reversible with each other, as demonstrated earlier. This indicates that the photoexcited electrons created in the above photoexcitation processes will not be annihilated by other unknown recombination processes. In other words, when the photoexcited electrons created in eq 1 are trapped by some trapping sites, all the trapped electrons are detrapped by irradiation of 225 nm light and are eventually captured by the same amount of the F2+ center, returning to the original concentration of the F2 center. The details of the above photoinduced trapping and detrapping processes will be discussed in Section 4.1. 3.2.4. The F22+ Center. In previously neutron-irradiated R-Al2O3 samples containing the F2 and F2+ centers, the doubly charged F2-type center, namely, the F22+ center, has also been found to exist;11,19,21,26 the center is characterized by an optical absorption band peaking at ∼450 nm (∼2.7 eV) and a corresponding PL emission band peaking at ∼560 nm (∼2.2 eV).11,26 However, we were unable to observe any trace of such PL features both in the sample prepared in vacuum and in the one prepared in Ar. Figure 8. PL characteristics of the F2+-center in the vacuum-prepared sample. (a) PL and PLE spectra. Excitation (λex) and emission monitoring (λem) wavelengths to obtain the respective PL and PLE spectra are shown. (b) PL decay curve monitored at 383 nm by excitation with the third harmonic of a pulsed Nd:YAG laser (355 nm). The thick line shows the result of least-squares fitting obtained by using a triple-exponential function, yielding the three decay-time constants of 4.5, 19, and 71 ns.

4. Discussion 4.1. Mechanism of Reversible Photoinduced Reactions. In the previous section, we have demonstrated that the R-Al2O3 sample prepared in vacuum exhibits the completely reversible photoinduced reactions associated with the F2 and F2+ centers, as described in eqs 1 and 2, by alternate irradiations with 303 and 225 nm lights. We should note that these photoinduced reactions depend strongly on the irradiation wavelength. It is reasonable that eq 1 is highly wavelength-dependent because the irradiation wavelength must be equal to the excitation energy (∼4.1 eV or ∼300 nm) of the first excitation state of the F2 center to induce expected photoionizatioin processes. It is interesting to note that eq 2 is also wavelength-dependent, as illustrated in the photorecovery and photobleaching spectra of

Figure 9. Changes in the F2+-center emission intensity (λem ) 383 nm) of the vacuum-prepared sample obtained by excitation with a 358 nm light before [(1)] and after alternate irradiations with a 303 nm light for 3 min [(2) and (4)] and a 225-nm light for 3 min [(3)]. In all these measurements, a monochromated Xe lamp (150 W) was used as an irradiation source.

ing and photorecovery of the F2-center emission, respectively (see Figure 6). It is also interesting to note that the photobleaching spectrum of the F2-center emission (see Figure 10) is inversely correlated with the photorecovery spectrum of the F2center emission shown in Figure 7. Thus, there certainly exists an anticorrelation between the changes in the F2-center and F2+center emission intensities caused by alternate irradiations with 303 and 225 nm lights. Therefore, we propose the following photoinduced reactions concerning the F2 and F2+ centers by irradiations with 303 and 225 nm lights:

Figure 10. Photobleaching spectrum [IPB(λ)] of the F2+-center emission band of the vacuum-prepared sample. To obtain this spectrum, the sample was first preirradiated with a 303 nm light for 3 min to obtain the maximum attainable intensity of the F2+-center emission band. Next, the sample was irradiated with a certain wavelength λ of light (200 < λ < 300 nm) for 3 min, and then, the F2+-center emission at 383 nm was monitored [IPB(λ)] at the instant of irradiation of a 358 nm light. The above cycles of measurements were performed every 3-5 nm from 200 to 300 nm.

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Figure 11. Changes in the F+-center emission intensity (λem ) 329 nm) of the vacuum-prepared sample obtained by excitation with a 260 nm light before [(1)] and after alternate irradiations with a 303 nm light for 3 min [(2) and (4)] and a 225 nm light for 3 min [(3)]. In all these measurements, a monochromated Xe lamp (150 W) was used as an irradiation source.

the F2 and F2+ centers shown in Figures 7 and 10, respectively. This implies that the photoexcited electrons created by irradiation with 225 nm light originate from a localized excited state of a certain defect center. Considering that the photorecovery and photobleaching spectra of the F2 and F2+ centers rather resemble the PLE spectrum of the F center shown in Figure 3a, we propose that the origin of the photoexcited electrons contributing to eq 2 is the photoexcitation process of the F center. This is consistent with the fact that the first excited state of the F center is located just below the conduction band edge,9,11 like in the case of the F2 center. Because the reverse reaction of the photoionization of the F center is the capture of an electron by the F+ center, the observed reversible reactions can be represented as follows: F2 + hν (303 nm) f F+ 2 + e

(3a)

e- + F+ f F

(3b) +

-

F + hν (225 nm) f F + e e- + F+ 2 f F2

Figure 12. Temporal fluctuations in emission intensity of the F (λem ) 413 nm) and F+ (λem ) 329 nm) centers in the vacuum-prepared sample during continuous excitations with 220 and 260 nm lights, respectively.

nism. Unfortunately, however, the corresponding change in the F-center emission intensity was difficult to confirm because, as shown in Figure 12, the F-center shows extraordinary wide temporal fluctuations (up to ∼20%) in PL intensities as compared with the other types of color centers investigated in this work. The observed fluctuation behavior of the F-center emission results most likely from a simultaneous occurrence of photoionization (F f F+ + e-) and recombination (F+ + e- f F) processes, which will decrease and increase the PL intensity, respectively, during photoexcitation of the F center. Previously, Pogatshnik, Chen and Evans26 have postulated a reaction mechanism similar to that proposed above to account for the observed photobleaching of several F- and F2-type centers in neutron-irradiated R-Al2O3.1,7 In neutron-irradiated samples, however, the F22+ center along with the F2 and F2+ centers exists in a considerable amount,21,22,26 whereas our samples hardly contain the F22+ center, as mentioned in Section 3.2.4. In addition to eqs 3a and 3b, Pogatshnik, Chen and Evans26 hence postulated the following reaction to account for the observed photobleaching process of the F2 center in neutron irradiated samples: + e- + F2+ 2 f F2

(3c)

(4a) (4b)

If the above reactions indeed occur, the F+- and F-center emission will also show a reversible change in intensity by alternate irradiations with 303 and 225 nm lights. This is because the irradiation with 303 nm light will result in a decrease (or increase) in the F+-center (or the F-center) concentration according to eq 3b, whereas the irradiation with 225 nm light will recover (or bleach) the concentration of the F+ center (or the F center) according to eq 4a. Previously, the optically induced interconversion of the F and F+ centers in R-Al2O3 has been demonstrated by several researchers,1,7 although its physical mechanism has not been fully understood. To confirm the proposed reaction schemes ourselves, we measured the F+center PL intensity after alternate irradiations with 303 and 225 nm lights (see Figure 11). As shown in Figure 11, the expected reversible changes in the F+-center emission intensity were actually observed, further corroborating the proposed mecha-

As mentioned earlier, the resulting F2+ center cannot be photoionized,11,26 indicating that the reverse reaction of eq 3c will not occur under normal UV excitation conditions. Consequently, in neutron-irradiated R-Al2O3, a complete reversible cycle of the photobleaching of the F2 center cannot be expected and has not actually been reported to date. We hence consider that the reversible photobleaching and photrecovery is a behavior peculiar to the present vacuum-prepared sample, in which only the F, F+, F2, and F2+ centers are present as the F- and F2-type centers. 4.2. Evaluation of the Concentration Ratio among Color Centers. With regard to a series of samples prepared in this work, we cannot exactly evaluate the absolute concentration of the color centers by measuring their absorption spectra. This is because our samples are generally millimeter-sized sphereshaped solids and are too complicated to obtain the analyzable absorption spectra. We should note, however, that the proposed photoinduced reactions allow us to estimate the initial F2/F+ concentration ratio of the as-prepared sample by analyzing photobleaching

Photoinduced Interconversion of Color Centers in R-Al2O3

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Figure 13. (a) Structure of corundum, R-Al2O3, projected on (21j1j0), after ref 37. The plane of the paper is presumed to pass through the centers of the aluminums (filled small circles), and the oxygens are located above (thick open large circles) or below (thin open large circles) this plane. (b) Schematic model of the oxygen monovacancies (F+ and F centers) and the expected alumina vacancies in the vacuum-prepared sample. (c) Schematic model of the oxygen divacancies (F2+ and F2 centers) and expected alumina vacancies in the vacuum-prepared sample.

characteristics of these color centers. As shown in Figure 11, the F+-center emission intensity of the vacuum-prepared sample was reduced by ∼10% after the 303 nm light irradiation for 3 min. On the other hand, the F2-center emission intensity was reduced by ∼90% on the same irradiation condition (see Figure 6). According to eqs 3a and 3b, the same number of photoexcited electrons contributes to the observed decrease in the F+and F2-center emission intensities. If we assume that the initial concentration of the F2 and F+ centers are C1 and C2, respectively, the numbers of electrons involved in eqs 3a and 3b are 0.9C1 and 0.1C2, respectively. Thus, the F2/F+ concentration ratio is estimated to be C1/C2 ) 0.1/0.9 ≈ 0.11. The above consideration can also be used to estimate the F2+/ + F and F2+/F2 concentration ratios. As shown in Figure 9, the F2+ emission intensity was increased by about five times after the 303 nm irradiation for 3 min. Provided that the initial concentration of F2+ is C3, 5C3 corresponds to 0.1C2. Thus, the F2+/F+ and F2+/F2 concentration ratios of the as-prepared sample are estimated to be 0.1/5 ) 0.02 and 0.9/5 ) 0.18, respectively. We should note that the above concentration ratio between the relevant color centers depends strongly on the irradiation history of ultraviolet light. From the changes in the relative emission intensities of the F2 and F2+ centers, we estimate that the F2+/F2 concentration ratio can be varied over a wide range, from 0.18 up to ∼11, by controlled irradiation of 303 and 225 nm lights. Such a wide variation in the concentration ratio of the F2-type centers may find an application in optical data storage and imaging devices. 4.3. Model of the Vacancies in r-Al2O3. As mentioned in the Introduction, two methods, namely, irradiation with energetic particles and thermochemical reduction in Ar, have been previously used to introduce oxygen vacancies in R-Al2O3 crystals. It is known that both methods mainly produce the F center rather than the F+ center.11 In particular, the F+/F concentration ratio is quite low, or sometimes zero, in thermochemically reduced R-Al2O3 crystals,2 basically in agreement with our results obtained for the sample prepared in Ar (see Figure 3b). For the sample prepared in vacuum, however, the F+ center is larger in PL intensity than the F center (see Figure 3a), suggesting that the concentration of the former center is comparable to or even larger than that of the latter.

Then, the following question arises: why does the vacuum assisted heating result in a larger F+/F concentration ratio as compared with the heating in Ar? Because the F+ center is the positively charged center, the introduction of the F+ center must, in principle, be accompanied by simultaneous introduction of some negatively charged centers. Answering the above question therefore requires us to identify the origin of expected negatively charged centers in the vacuum-prepared sample. One method to introduce such a negatively charged center is the substitution of Al3+ by a divalent cation, for example, Mg2+.23,24 In the present case, however, doping of divalent cations cannot account for the difference between vacuum- and Ar-prepared samples because we used the same high purity (99.999%) R-Al2O3 as starting materials for both cases. It is hence most likely that the observed difference originates from the difference in reactions between vacuum- and Ar-heating processes. In this work, a graphite crucible is used not only as a susceptor in the induction heating system but also as a reducing agent for both the vacuum- and Ar-heating procedures. It has previously been reported that, in the interaction of alumina with carbon, a noticeable reduction of Al2O3 commences at a temperature of ∼1600 °C.27,28 In the early stage of reduction, the vapor phase consists mainly of CO.27,28 As the reduction proceeds, vaporization of gaseous aluminum (Al) and aluminum suboxides29 such as AlO, Al2O, and Al2O2, also begins to take place, leading to volatilization of alumina especially in vacuum.27 This strongly suggests that the composition of R-Al2O3 can be fluctuated by heating in vacuum at temperatures more than 1600 °C. This composition fluctuation under vacuum may also account for the observed melting behavior of the starting R-Al2O3 powder even at ∼1900 °C, which is below the melting temperature of R-Al2O3 (2054 °C), because it has been reported that the melting temperature of R-Al2O3 is substantially lowered with Al addition.30 During heating under inert-gas atmosphere, however, vaporization tends to be suppressed because of the scattering and collision with residual gas atoms. Furthermore, a heat treatment under inert-gas atmosphere often results in the precipitation of aluminum carbide (Al4C3) and aluminum oxycarbide (Al2OC) depending on the Al/C ratio and the heating temperature.31-33 These results indicate that the effect of vaporization is more conspicuous in vacuum heating than in

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Ar heating. Indeed, we found that the heating of R-Al2O3 under the present vacuum condition generally results in a ∼20-30% decrease in total weight, although such an appreciable weight loss was not observed for the sample prepared in Ar. Considering the vaporization of CO, Al, and aluminum suboxides and the expected composition fluctuation under vacuum, we suggest that the vacuum heating procedure can introduce not only oxygen vacancies but also aluminum vacancies (VAl) in the resulting R-Al2O3 crystals. Previously, electron paramagnetic resonance (EPR) signals ascribed to Al3+ vacancies were reported in γ-irradiated R-Al2O3.34 The resulting ESR signals are characterized by S ) 1/2, g=2 and S ) 1, g=2 lines, which are attributed to one (VAl″) and two (VAl′) holes localized on oxygen ions adjacent to Al3+ vacancies.35 The charge states of the VAl″ and VAl′ centers are -2 and -1, respectively. We suggest that these aluminum vacancies can act as charge compensators for the positively charged oxygen vacancies, namely, the F+ and F2+ centers, hence accounting for rather a large F+/F concentration ratio in the vacuumprepared sample. Schematic structural models of aluminum vacancies along with oxygen vacancies in the vacuum prepared sample are shown in Figure 13. Thus, we assert that in the present vacuum heating process the introduction of positively charged oxygen vacancies is concomitant with the introduction of negatively charged aluminum vacancies, attaining local charge neutrality. If the assumed negatively charged aluminum vacancies coexisted with the oxygen vacancies, it would be possible to detect their presence by EPR measurements. However, our preliminary EPR measurements on the vacuum-prepared sample did not allow us to find any ESR signals over the entire magnetic-field range examined even when the measurements were carried out at 4 K. We, however, consider that this does necessarily mean that the positively charged oxygen vacancies and their charge compensating aluminum vacancies do not exist in the sample. This is because the identification of the F+ center in R-Al2O3 has been made difficult by the general absence of any confirmed EPR signals,4,9 like in the case of the present sample. A 13-line EPR spectrum ascribed to the F+ center has been reported only in heavily neutron-irradiated samples although no positive correlations was made with optical data.36 These results strongly imply that some of the F+ centers cannot be described as a simple oxygen-vacancy model occupied by one electron. To be more specific, we believe that the electron in the F+ center will interact with nearby electrons and/or holes in aluminum vacancies, forming an apparently ESR inactive state. However, further investigations will need to be done to clarify the details of the interaction between the oxygen and expected aluminum vacancies in R-Al2O3. 5. Conclusions We created sphere-shaped R-Al2O3 crystals containing a substantial amount of oxygen vacancies by heating a high purity R-Al2O3 powder under vacuum at ∼1900 °C with a carbon crucible. We confirmed the PL emission signals attributed to the F, F+, F2, and F2+ centers in the thus prepared sample. Heating the same R-Al2O3 powder in Ar, however, yielded a lesser amount of oxygen vacancies, resulting in no F-aggregate centers. As for the vacuum-prepared sample, we observed reversible photoinduced interconversions of the F- and F2-type centers by alternate irradiations with 303 and 225 nm lights. The observed phenomena were interpreted in terms of the photoionization and subsequent electron capturing processes of the relevant F- and

Itou et al. F2-type centers. Irradiation with a 303 nm light causes ionization of the F2 center, converting the F2 center into the F2+ center (eq 3a). The resulting photoexcited electrons in the conduction band will be captured by the F+ center, converting the F+ center into the F center (eq 3b). On the other hand, irradiation with a 225 nm light induces a completely reverse process, namely, the ionization of the F center (eq 4a) and the subsequent capturing of the photoexcited electrons by the F2+ center (eq 4b). From the changes in the PL intensities of the F- and F2-type centers involved in the above photoinduced reactions, we estimated that the initial F2/F+ concentration ratio in the vacuum-prepared sample is ∼0.1. Another interesting feature of the vacuum-prepared samples found in this study is that they contain a rather large amount of positively charged oxygen vacancies, namely, the F+ and F2+ centers, as compared with the samples prepared by a conventional thermochemical reduction method and irradiation with energetic particles. Considering that aluminum suboxides are likely to be evaporated during the vacuum heating process, we suggest that negatively charged aluminum vacancies, for example, the VAl″ and VAl′ centers, are introduced in the vacuumprepared sample and that these negatively charged centers can compensate and stabilize the positive charge of the F+ and F2+ centers (see Figure 13). Thus we have shown that the vacuum-heating procedure, although not well recognized, is quite promising for the creation of colored R-Al2O3 crystals. It is also interesting to note that this method can further be applied basically to other refractory materials, in which oxygen vacancies are difficult to introduce. Work on materials such as magnesia (MgO), magnesium spinel (MgAl2O4), and aluminum oxynitride spinel (AlON) is performed by the present authors, and the results will be reported in a forthcoming paper. References and Notes (1) Lee, K. H.; Crawford, J. H., Jr. Phys. ReV. B 1977, 15, 4065. (2) Lee, K. H.; Crawford, J. H., Jr. Appl. Phys. Lett. 1978, 33, 273. (3) Evans, B. D.; Stapelbroek, M. Phys. ReV. B 1978, 18, 7089. (4) Draeger, B. G.; Summers, G. P. Phys. ReV. B 1979, 19, 1172. (5) Lee, K. H.; Crawford, Jr., J. H. Phys. ReV. B 1979, 19, 3217. (6) Jeffries, B.; Summers, G. P.; Crawford, J. H., Jr. J. Appl. Phys. 1980, 51, 3984. (7) Jeffries, B. J.; Brewer, J. D.; Summers, G. P. Phys. ReV. B 1981, 24, 6074. (8) Hughes, R. C. Phys. ReV. B 1979, 19, 5318. (9) Brewer, J. D.; Jeffries, B. T.; Summers, G. P. Phys. ReV. B 1980, 22, 4900. (10) Caulfield, K. J.; Cooper, R.; Boas, J. F. Phys. ReV. B 1993, 47, 55. (11) Evans, B. D. J. Nucl. Mater. 1995, 219, 202. (12) Rieke, J. K.; Daniels, F. J. Phys. Chem. 1957, 61, 629. (13) McDougall, R. S.; Rudin, S. Health Phys. 1970, 19, 281. (14) Summers, G. P. Rad. Prot. Dosim. 1984, 8, 69. (15) Buckman, W. G. Health Phys. 1970, 22, 402. (16) McKeever, S. W. S.; Akselrod, M. S.; Colyott, L. E.; Larsen, N. A.; Polf, J. C.; Whitley, V. Radiat. Prot. Dosim. 1999, 84, 163. (17) McKeever, S. W. S. Nucl. Instrum. Methods Phys. Res. B 2001, 184, 29. (18) Bøtter-Jensen, L.; McKeever, S. W. S.; Wintle, A. G. Optically Stimulated Luminescence Dosimetry; Elsevier: Amsterdam, 2003. (19) Springs, M. J.; Valbis, J. A. Phys. Stat. Sol. B 1984, 123, 335. (20) Welch, L. S.; Hughes, A. E.; Pells, G. P. J. Phys. C: Solid State Phys. 1980, 13, 1805. (21) Evans, B. D.; Stapelbroek, M. Solid State Commun. 1980, 33, 765. (22) Atobe, K.; Nishimoto, N.; Nakagwa, M. Phys. Stat. Sol. A 1985, 89, 155. (23) Ramı´rez, R.; Tardı´o, M.; Gonza´lez, R.; Chen, Y.; Kokta, M. R. Appl. Phys. Lett. 2005, 86, 081914. (24) Ramı´rez, R.; Tardı´o, M.; Gonza´lez, R.; Santiuste, J. E. M.; Kokta, M. R. J. Appl. Phys. 2007, 101, 123520. (25) Surdo, A. I.; Pustovarov, V. A.; Kortov, V. S.; Kishka, A. S.; Zinin, E. I. Nucl. Instrum. Methods Phys. Res. A 2005, 543, 234. (26) Pogatshnik, G. J.; Chen, Y.; Evans, B. D. IEEE Trans. Nucl. Sci. 1978, NS-34, 1709.

Photoinduced Interconversion of Color Centers in R-Al2O3 (27) Cox, J. H.; Pidgeon, L. M. Can. J. Chem. 1963, 41, 671. (28) Folomeikin, Y. I.; Demonis, I. M.; Kablov, E. N.; Lopatin, S. I.; Stolyarova, V. L. Dokl. Chem. 2004, 399, 257. (29) Brewer, L.; Searcy, A. W. J. Am. Chem. Soc. 1951, 73, 5308. (30) Gitlesen, G.; Herstad, O.; Motzfeldt, K. In Selected Topics in High Temperature Chemistry; Forland, T., Grjotheim, K., Motzfeldt, K., Urnes, S. Eds.; Universitetsforlaget: Oslo, 1966; pp 179-196. (31) Lihrmann, J. M.; Zambetakis, T.; Daire, M. J. Am. Ceram. Soc. 1989, 72, 1704. (32) Qiu, C.; Metselaar, R. J. Am. Ceram. Soc. 1997, 80, 2013. (33) Zhao, J.; Lin, W.; Yamaguchi, A.; Ommyoji, J.; Sun, J. J. Ceram. Soc. Jpn. 2007, 115, 654.

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