Photoluminescent properties of nanoporous anodic alumina doped with manganese ions

Photoluminescent properties of nanoporous anodic alumina doped with manganese ions

Journal of Luminescence 185 (2017) ∎∎∎–∎∎∎ Contents lists available at ScienceDirect Journal of Luminescence journal homepage: www.elsevier.com/loca...

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Journal of Luminescence 185 (2017) ∎∎∎–∎∎∎

Contents lists available at ScienceDirect

Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

Photoluminescent properties of nanoporous anodic alumina doped with manganese ions I.V. Gasenkova a, N.I. Mukhurov a,n, S.P. Zhvavyi a, E.E. Kolesnik a, А.P. Stupak b a b

State Research and Production Association "Optic, Optoelectronic and Laser techniques”, 68 Nezavisimosti Ave., Minsk 220072, Belarus B.I.Stepanov Institute of Physics of the National Academy of Sciences of Belarus, 68 Nezavisimosti Ave., Minsk 220072, Belarus

art ic l e i nf o

a b s t r a c t

Article history: Received 13 October 2016 Received in revised form 17 January 2017 Accepted 23 January 2017

The results are presented of a comparative study of photoluminescent (PL) properties of unalloyed and Mn-alloyed porous anodic alumina (PAA) subjected to annealing at temperatures in the range of Тa ¼200–1300 °С. The possibility of alloying of PAA with metal atoms is illustrated through an example of Mn atoms, and the effect of this impurity on the optical properties of aluminum oxide is examined. Alloying of PAA with Mn ions leads to the formation of complex defects including manganese ions and oxygen vacancies. The difference observed in the spectral dependences of the PL intensity of alloyed and unalloyed specimens is explained by the change in the valence of manganese ions in the complex defects. A decrease has been discovered in the PL intensity of the PL bands and R-lines of Mn and Cr ions in the αphase under prolonged UV-exposure of the alloyed samples. & 2017 Elsevier B.V. All rights reserved.

Keywords: Porous anodic alumina Photoluminescence Manganese dopant Oxygen vacancies

1. Introduction Porous anodic alumina (PAA) of a unique structure whose parameters can be varied during growth process is used as membranes and templates for the synthesis of nanomaterials and nanostructures [1,2], photonic crystals [3] and metamaterials [4]. For this reason, great attention is paid to the study of optical properties of PAA films. When PAA is exposed to UV radiation, photoluminescence (PL) is observed in the visible spectrum with its characteristics depending on the electrolyte composition [5–7], type of anodizing and thermal treatment modes [8–10]. In spite of the large number of research works devoted to the PAA PL, no unambiguous view has been formed so far on the mechanism of origin of the PL. Thus, some authors assign a major role in the formation of PAA PL spectrum to electrolyte anions embedded into the oxide during anodizing process [7,11]. In refs [8,12–14]., the photoluminescence is deemed to be determined by oxygen vacancies with one (F þ -center) and two electrons (F-center) irrespective of the electrolyte type and PAA formation mode used. There is also a compromise point of view [10,15–17] according to which the PAA photoluminescence is due to the combined action of F þ -centers and impurities in the form of electrolyte anions. Refs. [18,19] report the detection of luminescence of chromium and manganese ions in the PAA α-phase obtained by high-temperature annealing, n

Corresponding author. E-mail address: [email protected] (N.I. Mukhurov).

http://dx.doi.org/10.1016/j.jlumin.2017.01.030 0022-2313/& 2017 Elsevier B.V. All rights reserved.

which is due to the presence of uncontrolled impurities. To clear up the role of impurity atoms in the formation of the PAA PL spectrum, we have investigated the photoluminescent properties of undoped and Mn-doped PAA specimens which were formed in a pure oxalic electrolyte and the oxalic electrolyte with addition of KMnO4 (PAA:Mn) and modified by subsequent annealing at high temperatures. The possibility of doping of PAA with metal atoms is illustrated through an example of Mn atoms, and the effect of this impurity on the optical properties of alumina is examined. The role of complex defects including manganese ions and oxygen vacancies on the formation of the PAA PL spectrum is discussed.

2. Specimen preparation and experimental methods Doping of the PAA was carried out by introducing alloy-containing compound into electrolyte in the process of aluminum anodizing, followed by annealing at temperatures of phase transitions from the initial amorphous state (by definition X-ray diffraction technique) to a crystalline state. PAA specimens were obtained by electrochemical oxidation of aluminum foil (99.99%) in an electrolyte based on oxalic acid (0.3 M) and in the electrolyte with the addition of KMnO4 (0.8 g/l). PAA 40 μm thick was separated from aluminum in the solution based on hydrochloric acid and copper chloride. The specimens were annealed at Ta ¼200, 400, 600, 800, 900, 1000 and 1300 °С and held at constant temperature for 30 min. The annealing of PAA specimens occurred sequentially: annealing at higher temperatures was

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preceded by annealing at 600 °С. The rate of temperature rise at the first stage (low temperature) was higher than that in the second stage (1 °С /min), and was equal to 5 °С/min. The phase composition of the samples was investigated by X-ray diffraction on a DRON-3 using CuKα emanation. Radiographs were taken in the angular range 2θ ¼ 15° up to 105° in increments 0.1°. Diffractometric data processing was performed using the WinVif1 program. Analysis of the diffraction patterns was carried out using the Crystallographica Search Match and tables of international diffraction data (JCPDS). The luminescence spectra were measured by using an SDL-2 automated spectrofluorometer that consisted of a high-aperture excitation monochromator MDR-12 and a recording monochromator MDR-23. Correction of the MDR-23–PMT recording system was performed by using a standard tungsten tape lamp TRSH 2850. The excitation wavelengths were λex ¼275 and 325 nm. The absorption spectra in the wavelength range from 200 to 800 nm were obtained by means of a spectrophotometer Cary 500 Scan (Varian).

3. Results As shown by the phase analysis in Fig. 1, all of the AA samples before annealing were X-ray amorphous (curve 1). The phase composition of the AA samples annealed at 800 °C corresponds to the γ-Al2O3 (curve 2) (JCPDS card no. 10-0425), and this is consistent with previous data [20], in which it was noted that the transition is not uniform, and even with γ-Al2O3 content at less than 50% a δ-Al2O3 begins to form. Identification radiographs of samples annealed at 900 °C and 1000 °C indicate a tetragonal δ-Al2O3 phase (curve 3, 4) (JCPDS card no. 46-1131). Blurred lines on the radiographs of these samples indicate the presence of γand θ- Al2O3, which may contribute to the peaks on the radiograph as a result of the narrowness of the interplanar distances [21,22]. The diffraction pattern of the sample annealed at 1300 °C corresponds to the rhombohedral phase α-Al2O3 (curve 5) (JCPDS card no. 46-1212). Narrow peaks indicate a coarse structure.

Fig. 2 shows the PL spectra of PAA (a, c) and PAA:Mn specimens (b, d) at the excitation wavelengths of λex ¼275 nm (a, b) and 325 nm (c, d). Spectra of the original specimens and those annealed at up to 600 °C are similar and represent wide bands in the range of 350–650 nm with a maximum at 450–500 nm. As the annealing temperature is increased the PL intensity rises reaching a maximum at 600 °C (curve 4). The ratio of PL areas of specimens annealed at Ta ¼600 °C to those of the original specimens is S600/Sor  3 for PAA: Mn, which is less than the increased ratio found for the undoped specimens (S600/Sor 4). For Тa Z800 °С, the PL spectra observed from the PAA and PAA:Mn specimens show a notable difference while having reduced intensities. The PAA is characterized by a shift of the PL band to shorter wavelengths and the formation of the second band in the red region with a maximum at about 760 nm. For the PAA:Mn specimens annealed at 800 °С, a decrease in the PL intensity is also observable but the bands become significantly expanded with the band maximum found at 415 nm for the excitation wavelength of λex ¼275 nm and at 500 nm for λex ¼325 nm (curve 5). For Ta ¼900 and 1000 °С, the PAA:Mn specimens did not show any PL when excited at these wavelengths. For excitation at λex ¼240 nm, a small luminescence intensity was observed (inset in Fig. 2d curve 6, 7). Fig. 3 shows the PL spectra of the PAA and PAA:Mn specimens о annealed at Ta ¼1300 С. According to [20,23] and X-ray data, annealing at this temperature results in the formation of a crystalline phase α-Al2O3. Here, two ranges can be identified. The first one is in the blue-green region with weak bands and maxima at 500 and 400 nm (inset in Fig. 3) for excitation at 275 and 325 nm, respectively. The second range lying in the red part of spectrum is characteristic of the R - lines of corundum crystals which correspond to radiative transitions (2Е - 4А2) in Mn4 þ ions (678 nm) [24] and Cr3 þ (694 nm) [25] which substitute Al3 þ ions in octahedral sites of the crystal lattice. Chromium impurity is typical of alumina crystals [26–28] and it manifests itself in PL spectra even at concentrations of less than 0.001% [29,30]. In our case, the presence of R-lines in the PL spectra is due to the occurrence of the elements in the initial aluminum and points to formation of αphase during annealing of PAA [26]. Comparison with the spectra of corundum [30,31] for which Cr3 þ is well established as impurity shows that the α-PAA spectra are similar to them and confirms embedding of chromium ions into the lattice. In case of α-PAA:Mn specimens, the peak intensity at 677 nm is higher than that at 693 nm regardless of the excitation wavelength. For the αPAA samples, the intensity of R-line of manganese ions is lower than that of chromium ions at λex ¼275 nm and is higher at λex ¼325 nm. Unlike α-PAA specimens, the α-PAA:Mn samples had a pinkish color testifying to the presence of Mn3 þ ions in the AA structure [32]. In addition, it has been found that irradiation of the PAA:Mn specimens annealed at Тa ¼600 °С at 275 and 325 nm wavelengths resulted in a decrease of PL with the irradiation time (Fig. 4 curve 1, 2). Long UV exposure of α-PAA:Mn also resulted in reduced intensity of R-lines of Mn4 þ and Сr3 þ (Fig. 4 curve 3, 4) and disappearance of the pink tint. With a prolonged exposure of the specimens to radiation at λ ¼405 nm (absorption band of Cr3 þ ions), no reduction of intensity of Сr3 þ R-line was observed. The dependence of the PL intensity reduction as a function of the exposure time is well described by a two-component exponential decay:

I ( t ) = I0 + I1exp ( −t /t1) + I2 exp ( −t /t2 )

Fig. 1. X-ray patterns for initial sample (1) and AA samples annealed at Ta ¼ 800 °C (2), 900 °C (3), 1000 °C (4), and 1300 °C (5).

(1)

where I0 is the stationary component of the intensity, I1 and I2 are the initial intensities of the fast and slow components, t1 and t2 are the corresponding decay constants.

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Fig. 2. The PL spectra of PAA (a, c) and PAA:Mn (b, d) from the original specimens (1) and specimens annealed at Ta ¼ 200 °C (2), 400 °C (3), 600 °C (4), 800 °C (5), 900 °C (6) and 1000 °C (7) for excitation at λex ¼ 275 (а, b) and 325 (c, d) nm.

Fig. 3. The PL spectra of PAA (a) and PAA:Mn (b) specimens annealed at Ta ¼1300 °C for excitation at 275 (1) and 325 (2) nm wavelengths.

Parameters of the Eq. (1) obtained by fitting the observed pendencies (Fig. 4) are shown in Table 1. A similar effect of UV radiation also reveals itself in the sorption spectra of α-PAA:Mn (Fig. 5.). After 15 minutes of exposure, the level of absorption in the range of 200‒400 decreased as compared to that found for initial sample.

deabUV nm

4. Discussion To identify the luminescence centers, photoluminescence excitation (PLE) spectra have been obtained by monitoring at the wavelengths corresponding to the PL maxima. It should be noted that profiles of the PLE spectra obtained from the PAA specimens

and PAA:Mn specimens annealed to Тa ¼600 °С are practically identical. The typical PLE spectrum is shown in Fig. 6. Here, we can distinguish between the bands at 270 and 350 nm related to F þ and F2+ -centers. As in ref. [33], these spectra were approximated by Gaussians with the corresponding values of the absorption bands of F þ -, F2- and F2+ - centers (Fig. 4) [34,35]. It can be seen that the superposition of the bands is in good agreement with the experimental spectra. The uniformity of types of the PLE spectra and the presence of a maximum in the range of 450–500 nm of the PL bands in both cases (for doped and undoped specimens) suggest that the excitation radiation is absorbed by oxygen vacancies in different charge states. Additional information about the luminescence centers has been obtained by analyzing optical absorption (OA) spectra (Fig. 7.)

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Fig. 4. The PL intensity at 450 nm from the PAA:Mn specimens annealed at Тa ¼ 600 °С (1, 2) and the intensities of R-lines of Mn4 þ (3) and Cr3 þ (4) ions versus the irradiation time at excitation wavelengths of λm ¼ 325 (1, 3) и 275 nm (2, 4). Table 1 Parameters of experimental dependencies of the PL intensity. Curve No

I0 (a.u.)

I1 (a.u.)

t1 (min)

I2 (a.u.)

t2 (min)

1 2 3 4

277.1 152.9 97.3 48.9

13.2 7.4 54.3 6.1

1.47 1.44 1.50 0.60

41.7 11.5 120.0 28.1

16.7 10.3 6.9 5.1

Fig. 5. The absorption spectra of α-РАА:Mn; 1–initial sample, 2–in 15 minutes.

of the PAA and PAA:Mn specimens. The OA spectra of both original specimens (curve 1) and annealed specimens differ significantly. For the doped specimens annealed up to 1000 °С, the presence of manganese ions in the AA structure leads to increased absorption (Fig. 7b). It should be noted here that the original specimen is characterized by increased absorption in the 210–300 nm band, whereas the specimen annealed at 800 °C shows it in the 200– 350 nm band (curve 2). In case of the α-PAA and α-PAA:Mn specimens (Ta ¼1300 °C), the inverse relationship is observed, i.e., the doped specimen shows a lower absorption (curve 5). Approximation of the OA spectra of the original PAA specimens and PAA: Mn specimens (Fig. 8) shows that their absorption is determined by F- and F2-centers in different charge states. As the annealing temperature is increased up to Тa ¼ 600 °С, a rise in the PL intensity is observed (Fig. 2). According to thermogravimetric studies [24], at the initial stage of annealing of PAA (Тa r150 °С) there occurs desorption of physically absorbed water

Fig. 6. The PL excitation spectrum of the original PAA specimen in case of monitoring at λm ¼ 440 nm.

the amount of which is 0.34% of the total mass of the oxide obtained by anodizing in oxalic electrolyte. At 150–800 ºС, changing of the mass takes place due to removal of chemically bound water from the anodic oxide structure. The amount of water calculated in terms of the loss of mass was equal to 1%. Growth of the PL intensity at this stage of annealing of specimens is caused by the formation of F- and F þ - centers as a result of dehydration and dehydroxylation of the anodic alumina. In this temperature range, anion vacancies have higher mobility, which contributes to their aggregation into F2-type centers in different charge states [35,37]. The PL intensity reaches maximum at annealing temperature of 600 °C, being accompanied by increased luminous intensity of all color centers. The ratio of the areas of PL bands of the specimens annealed at Тa ¼600 °С and the original specimens is S600/Sor  4 for PAA and  3 for PAA:Mn. This fact indicates that doping of the anodic oxide with manganese ions leads to a partial suppression of PL. Lower growth of the PL intensity observed in the doped specimens can be caused by a decrease in the number of vacancies and formation of a more perfect structure in the presence of manganese, which is believed as unlikely, or by the presence of complex defects including manganese ions, hydroxyl groups and oxygen vacancies [38]. Since the PAA:Mn specimens are characterized by the presence of manganese ions Mn2 þ , Mn3 þ и Mn4 þ [39], charge compensation of Mn ions, as they are embedded into the PAA structure, can occur due to the formation of both charge-compensating pairs Mn2 þ  Mn4 þ and Mn2 þ  Vo  Mn4 þ and more intricate defect complexes also comprising oxygen vacancies such as Mn3 þ – F þ – Mn4 þ , Mn4 þ – F – Mn4 þ , Mn4 þ – F22 þ – Mn4 þ . The excited state of the F- and F2- centers is located near the bottom of the conduction band (Δ o o kT) [34]. When UV radiation is absorbed some oxygen vacancies in the complex defects may participate in the changing of the valence state of manganese ions as a result of F- - F þ -center conversion, thermal ionization of photoexcited F*- and (F2)*- centers (Fþ hν-F*-F þ þ e-, F2 þ hν - (F2)*- F2 þ –e-) and photoionization (F þ hν-F þ þe-, F2 þhν - F2 þ –e-) [35,40], e.g., according to the scheme Mn4 þ – F –Mn4 þ þ hνex - Mn4 þ – F þ þ e-– Mn4 þ - Mn3 þ – F þ – Mn4 þ , (2a) Mn3 þ – F þ – Mn4 þ þ hνex - Mn2 þ – Vo – Mn4 þ . For the PAA specimens annealed at

(2b)

Тa ¼600 °С, the

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Fig. 7. The absorption spectra of PAA samples (a) and PAA:Mn (b). (1) - initial sample and samples annealed at Ta ¼ 800 °C (2), 900 °C (3), 1000 °C (4), and 1300 °C (5).

Fig. 8. Gaussian approximation of the absorption spectra of the PAA (а) and PAA:Mn (b) specimens.

concentration of F22 +-centers increases most significantly (the ratio of the band areas at 550 nm [37,41] is S600/Sor 4.7 and 6.4 at λex ¼275 and 375 nm, respectively), which results in the shifting of the PL band maximum to the red region up to 500 nm (Fig. 9). In case of the PAA:Mn specimens, such shifting of the band maximum is not observed, and the ratio of the band areas at 550 nm is almost equal to that of the PL bands (S600/Sor 3) and is  3.0 and 3.6 at λex ¼275 and 375 nm, respectively. Such difference may indicate that excitation of manganese ions in the Mn4 þ – F22 þ – Mn4 þ complex defect occurs through reabsorption of radiation of F- and F þ - centers (λem ¼420 and 480 nm) by F22 þ -centers (λabs ¼ 460 nm), i.e., the presence of manganese ions in the alumina structure leads to fewer F22 þ -centers participating in the PL. The PL intensity reduction found with a prolonged exposure of the PAA:Mn specimens to UV radiation (Fig. 4) also testifies to a decreased number of F- и F2- centers when the valence of manganese ions changes due to charge transfer in accordance with (2a, 2b). A sharp decrease in the PL intensity at Ta 4800 °C is caused by a loss of luminescence centers. At these annealing temperatures, there occurs an abrupt loss of mass which corresponds to sequential decomposition and removal from the PAA volume of the electrolyte anions and hydroxyl groups embedded in the structure during anodization (mass loss amounts to 7.6% of the initial mass [36,42]. This should lead to formation of oxygen vacancies [33]. However, due to diffusion of atmospheric oxygen into the PAA structure the concentration of oxygen vacancies decreases [8–10] and so does the PL intensity. The significant difference of the PL spectral characteristics observed in the doped samples as

compared with PAA samples in this temperature range is due to the presence of thermally stable complex defects, which is consistent with the PAA [38]. The formation of complex defects is assisted by thermally-stimulated migration of oxygen vacancies and their aggregation [35] in the PAA whose structure changes from amorphous to crystalline (amorphous PAA - γ- - δ- phase) in the range of Ta ¼ 800‒1000°C. The formation of oxygen vacancies of F- and F2 -types in different charge states near manganese ions as a result of dehydroxylation at Ta ¼ 800° C reveals itself in broadening of the PL spectrum. This spectrum is a superposition of the luminescence bands of both isolated F- and F2-type oxygen vacancies and those being part of complex defects. The absence of PL in the doped samples annealed at Ta ¼900 and 1000 °C under excitation at λex ¼275 and 325 nm is explained by the fact that in the complex defects the excited oxygen vacancies are involved in the changing of valence states of the manganese ions. At these annealing temperatures, the undoped samples are characterized by a shift of the PL band towards short-wave region, which is determined by the concentration of F þ - (λem ¼320 nm) and F2 þ - centers (λem ¼380 nm). Formation of γ- and δ- phase is accompanied by an increase in the specific surface area of the samples [36]. According to [43], luminescence spectra of the nanostructured alumina with a high specific surface area show the luminescence band of the surface Fsþ -centers with a maximum at 365 nm. However, the analysis of PL spectra of the samples in γand δ-phases has not established any presence of this characteristic luminescence band, which is consistent with the results of [10]. These surface centers arise due to a considerable non-

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Fig. 9. Gaussian approximation of the PL spectra of the original (a, b) specimens and specimens annealed Тa ¼600 °С (c, d) for PAA (a, c) and PAA:Mn (b, d) at λex ¼ 325 nm.

stoichiometry in the surface layers [43]. Under high-temperature annealing, the diffusion of atmospheric oxygen causes their annihilation. The PL excitation spectra of the α-PAA and α-PAA:Mn specimens (Fig. 10) when monitored at λm ¼ 678 (1) and 694 (2) nm show three intensive wide bands with the maxima at  315, 405 and 560 nm and a weak band at 470 nm (4А2 - 4Т2 transition in Mn4 þ ions) for λm ¼ 678 nm which is manifested more clearly in the α-PAA:Mn specimens. According to [25], the absorption spectra of corundum doped with chromium have two intensive wide bands with the maxima at λ E 410 nm (4А2 - 4Т1 transition) and λ E560 nm (4А2 - 4Т2). In [44], the band at 315 nm is attributed to the charge transfer band O2- - Mn4 þ . It was shown in [38] that excitation of luminescence of Mn4 þ ions are caused by absorption of radiation in the band of  300–325 nm by F-centers that interact with manganese ions in a complex defect. However, account should be taken here that this region of spectrum contains the absorption bands of the F2- and F2+ - centers with the maxima at 300 and 350 nm [34,35], respectively. The PL spectra of α-PAA:Mn in the blue-green region is well explained by the presence of oxygen vacancies, the predominant contribution of vacancies in different charge states being determined by the exciting radiation wavelength. Thus, with the PL excitation at 275 nm, luminescence in the 400‒600 nm band is mainly due to F-, F þ - and F22 +- centers, whereas with the excitation at 325 nm radiation in the 350–500 nm band is associated with F-, F þ - and F2+ - centers (Fig. 11). The PL excitation wavelengths located in the region of the absorption bands of F þ -, F2-, F2 þ - and F22 þ - centers. Thus, Mn4 þ

Fig. 10. The PL excitation spectra of α-PAA:Mn (а) for monitoring at λ ¼ 678 (1) and 694 (2) nm.

ions in the complex defect can be excited as a result of energy transfer from the oxygen vacancies and reabsorption of radiation of the F þ -centers (λem ¼480 nm [45]) by Mn4 þ ions in the absorption band at 470 nm. In addition, when UV radiation is absorbed by F- and F2- centers in the complex defects, changes in the valence state of manganese ions and the charge state of oxygen vacancies may occur, e.g., according to the Scheme (2a, 2b). In this case, disappearance of a pink color of the specimens due to lower concentration of Mn3 þ ions (Mn3 þ - Mn2 þ ) should be observed together with a decrease in the R-line intensity of manganese ions

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Fig. 11. Gaussian approximation of the PL spectra of α-PAA:Mn for excitation at λ ¼275 (a) and 325 (b) nm.

as a function of UV exposure time. Excitation of Cr3 þ ions in the absorption bands at 410 and 560 nm occurs via re-absorption of radiation of F- (λem ¼415 nm) and F22 þ - centers (560 nm [41]). With a prolonged UV exposure, the reduction of concentrations of individual F- and F22 þ - centers and those in complex defects can occur in accordance with the following reactions [35] F þ hν - F þ þ e-, F22 þ þ e- - F2 þ , leading to reduced R-line intensity of the chromium ions.

5. Conclusions The possibility has been demonstrated of doping of PAA with manganese ions in the process of electrochemical oxidation of an 99.99% aluminum in the oxalic acid (0.3 M) based electrolyte with addition of KMnO4 (0.8 g/l). It is shown that the spectral characteristics of the PAA are determined by F- and F2- centers in different charge states. Doping of the PAA with manganese ions in the process of anodizing leads to formation of complex defects in the alumina structure including manganese ions and oxygen vacancies. It has been established that although the observed spectra are generally similar, the PL intensity growth with increasing annealing temperature to Тa ¼600 °С is substantially higher for the undo ped specimens than for the doped ones. For Тa Z 800 °С, the PAA and PAA:Mn specimens exhibit differences in their PL spectra with simultaneous reduction in their intensities. Such changes in the spectra and reduced PL intensity in case of the doped specimens are associated with fact that part of the excited oxygen vacancies participates in changing the valence of Mn ions in the complex defects consisting of Mn ions and F- and F2- centers. The decrease in the PL intensity under a prolonged UV irradiation of the PAA:Mn specimens testifies to changes in the valence of manganese ions and the charge state of oxygen vacancies due to charge transfer in the intricate complex defect. The presence of these defects results in the growth of intensity of the R-line of Mn ions (678 nm) which substitute aluminum ions in octahedral environment in the PAA:Mn specimens annealed at 1300 °C as compared to the undoped ones. Excitation of luminescence of Mn4 þ ions occurs in the band at 315 nm owing to the absorption of radiation by oxygen vacancies located in the coordination sphere of the ions and reabsorption of emission of F þ -centers (λem ¼480 nm) by Mn4 þ ions in the absorption band at 470 nm.

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