Journal of Alloys and Compounds 651 (2015) 565e570
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Enhancement and quenching photoluminescence effects for rare earth e Doped lead bismuth gallate glasses _ Wojciech A. Pisarski*, Lidia Zur, Martyna Kowal, Joanna Pisarska University of Silesia, Institute of Chemistry, Szkolna 9, 40-007 Katowice, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 June 2015 Received in revised form 18 August 2015 Accepted 19 August 2015 Available online 20 August 2015
Lead bismuth gallate glasses doped with Eu3þ and Dy3þ ions were examined using excitation and luminescence measurements. From excitation spectrum of Eu3þ ions the electronephonon coupling strength and phonon energy of the glass host was calculated. Strong luminescence quenching of Dy3þ and enhancement of Eu3þ is well observed, which is due to presence of Bi3þ / Eu3þ and Dy3þ / Bi3þ energy transfer processes in lead bismuth gallate glass. Luminescence decay analysis and calculations based on InokutieHirayama model conﬁrms this hypothesis. © 2015 Elsevier B.V. All rights reserved.
Keywords: Heavy metal glasses Rare earth ions Luminescence Spectroscopic properties
1. Introduction Heavy metal oxide glasses (HMOG) have received great attention in the past due to their good chemical and physical properties, such as high stability against crystallization, ease of preparation, thermal conductivity and heat capacity. HMOG systems are promising glass hosts to incorporate trivalent lanthanides (Ln3þ), and what is especially important is that the radiative properties of Ln3þ ions in germanate [1e7], silicate [8e11], phosphate [12e16] and borate [17e20] glasses containing lead are excellent. Among inorganic HMOG glasses, lead bismuth gallate systems, referred as LBG, belong to low-phonon heavy metal oxide glass family  and they were characterized using various spectroscopic techniques . Especially, LBG system with molar composition of 57PbOe25Bi2O3e18Ga2O3 was selected as the glass host, which shows good mechanical and thermal properties, wide transmission window from visible to NIR spectral region (0.45e8 mm) and quite good chemical stability against crystallization. Also, trivalent lanthanides as the optically active ions were successfully incorporated into LBG glass host in relation to practical application in modern photonics and laser technology. They have been studied mainly for ﬁber-optic ampliﬁcation wavelength at the
* Corresponding author. University of Silesia, Institute of Chemistry, Szkolna 9 Street, 40-007 Katowice, Poland. E-mail address: [email protected]
(W.A. Pisarski). http://dx.doi.org/10.1016/j.jallcom.2015.08.160 0925-8388/© 2015 Elsevier B.V. All rights reserved.
near-infrared region. Strong broadband near-infrared luminescence has been observed in lead bismuth gallate glasses singly doped with Tm3þ , Er3þ [24,25], Nd3þ , Yb3þ  and Ho3þ [28,29] as well as doubly doped with Tm3þ e Ln3þ, where Ln ¼ Ho  and Er . The efﬁcient energy transfer from Yb3þ to Tm3þ , Er3þ  or Ho3þ  was also detected under 976 nm diodelaser excitation. These phenomena were investigated in details for up-conversion luminescence. The effects of GeO2 and/or PbF2 doping on local structure, thermal stability and spectroscopic properties of PbOeBi2O3eGa2O3 containing Ln3þ ions were also analyzed [35e37]. The experimental results indicate the potential possibility towards the development of the S-band optical ﬁber ampliﬁers TDFA  and blue-upconversion glass-ﬁber lasers . Moreover, waveguide structures were successfully produced inside PbOeBi2O3eGa2O3eGeO2 glass using by femtosecond laser pulses . In contrast to near-infrared luminescence and up-conversion processes, the experimental results including excitation and emission spectra for lanthanide doped lead bismuth gallate glasses in the visible spectral region are less documented in the literature. In this work, new optical results for Eu3þ and Dy3þ ions in ternary PbOeBi2O3eGa2O3 glass systems are presented. In recent years, the NIR (1.3 mm) luminescence spectra of Dy3þ ions in PbOeBi2O3eGa2O3 glasses were analyzed . It is well known that quite intense red (Eu3þ) and yellow/blue (Dy3þ) luminescence lines can be produced under direct excitation by commercial UV or blue LED in wide spectral region (340e480 nm). Our preliminary investigations for Ln3þ-doped lead bismuth gallate glass (Ln ¼ Eu, Dy)
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give rather unexpected results and they are extremely different in comparison to that one obtained for similar heavy metal oxide glass systems. The visible emission is signiﬁcantly enhanced (Eu3þ) or quenched (Dy3þ) in the studied glass host. The previously published results clearly indicate that the efﬁcient energy transfer Bi3þ / Ln3þ exist in glass systems, where Bi3þ ions together with Ln3þ were used as optical dopants [40,41]. It also suggests the presence of energy transfer processes between Bi3þ and Ln3þ ions in our system, where bismuth oxide is one of the main glass components. These aspects are examined here. 2. Experimental Leadebismuthegallate glasses wi0074h chemical formula (given in mol.%): 57PbOe25Bi2O3e17.5Ga2O3e0.5Ln2O3 (Ln ¼ Eu, Dy), were prepared by mixing and melting appropriate amounts of high-purity metal oxides (99.99%, Aldrich Chemical Co.). The samples were prepared in a glow box. A homogeneous mixture was heated in a protective atmosphere of dried argon. Mixed reagents were melted for 0.5 h at 1100 C. The nature of the studied samples was identiﬁed using the X-ray diffraction analysis (X'Pert X-ray diffractometer). The Fourier-transform infrared spectra FT-IR were performed by Bruker spectrometer using standard KBr disc techniques. The excitation and luminescence spectra as well as decay curves were measured with Jobin Yvon Fluoromax4 spectrophotometer. Spectral resolution was ±0.1 nm. Decay curves with accuracy ±1 ms were acquired. 3. Results and discussion Lead bismuth gallate glasses singly doped with Eu3þ and Dy3þ ions were prepared. In order to examine local structure of the obtained samples, the FT-IR spectroscopy was used. Typical FT-IR spectrum of lead bismuth gallate glass is shown in Fig. 1. In order to prove amorphous or semi-crystalline state of the obtained samples the phase analysis was also done with use of the X-ray diffraction. Inset of Fig. 1 shows X-ray diffraction pattern for the studied sample. The X-ray diffraction pattern reveals only two
Fig. 1. Typical FT-IR spectrum of lead bismuth gallate glass. Inset shows XRD pattern for lead bismuth gallate glass sample.
broad peaks, which are typical for glassy state. Independently on Ln3þ dopants (Eu3þ or Dy3þ), all the studied samples are fully amorphous. Several unresolved bands located in 400e800 cm1 frequency region are observed in the FT-IR spectrum. They were deconvoluted into Gaussian components. The band in 400e500 cm1 frequency region corresponds to PbeO stretching vibrations of the [PbO4] structural units along with the deformation modes of the BieO glass network. The additional FT-IR bands in the 500e800 cm1 frequency region are due to BieO bending and stretching vibrations in [BiO3] and [BiO6] units, respectively. Also, bands related to BieO vibrations in [BiO6] units near 860 cm1 and vibrations of PbOeOeBi and Bi(3)eOeBi(6) bridges near 920 cm1 are exist in similar glass systems containing PbO and Bi2O3 . In our case, these FT-IR bands located above 800 cm1 are practically not observed. Fig. 2 presents excitation spectrum for Eu3þ ions in lead bismuth gallate glass. Inset shows phonon sideband of Eu3þ (PSB) located near 520 nm. Excitation spectrum for trivalent europium ions was recorded in 300e500 nm ranges monitored at lem ¼ 612 nm (5D0 / 7F2 red transition of Eu3þ). Several bands can be assigned to the transitions originating from the 7F0 ground state to the 5D2, 5D3, 5 L6, 5L7, 5GJ and 5D4 excited states of Eu3þ. Phonon sideband (PSB) is associated with the pure 7F0 / 5D2 electronic transition (PET). Two important parameters like the electronephonon coupling strength g and the phonon energy of the glass host hu, were determined from phonon sideband measurement. The electronephonon coupling strength is the intensity ratio of the PSB, !IPSB dn, to the PET, !IPET dn, whereas the phonon energy is the difference between the position of the PSB and the one of the PET.
IPSB dn (1) IPET dn
hu ¼ PSB PET
Based on literature data [43e45] it is well known that the multiphonon relaxation rate, Wp(T), depends on the
Fig. 2. Excitation spectrum of Eu3þ ions in lead bismuth gallate glass.
W.A. Pisarski et al. / Journal of Alloys and Compounds 651 (2015) 565e570
electronephonon coupling strength and phonon energy of the glass host using the following relations:
Wp ðTÞ ¼ W0 ð0Þexpð aDEÞ
lnðp=gÞ 1 hu
where p denotes the phonon number, DE e the energy gap between neighboring energy states and W0(0) is the probability transition extrapolated to zero energy gap and zero phonon emission. The non-radiative multiphonon relaxation rate is often given as Wp(T)/W0(0) . The values of 1750 cm1 and 2500 cm1 as the 5 D1 e 5D0 and 5D2 e 5D1 energy gaps of Eu3þ are used for Wp(T)/ W0(0) calculations. The parameters from phonon sideband measurement are given in Table 1. The energy phonon (PSBePET) is in a good agreement with the value obtained from Raman spectrum . The electronephonon coupling strength and nonradiative relaxation rates from the 5D1 and 5D2 states are considerably smaller than that obtained previously by us for lead phosphate glass containing Eu3þ ions . Fig. 3 shows excitation and emission spectra for Dy3þ ions in lead bismuth gallate glasses. Luminescence decay from the 4F9/2 (Dy3þ) excited state of Ln3þ ions is also presented. The observed characteristic bands can be assigned to transitions originating from the 6H15/2 ground state to the 4F9/2 excited states of Dy3þ, when the excitation spectrum was monitored at lem ¼ 575 nm (main 4F9/ 6 3þ 2 / H13/2 yellow transition of Dy ). It is interesting to note that two broad excitation bands at about 260 nm and 300 nm are also quite well observed, when the spectrum is monitored at lem ¼ 545 nm. They correspond to 1S0 / 1P1 and 1S0 / 3P1 transitions of Bi3þ . The electronic transitions of bismuth ions in several glass host matrices were systematically investigated using different spectroscopic techniques [49e51]. Independently on excitation wavelength (Fig. 3), the emission spectra consist of the broad band centered at 545 nm with an intense tail extending up to nearly 800 nm, which is typical for 3P1 / 1S0 transition of Bi3þ . Additionally, the much more narrow band component centered at 575 nm characteristic for 4F9/2 / 6H13/2 transition of Dy3þ is clearly visible, when glass sample is excited at 480 nm. In this case, the subsystems of Bi3þ and Dy3þ contribute independently to observed luminescence in Dy3þ-doped lead bismuth gallate glass. Our previously published results  clearly indicate that the overlap between Bi3þ emission band and Dy3þ absorption band becomes negligible small in lead bismuth gallate glass making the nonradiative energy transfer from Bi3þ to Dy3þ inefﬁcient. The sensitization of Dy3þ emission by Bi3þ ions does not occur in the system under study. Comparison of luminescence features for the system under study to those reported earlier for Dy3þ-doped lead borate glass indicates that the luminescence of Dy3þ is efﬁciently quenched by Bi3þ ions . In comparison to Dy3þ-doped glass sample, luminescence spectra and their decays obtained for Eu3þ ions in lead bismuth Table 1 Parameters obtained from phonon sideband measurement. Parameters
PSBePET [cm1] Electronephonon coupling strength g  Nonradiative relaxation rate Wp(T)/W0(0)
550 11 3.55 107 (from 5D1 state) 1.21 1010 (from 5D2 state)
gallate glass are extremely different. Fig. 4 shows excitation and emission spectra for Eu3þ ions in lead bismuth gallate glasses. Luminescence decay from the 5D0 (Eu3þ) excited state of Ln3þ ions is also presented. The observed characteristic bands can be assigned to transitions originating from the 7F0 ground state to the 5 L6 and 5D2 excited states of Eu3þ, when the excitation spectrum was monitored at lem ¼ 612 nm (main 5D0 / 7F2 red transition of Eu3þ). In comparison to Dy3þ-doped glass sample, the excitation bands corresponding to 1S0 / 1P1 and 1S0 / 3P1 transitions of Bi3þ are less resolved, when the spectrum is monitored at lem ¼ 400 nm. Typical luminescence bands associated to 5D0 / 7FJ (J ¼ 0, 1, 2, 4) transitions of Eu3þ were detected upon excitation of 5L6 state (lexc ¼ 393 nm). When glass sample is excited at 300 nm (3P1 state of Bi3þ), we observe simultaneously low-intense broad emission band at about 400 nm due to 3P1 / 1S0 transition of Bi3þ and emission bands due to characteristic 4fe4f intraconﬁgurational transitions of Eu3þ. The intensity of the main red emission band due to 5D0 / 7F2 transition of Eu3þ is considerable higher under excitation of Bi3þ ions, which indicates that the energy transfer from Bi3þ to Eu3þ occurs. The similar Bi3þ / Eu3þ energy transfer effects were also observed for phosphors synthesized via the solid-state reaction, where Eu3þ and Bi3þ were incorporated as optical dopants . Emission decay curve analysis and calculations based on InokutieHirayama model  conﬁrms this hypothesis. The investigations clearly indicate that luminescence from the 5D0 state of Eu3þ is long-lived (2.05 ms), whereas measured lifetime for the 4F9/ 3þ is very short (38 ms). Our previous spectroscopic 2 state of Dy results  indicate that the 4F9/2 (Dy3þ) lifetime starts to decrease from 580 ms to 356 ms with increasing heavy metal concentration in PbOeB2O3 / PbOeP2O5 / PbOeSiO2 / PbOeGeO2 direction. In contrast to lead borate glass, the luminescence decay curve for lead bismuth gallate glass is non-exponential suggesting the energy transfer processes between Dy3þ and Bi3þ. The 4F9/2 luminescence lifetime of Dy3þ is close to 38 ms for the studied glass sample, value markedly smaller in comparison to that obtained for similar heavy metal oxide glasses. These ﬁndings imply the occurrence of energy transfer from Dy3þ to Bi3þ ions and consequently strong luminescence quenching of Dy3þ by Bi3þ ions in lead bismuth gallate glass. The non-exponential decay curve for 4F9/2 state of Dy3þ ions was applied to calculate the energy transfer probability WDA using the InokutieHirayama (IeH) model. In contrast to Dy3þ, the luminescence decay curve for 5D0 state of Eu3þ ions is nearly singleexponential. However, the same IeH model was also adopted successfully for Eu3þ-doped glass sample in order to compare the WDA values. The time evolution of Dy3þ and Eu3þ emission intensities was ﬁtted to that predicted by the formula:
i h IðtÞ ¼ I0 exp t=t aðt=tÞ3=s
where I0 is a constant, I(t) is emission intensity after pulse excitation, t e intrinsic lifetime of donors in the absence of acceptors, s ¼ 6 for a dipoleedipole interaction between the ions, and the a is the parameter given by the relation:
a ¼ 4=3 p Gð1 3=sÞN0 R30
where G e gamma function, N0 e concentration of acceptor ions and R0 is the critical transfer distance deﬁned as a donoreacceptor separation for which the rate of energy transfer between a donoreacceptor is equal to the rate of intrinsic decay rate t1. The critical transfer distance and lifetime were used to calculate the donoreacceptor interaction parameter CDA and the energy transfer probability WDA:
W.A. Pisarski et al. / Journal of Alloys and Compounds 651 (2015) 565e570
Fig. 3. Emission spectra for lead bismuth gallate glasses doped with Dy3þ ions under different excitation wavelengths. Insets show excitation spectra (on right) and luminescence decay from 4F9/2 state of Dy3þ (on left).
Fig. 4. Emission spectra for lead bismuth gallate glasses doped with Eu3þ ions under different excitation wavelengths. Insets show excitation spectra (on left) and luminescence decay from 5D0 state of Eu3þ (on right).
W.A. Pisarski et al. / Journal of Alloys and Compounds 651 (2015) 565e570
CDA ¼ R60 t1
WDA ¼ CDA R6 0
Results of the ﬁtting procedure using InokutieHirayama model are summarized in Table 2. Our results clearly indicate that the energy transfer probability WDA is extremely high for Dy3þ and considerably lower for Eu3þ. It suggests strong emission enhancement (Eu3þ) and quenching (Dy3þ) due to presence of Bi3þ / Eu3þ and Dy3þ / Bi3þ energy transfer processes in lead bismuth gallate glass. It is clearly seen, when 5D0 (Eu3þ) and 4F9/2 (Dy3þ) values of measured lifetimes are compared to that one obtained experimentally for lead borate glass, lead phosphate glass and PbOeGa2O3eXO2 (where X ¼ Te, Ge, Si) glasses [55e57]. In all compared glass systems, the molar ratio PbO to B2O3, PbO to P2O5 and PbO to XO2 was 1:1. Further investigations also indicate that some spectroscopic parameters (luminescence intensity ratios, measured lifetimes) for rare earth ions depend strongly on PbO/B2O3  and PbO/P2O5  ratios in glass composition. From literature data it is well known that the nonradiative multiphonon relaxation rates of rare earths increase exponentially with increasing phonon energy of the glass host in TeO2 / GeO2 / SiO2 / P2O5 / B2O3 direction . Therefore, luminescence lifetimes for excited states of rare earth ions usually reduce due to higher multiphonon relaxation rates and lower radiative quantum efﬁciencies. The quite opposite situation is observed here. In this case, radiative relaxation is a dominant transition due to large energy separation between 5D0 state and lower-lying 7F6 state of Eu3þ as well as 4F9/2 state and lower-lying 6F1/2 state of Dy3þ. The radiative relaxation rates for 5D0 state of Eu3þ and 4F9/2 state of Dy3þ decrease with reduction of heavy metal oxide content (TeO2 / GeO2 / SiO2 / P2O5 / B2O3) in glass composition. Thus, luminescence lifetime tm as an inverse of total radiative relaxation rate starts to increase from 0.65 ms to 1.85 ms for 5D0 state of Eu3þ and from 0.25 ms to 0.58 ms for 4F9/2 state of Dy3þ in PbOeTeO2 / PbOeGeO2 / PbOeSiO2 / PbOeP2O5 / PbOeB2O3 direction. The results are schematized on Fig. 5. Generally, the 5D0 (Eu3þ) and 4F9/2 (Dy3þ) measured lifetimes starts to reduce with decrease phonon energy of the glass host in B2O3 / P2O5/ SiO2 / GeO2 / TeO2 direction. Extremely different values of 5D0 (Eu3þ) and 4F9/2 (Dy3þ) measured lifetimes in PbOeBi2O3eGa2O3 glasses are due to presence of energy transfer processes between Bi3þ and Ln3þ. It is in a good agreement with the results obtained for Y2O3, which indicate that Ln3þ with lower reduction potentials (Eu3þ) can greatly quench Bi3þ emission, whereas Dy3þ cannot be efﬁciently sensitized by Bi3þ due to the forbidden nature of relevant absorption transitions of these ions and the effective cross relaxation resulting from diversity of energy levels of the ions . Table 2 Results of the ﬁtting of the luminescence decay curves from 4F9/2 (Dy3þ) and 5D0 (Eu3þ) states of Ln3þ ions in leadebismuthegallate glasses obtained using the InokutieHirayama model. The molar ion concentrations NA, the measured luminescence lifetimes t, the a values, the critical transfer distances R0, the donoreacceptor interaction parameters CDA and the energy transfer probabilities WDA are reported. Leadebismuthegallate glass NA [10 t [ms]
R0 [Å] CDA [cm6 s1] WDA [s1]
1.84 38 0.9 8.7 114 1040 26,290
1.95 2050 0.02 2.4 93 1045 488
Fig. 5. Measured lifetimes of 5D0 (Eu3þ) and 4F9/2 (Dy3þ) states as a function of glass host.
4. Conclusions In summary, lead bismuth gallate glasses doped with Eu3þ and Dy ions were examined using excitation and emission measurements. From excitation spectrum of Eu3þ some parameters like the electronephonon coupling strength and phonon energy of the glass host were determined. Strong luminescence quenching of Dy3þ and enhancement of Eu3þ is related to presence of Bi3þ / Eu3þ and Dy3þ / Bi3þ energy transfer processes in lead bismuth gallate glass. Luminescence decay analysis and calculations based on InokutieHirayama model conﬁrms this hypothesis. 3þ
References  Z. Pan, S.H. Morgan, A. Loper, V. King, B.H. Long, W.E. Collins, J. Appl. Phys. 77 (1995) 4688.  Z. Pan, S.H. Morgan, K. Dyer, A. Ueda, H. Liu, J. Appl. Phys. 79 (1996) 8906.  R. Balda, I. Saez de Ocariz, J. Fernandez, J.M. Fdez-Navarro, M.A. Arriandiaga, J. Phys. Condens. Matter 12 (2000) 10623.  L.P. Naranjo, C.B. de Araújo, O.L. Malta, P.A. Cruz, L.R. Kassab, Appl. Phys. Lett. 87 (2005) 241914.  L.R.P. Kassab, F.A. Bomﬁm, J.R. Martinelli, N.U. Wetter, J.J. Neto, C.B. de Araújo, Appl. Phys. B 94 (2009) 239.  P. Ghigna, C. Tomasi, A. Speghini, M. Bettinelli, M. Scavini, J. Appl. Phys. 105 (2009) 023519. nez de Castro, J.M. Fern  M. Jime andez Navarro, Appl. Phys. B 106 (2012) 669.  J.A. Capobianco, G. Prevost, P.P. Proulx, P. Kabro, M. Bettinelli, Opt. Mater. 6 (1996) 175.  B. Karmakar, R.N. Dwivedi, J. Non-Cryst. Solids 342 (2004) 132.  M. Bettinelli, A. Speghini, M. Brik, Opt. Mater. 32 (2010) 1592. _  L. Zur, J. Janek, M. Sołtys, J. Pisarska, W.A. Pisarski, Phys. Scr. T 157 (2013) 014035.  T.B. Brito, M.V.D. Vermelho, E.A. Gouveia, M.T. de Araujo, I. Guedes, C.K. Loong, L.A. Boatner, J. Appl. Phys. 102 (2007) 043113.  C.C. Santos, I. Guedes, J.P. Siqueira, L. Misoguti, S.C. Zilio, L.A. Boatner, Appl. Phys. B 99 (2010) 559.  C.C. Santos, I. Guedes, C.-K. Loong, L.A. Boatner, A.L. Moura, M.T. de Araujo, C. Jacinto, M.V.D. Vermelho, J. Phys. D. Appl. Phys. 43 (2010) 025102.  C.C. Santos, U. Rocha, I. Guedes, M.V.D. Vermelho, L.A. Boatner, C. Jacinto, J. Appl. Phys. 111 (2012) 123101. _  W.A. Pisarski, L. Zur, M. Sołtys, J. Pisarska, J. Appl. Phys. 113 (2013) 143504.  M.B. Saisudha, J. Ramakrishna, Phys. Rev. B 53 (1996) 6186.  C.K. Jayasankar, V. Venkatramu, S. Surendra Babu, P. Babu, J. Alloys Compd. 374 (2004) 22.  W.A. Pisarski, J. Pisarska, G. Dominiak-Dzik, W. Ryba-Romanowski, J. Phys.
                     
W.A. Pisarski et al. / Journal of Alloys and Compounds 651 (2015) 565e570 Condens. Matter 16 (2004) 6171. J. Pisarska, J. Phys. Condens. Matter 21 (2009) 285101. W.H. Dumbaugh, J.C. Lapp, J. Am. Ceram. Soc. 75 (1992) 2315. J. Heo, G.G. Kim, Y.S. Kim, J. Am. Ceram. Soc. 78 (1995) 1285. J. Heo, Y.B. Shin, J.N. Jang, Appl. Opt. 34 (1995) 4284. Y.G. Choi, K.H. Kim, J. Heo, J. Am. Ceram. Soc. 82 (1999) 2762. J. Coleman, S.D. Jackson, P. Golding, T. King, J. Opt. Soc. Am. B 19 (2002) 2927. L.R.P. Kassab, S.H. Tatumi, C.M.S. Mendes, L.C. Courrol, N.U. Wetter, Opt. Express 6 (2000) 104. L.R.P. Kassab, M.E. Fukumoto, V.D.D. Cacho, N.U. Wetter, N.I. Morimoto, Opt. Mater. 27 (2005) 1576. J. Heo, K.Y. Kim, Y.K. Kwon, J. Am. Ceram. Soc. 91 (2008) 938. B. Zhou, H. Lin, D. Yang, E.Y.B. Pun, Opt. Lett. 35 (2010) 211. Y.B. Shin, H.T. Lim, Y.G. Choi, Y.S. Kim, J. Heo, J. Am. Ceram. Soc. 83 (2000) 787. B. Zhou, E.Y.B. Pun, J. Phys. D. Appl. Phys. 44 (2011) 285404. Q.Y. Zhang, T. Li, Z.H. Jiang, X.H. Ji, S. Buddhudu, Appl. Phys. Lett. 87 (2005) 171911. L.R.P. Kassab, M.E. Fukumoto, N.U. Wetter, J. Opt. Soc. Am. B 22 (2005) 1255. B. Zhou, E.Y.B. Pun, H. Lin, D. Yang, L. Huang, J. Appl. Phys. 106 (2009) 103105. J.H. Song, J. Heo, S.H. Park, J. Appl. Phys. 93 (2003) 9441. H. Yamauchi, G.S. Murugan, Y. Ohishi, J. Appl. Phys. 96 (2004) 7212. Q.Y. Zhang, T. Li, D.M. Shi, G.F. Yang, Z.M. Yang, Z.H. Jiang, S. Buddhudu, J. Appl. Phys. 99 (2006) 033510. J. Siegel, J.M. Fern andez-Navarro, A. García-Navarro, V. Diez-Blanco, O. Sanz, J. Solis, F. Vega, J. Armengol, Appl. Phys. Lett. 86 (2005) 121109. Y.G. Choi, J. Heo, J. Non-Cryst. Solids 217 (1997) 189. B. Zhou, H. Lin, B. Chen, E.Y.B. Pun, Opt. Express 19 (2011) 6514. M. Peng, N. Zhang, L. Wondraczek, J. Qiu, Z. Yang, Q. Zhang, Opt. Express 19
(2011) 20799.  E. Culea, J. Non-Cryst. Solids 357 (2011) 50.  H. Ebendorff-Heidepriem, D. Ehrt, J. Non-Cryst. Solids 208 (1996) 205.  S. Surendra Babu, K. Jang, E.J. Cho, H. Lee, C.K. Jayasankar, J. Phys. D. Appl. Phys. 40 (2007) 5767.  T. Som, B. Karmakar, J. Phys. Condens. Matter 22 (2010) 035603.  G. Vijaya Prakash, R. Jagannathan, Spectrochim. Acta A 55 (1999) 1799.  Y.S. Han, J.H. Song, J. Heo, J. Appl. Phys. 94 (2003) 2817. _  W.A. Pisarski, L. Zur, T. Goryczka, M. Sołtys, J. Pisarska, J. Alloys Compd. 587 (2014) 90.  W. Xu, M. Peng, Z. Ma, G. Dong, J. Qiu, Opt. Express 20 (2012) 15692.  M. Peng, C. Zollfrank, L. Wondraczek, J. Phys. Condens. Matter 21 (2009) 285106.  B. Xu, S. Zhou, D. Tan, Z. Hong, J. Hao, J. Qiu, J. Appl. Phys. 113 (2013) 083503.  W.A. Pisarski, J. Pisarska, R. Lisiecki, G. Dominiak-Dzik, W. Ryba-Romanowski, Chem. Phys. Lett. 531 (2012) 114.  P. Yang, X. Yu, H. Yu, T. Jiang, X. Xu, Z. Yang, D. Zhou, Z. Song, Y. Yang, Z. Zhao, J. Qiu, J. Lumin. 135 (2013) 206.  M. Inokuti, F. Hirayama, J. Chem. Phys. 43 (1965) 1978. _  J. Pisarska, L. Zur, W.A. Pisarski, J. Mol. Struct. 993 (2011) 160. _  W.A. Pisarski, L. Zur, J. Pisarska, Opt. Lett. 36 (2011) 990. _  W.A. Pisarski, J. Pisarska, L. Zur, T. Goryczka, Opt. Mater. 35 (2013) 1051.  W.A. Pisarski, J. Pisarska, M. Ma˛ czka, R. Lisiecki, Ł. Grobelny, T. Goryczka, G. Dominiak-Dzik, W. Ryba-Romanowski, Spectrochim. Acta A 79 (2011) 696. _  M. Sołtys, J. Janek, L. Zur, J. Pisarska, W.A. Pisarski, Opt. Mater. 40 (2015) 91.  M. Shojiya, Y. Kawamoto, K. Kadono, J. Appl. Phys. 89 (2001) 4944.  G. Ju, Y. Hun, L. Chen, X. Wang, Z. Mu, H. Wu, F. Kang, J. Lumin. 132 (2012) 1853.