J. Phys. Chem. 1995, 99,
15963-15967
15963
Luminescent Properties of Nondoped and Rare Earth Metal Ion-Doped KaLaiTisOio with Layered Perovskite Structures: Importance of the Hole Trap Process Akihiko Kudo*’1 and Tadayoshi Sakata Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan Received: February 24, 1995; In Final Form: August 15, 1995®
ty^TisOio with
a layered perovskite structure showed photoluminescence at 475 nm by excitation at 315 K. On K^L^TisOio doped with rare earth metal ions, luminescence of the rare earth metal ions was observed by the host excitation (K2La2Ti30io). The efficiency of the energy transfer from the host to the rare earth ions led to the classification of the REMI-doped KLT into two groups. Luminescence of both the host and of the rare earth ions was observed on the Sm3+-, Dy3+-, Er3"1"-, and Tm3+-doped (1 mol % to La) K2La2Ti30io at 77 K by the host excitation. On the other hand, on Eu3+-, Tb3+-, and Pr3+-doped K2La2Ti3Oio, the host luminescence was almost completely quenched, and only the luminescence of the rare earth metal ions was observed. For the latter cases, Eu3+, Tb3+, and Pr3+ showed their luminescence even at 300 K by the host excitation. Moreover, the Tb3+- and Pr3+-doped K2La2TÍ30io showed long afterglows at 77 K and bright thermoluminescence. The long afterglows indicated the achievement of charge separation for a long time (> 10 min). It was concluded that the hole trap process accompanying the valency change of the Tb and Pr ions (3H—- 4+) played an important role for the characteristic behaviors of the Tb3+- and Pr3+-doped
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nm at 77
K2La2Ti30io·
Introduction
K2La2Ti30io and K2Lai .ggLno.oz^O i o (Ln = Pr, Sm, Eu, Tb, Dy, Er, and Tm) to explore the possibility of layered compounds as new photofunctional materials. The luminescent mechanism of REMI-doped K2La2TÍ30m is also discussed.
Layered compounds have been seen as attractive functional materials that serve two-dimensional microspaces. Recently, one of the groups of layered compounds, ion-exchangeable layered perovskite oxides (LPO), has been synthesized.1,2 Properties such as intercalation3-6 and acidity7 have been investigated. The oxide layers of LPO have the perovskite structure and alkali metal ions are forced at the interlayer by Coulombic force. Many LPOs consist of titanium and niobium oxides. Many compounds consisting of the titanium and niobium oxides, for example, T1O2 and SrTiOs, are known to have photoresponsibility such as photocatalytic activities. Therefore, the LPOs consisting of the titanium and niobium oxides can be expected to possess some photoactive properties. As their examples, photocatalytic activities of K2La2Ti30io8 and KLaNb2078 and photoluminescence of KLahrt^Oy9 have been reported. Rare earth metal ions (REMI) have been widely used as phosphors, and therefore, their luminescence properties have been extensively studied for a long time.10-13 The luminescent properties strongly depend on the host. Therefore, it is important to investigate the luminescent properties of the REMIs doped in various hosts from a viewpoint of the fundamental and applied research such as energy transfer and development of new luminescent materials. Many LPOs contain La3+ ions, and the compounds in which La3+ ions have been replaced with other lanthanoid ions have been synthesized.23 Therefore, LPOs seem to be interesting as new hosts for the REMI phosphors with two-dimensional structures. Only a few studies on the luminescent properties of europium ions doped in two-dimensional compounds have been reported.14,15 In the present paper, we investigated luminescent properties (photoluminescence, afterglows, and thermoluminescence) of
Experimental Section The following reagents were used as starting materials: K2CO3 (Kanto Chemical; 99.5%), T1O2 (Kojundo Chemical; 99.9%), La203 (Wako Pure Chemical; 99.99%), Pr60,i (Wako Pure Chemical; 99.5%), Sni2C>3 (Wako Pure Chemical; 99.9%),
Chemical; 99.9%), Tb-tO? (Kanto Chemical; 99.95%), Dy203 (Shin-etsu Chemical; 99.9%), Er203 (Wako Pure Chemical; 99.9%), and 2 3 (Wako Pure Chemical; 99.9%). K2La2Ti30io (KLT) and KLT doped with REMIs of 1 mol % to La (^Lai.ggLnomTisOio) were prepared by calcination of the mixture of suitable starting materials at 1323 K for 40 h with one grinding after precalcination at 1173 K for 5 h in air using a platinum crucible.23 All samples were confirmed to be single phases by X-ray diffraction. Commercial Gd2C>2S:Tb (Sylvania) was used as a standard phosphor for Tb3+ luminescence. Luminescence of powdered or pressed samples was measured in a quartz glass cell or a cryostat (Oxford) with a temperature controller in vacuo using a fluorometer (Spex, Fluoromax). Afterglows were measured after excitation at an appropriate wavelength at 77 K for 5 min. Glow curves were obtained by monitoring the thermoluminescence during elevating temperature from 77 to 300 K after the afterglow at 77 K had become negligible. A diffuse reflection spectrum was measured using a UV—vis spectrometer (Shimadzu: UV-2100PC) with an integrating sphere (ISR 260). The diffuse reflection spectrum was converted to the absorbance mode by the Kubelka—Munk method. EU2O3 (Shin-etsu
Results
+ Present Address: Department of Applied Chemistry, Faculty of Science, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162, Japan. ® Abstract published in Advance ACS Abstracts, September 15, 1995.
0022-3654/95/2099-15963$09.00/0
(1) Photoluminescent Properties of KiLaiTiaOio (KLT). Figure 1 shows excitation and luminescence spectra at 77 K ©
1995 American Chemical Society
15964
J. Phys. Chem.,
Vol 99, No. 43, 1995
Kudo and Sakata
Wavelength / nm
Wavelength / nm
Wavelength / nm
Wavelength / nm
Wavelength / nm
Figure 1. Photoluminescence (PL) and diffuse reflection (DRS) spectra of KaLaiTiaOio: (a) PL at 77 K (excited at 315 nm); (b) PL at 77K (monitored at 475 nm); (c) DRS at 300 K.
Figure 2. Temperature dependence of photoluminescence intensity of KaLaaTisOio. and a diffuse reflection spectrum at 300 K of nondoped KLT. Broad photoluminescence at 475 nm by excitation at 315 nm was observed. The excitation spectrum had an onset at 330 nm and agreed well with the diffuse reflection spectrum. The photoluminescence intensity increased remarkably below 120 K, as shown in Figure 2. (2) Photoluminescence of K2Lai.98Lno.o2TÍ30io. Figure 3a—g shows excitation and photoluminescence spectra of KLT doped with various REMIs of 1 mol % at 77 K. The REMI luminescence was dominant for the KLT doped with Sm3+, Dy3+, and Er3+, and the host luminescence was considerably quenched, as shown in Figure 3a—c. Their excitation spectra obtained by monitoring the REMI luminescence had a broad peak with the onset at 330 nm as well as sharp peaks due to the direct excitation of REMIs. Tm3+-doped K2La2Ti30io also showed the blue luminescence of Tm3+ by the host excitation
(Figure 3d). In contrast, for the Pr3+- and Tb3+-doped KLT, only the luminescence of REMIs was observed by the host excitation at 77 K (Figure 3e,f) and even at 300 K. The luminescent intensity of Tb3+-doped KLT was ca. one-tenth at 77 K compared with that of a commercial Gd2C>2S:Tb phosphor. When the amount of Tb3+ doped was small (3.16 Thus, the temperature dependence of Pr3+and Tb3+-doped KLT is quite different from that of nondoped
Photoluminescence of Ln-doped K2La2TÍ30io
J. Phys. Chem., Vol. 99, No. 43, 1995
Temperature / K
15965
Temperature / K
Figure 4. Temperature dependence of photoluminescence intensity of K2Lai ggPro.oaTiaO i and K2Lai,98Tbo.o2TÍ30io: (a) K2Lai gsPro.orTisO i o; o
excited at 315 nm, monitored at 489.5 nm; (b) K2Lai.98Tbo,o2TÍ30io; excited at 315 nm, monitored at 543.5 nm; (c) KiLai.ggPro.oiTisOio; excited at 449.5 nm, monitored at 489.5 nm.
Figure 6. Glow curves of K2Lai.98Pro.o2TÍ30io and K2Lai.98Tbo.o2TÍ30io after host excitation at 315 nm at 77 K: (a) K2Lai.98Pro.o2TÍ30io; monitored at 489.5 nm; (b) K2Lai.gsTbo.ccTisOio; monitored at 543.5 nm. These curves were measured after exciting the host for 5 min and maintaining it for more than 1 h at 77 K to eliminate afterglows.
afterglows, and glow curves. These facts indicate that these phenomena were due not only to the properties of Pr33" and Tb33" ions but also to that of the host.
Discussion
Time / sec
Figure 5. Afterglows of K2La2TÍ30io, K2Lai,98Pro.o2TÍ3010, and K2Lai.98Tbo.o2TÍ30io at 77 K after host excitation at 315 nm for 5 min: (a) K2La2Tt30io; monitored at 475 nm; (b) K2Lai.98Pro.o2TÍ30io; monitored at 489.5 nm; (c) K2Lai.98Tbo,o2TÍ30io; monitored at 543.5 nm.
TABLE 1: Decay Constants (r„) and Ratios of Pre-Exponential Factors (a„) of Afterglows of K2La2TÍ30io< K2Lai.9gPro.o2TÍ30io, and K2Lar.9gTbo.o2TÍ30io at 77 K“ nondoped Pr-doped Tb-doped a
I
ci]
til
ri (s)
«2
150
3.1
24 52
2.2
60 22 42
1.8
exp(-t/ri) +
a2
r2 (s) 13 11
8.3
ti3
12 6 9
T3
(S)
62 63 47
exp(-t/r2) + a} exp(-t/T3) +
ÜA 1 1 1
a4
r4(s) 640 830 680
exp(-t/
r4).
KLT. On the other hand, the temperature dependency of the luminescence intensity of Pr3+ by direct excitation was quite small, as shown in part c of Figure 4. Figure 5 shows afterglows of nondoped and Pr3+- and Tb3+doped KLT. In addition, the decay constants and the ratios of pre-exponential factors are shown in Table 1. These afterglows were measured after exciting the host for 5 min. The spectra of the afterglows of Pr3+- and Tb3+-doped KLT were confirmed to be the same as the Pr33· and Tb3+ luminescence by comparison with Figure 3e-f. In all cases, the afterglows were observed for longer than 10 min. Moreover, the characteristics of the afterglows of Pr3+- and Tb3+-doped KLT were similar to that
of nondoped KLT, although initial decays ( \ and 2) of Pr3+and Tb3+-doped KLT seem to be faster than nondoped KLT,
due to quenching by Pr3+ and Tb3+. Although long afterglows (several minutes) have been reported for Tb3+-doped galíate 18 as far as we systems such as YjGasOn and GdgGasOn,17 know, such a long afterglow has not been reported so far for titanate systems. Figure 6 shows glow curves of Pr3+- and Tb3+-doped KLT. The spectra of the thermoluminescence (TL) of Pr3+- and Tb3+doped KLT were also confirmed to be due to the Pr3+ and Tb3+ ions. In both cases, TL arose around 120 K and showed the maximum at 200 K when the temperature was elevated with a rate of ca. 15 K/min. Thus, Pr3+- and Tb3+-doped KLT showed similar tendencies for temperature dependence of the photoluminescence intensity,
(1) Luminescence of Nondoped KLT. The onset of absorption and excitation spectra of nondoped KLT was ca. 330 nm, as shown in Figure 1. Blasse et al. have discussed luminescence of perovskite-like titanates in detail.910 Compared with their data, the excitation energy of KLT is lower than those of La2Ti2C>7 and La2TiOs in which energy states were localized. This indicates that the energy state of KLT is delocalized, as observed in Y2Ti207, SrTiOj, Ca3Ti207, etc., suggesting that energy and/or electron transfer should take part in the luminescent phenomena of KLT. (2) Relation between Quenching of Host Luminescence and Chemical Properties of REMIs. REMIs doped in KLT showed efficient luminescence at 77 K by the host excitation, as shown in Figure 3a—g. The following mechanisms can be applied for the luminescence for the KLT system: (i) resonant energy transfer; (ii) trapping and successive recombination of e~ and h+.
First, let’s discuss the possibility of the resonant energy transfer mechanism. Requirements for this mechanism are (a) overlap of a luminescent spectrum of a host with an absorption spectrum of the guest (energy overlap) and (b) suitable configuration of atoms for overlap between orbitals (atomic configuration).19 In regard to the energy overlap, the luminescent spectrum of the KLT (Figure 1) overlapped with the excitation spectra of guest ions (Figure 3) by the direct excitation of Dy33", Sm3+, Er3+, Eu3+, and Pr3+. Moreover, absorption spectra of REMIs in a Na20‘3Si02 glass20 overlap with the luminescence spectra obtained in the present study, more or less. For example, the overlaps in cases of Tb3+ (group 2) and Tm33" (group 1) are relatively small, whereas those of Pr33" (group 2) and Er33" (group 1) are large. However, the degree of the energy overlap (roughly Er, Eu > Sm > Dy, Pr > Tb, Tm from their excitation spectra) does not agree with the order extent of >> Er, Sm, quenching of the host luminescence (Pr, Tb, Eu > the This fact on energy overlap strongly suggests Dy Tm). that, in cases of Pr33"- and Tb3+-doped KLT, the quenching of the host luminescence is due not only to the resonant energy transfer but also to other mechanisms. Next, let’s discuss the other mechanism, that is, trapping and successive recombination of e~ and h3". In general, trivalent ions are stable states for lanthanoids. Pr and Tb, which efficiently quenched the host luminescence (group 2), easily take tetravalency in the solid state. Valencies do not easily change for Sm, Dy, Er, and Tm, which belong to group 1. This fact suggests that, in the cases of Pr3+- and Tb3+-doped KLT, the Pr33" and Tb33" ions work as hole trap and recombination
15966
Kudo and Sakata
J. Phys. Chem., Vol. 99, No. 43, 1995
sites for efficient excitation energy transfer from the host to their ions.
Tb3+ + h+ Tb4+ + e~ *
—*
—
—
Tb4+
Tb3+*
Tb3+ + luminescence
(1) (2)
e'
"
--*n
1
’
Impurity level
* -
Step 3
Excitation Step
1
t
(3)
where e~ and h+ come from the excitation of the host lattice. Ghosh et al. have reported the hole trap mechanism by Cr3+ for the luminescence of the Cr3+-doped SrTiOs system.21 In principle, the present scheme is similar to theirs. They have evidenced that the luminescence of the Cr3+-doped SrTiOs system is mainly due to a charge transfer process, not but resonant energy transfer. As seen in Figures 2 and 4, Pr3+- and Tb3+-doped KLT showed efficient luminescence by host excitation around 150 K, where nondoped KLT did not show the luminescence. This result suggests that the nonradiative recombination of e_ with h+ is partly suppressed by the hole trap mechanism by Pr34" and Tb3+. It is notable that the recombination process of eand h+ is predominant for the REMI luminescence by energy transfer on the KLT system, while luminescence of REMIs doped in oxyacids such as vanadates and tungstates is mostly due to resonant energy transfer. Moreover, it is interesting that this process occurs in the two-dimensional perovskite layers. Domen et al. reported that KLT has high photocatalytic activity for Ha evolution from aqueous alcohol solution.8 This indicates that e~ and h+ are generated under irradiation, and the e~ and h+ play an important role for the luminescent process also for the present case. Incidentally, alcohol irreversibly works as a hole trap reagent in the photocatalytic process. The excitation spectrum of Eu3+-doped KLT was different from those of other REMI-doped KLT. If the excitation spectrum (around 330 nm) of Eu3+-doped KLT is due to the excitation of the host (KLT), the quenching of host luminescence would be due to the redox of Eu2+/3+; here, Eu3+ works as an electron-trapping and -recombination center. On the other hand, Blasse et al. have reported that, in the case of Eu3+-doped NaLaTiOi, there were two bands (290 and 325 nm) on the excitation spectrum and that the band at 325 nm was due to charge transfer of Eu3+-02-.14 The band at 330 nm of Eu3+doped KLT as well as that of the Eu3+-doped NaLaTi04 system may be due to charge transfer, although the wavelength of the band of Eu3+-doped KLT seems to be quite longer than those of other systems, for example, YaOsiEu3"1".22 (3) Mechanism of Afterglow and Thermoluminescence. As mentioned above, Pr3+ and Tb3+ can work as hole trap reagents because they easily become tetravalent ions. Some systems where Tb3+ plays an important role for the luminescence process as a hole trap have been reported. Yamamoto et al. reported that high efficient cathode luminescence was due to hole trapping by Pr3+ and Tb3+ in the Y2O2S host.23 Meulenkamp et al. proposed the hole trap process by Tb3+ in and/or on Ta20s for the electroluminescence in solutions using hole injection reagents (H2O2 and S2O82").24 On the other hand, in the cases of Cr^-doped SrTiOg21 and YgGasOn,25 thermoluminescence and afterglow that a hole trap process by Cr3+ takes part in have been reported. However, there are no detailed reports for the hole trap mechanism by lanthanoid ions. The present systematic study could strongly assert the important role of the hole trap process for efficient electron transfer. In acidic solutions, the redox potentials of Pr3+/4+ and Tb3+/4+ are +3.2 and +3.1 V vs NHE, respectively.26 These values would not exactly agree with those in the solid system but can
C*>-
Tb3*74* or
Pr34^*level
Luminescence
--·
jFigure 7. Luminescence Mechanism of K2Lai.9gPr0.o2TÍ30io and K2Lai.ggTbomTisOm.
be expected to be close. Moreover, valence band levels are known to be around +3 V vs NHE for many titanates for which the d-levels are filled completely.27 This situation could be available for the KLT system. The energy state of KLT as well as SrTiOa (n-type semiconductor) is delocalized, as discussed above. Moreover, KLT has photocatalytic activity for water reduction.8 Therefore, KLT can be assumed to have an n-type semiconductive characteristic, and the impurity levels such as oxygen vacancies that work as electron trap sites will exist just under the conduction band levels. These situations lead to the
luminescent mechanism (for photoluminescence, thermoluminescence, and afterglow) shown in Figure 7. Step 1: e~ is promoted from valence to conduction band by
host excitation. Step 2: Pr3+ or Tb3+ traps holes to be Pr44" or Tb4+, while e~ is trapped on impurity levels in the host. Step 3: e™ is thermally excited into the conduction band and
then reaches Pr44" or Tb4+ to show luminescence. At 77 K, the long afterglow (Figure 5) was explained by the gradual thermal excitation of trapped e~ into the conduction band. Overall characteristics of afterglows of Pr3+- and Tb3+doped KLT were similar to that of nondoped KLT, suggesting that the rate-determining step of the afterglow was the excitation of trapped e~ into the conduction band. It is remarkable that, at step 2, charge separation of e~ and h+ is maintained for a long time. On the measurement of glow curves, e~ is thermally excited from the impurity levels to the conduction band during elevating temperature. In cases of Pr34"- and Tb34"-doped KLT, most of h+ is thought to be trapped as Pr44" and Tb4+. Therefore, e~ in the conduction band can radiatively disappear only when it reaches Pr44" and Tb44", resulting in the suppression of nonradiative transition in the host. This can explain that thermoluminescence (Figure 6) was observed even around 200K, at which nondoped KLT did not show luminescence. Because the concentration of trapped e~ can be approximately constant near the onset of the glow curve of the low temperature side, the following Arrhenius type equation can be available to determine the trap (impurity) levels.
I(T)
oc
n
exp(—e/£7)
(4)
where I(T) and e represent the thermoluminescence intensity and the depth of trap levels, respectively. If one plots 1/ vs In I(T), e can be approximately determined from the slope.28 The trap levels of KLT were determined to be ca. 0.01—0.04 eV below the conduction band level by plotting the data of Figure 6. The reason that afterglow curves (Figure 5) consisted of some different exponential factors as shown Table 1 is probably due to the existence of several trap levels. Thus, the relation among the redox levels of Tb3+/4+ and Pr3+/4+, trap levels of e~, and conduction and valence band positions of the KLT is important for the efficient luminescence
Photoluminescence of Ln-doped KaLazTigOio
of
Pr3+ and Tb3+ by the host excitation, long afterglows, and on Pr3+- and Tb3+-doped KLT.
bright TL
Summary (1) KLT showed photoluminescence at 475 nm by excitation at 315 nm below 120 K. The relatively small value of excitation energy suggested the energy state of KLT was delocalized. (2) Sm3+-, Dy3+-, Er3+-, Tm3+-, Pr3+-, Tb3+-, and Eu3+-doped KLT showed luminescence by excitation of the host. They were classified into two groups by the degree of quenching of the
host luminescence. (3) The photoluminescence intensities of KaLai.ggPro.oiTigOio and KaLai.ggTbo.ozTigOio were saturated around 120 K when the temperature was lowered. (4) KaLai.ggPro.mTigOio and KiLai.ggTbo.oaTigOio showed afterglows for longer than 10 min at 77 K. (5) ^Lai.ggPro.ozTigOio and KaLai.ggTbo.ceTigOio showed TL with the maximum at 200 K. (6) The hole trapping and successive recombination with electrons played an important role for the luminescent properties of K2La,.ggPro.o2TÍ30,o and KaLai.ggTbo.mTigOio.
References and Notes (1) Dion, M.; Ganne, M.; Toumoux, M. Mater. Res. Bull. 1981, 16, 1429. Dion, M.; Ganne, M.; Toumoux, M. Rev. Chim. Miner. 1984, 21, 92. Dion, M.; Ganne, M.; Toumoux, M. Rev. Chim. Miner. 1986, 23, 61. (2) (a) Gopalakrishnan, J.; Bhat, V. Inorg. Chem. 1987, 26, 4299. (b) Gopalakrishnan, J.; Bhat, V. Mater. Res. Bull. 1987, 22, 413. (c) Subramanian, . A.; Gopalakrishnan, J.; Sleight, A. W. Mater. Res. Bull. 1988, 23, 837. (d) Uma, S.; Raju, A. R.; Gopalakrishnan, J. J. Mater. Chem. 1993, 3, 709. (3) Jacobson, A. J.; Lewandowski, J. T.; Johnson, J. W. Mater. Res. Bull. 1990, 25, 679. Jacobson, A. J.; Johnson, J. W.; Lewandowski, J. T. Inorg. Chem. 1985, 24, 3727. (4) Mohan Ram, R. A.; Clearfield, A. J. Solid State Chem. 1991, 94, 45. (5) Hardin, S.; Hay, D.; Millikan, M.; Sanders, J. V.; Tumey, T. W. Chem. Mater. 1991, 3, 977. (6) Sato, M.; Abo, J.; Jin, T.; Ohta, M. Solid State Ionics 1992, 51, 85. Sato, M.; Watanabe, J.; Uematsu, K. J. Solid State Chem. 1993, 107,
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460. Sato, M.; Toda, K.; Watanabe, J.; Uematsu, K. Nippon Kagaku-kaishi 1993, 640. Sato, M.; Jin, T.; Ueda, H. Chem. Lett. 1994, 161. (7) Matsuda, T.; Fujita, T.; Miyamae, N.; Takeuchi, M.; Kanda, K. Bull. Chem. Soc. Jpn. 1993, 66, 1548. Matsuda, T.; Fujita, T.; Miyamae, N. Catal. Today 1993, 16, 455. (8) Domen, K.; Yoshimura, J.; Sekine, T.; Tanaka, A.; Onishi, T. Catal. Lett. 1990, 4, 339. Domen, K.; Ebina, Y.; Sekine, T.; Tanaka, A.; Kondo, J.; Hirose, C. Catal. Today 1993,16, 479. Yoshimura, J.; Tanaka, A.; Kondo, J.; Domen, K. J. Phys. Chem. 1993, 97, 1970. (9) Hamoumi, M.; Wiegel, M.; Blasse, G. J. Solid State Chem. 1994, 108, 410. Wiegel, M.; Hamoumi, M.; Blasse, G. Mater. Chem. Phys. 1994, 36, 289. (10) Alarcon, J.; Blasse, G. Phys. Status Solidi A 1993,135, 627. Blasse, G. Prog. Solid State Chem. 1988, 18, 79, and references therein. (11) Powell, R. C.; Blasse, G. Struct. Bond. 1980,42, 43, and references therein. (12) Palilla, F. C; Levine, A. K.; Rinkevics, M. J. Electrochem. Soc. 1965, 112, 776. (13) Brixner, L. H.; Chen, H.-y. J. Electrochem. Soc. 1983, 130, 2435. Brixner, L. H. Mater. Chem. Phys. 1987, 16, 253. (14) Blasse, G.; Bril, A. J. Lumin. 1970, 3, 109. Alarcon, J.; Blasse, G. J. Phys. Chem. Solids 1992, 53, 677. Berdowski, P. A. M.; Blasse, G. J. Lumin. 1984, 29, 243. Blasse, G.; Bril, A. J. Chem. Phys. 1968, 48, 3652. (15) Endo, T.; Masuda, T.; Takizawa, H.; Shimada, M. J. Mater. Sci. Lett. 1992, 11, 1330. (16) De Haart, L. G. J.; De Vries, A. J.; Blasse, G. J. Solid State Chem. 1985, 59, 291. (17) Hoshina, T. J. Chem. Phys. 1969, 50, 5158. (18) Lammers, M. J. J.; Serevin, J. W.; Blasse, G. J. Electrochem. Soc. 1987, 134, 2356. (19) Blasse, G. J. Chem. Phys. 1966, 45, 2356. (20) Smith, H. L.; Cohen, A. J. Phys. Chem. Glasses 1963, 4, 173 (21) Ghosh, A. K.; Addiss, R. R.; Lauer, R. B. J. Appl. Phys. 1973, 44, 3798. (22) Blasse, G. Struct. Bond. 1976, 26, 43. (23) Yamamoto, H.; Kano, T. J. Electrochem. Soc. 1979, 126, 305. (24) Meulenkamp, E. A.; Kelly, J. J.; Blasse, G. J. Phys. Chem. 1992, 96, 1819. Meulenkamp, E. A.; Kelly, J. J.; Blasse, G. J. Electrochem. Soc. 1993, 140, 84. (25) Blasse, G.; Grabmaier, B. C.; Ostertag, M. J. Alloys Comp. 1993, 200, 17. (26) Shriver, D. F.; Atkins, P. W.; Langford, C. H. Inorganic Chemistry, 2nd ed.; Oxford University Press: Oxford, 1994; p B23. (27) Scaife, D. E. Sol. Energy 1980, 25, 41. (28) Kirsh, Y. Phys. Status Solidi A 1992, 129, 15.
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