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Reflection and photoluminescence spectra of CaS:Ho phosphors
REFLECTION
January 1990
MATERIALS LETTERS
Volume 9. number 2,3
AND P~OTOLU~INESC~NCE
SPECTRA OF CaS : Ho PHOSPHORS
S. BUDDHUDU Department
ofPhysics,
S. K University, Tirupati, India
F.J. BRYANT Department
of Physics, university of H&l, Hull HU6 lRX, UK
and Xi LUO Deportment
of Physics, Cha~gchu~ ~~st~t~te, Changchun,
China
Received 26 October 1989
Reflection and photoluminescence emission spectra of CaS: Ho phosphors of three different concentrations have been studied from the ultraviolet to the near infrared regions at room and liquid-nitrogen temperature. The spectral oscillator strengths for the nine bands observed have been accounted for in terms of three phenomenological intensity parameters. Treatment of the measured photoluminescence spectral data by the Judd-Ofelt theory of absolute intensities has allowed computation of radiative properties such as the transition probability, the relaxation rate and the branching ratio for the observed emission transition 5F5+‘Is. The emission cross-section value of this emission state has also been measured from the recorded photoluminescence spectra.
1. Introduction There has been renewed interest in rare-earthdoped CaS phosphors because of their high luminescence efficiencies [ l-31. In an earlier publication [4], we described a rapid and simple method of preparation of such phosphors. In the present paper, we report on their optical properties both at room temperature and liquid-nitrogen temperature as determined by reflection, excitation, photoluminescence emission and lifetime measurements (using a N’, laser as a source of excitation). We have also measured the emission cross sections for the luminescence transition ‘F5+‘Is of CaS: Ho phosphors.
2. Experimental
procedures
2. I. Material preparation Following
the procedures
given in our earlier pa-
0 167-577x/90/$ 03.50 0 Elsevier Science Publishers B.V.
per [4], we prepared CaS: Ho ferent Ho concentrations of 0.5 MO/o.The samples were annealed of Nz or Ar for about 20 min. fraction patterns confirmed that the salt lattice structure.
phosphors with difMO/o.I .O M% or 1.5 in a flowing stream X-ray powder difall the samples have
2.2. Measurements The reflection spectra of the CaS:Ho phosphors from 300 to 2000 nm were measured using a Unicam SP 720 spectrophotometer. Freshly prepared MgO samples were obtained by melting the Mg metal in the atmosphere for each of the three CaS: Ho phosphors investigated in the present work. Both the excitation and photoluminescence spectra of CaS: Ho phosphors were measured using an Hitachi MPF-4 spectrofluoriphotometer. We measured the lifetime of the ‘F5+‘Is emission transition (638 nm) of CaS: Ho (0.5 and 1.0 MO/o) at both 300 and 77 K by using an NRGO 9-5-30 Nz laser with 337 nm as the
( Nosh-Holland
)
109
Volume 9, number
2,3
MATERIALS
wavelength of excitation. An SPX 1403 double-grating monochromator was used with a Hamamatsu R938 photomultiplier and the signals were recorded by a PARC 162/ 165 boxcar. Refractive indices of the Cas: Ho phosphors were determined at the wavelengths of 4861, 5893 and 6563 A with a precision refractometer and by using a thin film of monobromonaphthalene liquid as a contact layer between the prism of the refractometer and the sample material studied.
LETTERS
January
as the hypersensitive level is remarkably more intense than any other transition of the ion studied. This particular transition satisfies the selection rules [71 AJ=2,
AL=2
indices (v) is a reciprocal
dispersion
intensities
In the 5000-30000 cm-’ region, the recorded reflection spectra revealed nine absorption states which originated from the *Is ground state. By using the relevant mathematical expressions given recently by Sibley et al. [ 5 ] and Reisfeld and Jerrgensen [ 61, we have successfully fitted the measured spectral intensities with values computed through the Judd-Ofelt (Q1) intensity parameters. The best fit has been evaluated in terms of the rms error parameter. The measured and computed absolute intensities of nine observed electric-dipole transitions of the CaS : Ho phosphors are given in table 1. For Ho3+ the transition 51s~5Gg, which is known
and calculated
Absorption levels from $
spectral
intensities
I (nm)
SIT Q6 5l=S 51=‘l Sl=S 51=Z 5(X =S JH6 rms deviation Judd-Ofelt parameters 1OzoQ2 (cm* ) lOW, (cm*) I 02’R6 (cm’)
1938 1153 645 540 485 474 452 416 360
where the subscripts D, F and C denote the index of refraction determined at the wavelengths 5893,486l and 6563 A respectively. The refractive indices at other wavelengths could easily be determined from a simple dispersion formula using v and nD [ 9 1. Optical electric fields associated with the propagation of the laser beam induce refractive index changes in the sample. The total refractive index is given by [ 91 n=n,+n,l, where n, is the linear index, n2 is the non-linear refractive index and I is the beam intensity (W cm-*).
( 106j-, and 104fe) of CaS: Ho phosphors 1.0 M%
1.5 M% fm
110
AS&O.
The Abbe number given by [ 81
3. Results and discussion
Table 1 The measured
and
The intensity of this level is very strongly influenced by the Judd-Ofelt (.Q,) parameter, as shown in table 1. 3.2. Refractive
3.1. Absolute
1990
f,
fm
I .43 I .06
1.40 0.90 2.00 2.55 1.00 0.70 22.00 1.40 3.85
2.19 2.65 1.16 0.66 22.51 1.46 3.92 k0.25
6.504 1.264 1.792
0.5 M% f,
0.97 0.63 1.25 1.75 0.80 0.35 14.80 1.00 2.60
0.99 0.73 1.51 1.83 0.81 0.46 15.53 1.07 2.70 kO.38
4.485 0.872 1.236
JT
fm 0.65 0.45 1.00 1.15 0.55 0.30 11.00 0.65 1.90
0.73 0.54 1.12 1.34 0.60 0.34 11.42 0.74 1.98 kO.21
3.297 0.641 0.910
January 1990
MATERIALS LETTERS
Volume 9, number 2,3
Table 2 Abbe number (c) and refractive index non-linear coefftcients ( ~CI’~Q(esu): 10”~ (~m~/W)) forCaS~Hophos~~lors
Through the measured absolute intensities, the radiative lifetime for 5F5+518 is obtained from [ IO]
T, =A7’ . Parameters
1.5 M%
1.OM%
0.5 M%
n,(,I=5983 ii) n&d=4861 A) nc(E.=6563 A) z 10’3nz (esu) lO’5o (cm’/W)
2.198 2.208 2.187 57 6.808
2.191 2.200 2.181 62 5.927
2.185 2.194 2.176 66 5.338
1.298
1.134
1.024
Through
The non-linear given
refractive
index coefftcient
the oscillator
strength
of the transition
1111
The radiative lifetime of the emission jF5 -+518 is obtained from [ 121
transition
nZ is
by
70(n,,--l)(n’D+2)” v[1,52+(n&+2)(n,+1)c/6n,]“2’
10’3n,(esu)=
Another non-linear is expressed as [ 9 ] v(cm2 W-r)=
refractive
4xx 10’ ~ n,(esu)
index coefficient
v
,
t/c = r,l T,
cnD
where (’ is the velocity of light. The measured values of the Abbe number (t’), and the non-linear refractive index coefftcients ( n2, v) for CaS: Ho are presented in table 2, together with the values of fib, nF and nc. 3.3. Luminescence properties Application of the Judd-Ofelt theory to the observed photoluminescence data of CaS: Ho ( 1.O and 0.5 MO/o) has allowed the following radiative properties to be determined [ lo]. The spontaneous emission probability for the observed emission from level 5F, to the other lowerlying levels has been computed from [ 101 ‘3 ( s - i )
zz
3~~T~~v,3j nD[(n20+2)2/91s.
---
The total transition
probability
of 5F5-‘518 is given
by ]tOl :IT(S-‘)=
1 A.
The fluorescent by [ 101
is given
RR =.-I /.s, .
branching
where M is the emission level width at half height. (Sois the emission cross section, and 1, is the emission level wavelength. The quantum efficiency characteristic parameter may be defined as the ratio of the measured lifetime to the computed hfetime [ 121
ratio (Bn) for ‘F5-t518
The quenching equation [ 131
property
is obtained
using
the
For the observed luminescent transition 5F5-+518 the induced emission cross section has been measured using the expression [ 141 10ZooE(cmZ)=
gKc::
#
ti..-t,
where J. is the luminescent state wavelength (nm) and hELis the bandwidth (nm), which has been determined by integrating the band shape and dividing by the intensity (relative) at i. In summary, the radiative properties which characterise the luminescence behaviour are: (i) the transition probability .-1(s-l ), (ii ) the total transition probability <+(s-‘), (iii) the branching ratio ( BR). (iv) the lifetimes ( TRf through three theoretical methods along with the measured values ( r,). both at 300 and 77 K, (v) the quantum efficiency ()I,), (vi ) the quenching property parameter (K,) and (vii) the stimulated emission cross section of 5F PSI*. Their values for the CaS: Ho phosphors are listed in table 3. 111
Volume 9, number 2,3
MATERIALS LETTERS
Table 3 The measured and computed radiative properties of the 5FS-+SI,r luminescent transition of CaS: Ho phosphors Optical parameters of the emission level SF+?s
CaS : Ho phosphor lM%
0.5 M%
1 (nm) M (nm)
638 16 900 1192 0.76 2.57 0.72 0.76 0.83 0.87 0.91 0.18 0.11
636 17 720 876 0.82 1.92 0.72 0.76 1.14 0.63 0.67 0.51 0.43
A (s-l)
AT (s-l) & 102’oE (cm2) r, (ms) at 300 K (meas.) Tf (ms) at 77 K (meas.) ra (ms) (theor.) tic (300 K) rk (77 K) Kc (300 K) K, (77 K)
January 1990
related with the theoretical lifetimes which are obtained through three different models, as explained in section 3 of this paper. Good agreement has been obtained between the measured and computed lifetimes with an error of IO%, as shown in table 3. From table 3 it is also noted that the other luminescence properties of the observed emission transition 5F5 -+ ‘1, of CaS : Ho phosphors are affected significantly by the change of holmium concentration and temperature.
Acknowledgement One of the authors (SB) expresses his grateful thanks to Professor S.V.J. Lakshman, Vice-Chancellor for his interest and extending full support in the present work.
4. conclusions References Study of the reflection spectra and photoluminescence excitation and emission spectra of CaS: Ho phosphors in three different Ho concentrations have allowed determination of absolute intensities and other luminescence properties. Application of the Judd-Ofelt model has resulted in a good lit, i.e. with a low rms error parameter of the absolute intensities of nine observed energy states. The observed bands, 51,,6, 5F5,4,3,2,5G6,5 and 3H6-‘s18, are considered to result from forced electric-dipole transitions. The influences of the different Ho3+ concentrations on the magnitudes of the oscillator strengths of the hypersensitive transition ( 51s-+5G6) have been found to be quite signi~cant (table 1). The non-linear refractive index coefficients, 10i3nz (esu), Y ( cm2 W-i ) and the Abbe number for the CaS: Ho phosphors have been determined by measuring the refractive indices nD, nF and n, and they are presented in table 2. The data collected in this table show clearly how the coefficients are affected by varying the Ho concentration. The lifetimes of the emission level 5Fg+5fg of CaS: Ho phosphors ( 1.O and 0.5 M%) have been measured at 300 and 77 K by using a Nz laser to excite the phosphors. The measured lifetimes are cor-
112
[ 11 N. Yamashi and S. Asano, J. Electrochem. Sot. 134 ( 1987) 2912. [ 21 F.T. Zhao, CL. Yun and X.R. Xu, J. Electrochem. Sot. 134 (1987) 3186. [3] Y. Kimura and M. Nallazawa, Japan. J. Appl. Phys. 26 (1987) L1253. 141 G. Garudaiah Naidu, S. Buddhudu, F.J. Bryant, X. Luo, B. Yu and S. Huang, Mater. Letters 8 ( 1989) 3 18. [ 51W.A. Sibley, M.D. Shinn, M.G. Drexhage and R.N. Brown, Phys. Rev. B 27 (1983) 6635. [ 6) R. Reisfeld and C.K. Jorgensen, J. Less-Common Met. I26 (1986) 187. [ 7 ] W.T. Carnall, H. Crosswhite, R. Rajnak and J.B. Mann, in: Systematics and properties of lanthanides, cd. S.P. Sinha (Reidel, Dordrecht, 1983). [ 81 M.J. Weber, J.E. Lynch, D.H. Blackbum and D.J. Cronin, IEEE J. Quantum Electron. 19 (1983) 1600. [9] M.J. Weber, Laser properties of rare earths doped materials (Springer, Berlin, 198 1). [ IO] W.A. Sibley and M.G. Drexhage, J. Appl. Phys. 62 ( 1987 ) 266. [ 111 D.C. Yeh and W.A. Sibley, J. Non-Cryst. Solids 88 ( 1986) 66. [ 121 R. Reisfeld, in: Spectroscopy of solid state laser type materials, ed. B. Dibartolo (Plenum Press, New York, 1987). [ 131 L.E. Ageeva, Soviet J. Glass Phys. Chem. 12 (1986) 155. [ 141 F. Gan and H. Zheng, J. Non-Cryst. Solids 95/96 (1987) 711.
January 1990
MATERIALS LETTERS
Volume 9. number 2,3
AND P~OTOLU~INESC~NCE
SPECTRA OF CaS : Ho PHOSPHORS
S. BUDDHUDU Department
ofPhysics,
S. K University, Tirupati, India
F.J. BRYANT Department
of Physics, university of H&l, Hull HU6 lRX, UK
and Xi LUO Deportment
of Physics, Cha~gchu~ ~~st~t~te, Changchun,
China
Received 26 October 1989
Reflection and photoluminescence emission spectra of CaS: Ho phosphors of three different concentrations have been studied from the ultraviolet to the near infrared regions at room and liquid-nitrogen temperature. The spectral oscillator strengths for the nine bands observed have been accounted for in terms of three phenomenological intensity parameters. Treatment of the measured photoluminescence spectral data by the Judd-Ofelt theory of absolute intensities has allowed computation of radiative properties such as the transition probability, the relaxation rate and the branching ratio for the observed emission transition 5F5+‘Is. The emission cross-section value of this emission state has also been measured from the recorded photoluminescence spectra.
1. Introduction There has been renewed interest in rare-earthdoped CaS phosphors because of their high luminescence efficiencies [ l-31. In an earlier publication [4], we described a rapid and simple method of preparation of such phosphors. In the present paper, we report on their optical properties both at room temperature and liquid-nitrogen temperature as determined by reflection, excitation, photoluminescence emission and lifetime measurements (using a N’, laser as a source of excitation). We have also measured the emission cross sections for the luminescence transition ‘F5+‘Is of CaS: Ho phosphors.
2. Experimental
procedures
2. I. Material preparation Following
the procedures
given in our earlier pa-
0 167-577x/90/$ 03.50 0 Elsevier Science Publishers B.V.
per [4], we prepared CaS: Ho ferent Ho concentrations of 0.5 MO/o.The samples were annealed of Nz or Ar for about 20 min. fraction patterns confirmed that the salt lattice structure.
phosphors with difMO/o.I .O M% or 1.5 in a flowing stream X-ray powder difall the samples have
2.2. Measurements The reflection spectra of the CaS:Ho phosphors from 300 to 2000 nm were measured using a Unicam SP 720 spectrophotometer. Freshly prepared MgO samples were obtained by melting the Mg metal in the atmosphere for each of the three CaS: Ho phosphors investigated in the present work. Both the excitation and photoluminescence spectra of CaS: Ho phosphors were measured using an Hitachi MPF-4 spectrofluoriphotometer. We measured the lifetime of the ‘F5+‘Is emission transition (638 nm) of CaS: Ho (0.5 and 1.0 MO/o) at both 300 and 77 K by using an NRGO 9-5-30 Nz laser with 337 nm as the
( Nosh-Holland
)
109
Volume 9, number
2,3
MATERIALS
wavelength of excitation. An SPX 1403 double-grating monochromator was used with a Hamamatsu R938 photomultiplier and the signals were recorded by a PARC 162/ 165 boxcar. Refractive indices of the Cas: Ho phosphors were determined at the wavelengths of 4861, 5893 and 6563 A with a precision refractometer and by using a thin film of monobromonaphthalene liquid as a contact layer between the prism of the refractometer and the sample material studied.
LETTERS
January
as the hypersensitive level is remarkably more intense than any other transition of the ion studied. This particular transition satisfies the selection rules [71 AJ=2,
AL=2
indices (v) is a reciprocal
dispersion
intensities
In the 5000-30000 cm-’ region, the recorded reflection spectra revealed nine absorption states which originated from the *Is ground state. By using the relevant mathematical expressions given recently by Sibley et al. [ 5 ] and Reisfeld and Jerrgensen [ 61, we have successfully fitted the measured spectral intensities with values computed through the Judd-Ofelt (Q1) intensity parameters. The best fit has been evaluated in terms of the rms error parameter. The measured and computed absolute intensities of nine observed electric-dipole transitions of the CaS : Ho phosphors are given in table 1. For Ho3+ the transition 51s~5Gg, which is known
and calculated
Absorption levels from $
spectral
intensities
I (nm)
SIT Q6 5l=S 51=‘l Sl=S 51=Z 5(X =S JH6 rms deviation Judd-Ofelt parameters 1OzoQ2 (cm* ) lOW, (cm*) I 02’R6 (cm’)
1938 1153 645 540 485 474 452 416 360
where the subscripts D, F and C denote the index of refraction determined at the wavelengths 5893,486l and 6563 A respectively. The refractive indices at other wavelengths could easily be determined from a simple dispersion formula using v and nD [ 9 1. Optical electric fields associated with the propagation of the laser beam induce refractive index changes in the sample. The total refractive index is given by [ 91 n=n,+n,l, where n, is the linear index, n2 is the non-linear refractive index and I is the beam intensity (W cm-*).
( 106j-, and 104fe) of CaS: Ho phosphors 1.0 M%
1.5 M% fm
110
AS&O.
The Abbe number given by [ 81
3. Results and discussion
Table 1 The measured
and
The intensity of this level is very strongly influenced by the Judd-Ofelt (.Q,) parameter, as shown in table 1. 3.2. Refractive
3.1. Absolute
1990
f,
fm
I .43 I .06
1.40 0.90 2.00 2.55 1.00 0.70 22.00 1.40 3.85
2.19 2.65 1.16 0.66 22.51 1.46 3.92 k0.25
6.504 1.264 1.792
0.5 M% f,
0.97 0.63 1.25 1.75 0.80 0.35 14.80 1.00 2.60
0.99 0.73 1.51 1.83 0.81 0.46 15.53 1.07 2.70 kO.38
4.485 0.872 1.236
JT
fm 0.65 0.45 1.00 1.15 0.55 0.30 11.00 0.65 1.90
0.73 0.54 1.12 1.34 0.60 0.34 11.42 0.74 1.98 kO.21
3.297 0.641 0.910
January 1990
MATERIALS LETTERS
Volume 9, number 2,3
Table 2 Abbe number (c) and refractive index non-linear coefftcients ( ~CI’~Q(esu): 10”~ (~m~/W)) forCaS~Hophos~~lors
Through the measured absolute intensities, the radiative lifetime for 5F5+518 is obtained from [ IO]
T, =A7’ . Parameters
1.5 M%
1.OM%
0.5 M%
n,(,I=5983 ii) n&d=4861 A) nc(E.=6563 A) z 10’3nz (esu) lO’5o (cm’/W)
2.198 2.208 2.187 57 6.808
2.191 2.200 2.181 62 5.927
2.185 2.194 2.176 66 5.338
1.298
1.134
1.024
Through
The non-linear given
refractive
index coefftcient
the oscillator
strength
of the transition
1111
The radiative lifetime of the emission jF5 -+518 is obtained from [ 121
transition
nZ is
by
70(n,,--l)(n’D+2)” v[1,52+(n&+2)(n,+1)c/6n,]“2’
10’3n,(esu)=
Another non-linear is expressed as [ 9 ] v(cm2 W-r)=
refractive
4xx 10’ ~ n,(esu)
index coefficient
v
,
t/c = r,l T,
cnD
where (’ is the velocity of light. The measured values of the Abbe number (t’), and the non-linear refractive index coefftcients ( n2, v) for CaS: Ho are presented in table 2, together with the values of fib, nF and nc. 3.3. Luminescence properties Application of the Judd-Ofelt theory to the observed photoluminescence data of CaS: Ho ( 1.O and 0.5 MO/o) has allowed the following radiative properties to be determined [ lo]. The spontaneous emission probability for the observed emission from level 5F, to the other lowerlying levels has been computed from [ 101 ‘3 ( s - i )
zz
3~~T~~v,3j nD[(n20+2)2/91s.
---
The total transition
probability
of 5F5-‘518 is given
by ]tOl :IT(S-‘)=
1 A.
The fluorescent by [ 101
is given
RR =.-I /.s, .
branching
where M is the emission level width at half height. (Sois the emission cross section, and 1, is the emission level wavelength. The quantum efficiency characteristic parameter may be defined as the ratio of the measured lifetime to the computed hfetime [ 121
ratio (Bn) for ‘F5-t518
The quenching equation [ 131
property
is obtained
using
the
For the observed luminescent transition 5F5-+518 the induced emission cross section has been measured using the expression [ 141 10ZooE(cmZ)=
gKc::
#
ti..-t,
where J. is the luminescent state wavelength (nm) and hELis the bandwidth (nm), which has been determined by integrating the band shape and dividing by the intensity (relative) at i. In summary, the radiative properties which characterise the luminescence behaviour are: (i) the transition probability .-1(s-l ), (ii ) the total transition probability <+(s-‘), (iii) the branching ratio ( BR). (iv) the lifetimes ( TRf through three theoretical methods along with the measured values ( r,). both at 300 and 77 K, (v) the quantum efficiency ()I,), (vi ) the quenching property parameter (K,) and (vii) the stimulated emission cross section of 5F PSI*. Their values for the CaS: Ho phosphors are listed in table 3. 111
Volume 9, number 2,3
MATERIALS LETTERS
Table 3 The measured and computed radiative properties of the 5FS-+SI,r luminescent transition of CaS: Ho phosphors Optical parameters of the emission level SF+?s
CaS : Ho phosphor lM%
0.5 M%
1 (nm) M (nm)
638 16 900 1192 0.76 2.57 0.72 0.76 0.83 0.87 0.91 0.18 0.11
636 17 720 876 0.82 1.92 0.72 0.76 1.14 0.63 0.67 0.51 0.43
A (s-l)
AT (s-l) & 102’oE (cm2) r, (ms) at 300 K (meas.) Tf (ms) at 77 K (meas.) ra (ms) (theor.) tic (300 K) rk (77 K) Kc (300 K) K, (77 K)
January 1990
related with the theoretical lifetimes which are obtained through three different models, as explained in section 3 of this paper. Good agreement has been obtained between the measured and computed lifetimes with an error of IO%, as shown in table 3. From table 3 it is also noted that the other luminescence properties of the observed emission transition 5F5 -+ ‘1, of CaS : Ho phosphors are affected significantly by the change of holmium concentration and temperature.
Acknowledgement One of the authors (SB) expresses his grateful thanks to Professor S.V.J. Lakshman, Vice-Chancellor for his interest and extending full support in the present work.
4. conclusions References Study of the reflection spectra and photoluminescence excitation and emission spectra of CaS: Ho phosphors in three different Ho concentrations have allowed determination of absolute intensities and other luminescence properties. Application of the Judd-Ofelt model has resulted in a good lit, i.e. with a low rms error parameter of the absolute intensities of nine observed energy states. The observed bands, 51,,6, 5F5,4,3,2,5G6,5 and 3H6-‘s18, are considered to result from forced electric-dipole transitions. The influences of the different Ho3+ concentrations on the magnitudes of the oscillator strengths of the hypersensitive transition ( 51s-+5G6) have been found to be quite signi~cant (table 1). The non-linear refractive index coefficients, 10i3nz (esu), Y ( cm2 W-i ) and the Abbe number for the CaS: Ho phosphors have been determined by measuring the refractive indices nD, nF and n, and they are presented in table 2. The data collected in this table show clearly how the coefficients are affected by varying the Ho concentration. The lifetimes of the emission level 5Fg+5fg of CaS: Ho phosphors ( 1.O and 0.5 M%) have been measured at 300 and 77 K by using a Nz laser to excite the phosphors. The measured lifetimes are cor-
112
[ 11 N. Yamashi and S. Asano, J. Electrochem. Sot. 134 ( 1987) 2912. [ 21 F.T. Zhao, CL. Yun and X.R. Xu, J. Electrochem. Sot. 134 (1987) 3186. [3] Y. Kimura and M. Nallazawa, Japan. J. Appl. Phys. 26 (1987) L1253. 141 G. Garudaiah Naidu, S. Buddhudu, F.J. Bryant, X. Luo, B. Yu and S. Huang, Mater. Letters 8 ( 1989) 3 18. [ 51W.A. Sibley, M.D. Shinn, M.G. Drexhage and R.N. Brown, Phys. Rev. B 27 (1983) 6635. [ 6) R. Reisfeld and C.K. Jorgensen, J. Less-Common Met. I26 (1986) 187. [ 7 ] W.T. Carnall, H. Crosswhite, R. Rajnak and J.B. Mann, in: Systematics and properties of lanthanides, cd. S.P. Sinha (Reidel, Dordrecht, 1983). [ 81 M.J. Weber, J.E. Lynch, D.H. Blackbum and D.J. Cronin, IEEE J. Quantum Electron. 19 (1983) 1600. [9] M.J. Weber, Laser properties of rare earths doped materials (Springer, Berlin, 198 1). [ IO] W.A. Sibley and M.G. Drexhage, J. Appl. Phys. 62 ( 1987 ) 266. [ 111 D.C. Yeh and W.A. Sibley, J. Non-Cryst. Solids 88 ( 1986) 66. [ 121 R. Reisfeld, in: Spectroscopy of solid state laser type materials, ed. B. Dibartolo (Plenum Press, New York, 1987). [ 131 L.E. Ageeva, Soviet J. Glass Phys. Chem. 12 (1986) 155. [ 141 F. Gan and H. Zheng, J. Non-Cryst. Solids 95/96 (1987) 711.