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Luminescence of Mn-activated SrLa2S4
Volume 185, number I,2
CHEMICAL PHYSICS LETTERS
11 October I99 I
Luminescence of Mn-activated SrLa&, * Paul L. Provenzano
’ and William
B. White
Marerials Research Laborafory, The Pennsylvania State Umversity, UniversityPark, PA 16802, US.4 Received 30 May 1991
The Th,P,structure ternary sulfide compound SrLalS,:0.01Mn2+ is shown to be a photophosphor with a deep crimson emission peaking at 675 nm ( 1.84 eV) at 300 K. Measurement of the temperature dependence of emission and excitation spectra reveals a systematic shift in band position at 200 K which is interpreted as a second-order transition in the host structure.
1. Introduction Chalcogenides with large band gaps such as those with the Th3P4 structure are of interest as phosphor hosts. Their luminescence behavior when activated with rare earth or transition metal ions should be intermediate between the higher band gap insulator phosphors such as oxides, phosphates and silicates, and the lower band gap semiconductor phosphors such as GaP and GaAs. Optical band gaps for many ternary sulfide compounds are in the range of 2.3 to 3.0eV [l]. Luminescence investigations of ternary sulfides are rather sparse. Compounds with the thiospinel structure [ 21 and selenides with the PbGa$e, structure (pseudo-orthorhombic) [ 31 have been investigated. Yim et al. [4] prepared a series of Th3P4 compounds, CaLn,S, (Ln=La through Dy ). They reported that CaCe&, exhibited a weak, green cathodoluminescence but that the other compounds were not luminescent. A report from this laboratory gives a reconnaissance of rare earth luminescence in several of the ternary sulfide host structures [ 5 1. The present paper reports the luminescence of Mn’+-activated alkaline earth, rare earth sulfides with the Th3P, structure and an unusual temperature dependence of the luminescence. The Th,P*
* Work supported by the National Science Foundation under Grant No. DMR-74-00340.
’ Present address: UnitedTechnologies Research Center, Silver Lane, East Hartford, CT 06108, USA. 0009-2614/91/$
structure is cubic, space group 143d, with cations in 8-fold coordination. It is the structure of the ternary compounds MLn,!$ where M=Ca, Sr, Ba, Pb, Ln=larger rare earth ions.
2. Experimental The host materials were prepared by reacting SrC03 or BaC03 with La20,, mixed as powders in a silica boat, at temperatures of 900-l 100°C in an atmosphere of flowing HIS for times of 1 to 5 days. Phase identification and phase purity were checked by X-ray powder diffraction. The activator was obtained as the oxide and converted into sulfide by firing in flowing H,S. The manganese sulfide was mixed with the sulfide host in the desired proportion. These mixtures were then refired in H,S at 700°C for times of approximately 12 h. Luminescence measurements were made with a laboratory-built spectrometer. The emission monochromator was a Jarrel-Ash I m instrument while the excitation monochromator was a MacPherson 0.3 m unit. Both a mercury discharge lamp and a xenon arc were used as excitation sources. The Hg source was used with filters (Corning type 7-60 for 365 nm and Corning 7-54 in conjunction with a NiSO, SOlution for 253.7 nm). The xenon source was dispersed through the excitation monochromater. The detector was a cooled RCA 31034 GaAs-cathode photomultiplier. Full details are given in the thesis
03.50 0 1991 Elsevier Science Publishers B.V. All rights reserved.
117
Volume
185, number
1,2
CHEMICAL
PHYSICS
11October
LETTERS
I99
1
from which this paper is drawn [ 6 1. All spectra were measured on powders compacted into a sample holder. Low-temperature spectra were obtained with the aid of an Air Products Cryo-Tip refrigerator down to 94 K. Optical absorption edge spectra were measured as packed powders in the diffuse reflectance attachment of a Beckman DK-2A spectrophotometer. Kodak BaSO, optical paint was used as a reference material and as a coating for the integrating sphere.
3. Results The host materials were slightly yellow powders. The diffuse reflectance spectra were featureless at wavelengths longer than the optical absorption edge. The absorption edges were smooth curves at 440 nm and 431 nm for SrLa& and BaLa,S,, respectively. The diffuse reflectances were converted to a Kubelka-Munk function which in turn was fitted to a McLean function [ 71
The derived band gaps were 2.9 1 eV for SrLa& and 2.35 eV for BaLa,S, with n= l/3. According to the assumptions of the McLean analysis, ?r= l/3 implies an indirect forbidden band gap transition. Both SrLa2S4:0.01Mn2+ and BaLa2S~:0.01MnZ+ yielded crimson-red luminescence under 365 nm excitation. Because the luminescence from strontium lanthanum sulfide was substantially more intense, detailed emission and excitation spectra as a function of temperature were measured for this compound. Emission and excitation spectra were measured as a function of temperature (fig. 1). At room temperature the broad-band emission was centered at 675 nm. The excitation band was asymmetric, with the maximum at 375 nm and a definite shoulder at 390 nm. As the temperature was reduced to 200 K, the emission band narrowed and the maximum shifted to 645 nm. From 200 to 94 K the emission maximum and band shaped remained essentially the same. With decreasing temperature the maximum in the excitation band gradually shifted to 390 nm until 200 K where the trend reversed. As the temperature was lowered still further, the location of the maximum 118
L
I
700
1
I
600
III
/ ”
I
460
‘94K
xx,
WAVELENGTH.NM
Fig. 1. Excitation
and emission
spectra for SrLa,S,:0.01Mn2+
measuredat the temperaturesindicated.
quickly shifted back to 375 nm where it remained to the lowest temperature reached, 94 K (fig. 2). The intensity of the excitation band remained approximately constant from 95 K to 200 K. Above this temperature, both band width and band intensity increased monotonically. Interpretation of the spectra poses certain difficulties. Excitation is apparently into the conduction band given that the 375 nm (3.31 eV) excitation band is at higher energy than the optical absorption edge measured by diffuse reflectance spectroscopy, 2.91 eV. The emission of Mn2+ is usually from the 4Tl,(G) state to the 6A,,(S) ground state. When Mn2+ is in 4-fold coordination as it is in ZnS: Mn2 +, the excitation spectrum usually reveals the crystal field levels of the Mn2+ ion, No evidence for Mni+
Volume 185, number I,2
CHEMlCAL PHYSICS LETTERS
crystal field states was observed in either excitation or absorption spectra of the SrLa2S.,:Mn2+ phosphor. A detailed interpretation of the Mn2+ emission on a crystal field model is not possible. Using ZnS: Mn’+ as a reference [ 81, Dq is 505 cm- ‘. Assuming that the Racah parameters B and C are similar in both sulfide phosphors, and taking advantage of the fact that the energy level structure for the dS configuration is the same in 4-fold and g-fold coordination, a Tanabe-Sugano diagram was constructed. Comparing the absorption level of ZnS:Mn’+ predicted from Dq with the observed emission level indicates a Stokes shift of about 2700 cm-’ for the offset of the 4TI emitting level. Projecting the observed room temperature emission band of SrLa&: Mn’+ onto the Stokes-shifted 4T, level gives an estimate of Dq for the SrLa$, phosphor of 800 cm-‘. Crystal field theory predicts that Dq for ions in &coordinated sites should be twice Dq for the same ion in 4-coordinated sites, but the increase is partially offset by the weakening of the crystal field due to the larger metal-sulfur distances in SrLa,S,. The most interesting feature is the change in luminescence spectra with temperature, shown in fig. 1, and plotted in fig. 2. The emission band peaks at
I I October 1991
645 nm, 1.92 eV from 94 K to about 190 K where the energy decreases continuously over the temperature range from 190 to 2 18 K and then abruptly flattens at 675 nm, 1.84 eV, and remains at this energy up to room temperature. Emission at low temperature should be from the lowest energy state, and thermal population of nearby higher levels would cause a shift of the emission band to higher, not lower, energy. The shape of the emission/temperature curve is characteristic of a second order phase transition. It is known [ 9,101 that the isostructural La3S4, Pr,S,, and Eu& undergo ‘a cubic to tetragonal phase transition at 90, 40, and 168 K respectively. It appears that the ternary Th,P,-structure SrLa$, also undergoes a displacive transition at 218 K.
References
[I] 0. Schevciw and W.B. White; Mat. Res. Bull. 18 (1983) 1059. [2] L. Suchow and N.R. Stemple. J. Electrochem. Sot.
11I
(1964) 191. [ 31 P.C. Donahue and J.E. Hanlon, J. Electrochem. Sot. 121 (1974) 137. [4] W.M. Yim, AK Fan and E.T. Stofito, J. Electrochem. Sot. 120(1973)441. [5] M. Matsumura, M.S. Thesis, The Pennsylvania State University, University Park, PA, USA ( 198 1). [6] P.L. Provenzano, Ph.D. Thesis, The Pennsylvania University, University Park, PA, USA (1976).
E- 1 6
EXCITATION
= 400 t 3
[7]T.P.
MAXIMA .
.
375 :
)_-------
‘““3 80
IO0
I20
140
160 180 200 220 TEMPERATURE,KELVIN
240
260
280
300
Fig. 2. Temperature dependence of the emission and excitation maxima.
McLean, in: Progress in semiconductors,
State
ed. A.F.
Gibson (Oliver and Boyd, London, 1969 ). [S] A. ,4nastassiadou, E. Liarokapis, S. Stoyanov, E. Anastassakis and W. Giriat, Solid State Commun. 67 ( 1988) 633. [9] P.D. Dernier, E. Bucher and L.D. Longinotti, J. Solid State Chem. 15 (1975) 203. [lo] 0. Berkooz, M. Malamud and S. Shtrikman, Solid State Commun. 6 (1968) 185.
119
CHEMICAL PHYSICS LETTERS
11 October I99 I
Luminescence of Mn-activated SrLa&, * Paul L. Provenzano
’ and William
B. White
Marerials Research Laborafory, The Pennsylvania State Umversity, UniversityPark, PA 16802, US.4 Received 30 May 1991
The Th,P,structure ternary sulfide compound SrLalS,:0.01Mn2+ is shown to be a photophosphor with a deep crimson emission peaking at 675 nm ( 1.84 eV) at 300 K. Measurement of the temperature dependence of emission and excitation spectra reveals a systematic shift in band position at 200 K which is interpreted as a second-order transition in the host structure.
1. Introduction Chalcogenides with large band gaps such as those with the Th3P4 structure are of interest as phosphor hosts. Their luminescence behavior when activated with rare earth or transition metal ions should be intermediate between the higher band gap insulator phosphors such as oxides, phosphates and silicates, and the lower band gap semiconductor phosphors such as GaP and GaAs. Optical band gaps for many ternary sulfide compounds are in the range of 2.3 to 3.0eV [l]. Luminescence investigations of ternary sulfides are rather sparse. Compounds with the thiospinel structure [ 21 and selenides with the PbGa$e, structure (pseudo-orthorhombic) [ 31 have been investigated. Yim et al. [4] prepared a series of Th3P4 compounds, CaLn,S, (Ln=La through Dy ). They reported that CaCe&, exhibited a weak, green cathodoluminescence but that the other compounds were not luminescent. A report from this laboratory gives a reconnaissance of rare earth luminescence in several of the ternary sulfide host structures [ 5 1. The present paper reports the luminescence of Mn’+-activated alkaline earth, rare earth sulfides with the Th3P, structure and an unusual temperature dependence of the luminescence. The Th,P*
* Work supported by the National Science Foundation under Grant No. DMR-74-00340.
’ Present address: UnitedTechnologies Research Center, Silver Lane, East Hartford, CT 06108, USA. 0009-2614/91/$
structure is cubic, space group 143d, with cations in 8-fold coordination. It is the structure of the ternary compounds MLn,!$ where M=Ca, Sr, Ba, Pb, Ln=larger rare earth ions.
2. Experimental The host materials were prepared by reacting SrC03 or BaC03 with La20,, mixed as powders in a silica boat, at temperatures of 900-l 100°C in an atmosphere of flowing HIS for times of 1 to 5 days. Phase identification and phase purity were checked by X-ray powder diffraction. The activator was obtained as the oxide and converted into sulfide by firing in flowing H,S. The manganese sulfide was mixed with the sulfide host in the desired proportion. These mixtures were then refired in H,S at 700°C for times of approximately 12 h. Luminescence measurements were made with a laboratory-built spectrometer. The emission monochromator was a Jarrel-Ash I m instrument while the excitation monochromator was a MacPherson 0.3 m unit. Both a mercury discharge lamp and a xenon arc were used as excitation sources. The Hg source was used with filters (Corning type 7-60 for 365 nm and Corning 7-54 in conjunction with a NiSO, SOlution for 253.7 nm). The xenon source was dispersed through the excitation monochromater. The detector was a cooled RCA 31034 GaAs-cathode photomultiplier. Full details are given in the thesis
03.50 0 1991 Elsevier Science Publishers B.V. All rights reserved.
117
Volume
185, number
1,2
CHEMICAL
PHYSICS
11October
LETTERS
I99
1
from which this paper is drawn [ 6 1. All spectra were measured on powders compacted into a sample holder. Low-temperature spectra were obtained with the aid of an Air Products Cryo-Tip refrigerator down to 94 K. Optical absorption edge spectra were measured as packed powders in the diffuse reflectance attachment of a Beckman DK-2A spectrophotometer. Kodak BaSO, optical paint was used as a reference material and as a coating for the integrating sphere.
3. Results The host materials were slightly yellow powders. The diffuse reflectance spectra were featureless at wavelengths longer than the optical absorption edge. The absorption edges were smooth curves at 440 nm and 431 nm for SrLa& and BaLa,S,, respectively. The diffuse reflectances were converted to a Kubelka-Munk function which in turn was fitted to a McLean function [ 71
The derived band gaps were 2.9 1 eV for SrLa& and 2.35 eV for BaLa,S, with n= l/3. According to the assumptions of the McLean analysis, ?r= l/3 implies an indirect forbidden band gap transition. Both SrLa2S4:0.01Mn2+ and BaLa2S~:0.01MnZ+ yielded crimson-red luminescence under 365 nm excitation. Because the luminescence from strontium lanthanum sulfide was substantially more intense, detailed emission and excitation spectra as a function of temperature were measured for this compound. Emission and excitation spectra were measured as a function of temperature (fig. 1). At room temperature the broad-band emission was centered at 675 nm. The excitation band was asymmetric, with the maximum at 375 nm and a definite shoulder at 390 nm. As the temperature was reduced to 200 K, the emission band narrowed and the maximum shifted to 645 nm. From 200 to 94 K the emission maximum and band shaped remained essentially the same. With decreasing temperature the maximum in the excitation band gradually shifted to 390 nm until 200 K where the trend reversed. As the temperature was lowered still further, the location of the maximum 118
L
I
700
1
I
600
III
/ ”
I
460
‘94K
xx,
WAVELENGTH.NM
Fig. 1. Excitation
and emission
spectra for SrLa,S,:0.01Mn2+
measuredat the temperaturesindicated.
quickly shifted back to 375 nm where it remained to the lowest temperature reached, 94 K (fig. 2). The intensity of the excitation band remained approximately constant from 95 K to 200 K. Above this temperature, both band width and band intensity increased monotonically. Interpretation of the spectra poses certain difficulties. Excitation is apparently into the conduction band given that the 375 nm (3.31 eV) excitation band is at higher energy than the optical absorption edge measured by diffuse reflectance spectroscopy, 2.91 eV. The emission of Mn2+ is usually from the 4Tl,(G) state to the 6A,,(S) ground state. When Mn2+ is in 4-fold coordination as it is in ZnS: Mn2 +, the excitation spectrum usually reveals the crystal field levels of the Mn2+ ion, No evidence for Mni+
Volume 185, number I,2
CHEMlCAL PHYSICS LETTERS
crystal field states was observed in either excitation or absorption spectra of the SrLa2S.,:Mn2+ phosphor. A detailed interpretation of the Mn2+ emission on a crystal field model is not possible. Using ZnS: Mn’+ as a reference [ 81, Dq is 505 cm- ‘. Assuming that the Racah parameters B and C are similar in both sulfide phosphors, and taking advantage of the fact that the energy level structure for the dS configuration is the same in 4-fold and g-fold coordination, a Tanabe-Sugano diagram was constructed. Comparing the absorption level of ZnS:Mn’+ predicted from Dq with the observed emission level indicates a Stokes shift of about 2700 cm-’ for the offset of the 4TI emitting level. Projecting the observed room temperature emission band of SrLa&: Mn’+ onto the Stokes-shifted 4T, level gives an estimate of Dq for the SrLa$, phosphor of 800 cm-‘. Crystal field theory predicts that Dq for ions in &coordinated sites should be twice Dq for the same ion in 4-coordinated sites, but the increase is partially offset by the weakening of the crystal field due to the larger metal-sulfur distances in SrLa,S,. The most interesting feature is the change in luminescence spectra with temperature, shown in fig. 1, and plotted in fig. 2. The emission band peaks at
I I October 1991
645 nm, 1.92 eV from 94 K to about 190 K where the energy decreases continuously over the temperature range from 190 to 2 18 K and then abruptly flattens at 675 nm, 1.84 eV, and remains at this energy up to room temperature. Emission at low temperature should be from the lowest energy state, and thermal population of nearby higher levels would cause a shift of the emission band to higher, not lower, energy. The shape of the emission/temperature curve is characteristic of a second order phase transition. It is known [ 9,101 that the isostructural La3S4, Pr,S,, and Eu& undergo ‘a cubic to tetragonal phase transition at 90, 40, and 168 K respectively. It appears that the ternary Th,P,-structure SrLa$, also undergoes a displacive transition at 218 K.
References
[I] 0. Schevciw and W.B. White; Mat. Res. Bull. 18 (1983) 1059. [2] L. Suchow and N.R. Stemple. J. Electrochem. Sot.
11I
(1964) 191. [ 31 P.C. Donahue and J.E. Hanlon, J. Electrochem. Sot. 121 (1974) 137. [4] W.M. Yim, AK Fan and E.T. Stofito, J. Electrochem. Sot. 120(1973)441. [5] M. Matsumura, M.S. Thesis, The Pennsylvania State University, University Park, PA, USA ( 198 1). [6] P.L. Provenzano, Ph.D. Thesis, The Pennsylvania University, University Park, PA, USA (1976).
E- 1 6
EXCITATION
= 400 t 3
[7]T.P.
MAXIMA .
.
375 :
)_-------
‘““3 80
IO0
I20
140
160 180 200 220 TEMPERATURE,KELVIN
240
260
280
300
Fig. 2. Temperature dependence of the emission and excitation maxima.
McLean, in: Progress in semiconductors,
State
ed. A.F.
Gibson (Oliver and Boyd, London, 1969 ). [S] A. ,4nastassiadou, E. Liarokapis, S. Stoyanov, E. Anastassakis and W. Giriat, Solid State Commun. 67 ( 1988) 633. [9] P.D. Dernier, E. Bucher and L.D. Longinotti, J. Solid State Chem. 15 (1975) 203. [lo] 0. Berkooz, M. Malamud and S. Shtrikman, Solid State Commun. 6 (1968) 185.
119