On the Host Lattice Dependence of the 4f n-1 5d 4f n Emission of Pr 3+ and Nd 3+ T. Jüstel, W. Mayr, P. J. Schmidt, D.U. Wiechert e-mail to: thomas.juestel@philips.com 1 st Int. Conf. Sci. Tech. Emissive Displays and Lighting San Diego, CA, November 2001
Outline Introduction - Discharge lamps Down conversion phosphors Luminescence spectra of Ce 3+, Nd 3+, and Pr 3+ Host lattice dependence of the Pr 3+ luminescence Comparison between Pr 3+ and Nd 3+ Conclusions
Discharge Lamps Mercury Sodium Rare-gas Sulphur Low pressure p < 10 mbar High pressure p > 1 bar Low pressure Low pressure High pressure Hg / Ar Hg / Ne 185 + 254 nm (Compact) Fluorescent lamps Phosphors Hg / Ar Metal halide lamps Single line emitter NaX / TlX / InX, X=I,Br Multi-line emitter NaX/ TlX/ LnX 3 (Ln = Dy, Ho, Tm, Sc) SnX 2 Na / Ar / Ne 589 nm High pressure Na / Hg / Xe Ne 74 nm Medium pressure (DBD, PDP) Xe / Ne 147 + 172 nm Phosphors S 2
Evolution of Discharge Lamp Efficiency
Efficiency vs. Colour Rendition of Light Sources a Low Pressure Sodium b High Pressure Sodium c Fluorescent d Compact Fluorescent e High Pressure Mercury f Halogen g Incandescent
Light Generation in Hg low-pressure Discharges Mechanism 254 nm 1,0 cathode e - 0,8 e - + Hg Hg + + 2 e - 0,6 Hg + + e - Hg*( 3 P 1, 1 P 1 ) Hg*( 3 P 1, 1 0,4 P 1 ) Hg + hν UV 0,2 185 nm hν UV + phosphor hν visible 365 nm 0,0 100 200 300 400 Emission intensity [a.u.] Wavelength [nm] Quantum deficit: QD = [λ discharge /λ phosphor ] = 0.46 ε = ε discharge * QD*QE ε discharge = 70% ε = 30% (100 lm/w el ) Highly efficient, but Hg is a toxic metal!
Light Generation in Xe Discharges Xe + e - Xe( 3 P 1 ) + e - Xe** Xe( 3 P 2 ) + e - Xe** Xe( 3 P 1 ) + hν (828 nm) Xe( 3 P 2 ) + hν (823 nm) Xe( 3 P 1 ) Xe + hν (147 nm) Xe( 3 P 1 ) + Xe + M Xe 2 * + M Xe 2 * 2 Xe + hν (150/172 nm) Discharge efficiency ~ 65% (elaborated driving scheme) 147 Wavelength / nm 172 Emission spectrum High Pressure 2 nd Continuum 1 st Continuum Resonance Line Low Pressure Energy 3Σ u + Xe* 2 - Excimer Energy Level Diagram 1Σ u + 1Σ g + 2 nd 1 u 3P 1 + 1 S 0 1 st Continuum 3P 2 + 1 S 0 Resonance Line 1S 0 + 1 S 0 2 3 4 5 6 7 Internuclear Distance (Å) B A X
Xe 2* -Excimer Discharge Lamps 185 + 254 nm Low Pressure 147 172 2 nd Continuum 1 st Continuum Resonance Line Wavelength [nm] 150 + 172 nm 450 nm 545 nm 610 nm Lamp tube Features of Xe 2 *-excimer lamps Hg free Instant light Temperature independent Phosphors for Xe-excimer lamps High band gap host lattice 4f-5d absorption of free ion Nd 3+ 72000 cm -1 (140 nm) Dimmable and fast switching cycles Pr 3+ 62000 cm -1 (160 nm) Large quantum deficit: QD = 0.31 Ce 3+ 50000 cm -1 (200 nm)
Photon-Cascade Emission (Pr 3+ ) YF 3 :Pr, NaYF 4 :Pr (Sommerdijk 1974) YF 3 :Tm (Pappalardo 1976) In oxidic lattice possible if E(5d states) > E( 1 S 0 ) weak crystal field Pr 3+ on lattice sites with high CN (> 8) LaMgB 5 O 10 :Pr, LaB 3 O 6 :Pr (Srivastava 1996) Down-conversion efficiency < 140% Absorption via 5d states of Pr 3+ (e.g. at 185 nm)
Down Conversion by interacting Ions - LiGdF 4 :Eu Energy level scheme Gd 202 nm Gd** Gd** + Eu Eu* CR Eu* + Gd* Eu + hν ET Gd* + Eu Gd + Eu* Eu* Eu + hν Down-conversion efficiency DCE = 195% (Meijerink 1996) Similar in LiGdF 4 :Er,Tb
Efficiency of LiGdF 4 :Eu Excitation and reflection spectrum Emission intensity [a.u.] 1,0 0,8 0,6 0,4 0,2 202 nm 273 nm Exc. wave. [nm] QE [%] 160 0.23 202 0.32* 273 0.64 *including down-conversion effect 0,0 150 200 250 300 350 Wavelength [nm] (J. Luminescence 92, 2001, 245) Low efficiency at 202 nm due to strong host lattice absorption Inefficient energy transfer from host lattice to 6 G J states of Gd 3+
Efficiency of Down-conversion Phosphors Maximum phosphor quantum yield 100 QEact100 QEact90 QEact80 80 QEact70 QEact60 QEact50 60 QEact40 QEact30 40 QEact20 QEact10 20 Max. external quantum efficiency 0 0,01 0,1 1 10 100 Absorption: A = A Gd3+ + A host latttice External quantum efficiency QE ext QE ext = QE Eu3+ /(A Gd3+ + A host lattice ) = QE Eu3+ /(1+A host lattice /A Gd3+ ) Weak absorption of Gd 3+ results in low external quantum efficiency Sensitisation is required to improve absorption + external QE Sensitiser must have high-lying states above 202 nm A host lattice /A Activator
Luminescence Spectra of Ce 3+, Nd 3+ and Pr 3+ 4f n -4f n-1 5d 1-4f n luminescence First spin-allowed 4f n -4f n-1 5d 1 transition of Ln 3+ : E(Nd 3+ ) = E(Pr 3+ ) + 10000 cm -1 = E(Ce 3+ ) + 22000 cm -1 (Dorenbos 2000) Emission band position indicates position of lowest 4f5d level: YPO 4 :Nd YPO 4 :Pr YPO 4 :Ce 190 nm 235 nm 335 nm Y 3 Al 5 O 12 :Nd Y 3 Al 5 O 12 :Pr Y 3 Al 5 O 12 :Ce visible and IR lines UV bands + visible lines 540 nm
Spectra of Pr 3+ Phosphors YF 3 :Pr YPO 4 :Pr Y 2 O 3 :Pr 1,0 1,0 1,0 0,8 0,8 0,8 0,6 0,6 0,6 0,4 0,4 0,4 0,2 0,2 0,2 0,0 100 200 300 400 500 600 700 Wavelength [nm] 4f 2-4f 2 line emission 0,0 100 200 300 400 500 600 700 Wavelength [nm] 4f 1 5d 1-4f 2 band emission 0,0 100 200 300 400 500 600 700 800 Wavelength [nm] 4f 2-4f 2 line emission Although Pr 3+ is in all cases located on Y-sites, the luminescence spectra are much different from each other
Term Scheme of the free Pr 3+ Ion The energetic position of the lowest level of the 4f 1 5d 1 configuration governs the emission spectrum: E(4f 1 5d 1 ) > E( 1 S 0 ) line emission photon cascade emission E(4f 1 5d 1 ) < E( 1 S 0 ) band emission 4f 1 5d 1-4f 2 ( 3 H J, 3 F J ) E(4f 1 5d 1 ) << E( 1 S 0 ) line emission ( 3 P 0-3 H 4, 1 D 2-3 H 4,...)
Energy Distance of 4f and 5d States in Ln 3+ Free ion Nephelauxetic effect Crystal field 5d Co-valency 10 Dq 4f - Covalency (Ln 3+ - ligand bonds) reduces 4f-5d energy gap - Crystal-field splitting of the five 5d levels further reduces the 4f- 5d energy gap
Investigated Host Lattices composition mineral name crystal system CN YF 3 - orthorhombic 8 YPO 4 xenotime tetragonal 8 YBO 3 vaterite trigonal 8 Y 2 SiO 5 - monoclinic 6* Y 2 Si 2 O 7 yttrialite monoclinic 6 Y 3 Al 5 O 12 garnet cubic 8 Y 2 O 3 bixbyite cubic 6* all crystal data are from the ICSD database (* two Y-sites)
XRDs of Phosphors 1000 YPO 4 :Pr Y 3 Al 5 O 12 :Pr 900 YPO 4 :Pr 3+ (1%) (HB-7) 4000 Y 3 Al 5 O 12 :Pr 3+ UV-C14/99 800 Counts [s -1 ] 700 600 500 400 300 200 Counts [s -1 ] 3000 2000 1000 100 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 2 Theta 2 Theta 10 20 30 40 50 60 70 80 90 YPO 4 (PDF 09-0377) 10 20 30 40 50 60 70 80 90 Y 3 Al 5 O 12 (PDF 33-0040)) XRD Analysis: All investigated phosphors are of single phase
Pr 3+ Luminescence in YF 3 1,0 4f 1 5d 1 1 S 0-1 G 4 1 S 0-1 I 6 1 S 0-3 F 4 0,8 0,6 0,4 Host lattice 3 P 0-3 H 4 3 P 0-3 H 6, 3 F 2 0,2 distorted square antiprismatic E Y-F distances 4x 2.28 Å 2x 2.30 Å 2x 2.31 Å 0,0 100 200 300 400 500 600 700 Wavelength [nm] small crystal-field splitting low covalency (fluoride) E(4f 1 5d 1 ) > E( 1 S 0 ) Photon cascade emission (4f 2-4f 2 transitions)
3 H 5 Pr 3+ Emission in YPO 4 1,0 4f 1 5d 1 3 H 4 0,8 0,6 Host lattice 3 H 6 0,4 3 F J 0,2 E dodecahedral d x2-y2 d z2 d xy d xz d yz Y-O distances 4x 2.236 Å 4x 2.241 Å 0,0 100 200 300 400 500 Wavelength [nm] large crystal-field splitting low covalency (phosphate) E(4f 1 5d 1 ) < E( 1 S 0 ) UV-C band emission (235 nm) (4f 1 5d 1-4f 2 transitions)
Pr 3+ Emission in YBO 3 1,0 3 H 4 0,8 0,6 Host lattice 4f 1 5d 1 3 H 5 0,4 3 H 6 3 F J 0,2 E 0,0 distorted cubic 100 200 300 400 500 Y-O distances 6x 2.31 Å 2x 2.32 Å Wavelength [nm] medium crystal-field splitting medium covalency (borate) E(4f 1 5d 1 ) < E( 1 S 0 ) UV-C band emission (265 nm) (4f 1 5d 1-4f 2 transitions)
Pr 3+ Emission in Y 3 Al 5 O 12 1,0 Host lattice 4f 1 5d 1 3 P 0-3 H 4 E dodecahedral Y-O distances 4x 2.30 Å 4x 2.44 Å Emission intensity [a.u.] 0,8 0,6 0,4 0,2 4f 1 5d - 3 H J 0,0 100 200 300 400 500 600 700 Wavelength [nm] large crystal-field splitting high covalency (aluminate) E(4f 1 5d 1 ) << E( 1 S 0 ) UV band emission (320 nm) (4f 1 5d 1-4f 2 transitions)
Spectra of Pr 3+ UV Phosphors Excitation spectra Emission spectra LaPO 1,0 4 :Pr Y 3 Al 5 O 12 :Pr 1,0 LaPO 4 :Pr Y 3 Al 5 O 12 :Pr Emission intensity [a.u.] 0,8 0,6 0,4 0,2 Emission intensity [a.u.] 0,8 0,6 0,4 0,2 0,0 150 200 250 300 350 0,0 200 250 300 350 400 450 Wavelength [nm] Wavelength [nm] Energy of 4f 2-4f 1 5d 1 absorption edge LaPO 4 > YPO 4 > YBO 3 :Pr > Y 3 Al 5 O 12 4f 1 5d 1-4f 2 ( 3 H 4 ) emission band position LaPO 4 > YPO 4 > YBO 3 :Pr> Y 3 Al 5 O 12
Term Schemes of investigated Pr 3+ Phosphors Non-radiative relaxation into the 3 P J levels is observed if the lowest level of the 4f 1 5d 1 configuration is below 37000 cm -1
Impact of the Host Lattice - CF splitting Crystal field theory - ionic interaction with negative point charges Energy of d-orbital splitting depends on anion charge/anion radius (spectrochemical series) I - < Br - < Cl - < S 2- < F - < O 2- < N 3- < C 4- symmetry (co-ordination number and site symmetry) octahedral > cubic, dodecahedral, square antiprismatic > tetrahedral metal ligand distance (strong distance dependence) D = 35Ze/4R 5 R = cation-anion distance Z = valency of the anions e = electron charge
Impact of the Host Lattice - Covalency of the Ln 3+ -Oxygen Bonds Polarizibility (type) of anions sulphides > nitrides > oxides > fluorides Charge density on the surrounding (oxygen) anions: Basicity Type of network former: aluminates > silicates > borates > phosphates > sulphates AlO 5-4 SiO 4-4 BO 3-3 PO 3-4 SO 2-4 Degree of connectivity SiO 4-4 > Si 2 O 6-7 ~ Si 3 O 6-9
Example Covalent Character of ionic Bonds Type of network former YPO 4 Y 3+ O O-P-O O 3- low basicity Y 3 Al 5 O 12 Y 3+ tetrahedral AlO 5-4 + octahedral AlO 9-6 P 5+ withdraws more charge from the oxygen anions than Al 3+ tetrahedral PO 4 3- O 5- O-Al-O O O O O-Al-O O O 9- high basicity
Electron Population of the Oxide Anions YPO 4 Y 3 Al 5 O 12 4 x O(1) 7.248 4 x O(2) 7.193 low electron population gross populations from EHTB band structure calculations 4 x O(1) 7.528 4 x O(2) 7.504 high electron population
Comparison between Pr 3+ and Nd 3+ Emission spectra of Pr 3+ Emission spectra of Nd 3+ Emission intensity [a.u.] 1,0 YPO 4 :Pr YBO 3 :Pr 3 H 4 0,8 0,6 0,4 0,2 3 H 5 3 H 6 3 F 2 Emission intensity [a.u.] 1,0 0,8 0,6 0,4 0,2 4f 2 5d -> 4 I J -> 4 F J -> 4 G J 2 G(2) 9/2-2S+1 L J YPO 4 :Nd YBO 3 :Nd 0,0 200 250 300 350 400 Wavelength [nm] 0,0 150 200 250 300 350 400 Wavelength [nm] YPO 4 YBO 3 Pr 3+ ( 3 H 4 ) 235 nm (42600 cm -1 ) 265 nm (37600 cm -1 ) Nd 3+ ( 4 I J ) 190 nm (52600 cm -1 ) 210 nm (47600 cm -1 ) 5d-4f 5d-4f and 4f-4f (Nd 3+ )
Emission intensity [a.u.] 0,8 0,6 0,4 0,2 Pr 3+ and Nd 3+ in Y 3 Al 5 O 12 (YAG) Excitation and emission spectra YAG:Pr YAG:Nd Host lattice 3 P 1,0 4f 1 0-3 H 4 5d 4f 1 5d - 3 H J Emission intensity [a.u.] 1,0 0,8 0,6 0,4 0,2 Host lattice 4f 2 5d 0,0 100 200 300 400 500 600 700 Wavelength [nm] 0,0 100 200 300 400 500 600 700 Wavelength [nm] YAG:Pr YAG:Nd 5d-4f at 320 nm (31200 cm -1 ) and 4f-4f emission lines 4f-4f emission lines (emission from 2 F(2) J states)
Term Schemes of investigated Nd 3+ Phosphors Energy [10 3 cm -1 ] 60 50 40 30 20 10 0 YPO 4 :Nd YBO 3 :Nd Y 2 SiO 5 :Nd d-states 2 G(2) J 2 F(2) J 4 I 9/2 d-states 2 G(2) J d-states 2 F(2) J 4 I 9/2 2 G(2) J 2 F(2) J 4 I 9/2 Non-radiative relaxation into the 2 G(2) J levels is observed if the lowest level of the 4f 2 5d 1 configuration is below 52000 cm -1 above the ground state Fluorides, sulphates, and phosphates show 4f 2 5d 1 4 I J emission above 200 nm Nd 3+ can be used as sensitizer for the 202 nm Gd 3+ line
Sensitisation of GdPO 4 by Nd 3+ Excitation and emission spectrum of GdPO 4 :Nd 3+ 4f 3-4f 2 5d 1 1,0 6 P 7/2 -> 8 S 7/2 Emission intensity [a.u.] 0,8 0,6 0,4 0,2 -> 6 G J -> 6 I J 0,0 150 200 250 300 350 400 450 500 550 600 650 700 750 Wavelength [nm] Absorption maximum is at 170 nm Efficient energy transfer from Nd 3+ to Gd 3+ in GdPO 4 Nd 3+ sensitisation of Gd 3+ also works in other host lattices, e.g. LiGdF 4
Conclusions The position of the lowest lying (emitting) level of the 4f n-1 5d 1 configuration is governed by covalency of Ln 3+ -oxygen bonds: sulphates > phosphates > borates > silicates > aluminates > oxides Type of obtained emission Pr 3+ Nd 3+ fluorides, sulphates photon cascade 5d-4f phosphates 5d-4f 5d-4f borates, silicates 5d-4f 4d-4f and 4f-4f aluminates 5d-4f and 4f-4f 4f-4f oxides 4f-4f 4f-4f Nd 3+ might be a useful sensitizer for Gd-Eu and Gd-Er-Tb based down-conversion phosphors (CR remains a problem)
Acknowledgement Helmut Blankefort Synthesis Maya Doytcheva Synthesis Hartmut Lade VUV Spectroscopy Dieter Wädow VUV Spectroscopy