On the Host Lattice Dependence of the 4f n-1 5d 4f n Emission of Pr 3+ and Nd 3+

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1 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 to: thomas.juestel@philips.com 1 st Int. Conf. Sci. Tech. Emissive Displays and Lighting San Diego, CA, November 2001

2 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

3 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 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 nm Phosphors S 2

4 Evolution of Discharge Lamp Efficiency

5 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

6 Light Generation in Hg low-pressure Discharges Mechanism 254 nm 1,0 cathode e - 0,8 e - + Hg Hg 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, 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!

7 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 S 0 1 st Continuum 3P S 0 Resonance Line 1S S Internuclear Distance (Å) B A X

8 Xe 2* -Excimer Discharge Lamps nm Low Pressure nd Continuum 1 st Continuum Resonance Line Wavelength [nm] 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 cm -1 (140 nm) Dimmable and fast switching cycles Pr cm -1 (160 nm) Large quantum deficit: QD = 0.31 Ce cm -1 (200 nm)

9 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)

10 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

11 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 [%] * *including down-conversion effect 0, 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+

12 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, 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

13 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+ ) cm -1 = E(Ce 3+ ) 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

14 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, Wavelength [nm] 4f 2-4f 2 line emission 0, Wavelength [nm] 4f 1 5d 1-4f 2 band emission 0, 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

15 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,...)

16 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

17 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)

18 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/ Counts [s -1 ] Counts [s -1 ] Theta 2 Theta YPO 4 (PDF ) Y 3 Al 5 O 12 (PDF )) 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.

19 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, 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)

20 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 Å 4x Å 0, 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)

21 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 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)

22 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, 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)

23 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, , 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

24 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 cm -1

25 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

26 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

27 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 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

193 low electron population gross populations from EHTB

28 Electron Population of the Oxide Anions YPO 4 Y 3 Al 5 O 12 4 x O(1) x O(2) low electron population gross populations from EHTB band structure calculations 4 x O(1) x O(2) high electron population

29 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, Wavelength [nm] 0, 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+ )

30 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 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, Wavelength [nm] 0, 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)

31 Term Schemes of investigated Nd 3+ Phosphors Energy [10 3 cm -1 ] 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 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

32 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, 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

33 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)

34 Acknowledgement Helmut Blankefort Synthesis Maya Doytcheva Synthesis Hartmut Lade VUV Spectroscopy Dieter Wädow VUV Spectroscopy

New blue and green emitting BAM Phosphors for Fluorescent Lamps and Plasma Displays

New blue and green emitting BAM Phosphors for Fluorescent Lamps and Plasma Displays T. Jüstel*, W. Busselt, P. Huppertz, W. Mayr, J. Meyer, P.J. Schmidt, D.U. Wiechert Philips Research Laboratories, D-52066