1 Thermally induced 4f – 5d transitions in LuAlO 3 :Ce (LuAP) A.J. Wojtowicz, S. Janus Institute...

A.J. Wojtowicz, SCINT 2007, June 2007

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Thermally induced 4f – 5d transitions in LuAlO3:Ce (LuAP)

A.J. Wojtowicz, S. JanusInstitute of Physics, N. Copernicus Univ. Toruń, POLAND

IEEE 9th International Conference on Inorganic Scintillators and their Applications,

Winston-Salem, NC USA

June 4 – 8, 2007


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INTRODUCTION

LuAP: 8.34 g/cm3, photofraction 0.3,

365 nm emission, 17 ns decay time,

LY over 2xBGO, yet problems

GOAL of this work

study, report and explain details of optical

transitions generating scintillation light


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SAMPLES

2 pixels 2x2x10 mm, grown in 2004 at Institute of

Electronic Materials Technology, Warsaw, Poland

by prof. Lukasiewicz et al

LuAlO3:0.07%at Ce, LuAlO3:0.15%at Ce

Emission and excitation spectra:

Superlumi station, I – beamline, Hasylab,

Hamburg, Germany (prof. Zimmerer)


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LuAlO3:Ce, luminescence spectrum


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Luminescence spectra, summary:

Two spin – orbit split bands (1600 cm-1)

lowest d → 2F5/2 357 nm

lowest d → 2F7/2 379 nm

Crystal field structure not resolved

more or less comparable intensities


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Corrected excitation spectra


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EXCITATION SPECTRA, summary:

Strong triple 2F → T2 band

two strong (306, 295 nm),

one weak transitions (276 nm)

34 000 ± 1800 cm-1

Weaker double 2F → E band

(226, 214 nm)

45 500 ± 1200 cm-1


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Stokes shift ~ 4700 cm-1

DOMINANT CUBIC FIELD (Oh);

10Dq ~ 11 500 cm-1

weak low symmetry field ~ 1500 cm-1

The intensity ratio of two d–bands

2F → T2 / 2F → E

12 K ~ 4.4

298 K ~ 3.8


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Both T2 and E d–bands strongly expand

toward longer wavelengths with increasing T

Intensities of 5d E subbands;

„230 nm” subband increases,

„215 nm” subband decreases with T,

„230 nm” subband peak shifts

toward lower λ


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E subbands positions vs T


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Corrected excitation spectra vs T


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THEORY - MODEL

D.J. Robbins, YAG J. Electrochem. Soc.

1979

Thermally activated transitions from the higher ground state

level

Different transition moments

BAND SHIFTS

p1 < p1’

p1 < p2

p2’ < p1

’


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5d E subbands intensities vs T

kTE

exp1

kTE

expp

p1

NpI,2

,1

,2215

kTE

exp1

kTE

expp

p1

NpI 2

1

2230


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„230 nm” intensity vs T, experiment and theory


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„215 nm” intensity vs T, experiment and theory


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ENERGY LEVEL DIAGRAM, 5d levels

Sequence of levels must agree with the experiment

Γ7, Γ8 doubled-valued representations of Oh


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ENERGY LEVEL DIAGRAM, 4f levels

Sequence of levels must agree with the experiment

Γ6, Γ7, Γ8 doubled-valued representations of Oh


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Transition moment matrix elements(line strenghts)

between

Γ6, Γ7, Γ8 states originating from 2F7/2 and Γ7, Γ8 states from 2F5/2 term of 4f configuration

and

Γ8, Γ7 states originating from T2 and E terms of 5d electron configuration

T. Hoshina, J. Phys. Soc. Jap., 1980


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5dT2 Γ8a 5dT2 Γ8b 5dT2 Γ7 5dE Γ8a 5dE Γ8b

4f 2F7/2 Γ6 8,5713 25,7142 0,00 25,7144 25,714

4f 2F7/2 Γ8b 12,2448 12,2449 24,4902 18,3674 18,3674

4f 2F7/2 Γ8a 12,2449 12,245 24,49 18,3674 18,3674

4f 2F7/2 Γ7 4,0816 4,0816 65,307 6,1224 6,1224

4f 2F5/2 Γ8b 6,8028 33,4696 2,1876 1,6326 41,6327

4f 2F5/2 Γ8a 33,469 6,8028 2,1768 41,6314 1,6326

4f 2F5/2 Γ7 34,014 34,0138 1,3605 8,1632 8,1632


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5d T2 5d E 5dE/5dT2

4f 2F5/2 Γ8b 42,46004 43,2653 1,02

4f 2F5/2 Γ8a 42,4486 43,264 1,02

4f 2F5/2 Γ7 69,38833 16,3264 0,24

The lowest 4f level (absorption), options:

0.24 is the only option where T2 takes larger share (4.17), consistent with the experiment

(3.8–4.4).


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The sequence of 5d T2 levels (absorption), relevant matrix elements:

Only 4f Γ7 ground state and 5d Γ8 below Γ7 are consistent with two strong and one weak

transitions generating 4f 2F5/2 → 5d T2 band

5dT2Γ8a 5dT2Γ8b 5dT2Γ7

4f2F5/2 Γ8b 6,8028 33,46962 2,18762

4f2F5/2 Γ8a 33,469 6,8028 2,1768

4f2F5/2 Γ7 34,014 34,0138 1,36053


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The lowest 5d level (emission), options:

Γ8a and Γ8b give reasonable values for the ratios of 2F7/2 and 2F5/2 bands – Γ8 below Γ7

4f2F7/2 4f2F5/2 TOTAL

5dT2 Γ8a 37,1426 74,2858 111,4284

5dT2 Γ8b 54,28574 74,28622 128,57196

5dT2 Γ7 114,2872 5,72495 120,01215

5dE Γ8a 68,5716 51,4272 119,9988

5dE Γ8b 68,5712 51,4285 119,9997


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Thermally induced transitions; line strengths between 5dEΓ8 states and two lowest 4f levels:

5dE Γ8a 5dE Γ8b

4f2F5/2 Γ8b 1,6326 41,6327

4f2F5/2 Γ8a 41,6314 1,6326

4f2F5/2 Γ7 8,1632 8,1632

p1/p2 = 0.2 (experiment 0.15±0.02)p’

1/p’2 = 5 (experiment 7±5)


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SUMMARY

Dominant cubic and low symmetry crystal field components

determine the ground and excited states of the Ce3+ ion in LuAP,

in order of increasing energy:

4f 2F5/2: 2F5/2Γ7, 2F5/2Γ8a, 2F5/2Γ8b

5d 2D: T2Γ8a, T2Γ8b, T2Γ7, EΓ8a, EΓ8b


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The transition moment matrix elements between these states are consistent with the

observed emission and absorption intensities and their temperature dependences

In contrast to Ce–activated YAG and BaF2,

in LuAP, YAP and LuYAP sequence of ground and excited state energy levels may

enhance self–absorption of Ce emission;

consequences for scintillation of large crystals; radiation trapping and energy

migration

1 Thermally induced 4f – 5d transitions in LuAlO 3 :Ce (LuAP) A.J. Wojtowicz, S. Janus Institute...

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