PHYSICAL REVIEW B 95, 014303 (2017)

Thermal ionization and thermally activated crossover quenching processes
for 5d-4 f luminescence in Y3 Al5−x Ga x O12 : Pr3+
Jumpei Ueda,1,2,3,4,* Andries Meijerink,4 Pieter Dorenbos,3 Adrie J. J. Bos,3 and Setsuhisa Tanabe1
1Graduate School of Human and Environmental Studies, Kyoto University, Kyoto 606-8501, Japan
2
Graduate School of Global Environmental Studies, Kyoto University, Kyoto 606-8501, Japan
3
Luminescence Materials Research Group, Section FAME-RST, Faculty of Applied Sciences,
Delft University of Technology, 2629 JB Delft, Netherlands
4
Debye Institute, Utrecht University, 3508 TA Utrecht, Netherlands(Received 25 July 2016; revised manuscript received 24 November 2016; published 11 January 2017)
We investigated thermally activated ionization and thermally activated crossover as the two possibilities ofquenching of 5d luminescence in Pr3+ -doped Y3 Al5−x Gax O12 . Varying the Ga content x gives the control
over the relative energy level location of the 5d and 4f 2 : 3PJ states of Pr3+ and the host conduction band
(CB). Temperature-dependent luminescence lifetime measurements show that the 5d luminescence quenchingtemperature T50% increases up to x = 2 and decreases with further increasing Ga content. This peculiar behavior
is explained by a unique transition between the two quenching mechanisms which have an opposite dependenceof thermal quenching on Ga content. For low Ga content, thermally activated crossover from the 4f 5d state to
the 4f 2 (3PJ ) states is the operative quenching mechanism. With increasing Ga content, the activation energy forthermally activated crossover becomes larger, as derived from the configuration coordinate diagram, while fromthe vacuum referred binding energy diagram the activation energy of thermal ionization becomes smaller. Basedon these results, we demonstrated that the thermal quenching of Pr3+ : 5d1 -4f luminescence in Y3 Al5−x Gax O12
with x = 0, 1, 2 is a thermally activated crossover while for x = 3, 4, 5 it results from the thermal ionization.
DOI: 10.1103/PhysRevB.95.014303

I. INTRODUCTION3+

3+

The 5d-4f luminescence of lanthanide ions Ce , Pr ,Eu2+ , and Yb2+ has attracted a great deal of attention.
The luminescence is crucial in optical applications such as
white LEDs, fluorescent tubes, displays, afterglow materials,
anticounterfeiting, and scintillators because of their intense
broad absorption, high quantum efficiency, fast decay lifetime,
and tenability of luminescence colors. For the optical devices
based on the 5d-4f luminescence, especially the high-power
white LEDs, the thermal quenching of the luminescence becomes a problem. Up until about the 1990s, the luminescencequenching of the 5d-4f transition was mainly explained by thethermally activated crossover quenching using configuration
coordinate (CC) diagrams [1]. However, in the 1990s, Yen et al.
demonstrated the existence of thermally activated ionization(thermal ionization) quenching from the Ce3+ : 5d excitedlevel in some compounds by photoconductivity analysis [2].
Since then both mechanisms are often discussed as being
responsible for thermal quenching of luminescence in whiteLEDs phosphors [3–7]. However, the mechanism proposed is
not always supported by the experimental results. To design
new phosphors with 5d-4f luminescence and to improve thethermal quenching behavior, it is necessary to elucidate whichluminescence quenching process is dominant in the different
phosphors.
To understand the two quenching processes, a schematic
diagram combining the CC diagram and the vacuum referredbinding energy (VRBE) diagram for Y3 Al5 O12 (YAG) dopedwith Ce3+ and Pr3+ are shown in Fig. 1. Note that the x axis*

ueda.jumpei.5r@kyoto-u.ac.jp2469-9950/2017/95(1)/014303(8)of the parabolas in the CC diagram is the distance between
Ln3+ and the coordinating anions (ligands) with a minimum
energy of the parabolas for the equilibrium distance. However,the x axis is not shown in this figure. A band diagram
is a one-electron energy diagram and relaxation processes
involved in optical excitation and emission cannot be included.
This incompatibility of band diagrams and configurational
coordinate diagrams can be the cause of misunderstanding.
Thermally activated crossover, as shown by the pink arrows
in the CC diagram of Fig. 1, is the nonradiative relaxation
process from the excited 5d potential curve to the lower 4f potential curve through the crossing point. At the crossing point,
the energy of the 5d vibronic state coupling with a few phonons
matches that of a higher 4f vibronic state and the resonanttransition to the 4f state is followed by rapid nonradiative
relaxation to lower vibronic 4f states. The high 4f vibrational
levels involved have their amplitude concentrated almost
exclusively at the extremes (turning points) of the parabola.
Thus the thermally activated crossover is often depicted as athermally activated process with the energy difference between
the lowest vibrational level of the excited state and the crossing
point of the parabola as activation energy. This activation
barrier decreases for a larger parabola offset and a smaller
energy difference between the states. Thermal ionization is thethermally activated electron transfer process from the 5d state
to the conduction band (CB), as shown by the red arrows in
Fig. 1. Note that not all the ionized electrons recombine withthe photo-oxidized luminescent center nonradiatively. Someof the ionized electrons can transfer back to the 5d excitedstate immediately or after trapping to intrinsic defects anddetrapping processes (persistent luminescence and delayed
recombination luminescence) [8,9]. In any cases, thermally
activated photoionization leads to a reduced light output. The

014303-1©2017 American Physical SocietyUEDA, MEIJERINK, DORENBOS, BOS, AND TANABE

VRBE (eV)-6fThermalcrossover

ΔE

-4

1

5d
1 -4

Thermalionization

ΔE

-2 5d

5d
1 -C

B

CB

PHYSICAL REVIEW B 95, 014303 (2017)

Ce3+
Ln3+:4f-8

Pr3+-10 VB
1

2

Ln3+ 4f electrons
FIG. 1. Schematic diagram combining configuration coordinate(CC) and VRBE diagrams for YAG:Ce3+ and YAG:Pr3+ explaining
thermal quenching of luminescence.

quenching temperature is determined by the energy difference
between the emitting excited state and the conduction bandedge.
To provide a better understanding of the quenching processes of the 5d-4f luminescence, the family of garnet
Ln3 M5 O12 (Ln = Gd, Y, Lu; M = Sc, Al, Ga) doped with
Ce3+ and Pr3+ serve as important model systems because of thepossibility to tune optical properties as a function of covalency,
size of the cation site, bandgap, vibrational energies, etc., byvariation of the ions on the three different cation sites in
the garnet hosts [5,10–13]. In the past, we have focused on
the garnet materials doped with Ce3+ for the analysis of thequenching mechanisms and for the development of persistentphosphors [14–17]. By performing photoconductivity andthermoluminescence (TL) measurements as a function oftemperature and excitation wavelength, we demonstrated
that the Ce3+ : 5d-4f luminescence quenching in Y3 Al5 O12 ,Y3 Al2 Ga3 O12 , and Y3 Ga5 O12 (YGG) is caused by the ionization process and not by the crossover [14,18,19]. For Pr3+ doped YAG and Lu3 Al5 O12 (LuAG), precise configuration
coordinate diagrams were constructed from low-temperature
spectroscopy data and the 5d-4f thermal quenching behavior
was related to the energy gap between the lowest 5d1 state
and the next lower 4f state [10]. On the basis of the good
agreement between the configuration coordinate diagramsand the quenching temperature, the quenching process of
the 5d-4f luminescence in these materials is attributed
to the thermally activated crossover process [10]. Hence,
YAG:Pr3+ shows thermally activated crossover quenching
while YAG:Ce3+ is characterized by the thermal ionizationquenching. This difference can be understood from the energy
diagram of YAG:Pr3+ and YAG:Ce3+ in Fig. 1. The energy
gap between the lowest 5d1 state and the CB (E5d1 −CB ) in
YAG:Ce and YAG:Pr are 1.4 and 1.1 eV, respectively. On
the other hand, the activation energy of thermally activatedcrossover in YAG:Pr and YAG:Ce is regarded to be totallydifferent because the energy gaps between the 5d1 state andthe next lower 4f level (E5d1 −4f ) are 3.1 eV for Ce3+ and
1.7 eV for Pr3+ . Based on the much smaller gap for Pr3+ ,thermally activated crossover quenching is expected to start ata much lower temperature.
In this study, to investigate the difference in thequenching process between Ce3+ and Pr3+ , we focus on the optical properties and thermal quenching of
Y3 Al5−x Gax O12 (YAGG):Pr3+ . In YAGG:Pr3+ , it is predictedthat E5d1 −CB decreases dramatically with increasing Ga
content, as was the case for YAGG:Ce3+ in our previous
paper [18,20]. However, the E5d1 −4f is expected to notvary as strongly compared with E5d1 −CB since the 4f levelis not sensitive to the environment around a lanthanide ion
and the energy shift of the 5d level is much smaller than the
conduction-band energy shift in YAGG:Pr3+ . Consequently,
the quenching process of Pr3+ : 5d1 -4f luminescence in
the YAGG host is expected to change with increasing Ga
content from the thermally activated crossover to the thermal
ionization at the point that the activation energy of thermal
ionization becomes much smaller than that of thermally
activated crossover. On the basis of experimental resultson the luminescence quenching behavior of the YAGG:Pr3+
and precise CC diagrams constructed using low-temperature
spectroscopy, thermoluminescence excitation (TLE) analysis,
and VRBE diagrams, we demonstrated that the quenchingof Pr3+ : 5d1 -4f luminescence in the x = 0, 1, 2 samples
is thermally activated crossover and that in the x = 3, 4, 5
samples is the thermal ionization.
II. EXPERIMENTAL PROCEDURE

For the synthesis of polycrystalline ceramics of
Y3 Al5−x Gax O12 : Pr3+ (0.2% at the Y site), Y2 O3 : Pr3+
(0.2%) and Ga2 O3 fine powder prepared using the coprecipitation method and commercial Al2 O3 (4 N) fine powder wereused as starting materials. The stoichiometric amounts of Y2 O3(4 N) and Pr6 O11 (4 N) powder mixture for Y2 O3 : Pr3+ (0.2%)
were dissolved in a mixture of hydrochloric acid and nitricacid, and then Y3+ and Pr3+ cation solutions (∼0.4 M)
were prepared. After adding a 30-mL cation solution witha 150-mL ammonium hydrogen carbonate (AHC) solution(1 M), precipitate was formed. For Ga2 O3 , a 30-mL Ga(NO3 )solution (∼0.4 M) was added to a 150-mL AHC solution,and then precipitate was formed. Both precipitates were
centrifuged and washed three times with deionized water,
and then dried and heat-treated at 700 ◦ C for 4 h. Finally,Y2 O3 : Pr3+ (0.2%) and Ga2 O3 fine powder were obtained.
The stoichiometric amounts of powders (Y2 O3 : Pr3+ , Ga2 O3 ,Al2 O3 ) for Y3 Al5−x Gax O12 : Pr3+ (0.2% at the Y site) weremixed in an alumina mortar with ethanol. The obtained
slurry was dried, pulverized, and sintered two times at
1500 ◦ C for 6 h in N2 . The YAG:Ce3+ (0.1%) sample wasprepared by solid-state reaction at 1600 ◦ C using CeO2 ,Y2 O3 , and Al2 O3 chemicals. The crystal phase was identified
as a single phase of the garnet structure using an x-ray
powder diffraction measurement system (see Fig. S1 in the
Supplemental Material [21]). Photoluminescence (PL) and PL
excitation (PLE) spectra were measured by detecting emission
using a monochromator (Princeton Instruments, Acton SP
2300) and a photomultiplier tube (Hamamatsu, H10330A-75).
For excitation either a Xe lamp (Newport, 66921) and a
monochromator (Horiba Jobin Yvon, GEMINI180) or a D2

014303-2

THERMAL IONIZATION AND THERMALLY ACTIVATED . . .

PHYSICAL REVIEW B 95, 014303 (2017)

lamp (Hamamatsu Photonics, L1835) and a monochromator(Acton Research Corporation, VM 502) was used. To control
temperature, a closed-cycle He cryostat (SHI-APD, DE204SLFF) was used. For high-resolution VUV spectroscopy,
the beam line BL3B of the UVSOR facility at the Institute for
Molecular Science, Okazaki, was used. Emission spectra weredetected using a combined spectroscopy system of a grating
monochromator (Princeton Instruments, Acton SP 2300i) anda CCD detector (Roper Scientific, LN/CCD-100EB-GI). The
excitation spectra were measured with a photomultiplier tube
(Hamamatsu, R928) attached at another output port of thegrating monochromator.
For the temperature dependence of decay curve measurements, decay curves were recorded using a time-correlated
single-photon counting (TCSPC) method under excitation
with a 270-nm PLS-270 LED (λex = 270 nm, pulse width∼200 ps) and using the detection system of an Edinburgh
Instruments FLS920 fluorescence spectrometer with a cooled
single-photon counting photomultiplier (Hamamatsu R928P)
and an Oxford Instruments liquid helium flow cryostat.
For the thermoluminescence excitation spectra, the samples
were illuminated with monochromatic light obtained from a
Xe lamp (Newport, 66921) and a monochromator (Newport,
74004), and after the illumination phase the thermoluminescence glow curve was measured using a RISØ TL/OSL reader
model DA-15 and a controller model DA-20. The TL wasdetected with a photomultiplier tube (EMI, 9635QA) filteredby UG-11 (transmittance window from 250 nm to 400 nm).

PLE of YAG:Ce3+ , two PLE bands were observed at 460 nm
and 340 nm. These two bands are attributed to the 4f 0 5d 1states split by the crystal field. In the PL of YAG:Pr3+ , broad
PL bands are observed at around 300, 320, and 360 nm, which
are attributed to the transition of Pr3+ from the 4f 1 5d 1 state
to the 4f 2 (3H4 ,3H5 ,3H6 +3FJ ) states, respectively. In addition
to broad PL bands, sharp PL peaks attributed to the Pr3+ :3
P0 → 3HJ , 3FJ and 1D2 → 3H4 f -f transitions are observed.
In the PLE of the 4f 1 5d 1 → 4f 2 (3H4 ) luminescence at 303 nm
in YAG:Pr3+ , two broad PLE bands are observed at around
280 and 240 nm, which are attributed to the transitions of Pr3+
from 4f 2 (3H4 ) to the first and second sets of 4f 1 5d 1 excitation
bands, respectively. Pr3+ includes the energetic 140 states of
the 4f 1 5d 1 configuration. Therefore, it is difficult to assign
all of the levels in the PLE spectrum of YAG:Pr3+ . However,compared with the 4f 0 5d 1 PLE bands of Ce3+ -doped YAG,
which is split by the crystal field term as shown in Fig. 2,
YAG:Pr3+ shows similar PLE bands. The energy differences(12 ) between the first and second set of PLE bands related
5d in YAG:Ce3+ and YAG:Pr3+ are 7641 and 7058 cm−1 ,respectively, at ambient temperature. From these results, thefirst and second set of 4f 1 5d 1 PLE bands in YAG:Pr3+ would
be split mainly by the crystal field but not by the spin-orbit
coupling.
Figure 3 shows an overview of the PL and PLE spectraof Y3 Al5−x Gax O12 : Pr3+ (x = 0, 1, 2, 3, 4) at 10 K. All
samples show the typical 5d-4f transitions of Pr3+ in both PL
and PLE, similar to those shown in Fig. 2. When the Ga content
increases in the host composition, the Pr3+ : 4f 1 5d 1 (firstset)-4f 2 (3H4 ) PL and PLE bands are shifted to higher energy.
For the second set of 4f 1 5d 1 , the band is shifted to lower
energies with increasing Ga content. The decrease of 12

III. RESULTS AND DISCUSSION
A. Photoluminescence

To illustrate the luminescence behavior of Ce3+ and Pr3+ inthe garnets, Fig. 2 shows the PL and PLE spectra of Y3 Al5 O12
doped with Ce3+ and Pr3+ . In the PL of YAG:Ce3+ , broad PL
bands are observed at around 540 nm which are attributed to thetransitions of Ce3+ from 4f 0 5d 1 to 4f 1 (2F5/2 and 2F7/2 ). In theNormalized photon fulx (cps/nm)

Wavelength (nm)
200

300

Photon flux (cps/nm)

YAG:Ce3+

400 500 600 800

2

0

1

0

1

4f1( F5/2)→ 4f 5d
4f 5d(1st set) 4f1(2F5/2, 2F7/2)
Δ12PL (λex=350 nm)
PLE (λem=550 nm)

YAG:Pr3+
PL (λex=288 nm)

4f05d1(2nd set)

PLE (λem=317 nm)
4f15d1 4f15d1
4f2(3H4)(1st set) 3
H4
Δ12
3
H
4f15d1(2nd set)

1
3

D2

P0
3

3

Wavenumber (103cm-1)

H4

H4

4f2(3H4)→

4f15d1(1st set)→
4f15d1(1st set) 4f2(3H4)

4f15d1(2nd set)

10K

4f2(3H5)
4f2(3H6,3FJ)

x=0
x=1
x=2
x=3
x=4
x=5

5

3

H5+3FJ

3

H5

220

240

260

280

300

320

340

360

380

Wavelength (nm)
50

45

40

35

30

25

20

15

Wavenumber (103cm-1)
FIG. 2. PL and PLE in Y3 Al5 O12 doped with 0.2%Pr3+ and
0.1%Ce3+ .

FIG. 3. PL (for excitation to the 5d1 state) and PLE (for 5d1 -4f
luminescence) of Y3 Al5−x Gax O12 : 0.2%Pr3+ (x = 0, 1, 2, 3, 4) at
10 K. For the x = 5 samples, PLE was measured by monitoring 3PJ
luminescence.

014303-3

UEDA, MEIJERINK, DORENBOS, BOS, AND TANABEPHYSICAL REVIEW B 95, 014303 (2017)

TABLE I. The parameters obtained for Y3 Al5−x Gax O12 : Pr3+ from the PL/PLE spectra and temperature dependence of PL intensity.
5d1 -4f 2 (3H4 ) emission peak (E5d1 Em ), 4f 2 -5d1 and 5d2 excitation peaks (E5d1 Ex , E5d2 Ex ), Stokes shift (SS), energy gap between 5d1 andthe crossing point with the next lower 4f level (E5d1 −CP ), and from temperature-dependent luminescence decay measurements: quenchingtemperature (T50% ), radiative rate (ν ), attempt rate of nonradiative process (0 ), and activation energy of thermal quenching (ETQ ).

x

E5d1 Em(cm−1 )

E5d1 Ex(cm−1 )

E5d2 Ex(cm−1 )

SS
(cm−1 )

E5d1−CP(cm−1 )

T50%
(K)

ν
(107 s−1 )0(109 s−1 )

ETQ
(eV)

0
1
2
3
4
5

31646
32154
32600
32949
33195
33384

34542
34843
35335
35747
35939
36331

42017
41754
41580
41237
41109
40816

2896
2689
2735
2798
2744
2947

17388
18487
19839
20713
22097
20857

321
377
407
316
173
149

4.63
5.04
5.70
6.26
7.77
10.2

31.3
46.7
305
59.6
7.97
5.05

0.17
0.21
0.29
0.18
0.07
0.05

with increasing Ga content is analogous to the Ga content
dependence of 12 observed for Ce3+ in Y3 Al5−x Gax O12[18,20]. The data obtained for the 4f 1 5d 1 -4f 2 PL peak energy(E5d1 Em ) and the first and second sets of 4f 1 5d 1 PLE peakenergies (E5d1 Ex and E5d2 Ex ) are listed in columns 2, 3, and
4 in Table I. The Stokes shift energy (SS) was estimated bysubtracting E5d1 Em from E5d1 Ex and is shown in column 5 ofTable I.
B. Zero phonon line, phonon progression, and
Huang-Rhys parameter

To be able to construct a reliable configurational coordinate
diagram and estimate the Huang-Rhys coupling parameter,
high-resolution spectra recorded at low temperature are required for samples with low dopant concentrations to prevent
inhomogeneous broadening by dopant disorder, spectral shifts
because of reabsorption, and saturation effects [3]. Figure 4

Wavelength (nm)
310

300

290

280

270

Y3Al5O12:Pr3+

240

230

(Photon flux × λ2)/Energy3PLE(λem=318nm)
PL(λex=280nm)

32

33

34

35

36

37

41
3

42

43

44

-1

Wavenumber (10 cm )FIG. 4. High-resolution PL spectrum (excited by 280 nm) and
PLE spectrum (of 318 nm luminescence) in YAG:0.2%Pr3+ at 8 K.
Vertical black lines are phonon lines.

shows the high-resolution PL and PLE spectra at 8 K in
YAG:0.2%Pr3+ . The luminescence spectra of YAG:0.2%Pr3+
shows a zero phonon line (ZPL) at 33 257 cm−1 and vibronic progression in both PL and PLE spectra, while other
YAGG:Pr3+ samples with different Ga content do not. The
absence of fine structure can be explained by the fact that PL
and PLE bands of YAGG:Pr3+ are affected by inhomogeneous
broadening which originates from the local environment
around Pr3+ with statistically varying number of Al and Ga in
the nearest octahedral and tetrahedral sites. For the transitions
between 4f 2 (3H4 ) and 4f 1 5d 1 in the high-resolution PLE and
PL of YAG:Pr3+ , the typical mirror symmetry is observed.
Similar mirror symmetry for YAG:Pr3+ was reported by before[10]. The typical mirror symmetry is one of the evidencesof phonon sidebands. The positions of the ZPLs in the PL
and PLE spectra have a small difference of 28 cm−1 , possibly
because of a small redshift of ZPL in the PL spectrum byreabsorption [22]. Alternatively, the small offset may be resultof a small calibration error. In the PLE spectrum, the zerophonon line at 33 257 cm−1 and two vibronic lines at 199 and391 cm−1 higher energy are observed. The vibronic lines are
assigned to a one- and two-phonon replica indicating that thef -d transition strongly couples with approximately 196-cm−1
vibration modes. For the transitions between 4f 1 (2F5/2 ) and
5d1 in YAG:Ce, coupling to similar vibrational modes of
200 cm−1 was reported [3]. Because the vibrational energy
in the same host material does not depend on the type of(Ln) dopant ion according to the experimental results andtheoretical calculation [23], the 196-cm−1 vibration mode
found in YAG:Pr3+ is in agreement with what is expected.
From these, the Huang-Rhys electron-phonon coupling
parameter (S) can be obtained using the relationship of
IZPL = I0 × exp(−S), where IZPL and I0 are the intensity of
ZPL and total emission intensity, respectively [3,24]. Thismethod can be valid even though there are 140 states of 4f 1 5d 1
configuration. This is because only the lowest 4f 1 5d 1 level canbe populated at 8 K, and other energy levels can be ignored
for the luminescence. The Huang-Rhys parameters obtained is5.5. The Huang- Rhys parameter reflects the change in bondlength for a transition between two states. According to thesimilar vibrational mode in PL and PLE spectra as shown in
Fig. 4, we can assume the 5d and 4f states as having thesame force constant for the CC diagram. This assumption isreasonable because the force constant can be expressed byk = μω2 , where μ is the reduced mass of the vibration and ω

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THERMAL IONIZATION AND THERMALLY ACTIVATED . . .

PHYSICAL REVIEW B 95, 014303 (2017)is the vibrational frequency, and these parameters are common
to both ground and excited vibrational coordinates in the same
host [25,26].

of ∼5, intermediate coupling, this is a valid assumption. In
this case, the Stokes shift is composed of the same phonon
relaxation energies in the 4f 1 5d 1 and 4f 1 state and Eq. (4)can be obtained. Because of the difference in excitation peakenergy E5d1 Ex and ZPL energy Ezp,4f −5d1 being equal to halfof the Stokes shift energy, Eq. (5) is also obtained. The CC
diagrams of other YAGG:Pr systems were constructed basedon the above equations and the data are collected in Table I.
The enlarged view around the crossing point between the 5d1
level and the next lower 4f 2 (3P2 ) level is shown in Fig. 5(b).
To visualize the tendency of activation energy clearly, only the
4f 2 (3P2 ) level of YAG:Pr3+ is drawn in Fig. 5(b) because thedifference of 3P2 energy levels in YAGG:Pr3+ is very small.
However, for the calculation of activation energy, we used each3
P2 energy level in each YAGG:Pr3+ sample (see Fig. S4 andTable S1 in the Supplemental Material [21]). For increasing Ga
content, the energy of the first set of 4f 1 5d 1 states is shifted
upwards so that the crossing point energy between the first set
of 4f 1 5d 1 and 4f 2 : 3P2 parabolas also increases, as shownin Fig. 5(b). The energy gap (E5d1 −CP ) between the bottomof the first set of 4f 1 5d 1 parabola and the crossing point wasestimated as shown in column 6 of Table I. The E5d1 −CP isoverestimated compared with the actual activation energy asshown later, because in actuality the anharmonicities of thepotential curve will reduce the energy barrier from the 5d state
to the crossing point. However, the obtained E5d1 −CP canbe compared among the YAGG:Pr samples. The increase inenergy gap E5d1 −CP suggests that the quenching temperature
increases with Ga content when the quenching process is dueto the thermally activated crossover.

C. Configuration coordinate (CC) diagram

On the basis of the obtained energies from the lowtemperature spectroscopy, the configuration coordinate diagram of YAG:Pr3+ was constructed as shown in Fig. 5(a) byusing Eqs. (1)–(4):
2

x
−1 ,
(1)
E5d1 (x) = Ezp,4f −5d1 + Sωa 2
xE3H4 (x) = Sω
,
(2)
a
 2
x
E3P2 (x) = E3P2Ex + Sω
,
(3)
a
SS
,
(4)
Sω =
2
SS
Ezp,4f −5d1 = E5d1 Ex −,(5)2
where E5d1 (x), E3H4 (x), and E3P2 (x) are the potential curvesof the first set of 4f 1 5d 1 , the 3H4 ground state, and 3P2
excited state of 4f 1 as a function of equilibrium internuclear
distance x, Ezp,4f −5d1 is the ZPL energy of the 4f -4f 1 5d 1transition, E3 P2Ex is the excitation peak of the 3P2 level, E5d1 Ex
is the excitation peak energy of the first set of 4f 1 5d 1 , S
is the Huang-Rhys parameter, ω is the vibrational energy,
SS is the Stokes shift energy, and a is the configuration
offset of the 5d potential curve from the bottom of the 4f
potential curve (taken as zero), as shown in Fig. 5(a) [1].
Here, we assumed that the 4f and 4f 1 5d 1 parabola haveequal force constants. For a Huang-Rhys coupling parameter

FIG. 5. (a) Configuration coordinate diagram showing the various
4f 2 and 4f 5d states of Pr3+ in YAG:Pr3+ and (b) enlarged view of
the CC diagram around the crossing point of the lowest energy 4f 5d1
state with the highest energy 4f 2 states (3P2 ) for Y3 Al5−x Gax O12 :Pr3+ (x = 0−4).

D. Temperature quenching behavior

To determine the luminescence quenching temperature
the temperature dependence of the lifetime of the Pr3+ :4f 1 (2F5/2 )5d luminescence was investigated as shown in
Fig. 6. The lifetimes were estimated by single exponential
fitting to luminescence decay curves (see Figs. S2 and S3
in the Supplemental Material [21]). In previous work it was
shown that the most accurate way to determine the luminescence quenching temperatures for YAG:Ce was to measure
lifelimes for materials with very low dopant concentrations[23] rather than relying on intensity measurements, which
can be influenced by changes in absorption strength withtemperature, or using higher dopant concentrations where
thermally activated concentration quenching obscures theobservation of quenching of the emitting center [3]. At 4.2 K,
the lifetime decreases with increasing Ga content. An increase
in refractive index with increasing Ga content can explain thistrend. The lifetime of the Pr3+ luminescence of all samples
decreases with increasing temperature, and all samples showthat luminescence becomes quenched below 500 K. From thelifetime data, the quenching temperature T50% , which is thetemperature at which the lifetime becomes 50% of that at
low temperatures, was estimated as shown in column 7 ofTable I. T50% increases up to x = 2 and then decreases forhigher Ga content. This behavior corresponds to the prediction
from the configuration coordinate diagram for the samples
with x = 0, 1, and 2 as shown in Fig. 5(b), but it is not inagreement with the results of the CC diagram for x = 3, 4,014303-5

UEDA, MEIJERINK, DORENBOS, BOS, AND TANABE
25

x=0
x=1
x=2
x=3
x=4
x=5

15

1400
1200

TL intensity (arb. unit)

20

Lifetime (ns)

PHYSICAL REVIEW B 95, 014303 (2017)

10

1000
800
600

5d1
x=0
x=1
x=2
x=3
x=4
x=5

400
200

5

5d2

0

0

180

200

220

240

260

280

300

320

Wavelength (nm)
0

100

200

300

400

500

Temperature (K)FIG. 6. Temperature dependence of lifetime in Y3 Al5−x Gax O12 :Pr3+ .

and 5. This result suggests that for samples with x = 3, 4,
and 5 another mechanism is responsible for the luminescencequenching. As discussed above, this can be expected, as thefirst set of 4f 1 5d 1 states moves up in energy for increasingx while the CB edge moves down in energy, thus making
the energy barrier for thermal ionization from the 5d state to
the CB smaller. These observations show that between x = 2
and x = 3 there is a transition from temperature quenchingby crossover to the 4f 2 (3P2 ) parabola in the CC diagram tophotoionization to the conduction band.
To further the quantitative explanation for the peculiar
trend in the quenching temperatures observed for the Pr3+ d-f
luminescence in Y3 Al5−x Gax O12 : Pr3+ (x = 0−5) with x,
activation energies, radiative rate, and attempt rate of the nonradiative process are derived from the temperature dependenceof lifetime according to the single barrier quenching model,
Eq. (6):
τ (T ) =1,ν + 0 exp(−ET Q /kT )(6)

where τ is the lifetime, ν is the radiative rate, 0 is the attempt
rate of the nonradiative process, ETQ is the activation energy,
k is the Boltzmann constant, and T is the temperature. Here,we applied the equation of one barrier quenching process
despite the possibility of two quenching processes. This isbecause if the activation energy of one nonradiative process
is smaller than that of another, the quenching curve mainly
depends on the parameters of the nonradiative with the smaller
activation energy. The fitting by one barrier quenching process
is a reasonable method to obtain the activation energy ofthe quenching process (see Fig. S5 and Table S2 in the
Supplemental Material [21]).
The parameters obtained for ν , 0 , and ETQ are listed incolumns 8, 9, and 10 of Table I. For comparison with ETQ
in other Pr3+ -doped compounds exploited in the scintillator
area, the unit of ETQ is reported in electronvolts [27,28].

FIG. 7. Integrated TL intensity between 300 K and 600 K as afunction of charging excitation wavelength from 180 nm to 320 nmwith 300 s charging time in Y3 Al5−x Gax O12 : Pr3+ .

ETQ increases up to x = 2 and then decreases with increasing
Ga content, following the same tendency of the Ga content
dependence of quenching temperature.
E. TLE analysis

To provide further insight into the quenching process,thermoluminescence excitation spectra were measured. TLE
spectroscopy is a powerful tool to investigate whether thermal
ionization from the excited 5d state occurs [18,19,29]. The
observation of a peak in a TL glow curve is a direct evidenceof thermal ionization, because the trap filling from the excitedstate proceeds by electron transport to the conduction bandand trapping of the conduction-band electrons. As a result, theobservation of thermoluminescence after excitation to the 5d1
state in the temperature range where luminescence quenching
is observed provides a clear signature of thermal ionization.
The thermal quenching for the d-f luminescence in all
Y3 Al5−x Gax O12 : Pr3+ (x = 0−5) samples starts to occur below 300 K, as shown in Fig. 6. Therefore, thermoluminescence
glow curves in the range between 300 and 600 K after charging
at 300 K were measured (see Fig. S6 in the Supplemental
Material [21]). In addition, the integrated intensity of TL glow
peaks was plotted as a function of charging wavelength, which
is TLE spectrum (see Fig. S7 in the Supplemental Material[21]). In the TLE spectra of Fig. 7, only x = 3 and x = 4
samples show a strong TLE band at 275 nm, which is attributed
to the Pr3+ : 4f -5d1 transition. This observation confirms thatfor the x = 3 and x = 4 samples the quenching mechanism isthermal ionization from the 5d1 to the conduction band.
F. VRBE diagram

We also tried to elucidate the quenching processes considering a different approach, i.e., by means of the vacuum referredbinding energy diagram, which is a helpful tool to estimate theenergy gap between 5d1 and the conduction band. First, theband-gap energies of Y3 Al5−x Gax O12 : Pr3+ were analyzed

014303-6

THERMAL IONIZATION AND THERMALLY ACTIVATED . . .

50

PHYSICAL REVIEW B 95, 014303 (2017)

CB-2

Pr3+:5d2

ΔE5d -CB

x

λem

0
1
2
3
4
5

317nm
313nm
309nm
306nm
486nm
486nm

Pr3+:5d1

1-4

VRBE (eV)(hν * intensity)2 (arb. unit)

Y3Al5-xGaxO12:Pr3+

3

PJ

1

G4

-6

1

D2

n

to

5d1

i
xc

5d2

3

HJ

te

Pr3+:4f

-8

os

H
0

VB
-10

4

5

6

7

Energy (eV)

x=0

x=1

x=2

x=3

x=4

x=5

FIG. 9. Stacked VRBE diagram of Y3 Al5−x Gax O12 : Pr3+ .

FIG. 8. Tauc plot of PLE spectra in the VUV region in
Y3 Al5−x Gax O12 : Pr3+ .

from the PLE spectra in the VUV region as shown in Fig. 8.
(The data related to VRBE diagrams are in electronvolt units
according to the conventional VRBE diagram [20,30,31].)
For the PLE spectra of the samples from x = 0 to x = 3,
the 5d1 -4f transition was monitored. Because of the weakluminescence efficiency of the 5d1 -4f transition for x = 4 and5, the 3P0 -3H4 transition was monitored instead. In the PLE
in the VUV region for all samples, the excitation band due tothe host exciton was observed from 5.5 to 7 eV. Beyond theabsorption edges at higher energy, the excitation is saturated.
The energy of the fundamental absorption edge decreases withincreasing Ga content. Using a Tauc plot of the direct allowedtransition, the fundamental absorption energies were estimated[32]. The host exciton energy E ex was estimated by adding thesimilar calibration value of 0.46 eV to fundamental absorption,
as in our previous paper [18] and shown in column 2 of Table II.
The obtained E ex is in good agreement with the reported E ex in
Y3 Al5−x Gax O12 : Ce3+ [18,20]. The stacked VRBE diagrams
were constructed mainly using the E CT parameters (chargetransfer energy of Eu3+ ) from the data reported by Jia et al.
[33] and U (6,A) (energy difference between Eu2+ and Eu3+ )reported by Dorenbos [34], as shown in columns 3 and 4 of
Table II. For the 5d1 and 5d2 energies of the Pr3+ -doped YAGGTABLE II. Experimental data on exciton energy (E ex ), chargetransfer (E CT ) and Coulomb repulsion energy [U(6, A)], andE5d1 −CB for Y3 Al5−x Gax O12 : Pr3+ .
x

E ex (eV)E CT (eV)U (6,A) (eV)E5d1 −CB (eV)

0
1
2
3
4
5

7.10
7.09
6.88
6.67
6.42
6.06

5.42 [33]5.34 [18]5.25 [33]5.19 [33]5.12 [33]5.05 [33]6.79 [34]6.79 [34]6.79 [34]6.79 [34]6.79 [34]6.79 [34]

1.41
1.37
1.18
0.96
0.74
0.37

system as shown in Fig. 9, the band maxima from this work
were used. The energy gap between 5d1 and the bottom of
the CB (E5d1 −CB ) was estimated from the VRBE diagram asshown in column 5 of Table II. E5d1 −CB drops significantly
from x = 3 with increasing Ga content. This tendency is
consistent with the Ga content dependence of the activationenergy ETQ and decrease in quenching temperature T50%
above x = 3. The VRBE diagram strongly supports that thequenching processes for x = 0, 1, and 2 samples and for
x = 3, 4, and 5 are caused by thermally activated crossoverand thermal ionization, respectively.

IV. CONCLUSIONS

High-resolution PL and PLE spectra of YAG:Pr3+ were
measured at 8 K. The energy of the ZPL (zero phonon line)
is estimated to be 33 257 cm−1 and the vibronic energy is
196 cm−1 . The estimated electron-lattice coupling HuangRhys parameter for the 5d and 4f states of Pr3+ are 5.5 asderived from the intensity ratio of ZPL and total transition.
From low-temperature spectroscopy, the configuration coordinate diagrams of Y3 Al5−x Gax O12 : Pr3+ were constructed.
From those CC diagrams, the activation energy of thermally
activated crossover was found to increase with increasing Ga
content. However, from the temperature dependence of the
lifetime of the Pr3+ : 5d1 level the quenching temperature(T50% ) increases up to x = 2 and decreases with increasing Ga
content. From the TL excitation (TLE) spectra, the Pr3+ : 5d1band (the lowest excited 5d level) was observed at around
300 nm in the samples above x = 3 Ga content. From the
VRBE diagram, the activation energy of thermal ionization decreases with increasing Ga content. Based on the CC diagram,
VRBE diagram, and TLE spectra, we demonstrated that thequenching of Pr3+ : 5d1 -4f luminescence in Y3 Al5−x Gax O12
with x = 0, 1, 2 is thermally activated crossover and that withx = 3, 4, 5 is the thermally activated ionization.

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UEDA, MEIJERINK, DORENBOS, BOS, AND TANABEPHYSICAL REVIEW B 95, 014303 (2017)

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