Direct synthesis of Sr3Al2(OH)12 from solution for preparation of fine-grained Sr3Al2O6 phosphors at low temperature

Article abstract of DOI:10.1016/j.jallcom.2016.04.005

The precursors Sr₃Al₂(OH)₁₂ can be directly precipitated from solution near room temperature under the normal pressure. The cubic Sr₃Al₂O₆ with fine grain size can be synthesized from Sr₃Al₂(OH)₁₂ at the temperature as low as 600 °C. Especially, the rare earth element, such as Eu³⁺ etc., as luminescent center, has been simultaneously integrated into the lattices of Sr₃Al₂O₆. The luminescent intensities of the samples annealed at a temperature over 800 °C are strong enough for phosphors, and increase with the annealing temperatures, reach a maximum at 1200 °C and then drop down at 1300 °C. The grain sizes of all the samples annealed below 1200 °C are smaller than 1 μm and suitable for mixing with other phosphors in the applications. The photoluminescent spectra of the Sr₃Al₂O₆:Eu³⁺ reveal that the intensities of both emission and excitation peaks increase with the dose of Eu³⁺, and reach maxima at the dose level about 1.2%, then decrease due to concentration quenching. A new excitation band at 230 nm has been observed in heavier doped samples due to the complex point defects produced by association.

Full text of DOI:10.1016/j.jallcom.2016.04.005

Journal of Alloys and Compounds 678 (2016) 421e426
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Direct synthesis of Sr3Al2(OH)12 from solution for preparation of
fine-grained Sr3Al2O6 phosphors at low temperature
,
Jianquan Qi a *, Xinyi Zhang b, Xiumei Han a, Yuanyuan Li b, Xueyang Wu b, Ruixia Zhong a,
Rui Guo a
a School of Nature Resources & Materials Science, Northeastern University at Qinhuangdao, Qinhuangdao, Hebei Province 066004, PR China
b School of Chemistry, Monash University, Clayton, Victoria 3800, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
The precursors Sr3Al2(OH)12 can be directly precipitated from solution near room temperature under the
normal pressure. The cubic Sr3Al2O6 with fine grain size can be synthesized from Sr3Al2(OH)12 at the
temperature as low as 600 ꢀC. Especially, the rare earth element, such as Eu3þ etc., as luminescent center,
has been simultaneously integrated into the lattices of Sr3Al2O6. The luminescent intensities of the
samples annealed at a temperature over 800 ꢀC are strong enough for phosphors, and increase with the
annealing temperatures, reach a maximum at 1200 ꢀC and then drop down at 1300 ꢀC. The grain sizes of
Received 14 December 2015
Received in revised form
24 March 2016
Accepted 1 April 2016
Available online 5 April 2016
all the samples annealed below 1200 ꢀC are smaller than 1
mm and suitable for mixing with other
Keywords:
Sr3Al2(OH)12
Sr3Al2O6
Phosphors
Direct synthesis
Fine-grain
phosphors in the applications. The photoluminescent spectra of the Sr3Al2O6:Eu3þ reveal that the in-
tensities of both emission and excitation peaks increase with the dose of Eu3þ, and reach maxima at the
dose level about 1.2%, then decrease due to concentration quenching. A new excitation band at 230 nm
has been observed in heavier doped samples due to the complex point defects produced by association.
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
with yellow orange light [25,29], Mn4þ with red light [38], Bi3þ
with greenish blue to blue light [7], and etc. Their luminescent
Owing to their unique properties, oxide phosphors have been
recognized as the most promising materials for light emitting de-
vices [1,2]. Among them, rare earth doped strontium aluminates are
particular important from practical point of view [3e10]. With
much abundant species of the aluminates, their compositions can
be much different, for examples, strontium aluminates can have the
formulae as Sr5Al2O8 [3], Sr4Al2O7 [4], Sr3Al2O6 [5e9], Sr12Al14O33
[11], SrAl2O4 [12e36], Sr4Al14O25 [37e40], SrAl4O7 [41], and
SrAl12O19 [42,43] in series, so does barium aluminates also. Among
them, Sr5Al2O8, Sr12Al14O33, and Sr4Al14O25 were not reported in
the early phase diagram, and Sr3Al2O6, SrAl2O4, Sr4Al14O25 are
much more documented as the hosts of phosphors. The rare earth
ions can act as luminescent centres in a certain aluminates host.
Different activate ions can have the different wave-bands emission
lights, such as Eu3þ with orange and red lights [3,4], Eu2þ with
dominant emission of green or red light [5,24,27,28,31,37], Ce3þ
with blue green light [6,9,29], Sm3þ with orange red light [10], Pr3þ
properties are associated not only with the ingredients, but also the
processing arts. There are several traditional methods to prepare
aluminates, such as high-temperature solid state reaction [3,6,8],
sol-gel [30,33,34], combustion [31,39], and hydrothermal methods
[44]. So far, high temperature annealing has been required for
synthesizing final phosphors products. Besides of the highly energy
consumption, another main issue is that the grain size of alumi-
nates may be coarsened due to the rapid grain growth at high
temperature, which may cause colour stain when they are mixed
with other phosphors for white-light emission. It is important to
decrease the synthetic temperature of phosphors powders in order
to depress the point defects in grain bulk to achieve the outstanding
luminescent properties and avoid the grain coarsening to improve
the processing properties for the stepped applications. In this study,
we developed a simple chemical method to prepare a stable hy-
droxide Sr3Al2(OH)12. We demonstrate that strontium aluminates
phosphors Eu3þ doped Sr3Al2O6 phosphors with controlled
composition can been obtained at a low temperature by using
Sr3Al2(OH)12 as the precursor. The photoluminescence of Eu3þ
doped Sr3Al2O6 phosphors strongly depends on the preparation
conditions and shows great promise for optical devices.
* Corresponding author.
E-mail address: jianquanqi@mail.tsinghua.edu.cn (J. Qi).
http://dx.doi.org/10.1016/j.jallcom.2016.04.005
0925-8388/© 2016 Elsevier B.V. All rights reserved.

422
J. Qi et al. / Journal of Alloys and Compounds 678 (2016) 421e426
SrCO
a
b
1300 C
1200 C
1000 C
800 C
Sr Al (OH) pdf:24-1186
SrCO pdf:5-0418
600 C
RT
10
20
30
40
50
60
70
10
20
30
40
50
60
70
2Theta/o
2Theta/o
Sr Al O
SrCO
SrCO3
c
d
Sr Al O 2H O
Sr Al O H O
Sr Al O pdf:24-1187
SrCO pdf:05-0418
10
20
30
40
50
60
70
20
30
40
50
60
70
2Theta/o
2Theta/o
Fig. 1. XRD patterns of (a) as-prepared samples, (b) the samples annealed at different temperature (doping level of Eu3þ as1.5%), (c) the samples annealed at 600 ꢀC and (d) the
sample annealed at 800 ꢀC.both of the pdf-cards are also shown accordingly.
2. Experimental
In this condition, the precipitations should be washed several times
to wipe off the sodium salts in the products. The as-prepared
powders were also annealed respectively at 600, 800, 1000, 1200
and 1300 ꢀC for 2 h to check the annealing effects of the products.
The white as-prepared powders and the samples annealed at
different temperature were checked by the X-ray diffraction (XRD, a
Philips Diffractometer, X'Pert-Pro MPD), Field emission scanning
electron microscopy (FE-SEM, Zeiss super 55, Germany) and their
photoluminescent spectra were characterized on spectro-
flurophotometer (PL, Hitachi, Ltd. F-7000FL, Japan).
Direct synthesis from solution method is a processing to direct
crystallization of oxides, hydroxides and some oxysalts through
solution chemical reaction under normal pressure and near room
temperature. The processing of Sr3Al2(OH)12 by the direct synthesis
from solution is similar as that of BaTiO3 [45], BST(Ba1-xSrxTiO3)
[46], BZT (BaZrxTi1-xO3) [47], and etc., as we reported previously. In
this study, Sr(OH)2$8H2O and aluminium resources, such as
aluminium ethoxide or sodium metaaluminate are adopted as raw
materials to synthesis Sr3Al2(OH)12 by direct synthesis from solu-
tion method. In a typical synthetic process, 13.288 g Sr(OH)2$8H2O
(BDH Chem. Ltd., UK) was dissolved in a sealed conical beaker
containing 200 ml absolute ethanol (International Laboratory, USA)
and 200 ml DI water as base solution. Then, 5.405 g (C2H5O)3Al
(BDH Chem. Ltd., UK) and a certain content (0, 0.5, 0.8, 1.0, 1.5, 1.8,
2.0, 2.5, 3.0%mol relative to Sr) of EuCl3(BDH Chem. Ltd., UK) were
dissolved in 200 ml absolute ethanol and transferred into a buret as
titrating solution. The conical beaker was linked to a buffer vessel
with a perfusion tube. Another perfusion tube was used to link the
buffer vessel and anther beaker contained with sodium hydroxide
solution to exclude CO2 in air from our synthesis system. After the
base solution was heated to a certain temperature (i.e. 60 ꢀC), the
titrating solution was dripped into the base solution in conical
beaker dropwise under vigorous stirring. The white precipitations
were observed in the solution instantaneously. After the comple-
tion of the titration and followed by ageing for 3 h, the white
precipitation was obtained and baked at 60 ꢀC for 24 h in an oven.
To compare the effect of aluminium resources, the titrating solution
was replaced as sodium metaaluminate water solution such as
2.732 g NaAlO2 (BDH Chem. Ltd., UK) was solved in 200 ml DI water.
3. Results and discussion
Fig. 1(a) showed the XRD pattern of the as-prepared powders
under the condition of the aluminium resources as aluminium
ethoxide. A cubic Sr3Al2(OH)12(pdf: 24-1186) was indexed as main
phase, except a slight of orthorhombic SrCO3(pdf: 05-0418) with
very weak peaks as marked as red spots (near 25ꢀ, 36ꢀ, 43ꢀ). In fact,
the XRD pattern is very similar under the condition of the
aluminium resources as sodium metaaluminate. The different
aluminium resources can result in the same prescur Sr3Al2(OH)12
.
However, the rare earth elements cannot be simultaneously inte-
grated into the lattices and impurity of sodium ion is unavoidable to
decrease the luminescence properties of final products when so-
dium metaaluminat is adopted as raw materials. Therefore, in the
following discussion, we just discussed the condition of the
aluminium resources as aluminium ethoxide. In the XRD pattern,
the derivation of the small amount of SrCO3 in the as-prepared
powders possibly may result from the reaction of the strontium
hydroxides with CO2 in the air during its storage. It is confirmed
that the powders of Sr3Al2(OH)12 can be easily prepared by DSS

J. Qi et al. / Journal of Alloys and Compounds 678 (2016) 421e426
423
method. Theoretically, Sr3Al2(OH)12 can decompose to strontium
Sr3Al2ðOHÞ12/SrðOHÞ2 þ AlðOHÞ3
Strontium hydroxide can react with CO2 in the air consequen-
tially as
(1)
aluminates and H2O without any impurities at a suitable temper-
ature so that a convenient rout to strontium aluminates can be
constructed through it. It is noted that Sr3Al2(OH)12 can be directly
synthesized from solution not only from Sr(OH)2$8H2O and
C2H5O)3Al as raw materials, but also from other raw materials such
as NaAlO2, NaOH, and strontium salts although the impurities may
be introduced. Fig. 1(b) shows the XRD patterns of the as-prepared
powders and the samples annealing at different temperature
(doping level of Eu3þ as1.5%) It can be clearly seen that the cubic
Sr3Al2O6 can be observed in the sample annealed at the tempera-
ture as low as 600 ꢀC, indexed as a main phase in that annealed at
800 ꢀC and as single pure phase annealed at the temperature over
800 ꢀC.
To analyse the decomposition processing of Sr3Al2(OH)12, the
XRD pattern of the sample annealed at 600 ꢀC is shown in Fig. 1(c).
As mentioned above, our targeted phase Sr3Al2O6 can be observed
clearly, where its strong peaks are marked as black dots. In this
situation, aluminates can be prepared at the temperature as low as
600 ꢀC by our method in which Sr3Al2(OH)12 is synthesized at first
and then decomposed subsequently. Besides of Sr3Al2O6, three
impure phases are also indexed in this sample, such as, SrCO3 as
main phase and other two slight amount of phases of hydrated
aluminates (Sr3Al2O6$2H2O, Sr3Al2O6$H2O). It is noticed that the
amount of SrCO3 obviously increases compare with that in as-
prepared samples. The products of decomposition of Sr3Al2(OH)12
can be Sr(OH)2 and Al(OH)3 as the rout of
SrðOHÞ2 þ CO2/SrCO3 þ H2O
(2)
Therefore, a quantum of SrCO3 is observed in the samples. If the
precursor was annealed hermetically from air to prevent the
introduction of CO2, the above reaction would not be happen. There
is another decomposition rout as equation (3)e(5) to compete with
equation (1)
Sr3Al2ðOHÞ12/Sr3Al2O6$2H2O þ 4H2O
(3)
(4)
(5)
and
Sr3Al2O6$2H2O/Sr3Al2O6$H2O þ H2O
then
Sr3Al2O6$H2O/Sr3Al2O6 þ H2O
The results are consistent with the previous report in which
Sr3Al2(OH)12 was synthesized by hydrothermal method [34].
The XRD pattern of the sample annealed at 800 ꢀC is shown in
Fig.1(d) and the strong peaks are indexed as Sr3Al2O6. Only slight of
carbonate is residual. Compared with that annealed at 600 ꢀC, the
Fig. 2. SEM images of the samples prepared at different temperature: (a) as prepared, (b) 600 ꢀC, (c) 800 ꢀC, (d) 1000 ꢀC, (e) 1200 ꢀC, (f) 1300 ꢀC.

424
J. Qi et al. / Journal of Alloys and Compounds 678 (2016) 421e426
7000
612nm,
618nm
6000
5000
4000
3000
2000
1000
0
b
a
λem=613nm
254nm
6000
λex=254nm
593nm
5D0-7F1
5D0-7F2
5000
4000
3000
2000
1000
0
579nm
5D0-7F0
653nm
5D0-7F3
685nm
5D0-7F4
394nm
400
532nm
465nm
200
300
500
600
500
550
600
650
700
wavelenth/nm
wavelenth/nm
5000
4000
3000
2000
1000
0
200
150
100
50
c
λex=394nm
d
λem=593nm
0
200
300
400
500
600
500
550
600
650
700
wavelenth/nm
wavelenth/nm
Fig. 3. The photoluminescent spectra of the sample annealed at 1200 ꢀC. (a) Excitation spectra monitored at Eu3þ emission at 613 nm, (b) Emission spectra by exciting at 254 nm, (c)
Excitation spectra monitored at Eu3þ emission at 593 nm, (d) Emission spectra by exciting at 394 nm.
most of carbonate and intermediate hydrated aluminates are dis-
appeared. It is expected that SrCO3 can react with Al(OH)3 to syn-
thesize Sr3Al2O6 at the temperature below 800 ꢀC due to their
chemical activity as produced freshly and very fine grain size. The
reaction is expressed as following equation (6)
Therefore, a high temperature annealing leads to grains coarsening
and/or slight sintering, and thus degrade the processing properties.
To decrease the synthesis temperature is much necessary for the
stepped applications of the phosphors powders.
The photoluminescent properties of all annealed samples with
the same amount are characterized according to wavelength
scanning under the rate of 1200 nm/min, the slit of both Ex and Em
as 2.5 nm. The photoluminescent spectra of both Eu3þ and Eu2þ
doped aluminates are studied by many researchers, and their
spectra have been clear identified [4,10,18], such as, in Eu3þ doped
Sr3Al2O6 phosphors, charge transfer band (CTB) from the fully filled
2p orbital of O2ꢁ to the partially filled 4f6 level of Eu3þ indexed at
3SrCO3 þ 2AlðOHÞ3/Sr3Al2O6 þ 3H2O þ 3CO2
(6)
As we mentioned above, there is only a single pure phase
Sr3Al2O6 indexed in the samples annealed above 800 ꢀC, such as
1000, 1200, 1300 ꢀC as shown in Fig 1(b). Compared with the
standard card (JCPDS: No.24-1187), all the peaks have a slight right-
shift as shown clearly in Fig. 1(d). It indicates that the lattice has
shrunk and thus the Eu3þ entered into the lattice to replace Sr2þ
because Eu3þ (0.0947 nm) is smaller than Sr2þ(0.112 nm).
The SEM pictures of the as-prepared samples prepared at
different temperatures are shown in Fig. 2. There are many ag-
glomerations aggregated by much fine nanoparticles with the grain
size of 20 nm as shown in Fig. 2 (a) in our as-prepared precursors.
The two morphologies are observed in the samples annealed at
600 ꢀC as shown in Fig. 2 (b). The particles with the size of
50e200 nm are aluminates and hydrated aluminates, and rods with
7
254 nm, the transition from the 7F0, 1 / 5L6, F0, 1 / 5D2, and 7F0,
1 / 5D1 indexed at 395, 466, and 532 nm respectively in excitation
spectra. The strong and sharp peaks were attributed to the
5D0/7Fj(j ¼ 0e4) emission transition in emission spectra. Fig. 3(a)
shown the photoluminescent spectra of the sample annealed at
1200 ꢀC. The excitation spectra are shown in Fig. 3 (a) and (c) by
monitoring the Eu3þ emissions at 613 nm and 593 nm respectively.
A strong band at 254 nm is observed as CTB and three weak bands
at 394, 465, 532 nm as 7F0, 1 / 5L6, 7F0, 1 / 5D2, and 7F0, 1 / 5D1.
The emission spectra are shown in Fig. 3 (b) and (d) by exciting at
254 nm and 394 nm. The emission intensity of the spectra by
exciting at 254 nm as (b) is much strong than that at 394 nm as (d).
Five sharp peaks at 579, 593, 613, 653 and 685 nm are attributed to
the 5D0/7Fj(j ¼ 0e4) emission transitions, respectively.
the length of 2
mm and 200 nm in diameters are SrCO3 phases
which are indexed both by their crystallographic characteristics
and EDS. In the samples annealed at 800 ꢀC, the graingrowth
happens and the grain size of the particles increases upto ~300 nm.
Very few rods are residual as SrCO3. When the samples annealed at
1000 ꢀC, 1200 ꢀC and 1300 ꢀC, only particles are observed and the
size of them increase with the annealed temperature as about 500,
800, 800 nm respectively. All of the observations are perfect in
accord with the results of XRD above. The grain size of the sample
annealed at 1200 ꢀC and that at 1300 ꢀC increases slightly, but the
adhesion among the grains as sintering phenomenon is clear
observed in the sample annealed at 1300 ꢀC as shown in Fig. 2(f).
The emission spectra of the samples annealed at different
temperature by exciting at 254 nm are shown in Fig. 4(a), the
dependence of emission intensity at 613 nm on annealing tem-
perature is also inserted as inset. The spectra of the different
samples have the same emission bands as indexed in Fig. 3(b). Two
strong emission peaks at 613 nm and 593 nm can be observed in
the sample annealed at the temperature as low as 600 ꢀC, and thus,
the aluminates can be formed as confirmed by XRD while Eu3þ can

J. Qi et al. / Journal of Alloys and Compounds 678 (2016) 421e426
425
temperature over 800 ꢀC are strong enough for phosphors. The
emission intensities at their character bands increase quickly with
the annealing temperature of the different samples, and reach a
maximum at 1200 ꢀC, then decrease steeply. Too high annealing
temperature can lead to sintering as shown in Fig. 2(f), producing
stress, increasing point defects, and thus quenching the emissions.
The emission spectra of the Sr3Al2O6:Eu3þ samples annealed at
1200 ꢀC with different doping level by exciting at 254 nm are
shown in Fig. 4(b), the dependence of emission intensity at 613 nm
and 593 nm on doping level is also inserted as inset. Both of the
emission intensities at 613 nm and 593 nm increase with the dose
of Eu3þ, and reach maxima at the dose level about 1.2%, then
decrease due to concentration quenching. The excitation spectra of
the Sr3Al2O6:Eu3þ samples annealed at 1200 ꢀC with different
doping level by monitoring the emission at 593 nm are shown in
Fig. 4(c). The dependence of emission intensity at excitation max-
ima on doping level is also shown in inset and the tendency is as the
same as that in emission spectra. There is an excitation band at
254 nm indexed as CTB when the doping level is lower than 2%.
Two excitation bands at 254 and 230 nm coexist in the sample
doped with the dose of 2%. Above the doping level of 2%, the
excitation band at 230 nm dominates the spectra. The excitation
band at 230 nm in Eu3þ doped aluminates is rarely documented,
the heavier dopants would respond for it. It is suggested that cation
vacancy will compensate the impurity trivalent rare earth dopant
in Sr3Al2O6:Eu3þ as
3Eu2O3 þ 3Al2O3/6EuꢂSr þ 3V0S0 r þ 6AlAꢃl þ 18OꢃO
(7)
or
000
ꢃ
3Eu2O3 þ Al2O3/6EuꢂSr þ 2VAl þ 2AlAl þ 12OOꢃ
(8)
Therefore, which kind of cation vacancy can be determined by
either its formation energy or the stoichimetry of Sr3Al2O6. The
00
cation vacancy as V is preferred to form in Al-riched matrix as
Sr
000
equation (7), and that as VAl referred in Sr-riched one as equation
(8). When the doping level of Eu3þ is lower than 2%, the cation
vacancy cannot influence on the charge transfer distinctly, and the
CTB shift slightly. On the other hand, in heavier doped samples, the
complex point defects can be produced by association as
00
00
000
000
EuSꢂr þ VSr/EuSꢂrVSr and EuSꢂr þ VAl/EuSꢂrVAl:
(10)
At this time, charge transfer process can be changed from
OꢃO / EuꢂS
(11)
r
e0
to
00
000
OOꢃ / EuꢂS VSr or OOꢃ / EuꢂS VAl:
(12)
r
r
e0
e0
Because the point defect of Eu$Sr has positive charge whereas
000
Eu$SrVAl has negative charge, the transfer energy will increase and
CTB shift towards to short wavelength. Therefore, excitation band
at 230 nm can be observed in the heavier doped samples.
Fig. 4. (a) The emission spectra of the samples annealed at different temperature by
exciting at 254 nm, inset: the dependence of emission intensity at 613 nm on
annealing temperature. (b) The emission spectra of the Sr3Al2O6:Eu3þ samples
annealed at 1200 ꢀC with different doping level by exciting at 254 nm, inset: the
dependence of emission intensity at 613 nm. (c) The activation spectra of the
Sr3Al2O6:Eu3þ samples annealed at 1200 ꢀC with different doping level by monitoring
Eu3þ emission at 593 nm, inset: the dependence of emission intensity on the dose of
4. Conclusions
Eu3þ
.
In summary, we have developed a facile and effective approach
for the synthesis of a cubic hydroxide Sr3Al2(OH)12, which can be
directly transformed to cubic Sr3Al2O6 at the temperature as low as
600 ꢀC.The method of direct synthesis from solution is simple,
controllable, efficient and pollution free. The cubic Sr3Al2O6 can be
obtained as main phase by annealing at the temperate of 800 ꢀC and
be synthesized into their lattice cell at this time. The reason of the
emission intensities are weak in this sample is mainly due to the
adsorption of the emissions by so many impure phases, especially
as SrCO3. The luminescent intensities of the samples annealed at a

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J. Qi et al. / Journal of Alloys and Compounds 678 (2016) 421e426
as single pure phase by that over 800 ꢀC. The rare earth element,
such as Eu3þ etc., as luminescent center, has been simultaneously
dopped into the lattices. The luminescent intensities of the samples
which anneals at a temperature over 800 ꢀC are strong enough for
phosphors, increase with the calcinations temperatures, reach a
maximum at annealing temperature of 1200 ꢀC and then drop
down at 1300 ꢀC. Therefore, decrease of the synthetic temperature
can either depressing the point defects in grain bulk or avoid the
grain coarsening and achieve the outstanding phosphors proper-
ties. The grain sizes of all the samples annealed below 1200 ꢀC are
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smaller than 1 mm and suitable for mixing with other phosphors in
the applications. Therefore the precursor is very useful for prepa-
ration of strontium aluminate phosphors at low temperature. The
photoluminescent spectra of the Sr3Al2O6:Eu3þ reveal that the in-
tensities of both emission and excitation peaks increase with the
dose of Eu3þ, and reach maxima at the dose level about 1.2%, then
decrease due to concentration quenching. When the doping level of
Eu3þ is lower than 2%, the cation vacancy cannot influence on the
charge transfer distinctly, and the CTB shift slightly. In heavier
doped samples, the complex point defects can be produced by as-
sociation, the transfer energy will increase, and CTB shift towards to
short wavelength. Therefore, a new excitation band at 230 nm can
be observed in heavier doped samples.
Acknowledgements
The authors are grateful the supports of Basic Key Program of
Applied Basic Research of Science and Technology Commission
Foundation of Hebei Province in China (Grant No. 14961108D and
Grant No. 15961005D) and Open Project of State Key Laboratory of
New Ceramic and Fine Processing, Tsinghua University(No.
KF201410).
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