Journal of The Electrochemical Society, 158 (12) J377-J382 (2011)
0013-4651/2011/158(12)/J377/6/$28.00 © The Electrochemical Society
J377
Charge-Transfer Luminescence and Energy Transfer
in Eu2+ -Doped Barium Zirconosilicates
De-Yin Wang,a Ling-Li Wang,b Te-Ju Lee,a Teng-Ming Chen,a,z and Bing-Ming Chengc
a Phosphors
Research Laboratory and Department of Applied Chemistry, National Chiao Tung University,
Hsinchu 30010 Taiwan
Institute of Non-ferrous Metals, Guangzhou 510651, China
Radiation Research Center, Hsinchu 30076, Taiwan
b Guangzhou Research
c National Synchrotron
Although zirconates are known to show ultraviolet luminescence under X-ray excitation, few have studied the vacuum ultraviolet
(VUV) excited luminescence of zirconate compounds. We have investigated the VUV-excited luminescence of zirconate in BaZrSi3 O9
and Ba2 Zr2 Si3 O12 using synchrotron radiation, and examined the possibility of sensitizing Eu2+ by hosts that contain zirconate. Upon
VUV excitation, BaZrSi3 O9 and Ba2 Zr2 Si3 O12 show self-activated emission peaking at 285 nm and 334 nm, which are assigned
to Zr4+ -O2− charge transfer (CT) luminescence. The evidence for the occurrence of Zr4+ -O2− charge transfer in BaZrSi3 O9 and
Ba2 Zr2 Si3 O12 was further supported by density of state (DOS) calculation. Energy transfer from host excitation (∼172 nm) to Eu2+
was observed in BaZrSi3 O9 :Eu2+ and Ba2 Zr2 Si3 O12 :Eu2+ , showing that host sensitization of Eu2+ occurs in these materials. Despite
that the host emission overlaps with the Eu2+ 4f-5d absorption band significantly, host -to-Eu2+ energy transfer in Ba2 Zr2 Si3 O12 :Eu2+
was found to be less efficient than that in BaZrSi3 O9 :Eu2+ and Eu2+ luminescence was found to be inferior in Ba2 Zr2 Si3 O12 :Eu2+ .
These observations are ascribed to a smaller Zr-O-Ba(Eu) angles and a lower Eu2+ -Zr4+ metal-to-metal charge transfer energy in
Ba2 Zr2 Si3 O12 :Eu2+ .
© 2011 The Electrochemical Society. [DOI: 10.1149/2.008112jes] All rights reserved.
Manuscript submitted April 1, 2011; revised manuscript received August 30, 2011. Published October 31, 2011.
The development of plasma display panel (PDP) technology, as
well as the necessity to replace mercury lamps by mercury-free lamps
for environment concern, has made the spectral property study of
rare earth-doped phosphors in the vacuum ultraviolet region (VUV,
λ < 200 nm) an important field.1–4 In both PDP and mercury-free
lamps, Xe discharge is used for generating VUV radiation, then the
generated VUV radiation is converted into visible light (or UV light
for Hg-free lamps used in certain case4 ) by a phosphor. For this reason, phosphors with high VUV absorption should be given priority
when it comes to practical use. If the donor center or the host itself
emits and their emission overlaps the absorption band of an activator,
then resonant energy transfer from donor centers or host to activator could occur, resulting in an efficient luminescence. In this sense,
efficient VUV phosphor could be achieved in such way that absorption of light by donor centers or by the host lattice itself followed
by nonradiative transfer to acceptor rare earths ions.5 Zirconates are
known to show ultraviolet luminescence under X-ray excitation,6, 7
and some zirconate compounds were found to show UV emission under VUV excitation recently.8, 9 Furthermore, Eu2+ 4f-5d absorption
located in the UV region.10 Therefore, upon VUV excitation, host sensitization of Eu2+ would occur in Eu2+ -doped zirconium-containing
compounds, and efficient luminescence from Eu2+ would be obtained
in such compounds.
In the BaO-ZrO2 –SiO2 ternary system, there exist two alkali-metal
zirconosilicates, viz., BaZrSi3 O9 and Ba2 Zr2 Si3 O12 .11 BaZrSi3 O9 is
hexagonal, belonging to the space group D2 3h -P6c2 with two formula units per unit cell.12–14 The crystal structure of BaZrSi3 O9 is
based upon three SiO4 tetrahedra, each sharing two of their oxygen
atoms to form Si3 O9 rings (see Fig. 1a). Both Ba2+ and Zr4+ ions
are coordinated by six oxygens and they lie in a parallel row along
c axis;12–14 Ba2 Zr2 Si3 O12 crystallizes in a cubic structure, belonging
the space group P21 3 (198) with four formula units per unit cell.
Its structure is formed from the framework of SiO4 tetrahedras and
ZrO6 octahedras interlinked via vertexes as shown in Fig. 1b,15 in
which Ba2+ ions are coordinated by nine and twelve oxygens, repectively. In the pioneering work of Blasse et al., host emission with
maximum at 285 nm was observed in BaZrSi3 O9 under cathoderay excitation,11 which prompts us to believe that the same emission
would occur in BaZrSi3 O9 upon VUV excitation. Meanwhile, because
both BaZrSi3 O9 and Ba2 Zr2 Si3 O12 contain SiO4 tetrahedra and ZrO6
z
E-mail: [email protected].
octahedra, intrinsic UV emission is expected in Ba2 Zr2 Si3 O12 as well.
This paper is devoted to study the VUV-excited spectral properties
of the undoped and Eu2+ -doped BaZrSi3 O9 and Ba2 Zr2 Si3 O12 , and
examine the possibility of sensitizing Eu2+ by hosts that contains
zirconate, and investigate the host-to-Eu2+ energy transfer in them.
Experimental
Materials and synthesis.— Samples of undoped and Eu2+ -doped
BaZrSi3 O9 and Ba2 Zr2 Si3 O12 were prepared in polycrystalline powder via a solid state reaction route. Stoichiometric amounts of BaCO3
(99.9%, Aldrich), ZrO2 (99.5%, Aldrich), SiO2 (99.6%, Aldrich) and
Eu2 O3 (99.9%, Aldrich) were ground in an agate mortar, then the obtained mixtures were calcined at 1400 ◦ C under a reducing atmosphere
(5%H2 +95%N2 ).
Materials characterization.— The phase purity of all BaZrSi3 O9 :
xEu2+ (0 ≤ x ≤ 8%) and Ba2 Zr2 Si3 O12 :yEu2+ (0 ≤ y ≤ 10%) samples
was checked by using powder X-ray diffraction (XRD) analysis with a
Bruker AXS D8 advanced automatic diffractometer operated at 40 kV
and 40 mA with CuKα radiation (λ = 1.5418 Å).
The VUV spectra were recorded at the Beamline 03A at National
Synchrotron Radiation Research Center in Taiwan. A 6-meter cylindrical monochromator equipped with a 1200 lines/mm grating and a
Hamamatsu R943-02 photomultiplier (PMT) were used for excitation
spectra measurement. The emission spectra were recorded using an
Acton HR-320 monochromator equipped with a 450 lines/mm grating
and a Hamamatsu R943-02 PMT. The relative VUV excitation intensities of the samples were corrected by measuring the photon flux of
a photodiode whose wavelength dependence on quantum yield has
been calibrated.
Band structure calculations.— The electronic structures of
BaZrSi3 O9 and Ba2 Zr2 Si3 O12 were calculated by using the program CASTEP (Cambridge Serial Total Energy Package),16, 17 which
is based on the density functional theory (DFT) in the gradientcorrected local density approximation (LDA).18, 19 The optimized
pseudopotential20, 21 in the Kleinman– Bylander form22 for Ba, Zr,
Si and O atoms allows us to use small plane-wave basis sets without
compromising the accuracy required by our study. A kinetic-energy
cut off of 300 eV was used throughout the calculation and its reliability
was further demonstrated in the results of the linear optical properties
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J378
Journal of The Electrochemical Society, 158 (12) J377-J382 (2011)
Figure 2. (a) Excitation spectrum of BaZrSi3 O9 obtained by monitoring the
host emission at 285 nm and (b) emission spectrum of BaZrSi3 O9 excited at
172 nm.
Figure 1. Crystal structures of (a) BaZrSi3 O9 and (b) Ba2 Zr2 Si3 O12 .
calculation. The Read and Needs correction23 was implemented to
ensure accurate optical matrix elements calculations for the nonlocal
pseudopotential-based method.
luminescence. The reorganization of the charge density distribution
around the Zr4+ ion is accompanied by an expansion of the Zr4+ -O2−
bonds in the excited state, giving rise to the observation of large Stokes
shifts and broad bands. One more reason contributed to the origin of
the excitation band is probably the absorption from SiO4 4− group.
This deduction can be evidenced by comparing the VUV excitation
spectra of some silicates,25–28 among which a good example is the
well-known green-emitting phosphor Zn2 SiO4 :Mn2+ being used in
PDP.28 Therefore, the excitation band is presumably an overlap of the
O2− -Zr4+ CT transition with the SiO4 4− group absorption.
Presented in Fig. 3 is the excitation spectrum (curve a) of
Ba2 Zr2 Si3 O12 obtained by monitoring the host emission at 334 nm
and its emission spectrum recorded under 172 nm excitation (curve
b). The excitation band with maximum at 184 nm is associated with
the absorption from the O2− -Zr4+ CT transition and the SiO4 4− group.
Under VUV excitation at a wavelength of 172 nm, a broad emission peaking at 334 nm was observed in Ba2 Zr2 Si3 O12 , which is
considered to originate from Zr4+ -O2− CT luminescence as that in
BaZrSi3 O9 . The shoulder at 420 nm observed in the emission spectrum of Ba2 Zr2 Si3 O12 may be due to signal anomaly from the optical
Results and Discussion
Spectral properties of BaZrSi3 O9 and Ba2 Zr2 Si3 O12 .— Shown in
Fig. 2 is the excitation spectrum of BaZrSi3 O9 obtained by monitoring the host emission at 285 nm (curve a) and the emission spectrum of BaZrSi3 O9 under host excitation at 172 nm (curve b). Under
172 nm excitation, the undoped BaZrSi3 O9 emits a broad emission
centered at 285 nm, which is assigned to the charge transfer (CT)
luminescence from zirconate group11 and can only be excited at very
short wavelength. The maximum of the excitation band was found
at about 172 nm, making this phosphor interesting for application in
Hg-free fluorescent lamps. The Stokes shift of the CT luminescence
derived from the maximum of the emission and excitation spectrum
is 23,100cm −1 , and the full width at half-maximum (FWHM) of
the CT emission is about 14,300 cm−1 . The broad excitation band is
partially originated from the CT transition from O2− to Zr4+ , i.e., an
electron is excited from the 2p orbitals of the surrounding O2− ions
into the empty 4d orbitals of Zr4+ in the zirconium-oxygen octahedral
units.11 The reverse process of such transition is the Zr4+ -O2− CT
luminescence. When dealing with the Yb3+ -O2− CT luminescence,
Pieterson et al. pointed out that the CT transition doesn’t involve the
transfer of one electron, but involves a considerable reorganization of
the charge density distribution around the metal ion in reality.24 The
same situation is considered to occur in the case of the Zr4+ -O2− CT
Figure 3. (a) Excitation spectrum of Ba2 Zr2 Si3 O12 obtained by monitoring
the host emission at 334 nm and (b) emission spectrum of Ba2 Zr2 Si3 O12
excited at 172 nm.
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Journal of The Electrochemical Society, 158 (12) J377-J382 (2011)
J379
Table I. Selected interatomic distances (Å) and bond angles (o ) for BaZrSi3 O9 and Ba2 Zr2 Si3 O12 a .
Space group
(system)
BaZrSi3 O9
Ba2 Zr2 Si3 O12
a
D2 3h -P6c2 (hexagonal)
P21 3 (cubic)
Zr-O distances (Å)
Zr-O2(×6)
Zr1-O3(×3)
Zr1-O4(×3)
Zr2-O2(×3)
Zr2-O1(×3)
2.0421
2.1110
2.1137
2.1170
2.1398
Ba-Zr distances (Å)
Ba-Zr
Ba1-Zr2
Ba2-Zr2
Ba2-Zr1
3.8926
3.9439
3.8517
3.8897
Ba-O-Zr angles (o )
<Ba-O2-Zr>
<Ba2-O2-Zr2>
<Ba2-O3-Zr1>
<Ba1-O2-Zr2>
<Ba2-O4-Zr1>
107.1
93.2
98.8
96.5
91.3
These data are derived from the crystal structure data of BaZrSi3 O9 (ICSD 70105) and Ba2 Zr2 Si3 O12 (Pearson’s Crystal
Data. Data Sheet of- 377419)
gratings, because in our recent study a similar emission shoulder has
been observed in the emission spectra of some borate hosts (unpublished). The Stokes shift and FWHM of the Zr4+ -O2− CT luminescence in this sample is determined to be 24,400 cm−1 and 14,400 cm−1 ,
respectively. Although Zr4+ ions have the same coordination number
(CN) in both BaZrSi3 O9 (CN = 6) and Ba2 Zr2 Si3 O12 (CN = 6), it was
found the CT absorption of Ba2 Zr2 Si3 O12 located at a lower energy
than that of BaZrSi3 O9 , suggesting that it is easier for an electron to
be transferred from O2− 2p orbital to Zr4+ empty 4d orbital in the case
of Ba2 Zr2 Si3 O12 . This due to the fact that two zirconosilicates have
different crystal structures and bond lengths, then the CT energies of
O2− -Zr4+ are different consequently. Specifically, the crystal structure of Ba2 Zr2 Si3 O12 is cubic while that of BaZrSi3 O9 is hexagonal;
the mean O2− -Zr4+ bond length in Ba2 Zr2 Si3 O12 is 2.12 Å, while
that in BaZrSi3 O9 is 2.04 Å (see Table I). The longer O2− -Zr4+ bond
length in Ba2 Zr2 Si3 O12 results in the weaker O2− -Zr4+ bond strength
in Ba2 Zr2 Si3 O12 ; as a consequence, it will take less energy to remove
one electron from O2− ligands into the Zr4+ ions in Ba2 Zr2 Si3 O12 .
Furthermore, Shi et al. established a quantitative relationship between
the O2− -Zr4+ CT energy and the crystal structure recently by the
following empirical formula:
E CT = A + Be−the
[1]
where A, B and t are constants, he , called environmental factor, is
related to three chemical bond parameters (bond covalency (fc μ ), bond
volume polarization (αb μ ) and the presented charge of the nearest
anion in the chemical bond (Qμ )) by:29
1/2
he =
[2]
f cμ αμb (Q μ )2
The parameter calculations were based on the dielectric theory of
complex crystals,29 and the calculated values of O2− -Zr4+ CT absorption were in good agreement with the experimental results. However,
because the parameters calculations are complicated, which is beyond
our knowledge, we can’t predict the magnitude of O2− -Zr4+ CT absorption for Ba2 Zr2 Si3 O12 and BaZrSi3 O9 at present. Nevertheless,
Equations 1 and 2 indicated that O2− -Zr4+ CT energy is not only determined by the bond length but also greatly influenced by the bond
covalency, bond volume polarization and the presented charge of the
nearest anion in the chemical bond.
Electronic band structures of BaZrSi3 O9 and Ba2 Zr2 Si3 O12 .— To
support the above assignments regarding the possible origins of the
excitation spectra of BaZrSi3 O9 and Ba2 Zr2 Si3 O12 in the region below
200 nm, we have revealed the possible electronic transitions involves
in these two compounds. Further evidence to support occurrence of
O2− -Zr4+ charge transfer, theoretical calculations on the band structure, total density of states (DOS) and partial density of states (PDOS)
for both BaZrSi3 O9 and Ba2 Zr2 Si3 O12 were carried out and the results
are illustrated in Fig. 4. For BaZrSi3 O9 , an indirect band gap of 6.98 eV
can be concluded from the top of the valence band is at M and the bottom of the conduction band is at K. For Ba2 Zr2 Si3 O12, an indirect band
gap of 6.47 eV is concluded from the data of the top of the valence
band is at M and the bottom of the conduction band is at G. These
calculated band gap values are consistent with that reflected in the
excitation spectra BaZrSi3 O9 (absorption with maximum at 172 nm
corresponding to an energy about 7.22 eV) and Ba2 Zr2 Si3 O12 (absorption with maximum at 184 nm corresponding to an energy about
6.75 eV). According to the DOS and PDOS curves in Fig. 4, the top
valence bands for both BaZrSi3 O9 and Ba2 Zr2 Si3 O12 are dominated
by O 2p orbitals, and the bottom region of their conduction band are
mainly composed of the Ba (5d) and Zr (4d) orbitals. These results indicating charge transfer might occur not only in O(2p)-Zr(4d) but also
in O(2p)-Ba(5d) charge transfer. Therefore, besides the absorption of
O2− -Zr4+ CT transition and SiO4 4− group, the VUV excitation spectra
of BaZrSi3 O9 and Ba2 Zr2 Si3 O12 in the region below 200 nm may also
include a contribution from O2− -Ba2+ charge transfer transition.
Energy transfer from host excitations to Eu2+ in BaZrSi3 O9 :Eu2+
and Ba2 Zr2 Si3 O12 :Eu2+ .— The emission spectra of (Ba1-x Eux )
ZrSi3 O9 (0.5% ≤ x ≤ 8%) under excitation at 172 nm are shown in
Fig. 5. When Eu2+ is introduced into the host lattice of BaZrSi3 O9 , it
was found that the wavelength of host UV emission attributed to the
Zr4+ -O2− CT luminescence shifted to 276 nm and the UV emission
intensity was found to decrease with increasing Eu2+ concentration.
Accompanied by the decease of the UV emission intensity is an increasing of Eu2+ 4f 6 5d→4f 7 emission intensity peaking at 475 nm,
indicating that the energy transfer from host (Zr4+ -O2− CT luminescence) to Eu2+ ions occurs. As a result of energy transfer, the
Zr4+ -O2− CT luminescence is nearly quenched when Eu2+ concentration is raised to 8%; due to the concentration quenching effect, the
Eu2+ 4f 6 5d→4f 7 cyan emission starts to decrease when Eu2+ concentration exceeds 4%. In order to give further evidence to support
the occurrence of host-to-Eu2+ energy transfer via Zr4+ -O2− CT luminescence, we have measured the excitation spectra for a sample
of BaZrSi3 O9 :2%Eu2+ and the results are presented in Fig. 6. The
VUV excitation spectra were detected by monitoring the host emission at 276 nm and Eu2+ emission at 475 nm, respectively. The VUV
excitation spectra below 200 nm are almost the same, both are composed by a broad band with maximum at about 172 nm, which are
associated with the absorption of O2− -Zr4+ CT transition and SiO4 4−
group as discussed in BaZrSi3 O9 :Eu2+ . The presence of the O2− -Zr4+
CT transition in the excitation spectrum monitored within the Eu2+
transitions further indicates that the energy transfer from host to Eu2+
ions via Zr4+ -O2− CT luminescence has occurred. In the case of the
VUV excitation spectrum obtained by monitoring the Eu2+ emission
at 475 nm, there are two other bands located above 200 nm, which
correspond to the transitions from the ground 4f7 electronic configuration of Eu2+ ion to its excited 4f6 5d1 state split by the crystal field.
In BaZrSi3 O9 :Eu2+ , Ba2+ ions are coordinated by six oxygens,13 in
view of the ionic charges and ionic radii with six coordination (rBa 2+
= 135 pm, rZr 4+ = 72 pm and rEu 2+ = 117 pm),30 the incorporated
Eu2+ ions in BaZrSi3 O9 :Eu2+ are in the Ba2+ sites. According to
the crystal field theory, the five-fold 5d degenerate energy states of
Eu2+ would be split into two sets of energy levels in an octahedral
environments,31, 32 viz. a triply degenerate one designated by t2g (the
lowest) and a doubly degenerate one designated by eg (the highest).
Thus, the two bands located above 200 nm in the excitation spectrum of BaZrSi3 O9 :Eu2+ can ascribed to the transition from Eu2+
ground 4f7 state to its excited eg and t2g states. Furthermore, the
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Journal of The Electrochemical Society, 158 (12) J377-J382 (2011)
Figure 4. Electronic band structures of (a) BaZrSi3 O9 and (b) Ba2 Zr2 Si3 O12 as well as DOS and PDOS curves for (c) BaZrSi3 O9 and (d) Ba2 Zr2 Si3 O12 .
longer-wavelength portion of the host emission band (Zr4+ -O2− CT
luminescence) overlaps considerably with the Eu2+ 4f-5d absorption,
providing the beneficial condition for energy transfer from host to
Eu2+ via Zr4+ -O2− CT luminescence. The absorption of host emission
by Eu2+ would lead host emission shift to short wavelength side.
Figure 5. Emission spectra of BaZrSi3 O9 :xEu2+ (0.5% ≤ x ≤ 8%) excited at
172 nm.
Shown in Fig. 7 are the emission spectra of (Ba1-y Euy )2 Zr2 Si3 O12
(0.5% ≤ y ≤ 10%) under 172 nm excitation. The emission spectra of
Ba2 Zr2 Si3 O12 :yEu2+ are composed of Zr4+ -O2− CT luminescence at
335 nm and Eu2+ 4f 6 5d→4f 7 emission at 485 nm. The intensity of
Figure 6. VUV excitation spectra of BaZrSi3 O9 :2%Eu2+ monitored at
276 nm of the host emission and Eu2+ emission at 475 nm. The excitation
spectra are normalized on host absorption intensity.
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Journal of The Electrochemical Society, 158 (12) J377-J382 (2011)
Figure 7. Emission spectra of Ba2 Zr2 Si3 O12 :yEu2+ (0.5% ≤ y ≤ 10%) excited at 172 nm.
Zr4+ -O2− CT emission was found to decrease with increasing Eu2+
concentration, while the intensity of Eu2+ emission at 485 nm was
found to increase with increasing Eu2+ content, indicating that energy transfer from host to Eu2+ takes place in Ba2 Zr2 Si3 O12 :yEu2+ .
Due to the occurrence of energy transfer from the zirconate host to
Eu2+ , Zr4+ -O2− CT luminescence is reduced significantly when Eu2+
concentration is increased to 10%. Figure 8 is the VUV excitation
spectra of Ba2 Zr2 Si3 O12 :4%Eu2+ , which were measured by monitoring the host emission at 335 nm and Eu2+ emission at 485 nm,
respectively. The excitation spectra can be roughly divided into two
parts: a host lattice excitation, attributed to the O2− -Zr4+ CT transition
and SiO4 4− absorption, for spectral region with wavelengths shorter
than 215 nm, and a direct Eu2+ excitation region for wavelengths
longer than 215 nm. The presence of the O2− -Zr4+ CT transition in
the excitation spectrum monitored within the Eu2+ transitions further
proves that the occurrence of energy transfer from host via Zr4+ -O2−
CT luminescence to Eu2+ ions in Ba2 Zr2 Si3 O12 : Eu2+ .
Although the host emission (Zr4+ -O2− CT luminescence) overlaps significantly with Eu2+ absorption both in BaZrSi3 O9 :Eu2+ and
Ba2 Zr2 Si3 O12 :Eu2+ , and the average nearest neighboring Zr4+ -Ba2−
J381
distance (∼3.90 Å) in BaZrSi3 O9 :Eu2+ is close to that (∼3.89 Å)
in Ba2 Zr2 Si3 O12 :Eu2+ (see Table I), it is found that energy transfer from host to Eu2+ in Ba2 Zr2 Si3 O12 :Eu2+ is less efficient than
that in BaZrSi3 O9 :Eu2+ , and Eu2+ luminescence is inferior to that
of Ba2 Zr2 Si3 O12 :Eu2+ . It has been argued by Dexter and confirmed
by Blasse and Brilll that the energy transfer from donor to acceptor
in oxides was determined by wavefunction overlap (orbitals overlap of donor and acceptor using the O2− ion as an intermediary)
and energy overlap (spectral overlap of donor emission and acceptor
absorption).33 In both BaZrSi3 O9 :Eu2+ and Ba2 Zr2 Si3 O12 :Eu2+ , the
host emission (Zr4+ -O2− luminescence) overlaps considerably with
the Eu2+ absorption, then the energy transfer from host to Eu2+ ion
will be determined by orbital overlap. Blass et al. claimed that the
orbital overlap in rare earth ions doped oxides phosphors strongly
depended on the angle between the cation of the absorbing group
(donor), the O2− ion, and the rare-earth ions (acceptor). If this angle was 90◦ , the orbital overlap was assumed to be small (using π
bonding) and would lead a lower efficiency; if it was 180◦ , the orbital
overlap was assumed to be much larger (using σ bonding) and would
lead a higher efficiency.33 This hypothesis has been confirmed in
many phosphors contain transition metals, e.g., Gd2 WO4 :Eu3+ (low
efficiency, Gd-O-W angle is 90◦ ), Gd2 Ti2 O7 :Eu3+ (low efficiency,
Gd-O-Ti angle is 100◦ ), YVO4 :Eu3+ (high efficiency, V-O-Y angle
is 170◦ ) and Y2 WO4 :Eu3+ (high efficiency, Y-O-W is 180◦ ).33 In
BaZrSi3 O9 :Eu2+ the Ba-O-Zr angle is approximately 107◦ , whereas
in Ba2 Zr2 Si3 O9 :Eu2+ most Ba-O-Zr angles are below 100◦ and some
are close to 90◦ (see Table I). One more possible reason accounting
for the weaker Eu2+ luminescence in Ba2 Zr2 Si3 O12 :Eu2+ may due to
the presence of Eu2+ -Zr4+ metal-to-metal charge transfer (MMCT),
which may be viewed as transfer of an electron from Eu2+ to Zr4+
upon excitation, forming Eu3+ -Zr3+ in the excited state. The oxidation of Eu2+ is well possible. Since O2− -Zr4+ CT transition located
on a higher energy side in present case, Eu2+ -Zr4+ MMCT transition
is considered to locate at higher energy, which is accompanied by a
possible large relaxation and non-radiative return to the ground state.
Upon host excitation, the system crosses over into the MMCT state
and Eu2+ luminescence will be quenched. In Ba2 Zr2 Si3 O12 :Eu2+ , the
Eu2+ -Zr4+ MMCT energy may be relatively low, so Eu2+ emission is
largely quenched. This explanation is analogous to Delosh’s explanation to the quenching of Tb3+ emission in YVO4 :Tb3+34 and Blasse’s
explanation to quenching Ce3+ emission in YTaO4 :Ce3+ .35 Therefore,
the smaller Ba-O-Zr angle and the lower Eu2+ -Zr4+ MMCT energy
in Ba2 Zr2 Si3 O12 :Eu2+ may be responsible for the inefficient energy
transfer and Eu2+ luminescence observed in Ba2 Zr2 Si3 O12 :Eu2+ .
Conclusions
Figure 8. VUV excitation spectra of Ba2 Zr2 Si3 O12 :4%Eu2+ monitored at
335 nm of the host emission and Eu2+ emission at 485 nm. The excitation
spectra are normalized on host absorption intensity.
In summary, the photoluminescence properties of undoped and
Eu2+ -doped BaZrSi3 O9 and Ba2 Zr2 Si3 O12 have been studied under
vacuum ultraviolet excitation. Under host excitation at 172 nm, both
BaZrSi3 O9 and Ba2 Zr2 Si3 O12 show host emission at 285 nm and
334 nm, respectively, which were assigned to the Zr4+ -O2− charge
transfer luminescence. Although Zr4+ ions have the same coordination numbers in BaZrSi3 O9 and Ba2 Zr2 Si3 O12 , the energy of Zr4+ -O2−
charge transfer absorption is different, which can be rationalized by
considering the difference in crystal structures and Zr-O bond lengths.
Band structure calculations reveal that BaZrSi3 O9 and Ba2 Zr2 Si3 O12
have an indirect band gap about 6.98 eV and 6.47 eV, respectively,
which are consistent with those obtained from their VUV excitation spectra. Energy transfer from host to Eu2+ was observed in
BaZrSi3 O9 :Eu2+ and Ba2 Zr2 Si3 O12 :Eu2+ , showing that host sensitization occurs in these materials. Although the host emission (Zr4+ -O2−
CT luminescence) band overlaps significantly with the Eu2+ absorption band in both zirconosilicates and the average nearest Zr4+ -Ba2−
distances are close in both compounds. The energy transfer from host
emission to Eu2+ in Ba2 Zr2 Si3 O12 :Eu2+ is however less efficient than
that in BaZrSi3 O9 :Eu2+ and the Eu2+ luminescence was found weaker
in Ba2 Zr2 Si3 O12 :Eu2+ . These observations can be explained as that a
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Journal of The Electrochemical Society, 158 (12) J377-J382 (2011)
smaller Zr-O-Ba (Eu) angle in Ba2 Zr2 Si3 O12 :Eu2+ results in a smaller
Ba(Eu)-O-Zr orbital overlap, and a lower Eu2+ -Zr4+ metal-to-metal
charge transfer energy causes the Eu2+ luminescence to be largely
quenched in Ba2 Zr2 Si3 O12 :Eu2+ .
Acknowledgments
We gratefully thank the National Science Council of Taiwan
for financial support under Contract nos. NSC99-2811-M-009-052
(D.-Y. W.) and NSC98-2113-M-009-005-MY3 (T.-M. C.).
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