Luminescence of YPO4:Zr and YPO4:Zr,Mn under vacuum ultraviolet excitation

Article abstract of DOI:10.1149/1.1914750

Novel UV and blue phosphors, YPO₄:Zr and YPO₄:Zr,Mn, respectively, are synthesized and investigated. YPO₄:Zr shows the emission about 290 nm assigned to the charge transfer transition of O-Zr. The excitation spectrum of th

Full text of DOI:10.1149/1.1914750

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Journal of The Electrochemical Society, 152 ͑6͒ H80-H83 ͑2005͒
0013-4651/2005/152͑6͒/H80/4/$7.00 © The Electrochemical Society, Inc.
Luminescence of YPO4:Zr and YPO4:Zr,Mn under Vacuum
Ultraviolet Excitation
Masami Kaneyoshia,z and Eiichiro Nakazawab
aShin-Etsu Chemical Company, Limited, Takefu Plant, Takefu, Fukui 915-8515, Japan
bDepartment of Electrical Engineering and Electronics, Kogakuin University, Hachioji, Tokyo 192-0015,
Japan
Novel UV and blue phosphors, YPO4:Zr and YPO4:Zr,Mn, respectively, are synthesized and investigated. YPO4:Zr shows the
emission about 290 nm assigned to the charge transfer transition of O-Zr. The excitation spectrum of the emission has main band
peaked at 175 nm. The codoped phosphor YPO4:Zr,Mn shows the emission peaked at 477 nm assigned to the d-d transition of
Mn2+ within 3d5 configuration. The excitation bands of the blue emission are around 152 and 193 nm. YPO4:Mn does not show
any emission and the 290 nm emission observed for YPO4:Zr is diminished for YPO4:Zr,Mn. It is suggested that Zr plays an
important role in the excitation of the Mn blue emission of YPO4:Zr,Mn. The emission wavelength of YPO4:Zr,Mn is remarkably
short compared with the usual Mn2+ activated phosphors.
© 2005 The Electrochemical Society. ͓DOI: 10.1149/1.1914750͔ All rights reserved.
Manuscript submitted August 5, 2004; revised manuscript received January 10, 2005. Available electronically April 26, 2005.
Recently, the luminescence of the phosphors in the vacuum ul-
traviolet ͑VUV͒ region has become important for the improvement
of plasma display panels ͑PDP͒ and the development of the Hg-free,
rare-gas discharge lamps. The light of 147 and 172 nm emitted by
Xe plasma is used in both devices for the excitation of phosphors. In
the case of the 147 nm excitation, since the wavelength is shorter
than the absorption edge of the host crystal, the penetration depth of
the excitation light into phosphor particles is so shallow that most of
the excitation light is absorbed by the host crystal. Therefore it is
necessary for VUV phosphors to have a high energy-transfer rate
from the host to the activator.
On the other hand, at the present time PDP blue-emitting phos-
phors activated with Eu2+ are used, for the phosphors emit brightly
and have a suitable color. Among such phosphors, the most popu-
larly used is BaMgAl10O17:Eu ͑BAM͒. BAM has the problem of
how to reduce the drop of luminance during the panel fabrication
process and in the use of the panels. Some approaches to solve this
problem have been made through the improvement of the process of
preparation of the phosphor.1 An alternative host activated with
Eu2+, CaMgSi2O6, e.g., has also been proposed.2 The authors have
the question, however, whether an activator other than Eu2+ is pos-
sible for the VUV-excited blue phosphor.
͑NH4͒2HPO4 were mixed in a mortar. The mixture was fired in
nitrogen flow at 1473 K for 3 h. All samples were a white-powder
although some that contained a large amount of Mn were slightly
pink, which is evidence that manganese is doped as Mn2+. For the
purpose of comparison, the samples, YPO4:Zr,Mg and
YPO4:Zr,Zn, were also prepared, using MgO or ZnO instead of
MnC2O4. ZrP2O7:Mn and Mn2P2O7 samples were also prepared, in
a similar method except the calcination temperature was 1273 K.
Powder X-ray diffraction was carried out for the YPO4-based
phosphor samples. All the pattern was composed mainly of the
peaks assigned to YPO4 ͑xenotime͒. The pattern of the sample that
has 2.5 mol % Zr/Y + Zr, however, contains the trace of ZrP2O7,
and the ZrP2O7 peaks are obvious in the pattern of the sample with
5.0 mol % of Zr. Thus it is considered that the solubility of Zr into
YPO4 is limited to the extent of a few mol percents of the Y sites.
On the other hand, the peaks assigned to Mn2P2O7 appear in the
pattern of the sample that contain 5.0 mol % of Mn. The solubility
of Mn into YPO4 is estimated at less than 5 mol % of Y sites.
Measurement of luminescence.— Luminescence emission spec-
tra and excitation spectra of each samples were recorded using the
vacuum ultraviolet spectrofluorometer ͑Bunko Keiki Co., Ltd.͒,
which was equiped with a deuterium lamp and the Seya-Namioka-
type monochromator. All the measurements were carried out at room
temperature. The luminescence of the commercial BAM:Eu phos-
phor ͑NP-107-06, Nichia Corporation͒ was also measurerd. The ex-
citation spectra were calibrated, and the influence of the spectrum
distribution of the light source is compensated, using sodium salicy-
late as the standard.
To our knowledge, there are not many elements that show effi-
cient light emission in the visible range when doped with transparent
host crystals. Mn2+ seems to be a candidate for alternative to Eu2+
.
The emission wavelength of the known Mn-activated phosphors is
distributed in a rather wide range, from green to red.3 In addition,
there are some efficient Mn-activated phosphors known, although
some of them need to be codoped with an element ͑sensitizer͒ to-
gether with Mn. In this report, the luminescence of YPO4 phosphors
activated by Zr4+ and/or Mn2+ under VUV excitation is investigated.
Results and Discussion
YPO4:Zr.—Yttrium phosphate ͑YPO4͒ activated only with Zr
shows a broad emission in the ultraviolet region under vacuum ul-
traviolet excitation. Figure 1 shows the emission spectra of the
sample with a Zr content of 2.5 mol % ͑Zr/Y + Zr͒. The peak of the
emission is at 291 nm. The luminescence excitation spectrum of the
sample, which was measured for the emission peak, is shown in Fig.
2, in which the excitation peak is at 176 nm and the shoulder is at
about 155 nm.
The luminescence is obviously originated from Zr. YPO4 has the
zircon-type crystal structure ͑space group I41/amd͒.4 The Zr ion
doped in YPO4 is thought to replace the Y site eightfold coordinated
by oxygen ions, as it occupies in isostructural ZrSiO4 ͑zircon͒. The
luminescence of Zr as an emitting center has been reported for
Cs2ZrCl6,5 although there is no such example of the oxide materials.
The emission peak and excitation peak of the chloride have been
reported to be 451 and 258 nm. The luminescence of the present
study can be understood by analogy with luminescence of oxygen-
Experimental
Preparation of phosphors.—As a starting material for the phos-
phor preparation, yttrium phosphate ͑YPO4͒ was precipitated by
mixing an aqueous solution of yttrium chloride into a solution of
phosphoric acid that contains an excess of the acid at 350 K. The
precipitate was then filtrated, dried, and calcined in air at 1073 K.
Manganese oxalate ͑MnC2O4͒ was obtained as the precipitate
through the reaction of manganese chloride and ammonium oxalate
in aqueous solution. The precipitate was air-dried. The chemical
analyses of the prepared materials were carried out in order to quan-
tify the content of Y, Mn and P in the materials.
Phosphor samples, YPO4:Zr and/or Mn, were synthesized as fol-
lows. Calculated amounts of above obtained materials, ZrO2 and
z E-mail: masami.k@shinetsu.jp
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Journal of The Electrochemical Society, 152 ͑6͒ H80-H83 ͑2005͒
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Figure 1. Emission spectra of YPO4:Zr ͑2.5 mol %͒: ͑a͒ ␭ = 176.0 nm
ex
Figure 3. Concentration dependence of the emission intensity of YPO4:Zr.
͑solid line͒, ͑b͒ ␭ = 147.0 nm ͑broken line͒.
ex
290 nm emission, ␭ = 175 nm.
ex
coordinated, closed-shell-type transition-metals ͑Mm+ or MOxn− M:
V, W, Nb, Mo, Ti, etc.͒.6-8 It is known that their emission peaks are
mainly in blue region. The luminescence of the present study is
ascribed to the charge-transfer ͑CT͒ transition, the excitation to elec-
tron transfer O → Zr and the emission to Zr → O. Both the emis-
sion and excitation band are situated in a shorter wavelength region
than in the above M-O system. The reason is thought to be that Zr4+
attracts electrons less than W6+, Nb5+, etc., and even Ti4+ because of
its smaller positive charge or shielding effect by 3d, 4p, 5s electrons.
Compared with the Zr-Cl system, the fact that O is more electrone-
gative than Cl results in the larger transition energy and shorter
luminescent wavelength in this work. Another example of the VUV
excited luminescence of the closed-shell cation, has recently been
reported for BO33−, which emits at 310 nm.9
under the VUV or UV excitation from 130 to 330 nm. The reason is
thought to be that the absorption by which Mn2+ can be excited to
the emitting level of luminescence is very weak, because it is a
forbidden d-d transition. It is not the rare case that a phosphor acti-
vated only with Mn, without any sensitizer, does not show photolu-
minescence and only emits under excitation by electrons.3
YPO4:Zr,Mn.— YPO4 activated with both Zr and Mn shows
ultraviolet and blue emission under VUV excitation. The emission
spectra of a sample are shown in Fig. 4. The peak of the blue
emission is at 477 nm and that of the UV emission is at 291 nm. The
ratios of the height of the two peaks change under the excitation at
different wavelengths. The excitation spectra of the same sample are
shown in Fig. 5. The spectrum for the blue emission has two bands,
the peaks of which are 152 and 193 nm. Additionally, the emission
intensity is rising at 130 nm, the limit of measurement, which sug-
gests that possibly there is another excitation band at Ͻ130 nm. The
UV emission has an excitation band around 175 nm.
The Zr concentration dependence of emission intensities of
YPO4:Zr is shown in Fig. 3. The maximum of the intensity is situ-
ated around the point that Zr/Y + Zr = 2.5 mol %, and beyond that
point, the intensity is gradually weakened.
First, the blue emission is considered to originate from Mn2+ that
replaces the Y site of YPO4. A confirmation is the fact that neither
YPO4:Mn.— YPO4 activated with Mn, concentrations of 0.5 and
2.5 mol %, were prepared and their luminescence studied. They
both did not show any emission in the range from 250 to 750 nm
Figure 4. Emission spectra of YPO4:Zr ͑1.1 mol %͒, Mn ͑1.1 mol %͒: ͑a͒
␭
= 192.6 nm ͑solid line͒, ͑b͒ ␭ = 175.2 nm ͑dotted line͒, ͑c͒ ␭
ex
e
x
e
x
Figure 2. Excitation spectrum of YPO4:Zr ͑2.5 mol %͒. ␭ = 290.2 nm.
= 147.0 nm ͑dash-dotted line͒.
em
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Journal of The Electrochemical Society, 152 ͑6͒ H80-H83 ͑2005͒
Figure 5. Excitation spectra of YPO4:Zr ͑1.1 mol %͒, Mn ͑1.1 mol %͒: ͑a͒
= 477.0 nm ͑solid line͒, ͑b͒ ␭ = 294.2 nm, three times as measured
data ͑broken line͒.
Figure 6. Mn concentration dependences of emission of YPO4:Zr, Mn
␭
em
em
͑␭ = 193 nm͒. Triangles are for blue emissions, and circles are for UV
ex
emissions. Zr conc. = 2.5 mol %. Note that 193 nm is not at the peak but
skirts the excitation band for the UV emission.
ZrP2O7:Mn nor Mn2P2O7 show this blue emission. In addition, the
blue emission is not likely to originate from the Zr center of
YPO4:Zr,Mn. Luminescence of YPO4:Zr ͑2 mol %͒, Mg
͑2 mol %͒, and YPO4:Zr ͑2 mol %͒, Zn ͑2 mol %͒ was measured.
Both samples showed emissions only in the UV region under VUV
excitation, which is very similar to the emission of YPO4:Zr, though
the intensity of the emission was weaker. Neither sample showed a
blue emission like YPO4:Zr,Mn. Therefore the possibility that the
circumstances of the Zr center were changed by the addition of Mn
and the wavelength of Zr emission shifted is rejected. For the incor-
The Mn concentration dependence of emission intensities of
YPO4:Zr,Mn is shown in Fig. 6. The intensity of the blue emission
shows its maximum at 1.1 mol % Mn, and beyond that point, the
intensity is gradually weakened. The UV emission intensity de-
creases rapidly as the Mn concentration increases. The Zr concen-
tration dependence of emission intensities of YPO4:Zr,Mn is shown
in Fig. 7. The intensity of the blue emission increases rapidly in low
Zr concentration and is saturated in at 0.6 mol %.
poration of Mg2+ or Zn2+, which has an ionic radius similar to Mn2+
,
The emission under 147 nm excitation of the commercial
BAM:Eu phosphor was measured. The emission peak is at 456 nm
and the half width is 55 nm. When the Zr ͑1.1 mol %͒, Mn
͑1.1 mol %͒ sample ͑see Fig. 4͒, which shows the brightest blue
emission among the series of YPO4:Zr,Mn, is compared with the
commercial BAM:Eu for the emission under 147 nm excitation, the
peak height is 25% of the BAM:Eu one and the half width is 63%.
In order to be evaluated for practical use, YPO4:Zr,Mn should im-
is likely to give a similar change to the circumstances of Zr center.
There are numerous examples of the phosphor for practical use that
employ Mn2+ as the activator.3 The emission is usually explained as
the transition T1͑4G͒ → A1͑6S͒ of Mn2+ within the 3d5 configu-
ration. It is known that the peak wavelength of the emission of Mn2+
phosphor varies according to its host crystal. The emission peak in
the present study, 477 nm, is one of the shortest, very close to
471 nm for SrSb2O6:Mn, the shortest that is reported.10 On the basis
of what the crystal field theory teaches, the weaker the field strength
at the site which Mn2+ occupies, the shorter the emission wave-
length is.11-14 On the other hand, the crystal field at the site of YPO4
was reported to be rather weak, in connection with the absorption
and emission of YPO4:Ce.15
4
6
Second, the excitation bands for the blue emission are discussed.
The band peaking at 152 nm corresponds to the absorption of the
host crystal or PO4 group.16 The other excitation band is located
around 193 nm. This is near to the excitation band of YPO4:Zr but
does not overlap with it. One should remember the fact that
YPO4:Mn without Zr does not show any emission. In addition, the
codoping of Mn to YPO4:Zr has resulted in diminishing the Zr
emission at 290 nm ͑see also Fig. 6͒. Therefore we claim that Zr
plays an important role in the excitation of the Mn blue emission of
YPO4:Zr,Mn, although the mechanism for the excitation cannot be
determined only by the data of emission and excitation spectra. The
possibility that the excitation bands originate from ZrP2O7:Mn or
MnP2O7 is again rejected, for those samples do not show a blue
emission.
Third, the UV emission band in Fig. 4 is quite similar to that of
YPO4:Zr in Fig. 1, and the excitation spectra for these UV emis-
sions in Fig. 2 and Fig. 5 are also similar to each other. We claim
that the UV emission of YPO4:Zr,Mn occurs from Zr4+ that does
not have a Mn neighbor and thus its environment is similar to Zr4+
in YPO4:Zr.
Figure 7. Zr concentration dependences of emission of YPO4:Zr, Mn ͑␭
= 193 nm͒. Mn conc. = 1.1 mol %. See captions of Fig. 6.
ex
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Journal of The Electrochemical Society, 152 ͑6͒ H80-H83 ͑2005͒
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Conclusions
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excitation spectrum of this emission has the main band peaked at
175 nm and with a shoulder at 155 nm. This luminescence is as-
signed to the CT transition of O-Zr.
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The Zr, Mn codoped material, YPO4:Zr,Mn shows a blue emis-
sion peaked at 477 nm, which is assigned to the transition
6
4T1͑4G͒ → A1͑6S͒ of Mn2+ within 3d5 configuration. The excita-
tion spectrum of the emission has two bands, the peaks of which are
152 and 193 nm. The excitation band at 152 nm corresponds to the
absorption of the host crystal. YPO4:Mn does not show any emis-
sion, and the 290 nm emission due to Zr is diminished in
YPO4:Zr,Mn. It is suggested that Zr plays an important role in the
excitation of the Mn blue emission of YPO4:Zr,Mn.
14. K. Narita, J. Phys. Soc. Jpn., 18, 79 ͑1963͒.
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The emission wavelength ͑477 nm͒ of YPO4:Zr,Mn is remark-
ably short, which is similar to the known shortest record.
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