Luminescence and photometric characterization of K+ compensated CaMoO4:Dy3+ nanophosphors

Article abstract of DOI:10.1039/c3dt50780g

A series of CaMoO₄ phosphors doped with trivalent dysprosium (Dy³⁺) and codoped with potassium (K⁺) ions were prepared by hydrothermal method. The nanostructures of the as-synthesized phosphors were investigated by X-ray diffraction (XRD). The results reveal that the obtained powder phosphors are single-phase scheelite structure with tetragonal symmetry and the crystallite size is in the range of 10-60 nm. The emission spectra show a bright yellow emission at 576 nm and blue emission at 487 nm. As a charge compensator, K⁺ ions were incorporated into CaMoO₄:Dy ³⁺ phosphors, which enhance the PL intensities depending on the doping concentration of K⁺. The CIE parameters such as colour coordinates, colour correlated temperature and luminous efficacy of radiation were calculated using spectral energy distribution functions and McCamy's empirical formula. Photometric characterization indicates the suitability of K⁺ compensated CaMoO₄:Dy³⁺ phosphor for white LED applications. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3dt50780g

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Luminescence and photometric characterization of K+
compensated CaMoO4:Dy3+ nanophosphors
Cite this: DOI: 10.1039/c3dt50780g
S. Dutta, S. Som and S. K. Sharma*
A series of CaMoO4 phosphors doped with trivalent dysprosium (Dy3+) and codoped with potassium (K+)
ions were prepared by hydrothermal method. The nanostructures of the as-synthesized phosphors were
investigated by X-ray diffraction (XRD). The results reveal that the obtained powder phosphors are single-
phase scheelite structure with tetragonal symmetry and the crystallite size is in the range of 10–60 nm.
The emission spectra show a bright yellow emission at 576 nm and blue emission at 487 nm. As a charge
compensator, K+ ions were incorporated into CaMoO4:Dy3+ phosphors, which enhance the PL intensities
depending on the doping concentration of K+. The CIE parameters such as colour coordinates, colour cor-
related temperature and luminous efficacy of radiation were calculated using spectral energy distribution
functions and McCamy’s empirical formula. Photometric characterization indicates the suitability of
K+ compensated CaMoO4:Dy3+ phosphor for white LED applications.
Received 23rd March 2013,
Accepted 15th April 2013
DOI: 10.1039/c3dt50780g
www.rsc.org/dalton
yellow-to-blue intensity ratio (Y/B) appropriately, it is possible
to obtain pure white light from Dy3+ activated CaMoO4.5
1. Introduction
White light-emitting diodes (LEDs) are in high demand in
Currently nanotechnology and nanomaterials have attracted
solid-state lighting technology as a replacement for conven- several researches from the field of luminescence due to their
tional incandescent and fluorescent lamps.1 There are basi- potential applications in different light emitting devices.
cally two approaches to generate white light from LEDs. One is Different controlled approaches have been successfully carried
by mixing light of different colors emitted by several chips out recently to synthesize nanoparticles.6,7 In the present
called multichip LED and the other is to convert the light work, a series of Dy3+ doped CaMoO4 nanophosphors are pre-
emitted from a blue or ultraviolet (UV) LED to a longer wave- pared by hydrothermal synthesis and their luminescence and
length light using phosphors, called phosphor-converted (pc)- photometric properties are investigated. The codoping with K+
LED.2 Molybdates have attracted huge interest for pc-LEDs was done as charge compensator in order to obtain pure white
because of strong and broad absorption band of Mo–O bond light from this phosphor.8,9
in the near ultraviolet region. (MoO4)2− transfer their absorbed
energy to the dopant ions, therefore being an excellent host for
phosphor materials.3
2. Experimental
2.1. Sample synthesis
CaMoO4 has a broad emission from 330 to 550 nm with a
high absorption cross section (∼10 cm−1) due to its large band
3+
gap. Since the emission band is very broad, it is difficult to
tune the color for lighting and display applications. On the
other hand, because of the forbidden f–f transitions of Dy3+
Ca1−xMoO4:Dyx (x = 0, 0.01, 0.015, 0.02, 0.03, 0.04, 0.05)
3+
+
doped phosphors and Ca1−x−yMoO4:Dyx Ky (x = 0.02, y =
0.01, 0.02, 0.03, 0.04, 0.05) codoped phosphors were pre-
pared by hydrothermal synthesis. The reactants CaCO3,
(NH4)6Mo7O24·4H2O, Dy2O3 and K2CO3 were weighted in an
appropriate stoichiometric ratio. All reagents were analytical
grade and used without further purification. In a typical syn-
thesis, solution A was prepared by dissolving CaCO3 and Dy2O3
in diluted nitric acid. The solution was heated to drive away
the unreacted nitric acid, and the residue was redissolved in
10 mL of deionized water and stirred for 1 hour at room temp-
erature. (NH4)6Mo7O24·4H2O was dissolved in 50 mL of de-
ionized water named solution B. After stirring for about
15 minutes, solution B was added dropwise into the solution A
ions, they have low absorption cross-section (∼0.01–1 cm−1
)
and hence show poor luminescence emission. The forbidden
transitions of Dy3+ ion can be relaxed by the crystal field (sur-
roundings) when these ions occupy crystallographic sites of
the CaMoO4 lattice.4 Dy3+ exhibits, usually, two principal emis-
sions in the blue and yellow regions corresponding to the
4F9/2→6H15/2 and 4F9/2→6H13/2 transitions. By adjusting the
Department of Applied Physics, Indian School of Mines, Dhanbad, 826004, India.
E-mail: sksharma.ism@gmail.com; Fax: +91 3262296563; Tel: +91 3262235412
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under vigorous stirring. Then the pH value of the solution was luminous efficacy of radiation (LER), which is the ratio of lumi-
adjusted between 8–9 using NaOH solution (1 N) under stir- nous flux (lumen) emitted by the phosphor to the input electri-
ring and the resulting precursor solutions were further stirred cal power (watt).
for half an hour. After that, the solution was poured into an
80 mL capacity Teflon-lined stainless steel autoclave and
heated subsequently to 180 °C for 12 hours. After cooling
down to room temperature naturally, the nanophosphors were
3. Results and discussions
directly collected at the bottom of the vessel. The phosphors
3.1. Structural characterization
were filtered, washed several times with deionized water and
absolute ethanol, and dried in an oven at 100 °C for 5 hours.10
3.1.1. XRD studies. To investigate the structure of pre-
pared nanophosphors and to see the influence of K+ and Dy3+
doping in the host calcium molybdate lattice, X-ray diffraction
(XRD) pattern of the phosphors were recorded (Fig. 1). The
figure shows the XRD spectra of undoped CaMoO4 phosphor,
2 mol% Dy3+ doped CaMoO4, Dy3+ (2 mol%) and K+ (1, 2, 3, 4
and 5 mol%) codoped CaMoO4 phosphors. The XRD patterns
show sharp and single diffraction peaks of tetragonal CaMoO4
crystal structure (JCPDS pdf no. 77-2244) with space group I41/
a (88) and no peaks of other phases were observed. The (h k l)
values of most prominent peaks are shown in the XRD pattern.
The lattice parameters of the unit cell are a = b = 5.123 Å and
c = 11.11 Å with α = β = γ = 90° are in correspondence with the
JCPDS database 77-2244. Additionally, all the powders with
different Dy3+ and K+ concentrations show the very similar
XRD patterns. This indicates that the substitution of Dy3+ and
K+ for Ca2+ did not influence the phase and crystal structure of
CaMoO4 suggesting that these dopants and codopants might
have occupied the cationic sites in the host lattice structure,
following the charge balancing:8 2Ca2+ = Dy3+ + K+.
For preparing K+ codoped phosphors, solution A was prepared
by dissolving CaCO3, Dy2O3 and K2CO3 in diluted nitric acid
and the rest of the procedure was same.
2.2. Sample characterization
X-Ray diffractogram of the prepared phosphors were recorded
in a wide range of Bragg angle 2θ (10° ≤ 2θ ≤ 80°) using
Bruker D8 Focus X-Ray diffractometer with Cu Kα radiation (λ =
0.154056 nm). The photoluminescence studies were carried
out using Hitachi make F-2500 Fluorescence Spectropho-
tometer in the wavelength range 220 nm–650 nm. All the
studies were carried out at room temperature. The CIE para-
meters11 were determined using the Colour calculator soft-
ware. The colour of any light source can be described just by
three variables called as color matching functions and are
ˉ
ˉ
ˉ
expressed by dimensional quantities x(λ), y(λ) and z(λ). For a
given power-spectral density P(λ), the degree of stimulation
required to match the colour of P(λ) is given by three equations
based on three tristimulus values X, Y and Z,
The enhancement in some of the XRD peak intensities is
due to the incorporation of K+ thus improving the crystallinity.
However, for more than 2 mol% of K+ codoping, the observed
diffraction peak intensities are relatively less since at such
high concentration of K+ ions it is difficult to replace the
Ca2+ sites by K+ for charge compensation; hence the excess
K+ ions induce defects which eventually decrease the crystal-
line nature.
ð
ˉ
X ¼ xðλÞPðλÞdλ
λ
ð
ˉ
Y ¼ yðλÞPðλÞdλ
λ
ð
ˉ
Z ¼ zðλÞPðλÞdλ
λ
The tristimulus values give the stimulation (i.e. power) for
each of the three primary red, green and blue colours needed
to match the color of P(λ). The color coordinates x and y were
then calculated from the tristimulus values according to
equations:
X
x ¼
X þ Y þ Z
Y
y ¼
X þ Y þ Z
The quality of white light in terms of colour correlated
temperature (CCT) is given by the McCamy empirical
formula:12
CCT ¼ ꢀ437n3 þ 3601n2 ꢀ 6861n þ 5514:31
where n = (x − xe)/(y − ye) is the inverse slope line and chroma-
ticity epicentre is at xe = 0.3320 and ye = 0.1858. The energy
efficiency of the phosphors were evaluated in terms of Fig. 1 XRD spectra.
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3.1.2. Structural parameters. The structural parameters
such as lattice constants, crystallite size, dislocation density,
3+
microstrain and inter-planar spacing of Ca1−xMoO4:Dyx (x =
3+
0, 0.01, 0.015, 0.02, 0.03, 0.04, 0.05) and Ca1−x−yMoO4:Dyx
+
Ky (x = 0.02, y = 0.01, 0.02, 0.03, 0.04, 0.05) were determined
from the XRD data.
The lattice constants were calculated using the following
formula:13
λ2
sin2 θ ¼ ½fðh2 þ k2Þ=a2g þ fl2=c2gꢁ
ð1Þ
4
The crystallite size (D)14 was determined using the Debye
Scherrer equation:
D ¼ 0:9λ=β cos θ ðDebye–Scherrer formulaÞ
ð2Þ
The strain and the crystallite size were also calculated from
the full-width at half-maximum (FWHMs) of the diffraction
peaks. The FWHMs (β) can be expressed as a linear combi-
nation of the contributions from the strain (ε) and crystallite
size (D) through the Hall and Williamson relation:14
β cos θ=λ ¼ 1=D þ ε sin θ=λ ðHall–Williamson equationÞ ð3Þ
Fig. 2(a) and (b) shows the plot of β cos θ/λ versus sin θ/λ for
all the doped/codoped phosphors, which are almost straight
line. The reciprocal of intercept on the β cos θ/λ axis gave the
average crystallite size. The slope of this straight line plot indi-
cates the microstrain present in the phosphors.
The dislocation density (δ) and microstrain (ε) were calcu-
lated using following relations:14
Fig. 2 Hall Williamson plot.
1
D2
δ ¼
ð4Þ
β cos θ
3.2. Luminescence characterization
ε ¼
ð5Þ
4
Fig. 4 shows the excitation and emission spectrum of undoped
CaMoO4. The excitation spectrum was resolved into two Gaus-
sian peaks as shown in the inset. The first peak (a) at about
245 nm is due to band to band transition and second peak (b)
at 277 nm is Mo–O charge transfer band in which an oxygen
2p electron goes into one of the empty molybdenum 4d orbi-
tals.4 The emission spectrum shows a broad band located at
about 490 nm, which is attributed to the intrinsic lumines-
cence of the host lattice.
The inter-planar spacing for the highest peak were calcu-
lated using the following formula:13
1
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
dhkl
¼
ð6Þ
h2 þ k2 l2
þ
a2
c2
The structural parameters are summarized in Table 1.
It was seen that crystallite size and microstrain vary slightly
in a random way after doping and codoping. Therefore it
shows that the little amount of dopant/co-dopant does not
affect the crystal structure and morphology.
Influence of Dy3+ doping: concentration quenching. Fig. 5
3+
shows the emission and excitation spectra of CaMoO4:Dyx
nanophosphors (x = 1, 1.5, 2, 3, 4, 5 mol%). In this case the
excitation spectrum was resolved into three Gaussian peaks
(inset of Fig. 5). The extra peak (c) for Dy doped CaMoO4 at
305 nm was observed due to the Dy3+ to O2− charge transfer.15
The peaks (d–i) at 351 nm (6H15/2→6P7/2), 365 nm
(6H15/2→6P5/2), 390 nm (6H15/2→4I13/2), 428 nm (6H15/2→4G11/2),
450 nm (6H15/2→4I15/2) and 475 nm (6H15/2→4F9/2) are due to
f–f transition of Dy3+ which is absent in the undoped phos-
phor.16 The emission spectra of the phosphors recorded at
351 nm excitation wavelength consist of two dominating emis-
sions at 489 and 579 nm, corresponding to 4F9/2→6H15/2 and
3.1.3. TEM Studies. TEM images of undoped, Dy3+
(2 mol%) doped and K+ (2 mol%) codoped phosphors are shown
in Fig. 3(a–c). The images of the hydrothermally synthesized
phosphors exhibit nanorod structures. The nanorods are rela-
tively straight and uniform in diameter ∼20 nm regardless of
their compositional differences. This is in agreement with
average crystallite size values calculated from Scherrer’s
equation and Hall–Williamson plot. The SAED pattern of the
codoped phosphor is given in Fig. 3d showing concentric rings
which indicate that the phosphor is crystalline in nature.
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Table 1 Structural parameters of the doped and codoped phosphors
Lattice
Parameters (Å)
Crystallite size
(nm)
Dislocation
Inter-planer
spacing
(dhkl) (Å)
Concentration
(mol%)
Scherrer
formula
W–H
plot
Microstrain
density (δ)
Phosphor
a = b
c
(ε) ( 10−3
)
(× 1014) (m−2
)
3+
Ca1−xMoO4:Dyx
JCPDS
x = 0
x = 1
x = 1.5
x = 2
x = 3
x = 4
x = 5
x = 2, y = 1
x = 2, y = 2
x = 2, y = 3
x = 2, y = 4
x = 2, y = 5
5.13
5.12
5.12
5.10
05.10
05.09
05.09
5.03
5.136
5.044
5.033
2.025
5.020
11.11
11.09
11.08
11.06
11.06
11.05
11.04
11.04
11.14
10.94
10.91
10.80
10.67
—
—
—
—
3.037
3.012
3.012
3.014
2.996
3.012
3.030
3.030
3.090
3.036
3.009
3.001
2.99
15–60
15–65
20–60
20–60
12–65
20–65
30–65
15–60
15–65
10–65
10–60
10–60
25.77
15.63
27.42
29.42
15.77
25.63
20.57
25.34
18.94
12.71
12.57
13.65
1.24
1.54
1.99
1.24
1.78
1.83
1.86
1.69
1.49
1.56
1.88
1.75
15.06
40.96
13.30
11.55
40.21
15.23
23.63
15.57
27.88
61.90
63.29
53.67
Ca1−x−yMoO4:Dyx3+, Ky
+
Fig. 4 Emission and excitation spectra of CaMoO4.
Fig. 3 TEM images of (a) CaMoO4, (b) Ca0.98MoO4:Dy0.023+, (c) Ca0.96MoO4:
Dy0.023+, K0.02+ and (d) SAED pattern.
4F9/2→6H13/2 transitions, respectively. It was observed that after
doping, the intrinsic emission peak is suppressed markedly.
This is because the strong blue emission band of Dy3+ ion at
2−
489 nm is very close to the intrinsic emission of MoO4 ions
(∼490 nm), so they can be effectively overlapped and thus gets
suppressed. The 4F9/2→6H15/2 transitions are magnetic dipole
transitions and are much less sensitive to the coordination
environment.
The 4F9/2→6H13/2 transition belongs to a forced electric
dipole transition, which is allowed only in the case that the
Dy3+ ions are located at the local sites with non inversion
center symmetry.17 The yellow emission (4F9/2→6H13/2) is stron-
ger than the blue emission (4F9/2→6H15/2), indicating that Dy3+
is located in more non centrosymmetric position in the
3+
Fig. 5 Emission and excitation spectra of Ca1−xMoO4:Dyx
.
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CaMoO4 host. As the radius of Dy3+ and Ca2+ are comparable, Considering the energy match rule, the following cross-relax-
Dy3+ can easily enter into the four fold coordination Ca2+ low ation channels (CRC1, CRC2 and CRC3) among Dy3+ are
symmetry sites (without an inversion centre).
responsible for population decrease of 4F9/2 level:
Fig. 6 is the schematic energy-level diagram showing
the excitation and emission transitions of Dy3+ in CaMoO4
host.
6
6
6
6
4F9=2 þ H15=2 ! H9=2= F11=2 þ F5=2
6
4F9=2 þ H15=2 ! H7=2= F9=2 þ F3=2
6
6
6
The effect of Dy3+ ion concentration on emission intensities
of CaMoO4 is shown in Fig. 7. As the concentration increases,
the emission intensity increases, and it reaches to maximum
at x = 0.02, which was taken as the optimum concentration.
Beyond this optimum concentration, the intensity
decreases due to the concentration quenching phenomena as
per the cross-relaxation mechanism (Fig. 6) resulting from res-
onance energy transfer between neighbouring Dy3+ ions.
6
4F9=2 þ H15=2 ! F1=2 þ H9=2= F11=2
6
6
6
This is the process where the excitation energy is trans-
ferred from a Dy3+ ion in highly excited state to a neighboring
Dy3+ ion and promotes the latter from the ground state to the
metastable level. The Dy3+ ions at F9/2 level are de-excited via
4
these three cross-relaxation processes while the ground state
6
Dy3+ ions will accept the energies from the Dy3+ at H15/2 level
simultaneously. Finally, all the Dy3+ ions will go in their
4
ground states and thus the luminescence related to F9/2 level
is quenched.18
The concentration quenching is mainly caused by the non-
radiative energy transfer among Dy3+ ions, which occurs as a
result of an exchange interaction, radiation reabsorption, or a
multipole–multipole interaction. The type of the interaction
mechanism can be identified by knowing the critical distance
(Rc) between the neighbouring Dy3+ ions. According to Blasse,
the critical distance can be expressed as:
ꢀ
ꢁ
1=3
3V
4πCoN
Rc ¼ 2
ð7Þ
where V is the volume of the unit cell, Co is the optimum con-
centration of the activator ions and N is the number of ions
per unit cell.19 For the CaMoO4 host, when N = 4 (Z = 1), Co=
0.02, and V = 291.58 Å3, the obtained Rc value between Dy3+–
Dy3+ is 19 Å. The value of Rc greater than 5 Å for the rare earth
ions indicates that the multipole–multipole interaction is
dominant and is the major cause of concentration quenching
of Dy3+ in the phosphors. There are several types of multi-
polar interactions involved in the energy transfer, such as
dipole–dipole (d–d), dipole–quadrupole (d–q), quadrupole–
quadrupole (q–q) interactions, and so on. According to the
theory of Dexter, the emission intensity (I) per activator ion is
given by:
Fig. 6 Energy levels of Dy3+ ions.
I
C
k
¼
ð8Þ
βCS=3
where C is the activator concentration, k and β are constants
for a particular interaction, and the s values corresponding to
exchange, d–d, d–q and q–q interactions are 6, 8 and 10
respectively. From the slope of eqn (8), the electric multipolar
character (s) can be obtained by the slope (−s/3) of the plot log
(I/C) vs. log C. It can be seen that the dependence of log(I/C)
on log C is linear and the slope is −2.03 (inset of Fig. 7). Thus,
the value of s can be calculated as 6.09 (very close to the
theoretical value 6 for the electric d–d interaction), which
means that the d–d interaction is the main mechanism for the
3+
Fig. 7 Concentration vs. intensity graph of Ca1−xMoO4 :Dyx
.
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Fig. 8 Emission and excitation spectra of K+ codoped phosphor.
Fig. 9 Comparison between doped and codoped phosphors.
concentration quenching of 4F9/2–6H13/2 transition of Dy3+ in
the CaMoO4:Dy3+ phosphor.
Influence of K+ codoping: charge compensation. Fig. 8
shows the excitation and emission spectra of the CaMoO4:Dy3+
phosphor after codoping with K+. The concentration of K+ was
varied keeping the optimum concentration of Dy3+ fixed at
2 mol%. It can be seen that after codoping with K+ ion, the
intensity of blue and yellow emissions increases manifold and
it reaches maximum for 2 mol% concentration of K+ ions.
Fig. 9 shows the comparative graph of CaMoO4:Dy3+ (2 mol%)
and CaMoO4:Dy3+, K+ (2 mol% each). This is due to the charge
compensation phenomena.
It was observed that the emission intensities of CaMoO4:
3+
Dy0.02 increase with K+ molar concentration until the molar
fraction of K+ equals that of Dy3+ i.e. 2 mol%. The charge neu-
trality demands removal of three Ca2+ ions to introduce two
rare earth (RE3+) ions for effective lattice substitution, which
could eventually restricts the RE3+ concentration.8 Therefore, it
is difficult to maintain the charge balance in the phosphor.
Alkali metal ions have one outer-shell electron and they tend to
become electropositive after losing this electron. Thus, they can
be used for compensating charge of Dy3+ in CaMoO4 phosphor.
According to the charge compensation mechanism two Ca2+
ions are replaced by one Dy3+ ion and one K+ ion, 2Ca2+ = Dy3+
+
K+. Lattice distortion and non-radiative transitions will appear if
more K+ ions are added, which reduces the emission intensity.
3.3. Photometric characterization
The Commission Internationale de l’Eclairage (CIE) para-
meters such as colour coordinates, luminescence efficacy of
radiation (LER) and colour correlated temperature (CCT) were
calculated in order to know the photometric characteristics of
the prepared phosphors. Fig. 10a and b illustrates the CIE
chromaticity diagram of Dy3+ doped CaMoO4 and K+ codoped
CaMoO4 phosphors with varying concentration. The para-
meters were calculated by the spectrophotometric method Fig. 10 CIE diagram of doped and codoped phosphors.
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Table 2 Photometric characteristics of prepared phosphors
Colour coordinates
Concentration
(mol%)
LER
Phosphor
x
y
(lm W−1
)
CCT (K)
Ca1−xMoO4:Dyx
x = 0
x = 1
x = 2
x = 3
x = 4
x = 5
0.22
0.22
0.28
0.31
0.27
0.21
0.24
0.20
0.27
0.32
0.26
0.18
245
176
256
303
237
159
Undefined
Undefined
11 225
6738
14 016
Undefined
Ca1−x−yMoO4:Dyx, Ky
x = 2, y = 1
x = 2, y = 2
x = 2, y = 3
x = 2, y = 4
x = 2, y = 5
0.19
0.30
0.34
0.31
0.28
0.12
0.30
0.35
0.29
0.27
213
291
292
202
192
3086
7729
5188
7127
11 227
using the spectral energy distribution of chromaticity diagram
and are summarized in Table 2.
Acknowledgements
The colour coordinates (x − y) were calculated using the The authors are thankful to the Department of Science and
standard procedure. The coordinates approaches from blue Technology, New Delhi (Government of India) for funding this
region to white region as the concentration of Dy3+ increases. work under the Project SR/FTP/PS-087/2010.
Thus colour tunability can be achieved by changing the
doping concentration.
After co-doping of K+ in the Dy doped CaMoO4 phosphor,
the point approaches the white region and white light is
achieved for 3 mol% of K+ as seen from Fig. 10b.
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4. Conclusion
A white-light emitting CaMoO4:Dy3+ phosphor with K+ codop-
ing was successfully synthesized via hydrothermal route. The
structural studies confirm that the Dy3+ ions are situated at the
cationic sites of Ca2+ and the site occupancy was compensated
by K+ ions without creating any new defects. The charge com-
pensator K+ can effectively increase the luminescence intensity
of phosphor. The results revealed the luminescence intensity
will reach maximum as the molar concentration of K+ equals
that of Dy3+ (2 mol%). Luminescence intensity increases with
Dy3+ up to 2 mol% and decreases with further increase of
9 Q. Li, J. Huang and D. Chen, Luminescence, 2010, 26, 349.
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light in blue and yellow regions is varying with doping concen- 11 J. S. Kumar, K. Pavani, A. M. Babu, N. K. Giri, S. B. Rai and
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these phosphors in tunable colour applications. White light
with suitable LER and CCT was achieved after K+ codoping
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559.
indicates the usability of this phosphor in commercial white 13 B. D. Cullity, Elements of X-Ray Diffraction, Addison-Wesley,
LEDs and other display devices.
Massachusetts, 1956, p. 312.
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