ORIGINAL PAPER

NaLi2PO4:Eu3+ based novel luminescent red phosphor

P D Sahare* and M Singh

Department of Physics and Astrophysics, University of Delhi, Delhi 110 007, India

Received: 06 November 2013 / Accepted: 13 February 2014 / Published online: 5 March 2014

Abstract: Eu3? doped NaLi2PO4 red luminescent phosphor has been synthesized by solid state reaction and its phase

purity has been confirmed by X-ray diffraction analysis. Optical properties of the phosphor have been investigated and the

effect of dopant (Eu3?) concentration (0.1–3.0 mol%) has been observed. Excitation spectra of the phosphor NaLi2-

PO4:Eu3? show a very efficient absorbance band corresponding to the common allowed transition 7F0 ? 5L6 occurring at

393 nm, while the luminescence spectra exhibit prominent emission peak centered at 702 nm (5D0 ? 7F4) in pure red

region. Color purity of this red phosphor has been calculated and found to be around 99.9 %, which is very close to that of

commercial red phosphor Y2O3:Eu3?. Luminescence studies of the phosphor show excellent stability with respect to

excitation energy. Chromatic investigations have also been performed using emission spectrum and some important

chromatic parameters are calculated using CIE-1931 color calculator in order to find potential application of the phosphor.

Keywords: NaLi2PO4:Eu3?; Photoluminescence; Red phosphor; Color purity; Color-coordinates; WLEDs

PACS Nos.: 07.07.Hj; 32.50.?d; 33.50.Dq

1. Introduction

In last few decades, a lot of research has been carried out on

rare-earth (RE) doped various inorganic/organic phosphors

for their various applications. Among the dopants, usually

RE and transition metal ions, Eu3? is attractive dopant for

their unique optical properties, such as, almost a narrow band

light emission, long lifetime and higher luminescence effi-

ciency, etc., as compared to other dopant materials [1–5].

Therefore, Eu3?/Eu2? doped phosphors have been studied

extensively for their potential applications in light emitting

devices, displays, solid state lasers, optical amplifiers, sen-

sors and optoelectronics devices etc. [6–12].

Orthophosphates ABPO4 (where A and B are mono and

divalent cations, respectively) and oxide glasses (such as

borates), phosphates and fluorophosphate doped with Eu3?/

Eu2? ions are reported as efficient optical materials [13–

21]. These phosphors emit red and bluish emissions when

doped with Eu3? and Eu2? ion species, respectively [22–

24]. Mostly, Eu3? ion doped phosphors are reported as red

light emitting phosphors, such as, PKBAEu [25],

M6AlP5O20 (M = Sr/Ba/Mg) [26], LAFB [27] and PKSA

[28], show dominant emission peaks in the spectral ranges

611–620 and 587–594 nm, corresponding to the electric-

dipole allowed 5D0 ? 7F2 and magnetic-dipole allowed5D0 ? 7F1 transitions, respectively. But the phosphors,

e.g., SABBL [29] and Eu3?-Doped BaFCl [30] exhibit a

dominant peak centered at around 700 nm attributed to the5D0 ? 7F4 transition. The spectral ranges (611–620 and

587–594 nm) come under the radish–orange visible spec-

tral region, while the pure red color ranges from 620 to

750 nm.

The present commercial phosphors used for a ‘‘cool

white’’ fluorescent lamps are two rare earth doped phos-

phors, one LaPO4:Tb3?, Ce3? for green and blue emissions

and another Y2O3:Eu3? for red. These are likely the most

common type of fluorescent lamp phosphors in use today.

This approach is little better, as it exhibits better color

rendering index (CRI 89) in comparison to the SUN light

(CRI 82) but still poor than highest achievable (CRI 100).

This may be due to lack of proper combination of the

emission colors of solid-state lighting (blue/green/red)

source(s) used in compact fluorescent lamps (CFLs) or in

white light emitting diodes (WLEDs), especially, red

phosphor. Now most of CFLs are being replaced by*Corresponding author, E-mail: pdsahare@physics.du.ac.in; pdsahare@

yahoo.co.in

Indian J Phys (June 2014) 88(6):621–630

DOI 10.1007/s12648-014-0459-9

� 2014 IACS

WLEDs as the later ones are energy efficient and envi-

ronmental friendly (mercury free). Expected improvement

in the next generation WLEDs would be through a com-

bination of an ultraviolet (UV) or blue-emitting chip and

phosphors as solid-state lighting sources. This has, there-

fore, attracted considerable interests. Recently, thus much

more attention has been focused on the generation of white

light, through a better combination of red, green and blue

phosphors with UV or near-UV LEDs. From this point of

view, it is important to improve the efficiency and color

render index of red, green and blue color emitting phos-

phors having their prominent excitation wavelengths in

range of 350–410 nm [31, 32].

In present work, we have reported an efficient pure red

(emission wavelength around 702 nm) phosphor based on

Eu3? doped NaLi2PO4 system synthesized through solid

state reaction. Generally, Eu3? ions doped phosphors show

most prominent emission peak at around 618 nm

(5D0 ? 7F2) which is in orange-red region while in present

orthophosphate most prominent emission peak is centered

at around 702 nm (5D0 ? 7F4) which is really a pure red

light. Both transitions are electric-dipole allowed transi-

tions and their emission response and efficiency depend on

crystal field of the host around Eu3? ions. Different nature

of the emission spectrum (peaking at around 702 nm) of

this phosphor has been attributed to different crystal field

exerted on Eu3? ions. The better chromatic coordinate

shows its novelty and potential use as a pure red phosphor

for better color rendering WLEDs.

2. Experimental details

NaLi2PO4:Eu3? phosphor was synthesized through solid

state reaction with starting materials LiOH�H2O (CDH,

99.5 %) and NaH2PO4�2H2O (CDH, 99 %) and the impu-

rity salt EuCl3�6H2O (CDH, 99.5 %). The samples were

prepared taking into consideration the following chemical

reaction:

2LiOH � H2O + NaH2PO4 + 2H2O

þ EuCl3 � mol%ð Þ �!heatingNaLi2PO4 :Eu3þ þ 6H2O:

AR grade LiOH and NaH2PO4 (molar ratio 2:1) and the

appropriate amount of the impurity (x = 0.1–3.0 mol%)

were mixed consistently using agate mortar and pestle in

presence of ethanol for better mixing. A temperature

controlled programmable furnace with temperature

stability better than ±1 K was used for the samples

synthesis. Mixture of the precursors was heated at 673 K

for 12 h in alumina boat and cooled slowly at room

temperature, grinded in the form of fine powder and heated

again at 1,073 K for the same period. The ingot thus

obtained was grinded again to get fine powder. All these

samples used for further characterization were in the form

of white powder.

X-ray diffraction (XRD) patterns of the powder samples

were recorded using a high-resolution (D8 Discover Bruker)

X-ray diffractometer, equipped with a point detector (scin-

tillation counter), employing monochromatized Cu-Ka1

radiation obtained through a Gobel mirror with a scan rate of

1.0 s/step and step size of 0.02 at room temperature. High-

resolution transmission electron microscopy (HRTEM)

images of the samples were taken using a (Philips Tecnai G2

30) transmission electron microscope (TEM) operating at an

accelerating voltage of 300 keV. Thermo gravimetric ana-

lysis of the samples was carried out using (Perkin Elmer

Diamond) TG/DTA system from room temperature to

1,273 K at a heating rate of 10 K/min. Fourier transforma-

tion infrared (FTIR) spectra were recorded on a FTIR spec-

trometer (Spectrum RX I Perkin Elmer). Conventional

excitation and emission spectra of the powder samples were

recorded using (Horiba Jobin–Yvon Fluorolog) modular

spectrofluorimeter at room temperature using a Xenon lamp

as continuous source. Fluorescence decay curves were also

taken on (Horiba Jobin–Yvon FluoroCube) single photon

counting system. The decay curves were theoretically fitted

by using the least square method. Lifetime was then calcu-

lated from these decay curves.

3. Results and discussion

Figure 1(a) is stick XRD pattern for the data from JCPDS #

45-1348 for NaLi2PO4 crystalline material, while

Fig. 1(b)–(e) show experimental patterns for the synthe-

sized Eu3? doped NaLi2PO4 crystalline materials with

impurity concentrations 0.1, 1.0, 2.0 and 3.0 mol%,

respectively. The diffraction patterns are indexed to

orthorhombic crystal system of NaLi2PO4 space group

Pmnb(62) and JCPDS # 45-1348 [33]. Stick pattern of the

standard data has also been plotted along with the experi-

mental data for matching and identifying impurity/sec-

ondary phases in the diffraction pattern of the powder

material. No diffraction peaks of any phase related to the

impurity have been observed when doping level is lower

than 1.0 mol%, as seen in Fig. 1. However, when the

impurity concentration increases to 2.0 mol% or more, new

diffraction peaks are found growing and intensity increases

with doping concentration. These new diffraction peaks

have been attributed to formation of impurity clusters

related to monoclinic phase of Eu2O3 (JCPDF# 43-1009).

In order to confirm the prospect of substitution of Eu3? for

Li/Na ions in as prepared NaLi2PO4 samples, angular shift

d(2h) for peak corresponding to the plane (2 2 0) reflections

as a function of dopant Eu3? ions concentration has been

622 P D Sahare and M Singh

observed and illustrated in Fig. 2(a). Magnified region

around peak corresponding to plane (2 2 0) is also shown in

Fig. 2(b). The peaks are plotted in different line styles and

are (solid curve) for 0.1, (dash with two doted curve) for

1.0, (dash with single dotted curve) for 2.0 and (dashed

curve) for 3.0 mol%. It could be clearly seen that the peak

position corresponding to this plane (2 2 0) has got shifted

significantly with concentration of Eu3? ions increasing. It

demonstrates effective substitution of Eu3? for Li/Na ions

in lattice of crystalline NaLi2PO4 powder. Rietveld

refinement has also been done in order to confirm the

possibility of substitution of Eu3? ions for host ions

(Li/Na) by analyzing variations in structure parameters of

powder material [34, 35]. Table 1 shows the variation in

cell parameters with dopant concentration. The results are

also compared with that of the nominally pure NaLi2PO4

material (JCPDS # 45-1348). It is observed that the cell

parameters decreased with dopant concentration. This

demonstrates the presence of an effective substitution of

Eu3? ions in host materials. It is also observed that inten-

sity of peaks and cell volume (Table 1) gradually

decreased due to decreased crystallinity of the materials

with addition of Eu2O3 phase [36]. On the basis of all

above results, it is observed that incorporation of more and

more Eu3? ions in NaLi2PO4 lattice replaces Na?/Li? ions

at low concentrations. At the same time oxygen molecules

from surrounding atmosphere diffuse due to high temper-

atures and impurity phase (Eu2O3) clusters are formed and

grow more in number and size with concentration and time.

Thus as the concentration increases (beyond the doping

level *2.0 mol%), they fall out of the matrix to form

clusters.

Fig. 1 (a) is the stick pattern for the data from the JCPDS # 45-1348.

XRD patterns for materials having impurity concentrations (b) 0.5,

(c) 1.0, (d)2.0 and (e) 3.0, respectively are also shown. The peaks

corresponding to the Eu2O3 impurity phase are indicated by the solid

square (filled diamond)

(a) (b)

Fig. 2 Shifts in peak positions in the XRD patterns of NaLi2PO4

phosphor for different Eu3? impurity concentration. (a) is plot of

d(2h) of the peak corresponding (2 2 0) plane as a function of Eu3?

impurity concentration. (b) is magnified region of XRD patterns

around the peak corresponding to plane (2 2 0) for different Eu3?

impurity concentration: (straight curve) for 0.1 (dashed with two

dotted curve) for 1.0 (dashed with single dotted curve) for 2.0 and

(dashed curve) for 3.0 mol%

Table 1 Variation in cell perimeters of NaLi2PO4 phosphor with

Eu3? dopant concentration

Sample Lattice parameters Cell volume

(cm3)a b c

Reported work

NaLi2PO4 [29] 6.8770 9.9770 4.9255 337.95

Present work

NaLi2PO4:Eu (x mol%)

x = 0.1 6.8798 9.9870 4.9275 338.56

x = 0.5 6.8725 9.9825 4.9295 338.18

x = 1.0 6.8735 9.9828 4.9265 338.04

x = 2.0 6.8730 9.9775 4.9276 337.97

x = 3.0 6.8685 9.9725 4.9252 337.36

Novel luminescent red phosphor 623

The morphology of doped NaLi2PO4:Eu3? has been

characterized by TEM. TEM micrographs are shown in

Fig. 3. Mono-dispersed star like particles could be seen

here. The average particles size is observed to be around

1.5 lm in Figs. 3(a) and 3(b) show better resolution of a

single particle. Selected area electron diffraction (SAED)

of the particle as shown in Fig. 3(c), exhibits regular dif-

fraction points well indexed to orthorhombic structure of

NaLi2PO4 and is in good agreement with the observed

XRD patterns. Incorporation of impurity in the material has

been confirmed by energy-dispersive spectroscopy (EDS)

spectra and all EDS peaks corresponding to impurity and

host elements could very well be observed in the EDS

spectrum given in Fig. 4. Wt% and at.% of constituent

atoms are also given in inset.

TG/DTA of mixture of precursors has been taken to

understand diffusion process during synthesis and to

determine temperatures for this process. TG/DTA curves

(recorded for temperature range 323–1,273 K) for prepar-

ing NaLi2PO4:Eu3? (2.0 mol%) phosphor is shown in

Figs. 5(a) and 5(b). It is observed in this figure that the

sample shows an endothermic peak for DTA in Fig. 5(a) in

temperature range from 323 to 473 K, which is consistent

with enormous weight loss (about 30 %) shown by TGA in

Fig. 5(b) in the same temperature range. This observation

could be attributed to thermal release of water of crystal-

lization from ingredients and also more water molecules

due to their decomposition. Upon increasing temperature

up to 1,273 K, two endothermic peaks have been observed,

one at around 855 K and another broad peak around

1,085 K in DTA curve. The peak centered at around 855 K

is related to solid state reaction occurring among precursors

Fig. 3 TEM image of Eu3?

doped NaLi2PO4 phosphor:

(a) is low resolution image;

(b) is high resolution (enlarged

view) of the image of a single

particle and (c) is a SAED

pattern

Fig. 4 A typical EDS pattern of Eu3? doped (2.0 mol%) NaLi2PO4

phosphor. Elemental analysis (in at.% and wt%) of material is also

given in the inset

60

65

70

75

80

85

90

95

100

375 500 625 750 875 1000 1125 1250

-16

-14

-12

-10

-8

-6

-4

-2

0

Hea

t flo

w (

mW

)

Mas

s lo

ss (

%)

Endo

350 K

855 K

1085 K

a

b

Temperature (K)

Fig. 5 TG/DTA curves of the mixture of precursors for preparing

Eu3? doped (2.0 mol%) NaLi2PO4 phosphor. (a), (b) are for DTA and

TGA, respectively

624 P D Sahare and M Singh

to form NaLi2PO4 phosphor and later one centered at

1,085 K is related to formation of the impurity phase

Eu2O3 for higher concentration of impurity. For nominally

pure sample 1,085 K peak is not observed. It could also be

seen from TGA curve that there is only slight weight loss

(0.91 %) at high temperature range (750–1,150 K) could

be attributed to release of water molecules formed during

reaction and their compensation by oxygen diffusion to

form Eu2O3 phase [37]. Formation of impurity phase

(Eu2O3) is also supported by XRD results.

FT-IR spectra of NaLi2PO4:Eu3? sample doped with

2.0 mol% and that of undoped sample are shown in

Figs. 6(a) and 6(b), respectively. Typically, IR absorption

band of (PO4)3- group has two absorption bands in ranges

of 1,120–940 and 650–540 cm-1 [38]. In present phosphor

the phosphate group exhibit strong characteristic absorption

bands centered at around 1,043 and 588 cm-1 which have

been assigned to stretching and bending vibration modes of

(PO4)3- group, respectively. However, it is observed from

FTIR spectra some irrelevant bands at 1,629 and

3,437 cm-1 that could be attributed to (OH)- content due to

adsorption of water molecules at powder surface when the

sample is in contact with atmospheric air [39–41].

Excitation spectra of Eu3? (2.0 mol%) doped NaLi2PO4

recorded at room temperature are shown in Figs. 7(a)–

7(c) for different characteristic major emission bands

peaking at 593, 618 and 702 nm. In these spectra common

f–f transition lines (as absorption bands) have been

observed at 318, 361, 376, 381, 393, 464, and 524 nm.

These bands have been assigned to direct absorption bands

of Eu3? ions from ground state 7F0 to excited states 5H4,5D4, 5G2, 5G3, 5L6, 5D2, and 5D1, respectively. These states

match well with the similar bands reported in literature

[42]. Among all absorbance bands, most intense band has

been found to be at around 393 nm corresponding to the

transition of 7F0 ? 5L6 energy level.

Normally, light corresponding to 393 and 465 nm bands

are used as exciting sources to obtained the emission

spectra of Eu3? ions doped phosphors which give promi-

nent emission peak at around 618 nm corresponding to

electric-dipole allowed 5D0 ? 7F2 hypersensitive allowed

transition that followed DJ = 2 selection rule [25, 27, 28,

43]. It is well known that host matrix environment around

Eu3? ions and excitation energy could significantly influ-

ence emission intensity of electric-dipole allowed5D0 ? 7F2 and 5D0 ? 7F4 transitions. In order to optimize

the appropriate excitation energy of phosphor, it has been

excited by different excitation wavelengths. The emission

spectra of NaLi2PO4:Eu3? excited by these wavelengths,

i.e., 361, 382, 393 and 465 nm are as shown in Figs. 8(a)–

8(d), respectively. The spectra consist of five characteris-

tics peaks centers at around 580, 593, 618, 653, and

702 nm corresponding to the allowed transition 5D0 ? 7FJ

(J = 0, 1, 2, 3, and 4). There is no significant shift in

emission peak positions of the emission spectra. However,

the corresponding intensities increased for excitation

wavelengths in the spectral range 361–393 nm but

decreased rapidly on excitation with 465 nm. The emission

spectra for samples with different impurity concentrations

in the range (0.1–3.0 mol%) are also shown in Fig. 9(a).

The spectra are plotted with different line styles and are

given by (dashed curve) for 0.1, (dotted curve) for 0.5,

(dash with single dotted curve) for 1.0, (solid curve) for 2.0

and (dash with double doted curve) for 3.0 mol%. Varia-

tions of respective intensities and the respective peak

intensity ratios of 702 nm peak to 618 nm peaks with

impurity concentrations have also been shown in Fig. 9(b).

The curves for intensity variations have been denoted by,

filled diamond, filled circle and filled square symbols for

618, 653 and 702 nm emission peaks, respectively, while

Fig. 6 FTIR spectra: (a), (b) represents spectra for doped (2.0 mol%)

and undoped NaLi2PO4 phosphors, respectively

Fig. 7 Excitation spectra of Eu3? doped (2.0 mol%) NaLi2PO4 at

different emission wavelengths: (a) 593, (b) 618 and (c) 702 nm

Novel luminescent red phosphor 625

the ratio curve is denoted by filled triangle symbol. It could

be seen here that not only intensities of respective peaks

vary and saturate after impurity concentration beyond

*2.0 mol% but also their ratio vary. Careful observation

reveals that there is also some red shift up to this con-

centration but for 3.0 mol% concentration it again shifts a

bit towards blue at which the impurity starts clustering.

These changes thus may be attributed to quantum con-

finement of these clusters. Amongst these, the selection

rules make 5D0 ? 7F4 transition (702 nm) is of particular

interest. It is very interesting that this 5D0 ? 7F4 transition

is electric-dipole allowed hypersensitive transition and

therefore, it is also sensitive to coordination environment of

Eu3? ions in this system, while intensity of the magnetic-

dipole allowed 5D0 ? 7F1 and 5D0 ? 7F3 transitions

hardly vary with crystal-field strength around the Eu3?

ions. Concentration of Eu3? ions has been varied from 0.1

to 3.0 mol% to optimize the appropriate dopant concen-

tration in matrix of NaLi2PO4 phosphor. On increasing

Eu3? dopant concentrations, luminescence centers

increased and enhanced luminescence intensity. It is

observed from Fig. 9b, that the sample doped with

2.0 mol% of Eu3? ions gives maximum emission intensity

and decreased beyond this concentration. With higher

dopant concentration, the emission intensity decreases due

to non-radiative relaxation process which is due to cross

relaxation among the neighboring Eu3? ions [44].

It is observed in Fig. 9(a) that only emissions from level5D0 are taking place in our material indicating that the

emissions from 5D2 and 5D1 are quenched. This may be

occurring due to a cross-relaxation process, which involves

two Eu3? ions depopulates 5DJ (J = 1, 2) level and thus

quenches 5DJ emission in favor of lower energy 5D0 level

emission. In process, Eu3? ions at 5DJ (J = 1, 2) states

transfer their energy to neighboring Eu3? ions at ground

state (7F0). This could be represented as (5DJ–5D0) ?

(7F0–7FJ).

Therefore, the possible cross-relaxations occurring for

Eu3? ions may be represented as follows [45, 46]:

Eu3þ 5D1

� �+ Eu3þ 7F0 ! Eu3þ 5D0

� �+ Eu3þ 7F3

� �and

Eu3þ 5D2

� �+ Eu3þ 7F0

� �! Eu3þ 5D0

� �+ Eu3þ 7F5

� �:

Thus, closely matched energy difference between 5D1, 5D2

and 5D0 levels and 7F3, 7F5 and 7F0 levels makes these

processes possible [45, 46]. Besides cross-relaxation pro-

cess, luminescence from 5DJ levels can be quenched also

by high-energy lattice phonons leading to multiphonon

relaxation process [47]. Cross-relaxation can occur

between energy levels of the same Eu3? ion which happens

to have two pairs of energy levels separated by same

amount of energy. By this process original system (in

Fig. 8 Emission spectra of Eu3? doped NaLi2PO4 for different

excitation wavelengths: (a) 361, (b) 382, (c) 393 and (d) 465 nm

(a)

(b)

Fig. 9 (a) shows the emission spectra (for kex = 393 nm) for

NaLi2PO4 samples with different impurity concentrations in the

range (0.1–3.0 mol%): (dashed line) for 0.1, (dotted curve) for 0.5,

(dashed with single dotted curve) for 1.0, (solid curve) for 2.0 and

(dashed with double dotted curve) for 3.0 mol%). (b) represents

variations of respective intensities and peak intensity ratios of 702 to

618 nm peaks with impurity concentrations. The curves for intensity

variations have been denoted by filled diamond, filled square and

filled circle symbols for 618, 653 and 702 nm emission peaks,

respectively, while the ratio curve is denoted by filled triangle

626 P D Sahare and M Singh

excited state) loses energy by obtaining lower state and

another system acquires energy by going to a higher state.

Two energy gaps may be equal or can be balanced by one

or two phonons. It is generally a dominating factor in

nonradiative relaxations at high concentrations.

Figure 10 shows schematic energy level diagram of

allowed transition levels for Eu3? ions involved in emis-

sion process. Excited electrons migrate into the first excited

metastable singlet state 5D0 from successive excited states

without any visible emissions. Non-radioactive energy

transfer has been attributed to high phonon energy of the

host and relatively small energy gaps between 5L6, 5D3, 5D2

and 5D1 states. Although, emission spectra show narrow

peaks of Eu3? due to shielding of 4f orbitals by nearest 5s2

and 5p6 orbitals [48]. Allowed transitions between 5D0

level to Stark levels of 7FJ are well resolved for low con-

centration level of doping (0.1–1.0 mol%). However, for

higher concentrations, beyond 2.0 mol%, Eu3? doped

phosphor shows Stark level peak positions for 5D0 ? 7Fj

transitions which are not well resolved, especially,5D0 ? 7F0,1,2 and they merge in a broad band due to

decreased energy level separations with an increasing

doping level.

A representative photoluminescent (PL) decay curve of

NaLi2PO4:Eu3? phosphor monitored at 702 nm is shown in

Fig. 11. PL decay curves taken at room temperature, have

been fitted to a single exponential function (dotted line) as

I = Io�exp(-t/s), where, Io is initial intensity and s is 1/e

lifetime of the transition level. Lifetimes corresponding to

Eu3? for 5D0 ? 7F4 (detected at 702 nm) have been

determined for different concentrations (0.5–3.0 mol%). It

is, however, found that there is no appreciable change in the

decay curves and thus in lifetimes with impurity concen-

tration in this range. 1/e lifetime (s) has been found to be

12.8 ns.

Emission spectra of phosphor with optimized impurity

concentration (2.0 mol%) and excited at wavelengths 361,

382, 393 and 465 nm have been investigated through the

Commission Internationale de l’Eclairage (CIE) 1931 color

calculator. The various color parameters such as chromatic

coordinates (x, y), correlated color temperature (CCT) and

distance to the Planckian (black body curve) locus (Duv),

etc. play an important role in designing a CFL lamp or a

white light emitting diode (WLED). These parameters were

evaluated using CIE 1931 color calculator [49]. Duv is a

measure of distance of a (x, y) point from black body locus

in UV space. For standard white light LEDs are generally

required Duv to be below 0.0020 and the chromaticity

coordinates (x = 0.331, y = 0.329). CCT is an important

characteristic parameter for evaluating visible light emitted

from a phosphor which is a specification of the color

appearance of light as compared to an ideal black-body

radiator reference source at a particular temperature.

Black-body spectrum is considered as an ideal spectrum of

day light. Therefore, distance Duv from Planckian locus to

color coordinate (U0, V0) indicates how close tested light

source is to the ideal source and its CCT [50].

Fig. 10 Schematic energy level diagram of Eu3? ions in the lattice of

NaLi2PO4

1.3x10-8 1.4x10-8 1.5x10-8

0

2000

4000

6000

8000

10000

Inte

nsity

(ar

b. u

nits

)

Time (s)

Fig. 11 Representative decay curve for NaLi2PO4:Eu3? (2.0 mol%).

The emission was monitored at 702 nm for recording the decay curve

Table 2 Color coordinates, Duv and correlated color temperature

(CCT) for the 361, 382, 393, and 465 nm excitation wavelengths

k (nm) (x, y) (U0, V0) CCT (K) Color

purity (%)

361 (0.6044, 0.3414) (0.4106, 0.5218) 1,172 86.2

382 (0.6109, 0.3430) (0.4146, 0.5237) 1,152 99.6

393 (0.6298, 0.3589) (0.4246, 0.5359) 1,106 99.9

465 (0.6421, 0.3534) (0.4312, 0.5340) 1,075 99.8

Novel luminescent red phosphor 627

Fig. 12 VIBGYOR emission spectra of Eu3? doped (2.0 mol%) NaLi2PO4. for different excitation wavelengths: (a) 361, (b) 382, (c) 393 and

(d) 465 nm, respectively

Fig. 13 CIE-1930 chromatic

color space diagram showing

the chromatic coordinates of the

Eu3? doped NaLi2PO4 phosphor

excited at different

wavelengths, filled circle 361,

filled inverted triangle 382,

filled square 393 and filled

triangle 465 nm

628 P D Sahare and M Singh

Color purity of phosphor can be calculated taking into

account average of (x, y) coordinates relative to the coor-

dinates of illuminates and dominant wavelength.

Color purity ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðx� xiÞ2 � ðy� yiÞ2

q

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðxd � xiÞ2 � ðyd � yiÞ2

q � 100

where (x, y) and (xi, yi) are color coordinates of light source

and CIE white illuminate Cs (0.3101, 0.3162), respectively

and (xd, yd) are color coordinates of dominant wavelength

kd [48]. Color coordinates, Duv, CCT and color purity for

different excitations (361, 382, 393 and 465 nm) have been

summarized in Table 2. Better color visualization can be

seen from VIBGYOR emission spectra excited by different

excitation wavelengths as shown in Fig. 12(a), for 361,

(b) for 382, (c) for 393 and (d) for 465 nm. It can be

observed only relative intensity change while the peak

positions do not change appreciably. Thus change in CCT

values could be attributed to the change in intensity of

orange-red lines in emission spectra rather than change in

their peak positions on excitation by different excitation

wavelengths. CIE chromatic diagram shows a minor

change in chromatic coordinates (x, y) in red region as

shown in Fig. 13. Enlarged view of relevant portion of the

diagram has also been shown for better clarity. Present

phosphor shows color stability, since emission color doesn’t

change with excitation energy. Color purity of synthesized

NaLi2(PO)4:Eu3? phosphor doped with 2.0 mol% impurity

concentration has been calculated to be 99.9 % which is

better than LaPO4:Eu (99 %) and ZnAl2O4:Eu (93 %) pre-

pared by different techniques [51, 52] and very close to the

standard red oxide phosphor Y2O3:Eu films (100 %) pre-

pared by spray pyrolysis technique [53].

4. Conclusions

Pure red emission phosphor based on Eu3? doped ortho-

phosphate NaLi2PO4 has been synthesized through high

temperature solid-state reaction. Sample doped with 2 mol%

of Eu3? give a curiously intense and narrow emission at

702 nm corresponding to 5D0–7F4 electric-dipole transition

when sample has been excited at 393 nm. On excitation by

different excitation energy, emission peak positions do not

change while the intensity is varied. From application point

of view, it is important that even if excitation source is

changed the color purity of the phosphor would not change;

the only change would be in its efficiency. From color

coordinate data, determined from emission spectra, show

that it could be a good candidate as a red phosphor for solid

state lighting, especially, chromaticity coordinates of this

phosphor approach to that of an ideal red light emitting

phosphor. It could be used for pure red LEDs or also a pure

red light emitting phosphor in combination with other blue

and green phosphor(s) for a good quality white LEDs.

Acknowledgments We are thankful to University of Delhi for

partial financial assistance through R & D grants. The author (MS) is

thankful to University Grant Commission (UGC) for Rajiv Gandhi

National fellowship (RGNF).

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