Introduction

Phosphor converted white light emitting diodes (Pc-wLEDs) has attained more significance in current times for their various applications such as flashlights, indicator lights, display backlighting and architectural lighting ^[1,2]. It is believed that nUV-LED+RGB luminescent materials can be considered as one of the best methods to produce highly efficient white light ^[3,4].

Ca₂PO₄Cl, known as Calcium Chloro phosphate was first investigated by Martha Greenblatt ^[5] et-al. for its crystal structure.  A. Meijerink and G. Blasse have reported the photoluminescence and thermoluminescence properties of Ca₂PO₄Cl:Eu^{2+ [6]}. Yi-Chen Chiu^[7] has reported the application of Ca₂PO₄Cl:Eu²⁺ as a guaranteed blue phosphor for white light emission diodes by exciting with near ultra violet light and  prepared by solid state reaction. Panlai Li et al., reported Ca₂PO₄Cl:Eu²⁺, Mn²⁺ as a novel, warm light emitting phosphor for white LEDs ^[8].

The photoluminescence of Ca₂PO₄Cl:Yb²⁺ has been investigated and compared with that of Ca₂PO₄Cl l:Eu²⁺, and the application of Ca₂PO₄Cl:Yb²⁺ for white LED has been evaluated by Di-Yin Wang et al^[9].  A series of Ce³⁺, Eu²⁺ and Ce³⁺/Eu²⁺ doped Ca₂PO₄Cl phosphors, prepared by a high temperature solid-state method were investigated by Panlai Li et al., and found an increase in the emission intensity of Ca₂PO₄Cl:Eu²⁺ without affecting the spectral profile ^[10].

Hence we are motivated to study Ca₂PO₄Cl:Eu phosphor, but via a different synthesis route. Combustion synthesis (CS) is another wet chemical method for synthesizing powdered materials ^[11]. Combustion synthesis processes have typical characteristics like high-temperatures, fast heating rates and small reaction times. These characteristics make this a striking method for the creation of technologically functional materials at cheap rates compared to other usual processes.

Therefore we are reporting the luminescence properties of Ca₂PO₄Cl: Eu and Ce phosphors, prepared by urea assisted combustion synthesis method.

Figure 1: SEM photo graphs of Ca2PO4Cl phosphor Click here to View

Experimental

The chloride based Eu³⁺ and Ce³⁺ activated Ca₂PO₄Cl halophosphor material were prepared by combustion synthesis method. Chemically homogenous, well and untainted powders are prepared by this method. The ingredients used to synthesize this halosulphate phosphor were of AR grade Calcium nitrate (Ca(NO₃)₂),  Ammonium Chloride (NH₄Cl), Ammonium  di- hydrogen phosphate (NH₄H₂PO₄),  ammonium Cerium nitrate [(NH₄)₂Ce(NO₃)₆], Europium oxide (Eu₂O₃) and Urea[NH₂-CO-NH₂]. The initial materials in the stoichoimetric proportion were combined together along with Urea in agate mortar by grinding. The solution of dopant in the nitrate form was then added to homogenous mixture to form a semiliquid paste. The stoichoimetric composition of all the components were calculated using the total oxidizing and reducing valencies of the chemicals which serve as the numerical coefficients so that the equivalence ratio is unity. The paste formed is moved to   silica crucible and kept inside a muffle furnace operating at 550°C. Combustion synthesis is a reaction which takes place with the evolution of heat and light. This high temperature plays a vital role in the formation and crystallization of required phosphor material. The mixture when heated rapidly at 550°C started boiling and generating combustible gases. The thin chips of white colored powder in the crucible were then mashed to powdered form.  The synthesized host material was characterized for their phase purity and crystalline nature by XRD using PAN-analytical diffractometer. The PL measurement of excitation and emission were recorded on Shimadzu RF-5301PC Spectrofluorophotometer.

Figure 2: PL Excitation spectra of  Ca2PO4Cl:Eu3+ monitored at 620 nm emission  wavelength. Click here to View

Figure 3: PL emission spectra of Ca2PO4Cl:  Eu3+ monitored at 396nm excitation  wavelength. (a) 0.2mol%, (b) 0.5mol%,  (c)1mol% Click here to view figure

Characterization

Structure of compound was established by taking the XRD pattern.  The Photoluminescence excitation and emission spectra of the samples were recorded using fluorescence spectrometer RF-5301PC, using a spectral slit width of 1.5nm. The surface morphology was studied by taking scanning electron microscopy photographs.

The XRD pattern of Ca₂PO₄Cl is in agreement with that of JCPDS file No. 72-0010. Calcium chlorophosphate, as reported by Greenblatt^[5] et al. ,crystallizes in the orthorhombic system with a space group of Pbcm ^[7], with four molecules  per unit cell. The dimensions of the unit cell are a = 6.1850 Å, b = 6.983 Å, and c = 10.816 Å ^[7].  The ionic radii of Ca²⁺ and Eu³⁺/ Ce³⁺ are approximately same. Therefore Eu³⁺/ Ce³⁺ ions may prefer to occupy Ca²⁺ sites in Ca₂PO₄Cl host lattice.

Figure 4:  CIE color coordinates of  1mol%   Eu3+ doped Ca2PO4Cl  phosphor  at 396nm    excitation wavelength  Click here to view

SEM characterization

The Fig.2 shows SEM images of Ca₂PO₄Cl phosphor. It is clear that the surface morphology is inhomogeneous. The particles are rod like of irregular sizes. The particle size varies from 5µm to 10 µm.

Result and Discussion

Ca₂PO₄Cl: Eu³⁺

Rare earth ions are identified to be present in a variety of valence states although the trivalent state is the most common ^{[7, 12]}.  Among rare earth ions, Eu, particularly is known to be steady in both trivalent and divalent states. The excitation spectra for Ca₂PO₄Cl: Eu³⁺ is shown in Fig. 2. The excitation spectra, watched at 620 nm emission wavelength illustrates a maximum at 396nm which is ascribed to ⁷F₀→⁵L₆ transition of Eu³⁺. The sharp excitation bands in the 315 - 450nm region belongs to the intraconfigurational 4f-4f transitions of Eu³⁺. Two low intensity broad bands peaking at 328nm and 345nm are also seen, which may be due to 4f⁷ - 4f⁶5d transitions of the Eu²⁺ ions [13]. The ⁷F₀→⁵L₆ at 396nm is the strongest absorption.  Thus we find that Eu ions dwell in two different sites in the host Ca₂PO₄Cl network.

Figure 5: PL  Excitation spectra  of  Ca2PO4Cl:Ce3+   monitored at 360  nm  emission  wavelength. Click here to view

Upon excitation with 396nm radiation, the emission spectra (Fig.3) consists of two types of  transitions  one is the broad emission at 440 nm lying in the  blue region of visible spectrum,  which can be ascribed to the transition from 4f⁶5d excited state to the 4f⁷ ground state of a Eu²⁺ ion. And the second is the sharp intense emissions at 594nm  and 620nm which are due to ⁵D₀→⁵F_J (J=1-4) of the ⁴f₆ configuration of Eu³⁺. The bands obtained can be represented as: ⁵D₀→⁷F₁ (593nm) and ⁵D₀→⁷F₂ (620 nm). Thus along with Eu³⁺ emission, a broad emission at around 440 nm attributable to Eu²⁺ emission could be seen in the emission spectra, pointing that the reduction of Eu³⁺ to Eu²⁺ ions is possible to be achieved in this host Ca₂PO₄Cl material. The rare earth ion Eu²⁺ emission is a consequence of two forms of transitions. Commonly the transition is due to ⁴f₆ ⁵d₁→⁴f₇ (⁸S_7/2). As the position of the band corresponding to ⁴f₆5d configuration is strongly affected by the host, the emission can stretch out anywhere between 365 nm  to 650 nm ^[14].

The intensity of emission line 594nm assigned to the  magnetic dipole ⁵D₀→⁷F₁ (MD) transition of the Eu³⁺ ions is slightly greater than the intensity of emission peak at 620 nm corresponding to the forced (ED) electric dipole ⁵D₀→⁷F₂ transition of Eu³⁺. The electric dipole transition ⁵D₀→⁷F₂ with ∆J=2 is hypersensitive, and the intensity can vary by the order of magnitude, depending on the neighboring environment ^[15]. It is recognized that when the Eu³⁺ ions are embedded in a site with inversion symmetry, ⁵D₀→⁷F₁ MD transition of Eu³⁺ is directed, whereas without  inversion symmetry the ⁵D₀→⁷F_2, ED transition is the strongest among the ⁵D₀→⁷F_J (J=1,2,3,4) transitions^[16].

The result of Eu³⁺ ions concentration on the emission intensity of Ca₂PO₄Cl is studied. The variation of emission intensities of Ca₂PO₄Cl: Eu³⁺ with different doping concentration of Eu ions can be seen from Fig. 3. The form of the emission curve was same for all the concentrations, but the emission intensity was found to increase with the increase in the Eu doping content. The maximum intensity was for 1mol% concentration of the activator Eu³⁺ ions. No concentration quenching was observed for europium activator ions up to 1mol%.

As the phosphor Ca₂PO₄Cl: Eu^, shows three distinct colors due to Eu²⁺ and Eu³⁺ emissions, at 445nm(blue), 544nm(yellow) and 620nm(red), we have calculated the CIE color coordinates to find out the resultant emission color from the spectral distribution of intensity. The CIE color coordinates are found to be (0.310, 0.217) which lies in the white region of the visible spectrum as shown in Fig.4.

Figure .6: Photoluminescence Emission spectra of Ca2PO4Cl:Ce3+  monitored at 316 nm excitation wavelength   Click here to view

Ca₂PO₄Cl: Ce³⁺

Fig. 5 illustrates the photoluminescence excitation (PLE) of Ce³⁺ doped Ca₂PO₄Cl phosphor. As shown in the figure, the excitation spectrum monitored at 360 nm emission wavelength, shows two  broad band centered at 267nm and 316nm, which can be attributed to the crystal field splitting of 5d states of Ce^{3+ [17]}.

The excitation at 316nm of Ca₂PO₄Cl: 1% Ce³⁺ agrees well with the emission of nUV chips. Fig. 6 shows the emission spectrum at 316 nm excitation wavelength. The emission spectrum shows a broad band which extends between 335 to 400 nm peaking at 360 nm, which can be attributed to characteristic 5d -4f emission of Ce³⁺. This emission could be utilized in scintillators and photo-therapy. Also it could act as a good sensitizer in energy transfer process for phosphors. From the emission spectrum we find that the intensity of emission at 360 nm increases with increase in dopant concentration.

Conclusion

Phosphor Ca₂PO₄Cl activated by rare earth ions   Eu³⁺   and Ce³⁺ were synthesized by Urea assisted combustion synthesis procedure. The powdered phosphors were described by XRD and SEM procedures. Phase purity of phosphors was confirmed by XRD. The prepared micro phosphors display good luminescence properties. The Ca₂PO₄Cl:Eu³⁺ phosphor exhibit emission in the white region of the visible spectrum due to three prominent emissions peaking at 445nm, 594nm and 620nm, upon excitation at 396nm wavelength. The CIE color coordinates of   Ca₂PO₄Cl:1mol%Eu was calculated to be (0.310, 0.217). By changing the dopant concentration, the phosphor Ca₂PO₄Cl: Eu can be further investigated for its potential application as a white light emitting phosphor for Lamp industry.

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