Structural and Optical Investigations of Ce³⁺/Mn²⁺-Doped LaPO₄ Phosphors

Abstract

Lanthanum orthophosphate (LaPO₄) and La_0.95−xCe_0.05Mn_xPO₄ (x = 0.00, 0.03, 0.10) phosphors were synthesized by a simple and cost-efficient co-precipitation method and the formation of LaPO₄ nanorods with a monoclinic P21/n crystal structure was observed. X-ray diffraction pattern analysis indicated a slight distortion of the LaPO₄ crystalline structure and an increase of the lattice strain as a consequence of the Mn²⁺ and Ce³⁺ dopants incorporation in the host matrix. Scanning electron microscopy revealed that the microstructure of all powders consists of agglomerations of nanorods, which are around 17 ± 3 nm in diameter and length ranging from 100 nm to 300 nm. Electron paramagnetic resonance measurements have indicated the presence of Mn²⁺ in isolated species, but also as agglomerates. Ce³⁺ and Mn²⁺ doping of LaPO₄ resulted also in a decrease of the band gap up to 4.70 eV compared to the un-doped sample. Because of an energy transfer effect from Ce³⁺ to Mn²⁺ ions, green emission of Mn²⁺ ions at around 550 nm was observed upon 275 nm excitation.

Introduction

Since the emergence of luminescent materials with significant efficiencies, many studies have been devoted to understanding physical mechanisms leading to the emission of several photons in the visible region by the absorption of a single photon of higher energy (down-conversion) or lower energy (up-conversion). These mechanisms are usually known as photonic conversion which has been demonstrated in rare-earth orthophosphates (REPO₄), with RE = La, Ce, Y, etc.1,2

Rare-earth (RE³⁺) or transition metal (Meⁿ⁺) ion-doped REPO₄ have attracted much attention due to their physicochemical inertia and optical properties.3,4,5 REPO₄ compounds exhibit various potential applications in the fields of plasma display panels,6 integrated optical systems,7,8 fluorescent lamps9 and solar cells.10,11 In particular, lanthanum orthophosphate (LaPO₄) is an important optical material for application in lamps and solar cells because of its thermal stability, high index of refraction and useful host lattice for luminescent RE³⁺ ions doping.12 Hence, the optical characteristics such as ultraviolet (UV) absorption and luminescence of single doped LaPO₄:Ce³⁺, LaPO₄:Eu³⁺, LaPO₄:Gd³⁺, LaPO₄:Tb³⁺ and LaPO₄:Sm³⁺, phosphors have been studied.13,14,15,16,17 In addition, LaPO₄ co-doping has often been used to improve the RE³⁺-luminescence properties.16,17,18 For instance, in other co-doped materials, Ce³⁺ ions act as good sensitizers, transferring a part of their energy to activator ions such as Eu²⁺, Tb³⁺ and Mn²⁺.16,17,18

The proper choice of the preparation method plays a critical role in reaching the desired properties. For the preparation of LaPO₄, various routes of synthesis were employed, the most common synthetic methods being: the precipitation reaction in an aqueous medium (wet process) and the solid phase reaction (dry process).19 There are also other methods such as solid–liquid,20 hydrothermal,21,22,23,24 sol–gel25 and combustion reaction.26 In addition to the synthesis method, the nature of the activator ion and the energy transfer (ET) process efficiency are very important. The RE ion transitions are sharp and parity is forbidden, and therefore, a viable option is Mn²⁺ ions that can also act as an activator to emit broadband green light. Until now, several materials doped with Ce³⁺ and Mn²⁺have been used, such as fluoride and phosphate crystals (CaF₂,27 CeF₃, CePO₄28 and CaS(PO₄)29) but also films and glasses such as phosphate glasses30 or Al₂O₃ and zinc metaphosphate31,32 have been studied and an energy transfer (ET) between Ce³⁺and Mn²⁺ ions has been observed. However, to our knowledge, there have been no similar reports on the synthesis of Ce³⁺-Mn²⁺ co-doped LaPO₄ phosphors by a simple, controllable and cost-efficient method such as co-precipitation and the luminescence properties.

Herein, we present the synthesis and characterization of the LaPO₄ and La_0.95−xCe_0.05Mn_xPO₄ (x = 0.00, 0.03, 0.10) phosphors. Detailed structural, morphological, optical, thermal and electron paramagnetic resonance (EPR) analyses are presented and discussed. In particular, we discussed the EPR and luminescence properties related to Mn²⁺ ion and the energy transfer process between Ce³⁺ sensitizer and Mn²⁺ activator.

Experimental Details

Preparation

The un-doped LaPO₄ and La_0.95−xCe_0.05Mn_xPO₄ (x = 0.00, 0.03, 0.10) powders were prepared by co-precipitation (the Ce³⁺ concentration was fixed at 5 mol.%). The chemicals employed as precursors in this synthesis are the following: La(NO₃)₃·6H₂O (Aldrich, 99.999%), KH₂PO₄ (Aldrich, ≥ 99.99%), Ce(NO₃)₃·6H₂O (Aldrich, 99.999%), Mn(NO₃)₂·4H₂O (Aldrich, ≥ 99.99%) and ammonia (Aldrich, 25% solution). Bidistilled water was used in all preparation steps.

As a general synthetic strategy for LaPO₄ powder, La(NO₃)₃·6H₂O (4.2735 mmol) and KH₂PO₄ (4.2735 mmol) were dissolved in 30 mL of bidistilled water and then precipitated with NH₄OH solution (25%) until a pH value of 8–9 was reached. The obtained slurry was stirred at 80°C for 30 min and another 24 h at room temperature. In order to eliminate residual ions formed from started materials, the obtained precipitate was filtered and washed several times with bidistilled water until a pH value of 7 was reached. Finally, the sample was dried at 100°C and then calcined in air at 800°C for 2 h, with a rate of 10°C/min. For the synthesis of La_0.95Ce_0.05PO₄, La_0.92Ce_0.05Mn_0.03PO₄ and La_0.85Ce_0.05Mn_0.10PO₄ powders, similar procedure was followed according to the stoichiometric ratios.

Characterization

The x-ray diffraction (XRD) of the samples were recorded using a Bruker D8 Advance diffractometer using the principle of Bragg’s law, with Cu-Kα radiation (λ = 1.5405 Å). The x-ray diffractograms were recorded from 10° to 70° with 0.02 step. For the estimation of the crystallite size and the lattice strain induced by the imperfection and distortion nanocrystals, we applied the Williamson–Hall (W–H) analysis.33,34 The crystallite size D and lattice strain ε were calculated from the y-intercept and slope, respectively, of the plot between β_size.cosθ and 4 sinθ. Equation 1 gives the width of the Bragg reflection β_size, and leads to a separation between the contribution of the sample and that of the instrument. The broadening due to lattice strain is given by Eq. 2:

$$ \beta_{size} = [\beta^{2}_{measured} - \beta^{2}_{instrumental} ]^{1/2} $$

(1)

$$ \beta_{size} \cos \theta = \frac{k\lambda }{D} + 4\varepsilon \sin \theta $$

(2)

where λ = 1.5405 Å is the x-ray wavelength for a copper anticathode, k is a dimensionless shape factor (a typical value of 0.9 is used), β_size (rad) is full width half maximum (FWHM) of XRD peaks and θ (°) is half of the Bragg’s angle (2θ).

Fourier transform infrared by attenuated total reflection (ATR-FTIR) spectra of the synthesized samples were recorded between 500 and 4000 cm⁻¹ with a Perkin Elmer spectrum BX II spectrometer. A Pike-MIRacle ATR attachment with diamond-ZnSe and/or Ge crystal plate was utilized, according to the measurement scope. No FTIR band was detected beyond 1200 cm⁻¹, hence an enlarged part of the FTIR spectra was presented with the range from 500 to 1200 cm⁻¹.

The thermal behavior of the samples is inferred from thermogravimetric and differential thermal analysis (TG/DTA), using a LABSYS Evo device; the measurements being performed until 1000°C, with 10°C/min heating rate, under an Argon flux.

The morphology of the powder samples was assessed by field emission scanning electron microscopy (FE-SEM), using a Carl Zeiss EVO 50 microscope; the samples were placed on slabs using adhesive carbon discs.

The electron paramagnetic resonance (EPR) measurements were recorded with a continuous wave X-Band Bruker EMX plus EPR spectrometer fitted with a Bruker X-SHQ 4119HS-W1 X-Band resonator. Unless otherwise specified, the EPR measurement parameters were set as follows: microwave frequency 9.4344 GHz, microwave power 6.23 mW, modulation amplitude 0.1 mT, conversion time 0.04 s, a time constant 0.02048 s and number of scans 10.

The optical characterizations were obtained by ultraviolet visible (UV–Vis) spectroscopy in the diffuse reflectance (DR) approach. In this aspect, the DR-UV–Vis spectra were performed using an AvaLight-Xe pulsed light source connected to an AvaSpec-ULS 2048L spectrometer equipped with an AvaSphere 80-REFL integration sphere.

The photoluminescence (PL) and excitation spectra measurements were performed on the LaPO₄ powder samples at room temperature by using a Fluoromax 4P spectrophotometer.

Results and Discussion

Thermal Analysis (TG-DTA)

The TG-DTA results of the un-doped LaPO₄ prepared by co-precipitation method indicate two stages of mass loss, associated with thermal phenomena, as shown in Fig. 1. The first mass loss of 14.06% is from 50°C to 260°C accompanied by an endothermic reaction of dissociation (large peak symbolized as A in the DTA curve). This is attributed to the removal of water in the as-prepared hexagonal lanthanum orthophosphate. The second mass loss is initiated around 260°C and continued up to 650°C, which clearly depicts an endothermic peak (B) in the DTA curve. This change of 3.13% is due to the progressive dehydration of the rhabdophane phase, and then the decomposition of organic molecules (confirmed also by FTIR results shown in Fig. 7).

Fig. 1

TG/DTA curves of LaPO₄·0.5H₂O powder under an argon atmosphere.

In the temperature range of 650–900°C, no mass loss was observed, with a transition to the monazite phase (monoclinic system) almost completed around 750°C. This behavior occurs after the total dehydration of the rhabdophane phase and its stabilization and therefore the calcination temperature was chosen to be 800°C for all samples. At this temperature, the un-doped LaPO₄ is already crystallized and the total weight loss is 17.18%. Moreover, the exothermic peak (C) in the DTA curve, can be associated to the lanthanum metaphosphate phase that probably appears beyond 925°C, this result agrees well with the phase diagram of Park and Kreidler.35

Structural Investigations

The XRD patterns of the materials prepared are given in Fig. 2. For the un-doped material dried at 100°C (Fig. 2a), it can be seen that all the diffraction peaks correspond to the hexagonal rhabdophane-type phase LaPO₄·0.5H₂O (JCPDS no. 46-1439) having the space group P6₂22. After calcination at 800°C, the hexagonal rhabdophane-type phase evolves as shown in Fig. 2b and characteristic peaks associated with the formation of a monoclinic monazite LaPO₄ structure (P2₁/n as the space group, JCPDS no. 84-0600) appear. The diffractograms of La_0.95−xCe_0.05Mn_xPO₄ (x = 0.00, 0.03, 0.10) calcinated at 800°C are depicted in Fig. 2c, d and e. The obtained materials show high-intensity peaks at 21.28°, 26.87°, 28.64°, 31.03°, 41.98° and 45.82° corresponding to the planes (−111), (200), (120), (012), (−103) and (212), respectively, which correspond to the same monoclinic monazite LaPO₄ structure, JCPDS no. 84-0600.

Fig. 2

XRD patterns of (a) hexagonal LaPO₄·0.5H₂O, (b) monoclinic LaPO₄ and (c–e) La_0.95−xCe_0.05Mn_xPO₄ (x = 0.00, 0.03, 0.10) samples calcinated at 800°C; (f) experimental (symbols) and fitted (lines) data at small 2θ positions using multiple Gaussian functions.

Figure 2f shows the XRD patterns at small 2θ positions using multiple Gaussian functions. It can be found that the (−101), (110) and (011) peaks are slightly shifted into lower positions by doping Ce³⁺ and co-doping Ce³⁺/Mn²⁺ ions in LaPO₄; e.g. for the un-doped LaPO₄, the XRD peak of (011) plan is at 18.86° and, on inserting (0.05Ce/0.10Mn), the corresponding XRD peak is shifted to 8.70° due to the substitution of La³⁺ ions by Ce³⁺ and Mn²⁺ ions.

The Williamson–Hall (W–H) plot for LaPO₄ and La_0.95−xCe_0.05Mn_xPO₄ samples is shown in Fig. 3a, b, c and d. The crystallite sizes are about the same for all the samples (Table I) indicating a weak influence of doping on the nucleation and growth process of the LaPO₄ nanocrystals. However, the lattice strain gradually increases with the doping due to the lattice distortion induced by the Mn²⁺ and Ce³⁺ dopants incorporation in the crystalline matrix. The preferential orientation for LaPO₄ has been previously reported36,37 but in the present case from the W-H plot we inferred that there is no preferential growth direction36,37,38,39,40 and the size distribution of the crystallites is relatively wide.41,42

Fig. 3

W–H plot of (a) LaPO₄, (b) La_0.95Ce_0.05PO₄, (c) La_0.92Ce_0.05Mn_0.03PO₄ and (d) La_0.85Ce_0.05Mn_0.10PO₄ samples.

Table I Lattice parameters, unit cell volumes, crystallite sizes and strains of un-doped LaPO₄ and La_0.95−xCe_0.05Mn_xPO₄ (x = 0.00, 0.03, 0.10) samples.

The structural calculation was carried out based on one-unit cell of LaPO₄ as shown in Fig. 4 and the calculated lattice values of all phosphors are summarized in Table I. As the differences of the ionic radius between La³⁺ (110 pm) and Ce³⁺ (107 pm) are small the cell volume remain about the same for un-doped and Ce³⁺-doped LaPO₄ samples. However, by Mn²⁺ co-doping we observed an evident shrinkage of the cell volume up to 0.43% (for 10% doping) due to the small ionic radii of Mn²⁺ (90 pm). The evident influence of the Mn²⁺ co-doping is also revealed by the small shift of the (−101), (110) and (011) peaks, as shown in Fig. 2f.

Fig. 4

The unit cell of LaPO₄.

Morphological Investigations

Morphological characteristics such as particle size and shape of the synthesized powder phosphors were evidenced by FE-SEM. Figure 5a, b and c showed the FE-SEM images of un-doped LaPO₄ and La_0.95−xCe_0.05Mn_xPO₄ (x = 0.00, 0.03). It has been observed that the microstructure of all the powders consists of an agglomeration of nanorods, which are around 17±3 nm in diameter and have lengths ranging from 100 to 300 nm. Image J software package was used to estimate the average diameter of the nanorods and a lognormal function was used as a distribution curve. In the case of un-doped LaPO₄, the average diameter of nanorods is ~ 20 nm, and diminishes by doping with Ce³⁺ (~ 17 nm) and co-doping with Mn²⁺ (~ 14 nm).

Fig. 5

FE-SEM images of (a) LaPO₄, (b) La_0.95Ce_0.05PO₄ and (c) La_0.92Ce_0.05Mn_0.03PO₄ samples calcined at 800°C; (d) EDX spectrum of La_0.92Ce_0.05Mn_0.03PO₄ sample. The inset of image (a) shows the nanorod diameter distribution of un-doped LaPO₄ based on a lognormal distribution.

The chemical composition of the powders was determined by energy dispersive x-ray (EDX) assay. Figure 5d shows the major chemical elements (La, P, Ce, O) of La_0.92Ce_0.05Mn_0.03PO₄ sample. The EDX analysis from the Table II have shown a good agreement with the initial concentrations of the elements from the starting chemicals (La, O, P) and for dopant elements (Ce, Mn).

Table II Atomic % of elements present in un-doped LaPO₄ and La_0.95−xCe_0.05Mn_xPO₄ (x = 0.00, 0.03, 0.10) nanostructures.

EPR Investigations

In order to confirm the presence of the dopants (Ce and Mn), EPR spectroscopy was performed. Figure 6 shows the EPR spectra of the un-doped and doped (Ce and Mn) LaPO₄ at room temperature. The un-doped LaPO₄ shows no EPR signal (Fig. 6a). From Fig. 6b, the La_0.95Ce_0.05PO₄ phosphor shows two weak and broad signals with g₁ = 1.9657 and g₂ =4.6976. The signal with g₁ = 1.9657 can be assigned to an F⁺⁺ center, i.e. an electron trapped at an anion vacancy in the LaPO₄ lattice. The other new signal compared to the un-doped EPR spectra of LaPO₄ with g₂ = 4.6976 was assigned to Ce that is not bound in the host, with a g value characteristic to metals. The F⁺⁺ center can be an O⁻ ion, which may have been incorporated into the lattice during the synthesis. Figure 6c and d shows the EPR result of the phosphors as a function of Mn²⁺ co-doping (x = 0.03 and 0.10). Both curves exhibit the resonance signal with the g value of 2.0091 due to the presence of the Mn²⁺ ion, indicating the expected six lines18,43,44 from the hyperfine interaction and additional splitting from the dipolar interaction. By co-doping with 3 mol.% of Mn²⁺, the hyperfine structure (HFS) of manganese is clearly observed as in Fig. 6c. For higher Mn²⁺ co-doping (10 mol.%) the EPR signal broadens due to the high dipolar interaction between the paramagnetic centers in the doped material, as shown in Fig. 6d. We noticed that for both Mn²⁺ co-doped samples, the Mn²⁺ related EPR signal is superimposed on a broad band structure assigned to the clustered Mn²⁺ ions.45,46,47 Hence, the EPR spectra from the Fig. 6c and d are due to isolated, substitutional Mn²⁺ ions but also clustered in the interstitial space of the crystal lattice.

Fig. 6

EPR spectra recorded on (a) LaPO₄, (b) La_0.95Ce_0.05PO₄, (c) La_0.92Ce_0.05Mn_0.03PO₄ and (d) La_0.85Ce_0.05Mn_0.10PO₄ samples.

Vibrational Spectroscopy (FTIR-ATR)

The FTIR-ATR spectra of the calcined LaPO₄ and La_0.95−xCe_0.05Mn_xPO₄ (x = 0.00, 0.03, 0.10) show similar vibration bands as depicted in Fig. 7a, b, c and d. The bands located at around 536, 561, 576 and 611 cm⁻¹ belong to the bending vibrations (υ₄ label) of the O–P–O of the PO₄³⁻ groups. However, the main bands appearing at about 950, 987, 1009 and 1053 cm⁻¹ are associated with the stretching vibrations (υ₃ label) of the P–O of PO₄³⁻ groups.48,49,50 The broad FTIR peak of the O–H bond was not noticed at 1638 and 3394 cm⁻¹ in the monoclinic LaPO₄; this can be explained by removing the water molecules and organic molecules in the sample after calcination at 800°C, which is in good agreement with TG-DTA and XRD results (see Figs. 1 and 2b).

Fig. 7

FTIR-ATR spectra of (a) LaPO₄, (b) La_0.95Ce_0.05PO₄, (c) La_0.92Ce_0.05Mn_0.03PO₄ and (d) La_0.85Ce_0.05Mn_0.10PO₄ samples.

UV–Vis Spectroscopy

Figure 8 shows the reflectance spectra of LaPO₄ and La_0.95−xCe_0.05Mn_xPO₄ phosphors. The un-doped sample shows a sharp increase at around 220 nm (approx. 5.64 eV) which is due to band-to-band transitions of the LaPO₄ host lattice.51

Fig. 8

UV–Vis reflectance spectra of (a) LaPO₄, (b) La_0.95Ce_0.05PO₄, (c) La_0.92Ce_0.05Mn_0.03PO₄ and (d) La_0.85Ce_0.05Mn_0.10PO₄ samples.

In the case of doped samples, the absorption edge is shifted to about 245 nm due to Ce³⁺ ions absorption (4f → 5d). No absorption peaks associated with Mn²⁺ were detected in the (Ce, Mn)-co-doped samples because both spin and parity transitions are forbidden. The La_0.95−xCe_0.05Mn_xPO₄ (x = 0.03, 0.10) phosphors show less reflection due to their dark color compared to the un-doped LaPO₄ (white color) and La_0.95Ce_0.05PO₄ (yellow color).

Optical band gap of the samples were obtained from the diffuse reflectance (DR) data by using the Kubelka–Munk function (Eq. 3):52,53

$$ F(R) = \frac{\alpha }{s} = \frac{(1 - R)}{{2R}} $$

(3)

where R, α and s are the reflection, the absorption and the scattering coefficient, respectively.

Generally, the absorption coefficient (α) is related to photon energy (E) by the Eq. 4:54

$$ \alpha .E = M.(E - E_{g} )^{1/k} $$

(4)

where M is defined as energy constant-independent and E_g is the band gap. The exponent k is obtained by the form of transition. Since F(R) is proportional to the absorption α (see eq. 3); the exponent k could be determined by plotting (F(R).E)^k versus E.52,54 The optimal fit to a straight line near the absorption edge was found for k = 2 indicating a direct allow transition. Consequently, Eq. 4 can be transformed to Eq. 5:52,54

$$ F(R).E = M.(E - E_{g} )^{1/2} $$

(5)

Hence, the band gap energy was estimated from the graph of (F(R).E)² versus energy using the extrapolation of the slope to F(R) → 0.52,54,55 The optical band gap of un-doped LaPO₄ was 5.56 eV (approx. 223 nm, Fig. 9a), whereas the band gap energy of La_0.95Ce_0.05PO₄ was 5.11 eV (approx. 242 nm, Fig. 9b).51,56,57 This small shift of the E_g value is related to the Ce doping. The band gap of La_0.92Ce_0.05Mn_0.03PO₄ was 4.93 eV (approx. 251.5 nm, Fig. 9c) and La_0.85Ce_0.05Mn_0.10PO₄ was 4.70 eV (approx. 264 nm, Fig. 9d). By increasing the manganese concentration, the band gap of the materials decreases, most probably due to the introduction of a supplementary energy level. This behavior can be assigned to the presence of the Mn co-dopants at the site of the host matrix, but also to accompanying defects.

Fig. 9

The [F(R). E]² versus E plot of (a) LaPO₄, (b) La_0.95Ce_0.05PO₄, (c) La_0.92Ce_0.05Mn_0.03PO₄ and (d) La_0.85Ce_0.05Mn_0.10PO₄ samples.

Photoluminescence Properties

Figure 10a shows the excitation and emission spectra recorded on Ce³⁺ in La_0.95Ce_0.05PO₄. Photoluminescence spectrum excited at 275 nm (red line) shows the characteristic Ce³⁺ luminescence doublet (at around 319, 340 nm) due to the 5d → ²F_7/2, _5/2 transitions, accompanied by a weak shoulder at about 380 nm. The emission doublet can be decomposed into two components with peak centers at about 31,348 cm⁻¹ (319 nm) and 29,412 cm⁻¹ on an energy scale, with an energy difference of 1936 cm⁻¹, which is close to the theoretical difference between the ²F_5/2 and ²F_7/2 levels (2000 cm⁻¹).58,59 The excitation spectrum (black line) of the 340 nm luminescence consists of a strong excitation peak of Ce³⁺ ion at 275 nm with a shoulder at 260 nm, which is attributed to the Ce³⁺ ion transitions from the ²F_5/2 ground state to the 5d excited states.

Fig. 10

(a) Photoluminescence spectrum recorded in La_0.95Ce_0.05PO₄ under 275 nm excitation (red line) and the corresponding excitation spectrum monitored for the 340 nm emission (black line); (b) photoluminescence spectrum excited at 275 nm recorded in La_0.92Ce_0.05Mn_0.03PO₄ .

The emission spectrum of the La_0.92Ce_0.05Mn_0.03PO₄ phosphors excited by 275 nm UV light is represented in Fig. 10b. The characteristic Ce³⁺ UV luminescence doublet (5d → ²F_7/2, _5/2) is easily recognizable and accompanied by a weak and broad visible luminescence band at about 550 nm assigned to the ⁴T₁(G) → ⁶A₁(S) transition of Mn²⁺ ions (inset of Fig. 10b). As an energy transfer between the Ce³⁺ and Mn²⁺ ions has been observed in many other doped fluoride or phosphate crystals such as CaF₂,27 CeF₃, CePO₄28 and CaS(PO₄)29 it is likely that a similar mechanism is acting in the present case. Hence, we suppose that under UV irradiation in the Ce³⁺ absorption band, an energy transfer process Ce³⁺-Mn²⁺ triggers the Mn²⁺ luminescence at 550 nm.

The energy levels scheme and energy transfer from Ce³⁺ to Mn²⁺ in LaPO₄ phosphor is depicted in Fig. 11. Under UV-light excitation at 275 nm, the Ce³⁺ ions are excited on the 5d levels followed by the relaxation to the bottom component of the 5d level with the typical emission of UV light at about 319, 340 nm. According to the photoluminescence measurements, there is an energy transfer mechanism from Ce³⁺ to Mn²⁺ upper excited levels which then relaxes to ⁴T₁(⁴G) level with the typical Mn²⁺ green emission ⁴T₁(⁴G) → ⁶A₁(S),60,61 as also indicated in Fig. 10.

Fig. 11

The schematic level diagram for the energy transfer process in the LaPO₄: Ce³⁺-Mn²⁺ phosphors.

The visible luminescence is very weak because of the low number of the Mn²⁺ ions substituting the La³⁺ ions due to the charge inconsistency between the ions, a behavior also observed in the case of doped YPO₄.45 The Mn²⁺ ions are dominantly present as interstitial ions aggregates within the LaPO₄ lattice (Fig. 6) and are luminescent inactive.45 The Mn²⁺ luminescence from 550 nm is just slightly higher for the La_0.85Ce_0.05Mn_0.10PO₄ sample (data not showed), probably because of the concentration quenching effect.

The energy transfer efficiency (η_ETE) has been computed using the intensities of Ce³⁺ luminescence signals in the absence (I₀) and presence (I) of activator ions and the Eq. 6:62

$$ \eta_{ETE} = 1 - \frac{I}{{I_{0} }} $$

(6)

The ET efficiency (η_ETE) was found to be about 0.4% as was previous reported in Mn²⁺-doped CePO₄.28 In the last case the Ce³⁺-Mn²⁺ pairs are easily formed and result in an efficient energy transfer28 but in the present case the pairs number is low due the low number of isolated (substitutional) Mn²⁺ ions. However, it was shown that a high increase of the energy transfer efficiency (and Mn²⁺ luminescence efficiency) up to two orders of magnitude can be reached by Zr⁴⁺ co-doping, element which acts as a stabilizer of isolated Mn²⁺ ions in the regular lattice site through charge compensation.45,46

Conclusions

In summary, the LaPO₄ and La_0.95−xCe_0.05Mn_xPO₄ phosphors were obtained by a simple and cost-efficient co-precipitation method. The XRD results reveal that all La_0.95−xCe_0.05Mn_xPO₄ phosphors presented monoclinic monazite type structure of the LaPO₄ phase and the analysis indicated the incorporation of the Ce³⁺ to Mn²⁺ co-dopants in the LaPO₄ lattice, as was sustained also by the EPR measurements. The Ce³⁺ and Mn²⁺ doping of LaPO₄ induces a slight distortion of the crystalline structure without influence of the nanowire morphology. The band gap of Ce³⁺/Mn²⁺ doped LaPO₄ decrease down to 4.70 eV, compared to the 5.56 eV for undoped sample, due to Ce³⁺ absorption and supplementary energy levels introduced by the Mn²⁺. Upon UV excitation in the Ce³⁺ absorption band at 275 nm we observed the green emission of Mn²⁺ ions at around 550 nm due to an energy transfer effect, but the efficiency is very small, only about 0.4%. However, further investigations and optimizations of the obtained materials are required to improve the luminescence efficiency for practical application.

Conflict of interest

On behalf of all authors, the corresponding author states that there is no conflict of interest.