Emission Enhancement and Energy Transfers in YV0.5P0.5O4 Nanoparticles Codoped with Eu3+ and Bi3+ Ions.

In this study, solid-state solutions of yttrium orthovanadate-phosphate with varying concentrations of codopants (Eu3+, Bi3+) have been obtained via coprecipitation. An ionic radii mismatch between V5+ and P5+ substituents is manifested in broad XRD lines. The sharpening of the XRD lines is observed with increasing bismuth ions concentration in the Eu3+ codoped YV0.5P0.5O4 matrix. The difference in the number of the Stark components for the 5D0 → 7FJ transitions indicates changes in the lattice and a number of possible Eu3+ sites. A thorough, systematic spectroscopic analysis of YV0.5P0.5O4: x mol % Eu3+, y mol % Bi3+ was conducted at room temperature and 5 K. Metal-to-metal energy transfers occurring between Eu3+, V5+, and Bi3+ optically active ions have been investigated. Additionally, efficiency of the Bi3+-Eu3+ energy transfer (ET) was calculated.

Introduction

The yttrium orthovanadate and yttrium orthophosphate matrices, doped with europium ions, are popular luminescent phosphors. This is due to their potential applications as laser host materials, polarizers, solar cells, light emitting diodes, host materials for optically active ions, etc.[1−7]

YVO4 and YPO4 crystallize in the zircon tetragonal system, within the space group I4/amd.[8,9] Hence, a solid-state solution of yttrium orthovanadate-phosphate can be formed.[10] Considering YP0.5V0.5O4, its unit cell is composed of 50 mol % vanadium tetrahedral and 50 mol % phosphate tetrahedral groups, statistically substituted. Furthermore, in this work, yttrium ions in the lattice are statistically substituted with europium and bismuth ions.

In the present work, the fraction of the YPO4–YVO4–BiVO4–BiPO4 pseudoquaternary diagram (more precisely the shaded area of the YP0.5V0.5O4–BiVO4–BiPO4 pseudo ternary subdiagram) is investigated (Figure ). The bond valence sums (BVS) were obtained from VESTA,[11] and bond valence parameters were compiled in ref (12). The compounds in the shaded area have a disordered zircon-like crystal structure due to the inner structural characteristics of YPO4 and YVO4. Optically active europium ions are incorporated in YP0.5V0.5O4 and (Y,Bi)P0.5V0.5O4 for two purposes: to collect information on the local crystal structure and to investigate energy transfer processes involving Bi3+. This paper constitutes an extension of previous reports[2,13−16] with more systematic, thorough spectroscopic analysis. Previous works[2,13,14] focus on parameters affecting the Eu3+ emission intensity in micron-sized[14] and nanosized[2,13] systems. It was previously established that codoping YVO4:Eu3+ with P5+, Bi3+, and Gd3+ greatly enhances the europium ions’ emission intensity. The compositions which maximize Eu3+ emission intensity are typically Y0.9Bi0.05Eu0.05P0.5V0.5O4[13] or Y0.45Gd0.45Bi0.05Eu0.05P0.5V0.5O4.[2] The reasons for this remain obscure.

Figure 1

YPO4–YVO4–BiVO4–BiPO4 pseudoquaternary diagram. BVS = bond valence sum.

Trivalent bismuth ion is known as an attractive activator in zircon vanadates.[15,17−23] Furthermore, it has been observed that Bi3+ is an efficient luminescence sensitizer for trivalent lanthanide ions.[3,24−28] In this study bismuth and europium ions are chosen as codopants, bismuth ions improve the photoluminescence intensity of Eu3+. This phenomenon occurs as a result of UV excitation. This is due to the CT transitions from the Bi3+ 6s energy level to the 5d levels of the vanadate and subsequent energy transfer (ET) to Eu3+ ions 4f orbitals.[24,29−33]

This work emphasizes characterization of the luminescent properties of these two dopants incorporated into YV0.5P0.5O4. This matrix has a disordered structure as a result of phosphate and vanadate units being randomly dispersed throughout the lattice. This occurs because the vanadate units are ∼8% larger than the phosphates. The extent to which this disorder contributes to the efficiency of Bi3+-Eu3+ ET will be investigated in this work. To this end, two series of materials were synthesized. The first one was doped with varying amounts of bismuth ions, while the second was doped with varying amounts of europium ions. The chemical compositions involved doping and codoping YV0.5P0.5O4 with xBi3+, yEu3+ where x = 0, 1, 3, 5, 10, 15 mol % and y = 0.5, 1, 2, 5 mol %. The solid state solutions were obtained by the wet chemistry synthesis-coprecipitation method with additional heat-treatment at 800 °C for 3 h.

Experimental Methods

Materials Synthesis

Yttrium orthovanadate-phosphate powders, codoped with europium and bismuth ions, were obtained by the coprecipitation method. The concentrations of vanadium and phosphorus were fixed to 50 mol % each. Two series of materials were obtained: one in a function of bismuth concentration with fixed concentration of europium and vice versa. First, the concentration of europium ions was set to 1 mol %, while bismuth ion concentration changed from 0, 1, 3, 5, 10 up to 15 mol %. In the second series, the concentration of bismuth ions was set to 10 mol %, with concentrations of europium ion varying from 0.5, 1, 2, up to 5 mol %. Stoichiometric amounts of analytical grade Y2O3 (Alfa Aesar, 99.99%), Bi2O3 (Sigma-Aldrich, 99.9%), Eu2O3 (Alfa Aesar, 99.99%), (NH4)2HPO4 (ACROS Organics, >98%) and NH4VO3 (Sigma-Aldrich, 99.5%) were used in this synthesis process.

The lanthanide and bismuth oxides were converted into nitrate salts through digestion with an excess of 65% HNO3. Thereafter, the formed lanthanide and bismuth nitrates were recrystallized, and the HNO3 excess was removed. Using deionized water as a solvent, separate aqueous solutions of diammonium phosphate and ammonium metavanadate were made. The vanadium and phosphorus ion sources (NH4VO3 and (NH4)2HPO4) were mixed, followed by the nitrates (Y(NO3)3, Bi(NO3)3, Eu(NO3)3). The liquid mixture was stirred for 1.5 h at approximately 70 °C. Aqueous ammonia was used to maintain a pH of 9 during the reaction. The as-prepared precipitates were then washed and centrifuged at least three times, until neutral pH was reached. They were then dried for 24 h at 70 °C. The powders were finally crystallized by heat-treatment at 800 °C for 3 h in air.

ICP, XRD, SEM, and TEM Analyses

The crystal structure of synthesized materials was characterized by the X-ray Diffraction (XRD) technique using an X’Pert PRO X-ray diffractometer (Cu Kα1, 1.54060 Å) (PANalytical). Measured XRD patterns were compared to standards of YVO4 (no. 78074) and YPO4 (no. 79754) found in the Inorganic Crystal Structure Database (ICSD). Microstructural analyses (particle size, morphology) were performed by electron microscopy. SEM was carried out using an FEI Nova NanoSEM 230. High resolution transmission electron microscopy (HR-TEM) was performed using a Philips CM-20 Super Twin microscope. ICP-OES measurements were conducted on Thermo Scientific ICAP 7000 SERIES.

Spectroscopic Analysis

The Nicolet iS50 FT-IR from Thermo Scientific was used to collect infrared spectra at 300 K of samples processed in KBr pellets. The room-temperature emission spectra utilized excitation at 397, 340, and 300 nm. These spectra were collected using a FLS1000 photoluminescence spectrometer from Edinburgh Instruments. The same apparatus was used to collect the excitation spectra. The 5D0 → 7F2 transition at 619 nm was monitored at room temperature for the excitation spectra measurements. Emission spectra were also recorded in response to the 266 nm excitation of a laser diode (CW) at room temperature and detected using the Hamamatsu PMA-12 photonic multichannel analyzer. The emission decay profiles were measured at 300 K using either a Ti:sapphire tunable laser or a Nd:YAG laser, a Hamamatsu R928 photomultiplier, a Jobin-Yvon THR 1000 spectrophotometer, and a digital LeCroy WaveSurfer oscilloscope. Excitation and emission spectra were collected at low temperature (5 K) using a temperature-controlled, continuous-flow liquid helium cryostat: Oxford Model CF 1204. Low temperature excitation spectra were measured with a Dongwoo Optron DM151i monochromator and a 150W ozone free lamp. The low temperature emission spectra were measured using a Dongwoo Optron DM750 monochromator, an Electro-Optical System INC PbS photodiode, or a Hamamatsu R928 photomultiplier.

Results and Discussion

Structure and Morphology

The chemical composition of all samples, analyzed by ICP-OES, is given in Table . It is verified that the nominal compositions are congruous with actual compositions.

Table 1

Elemental Composition of the Europium and Bismuth Codoped YV0.5P0.5O4 in Molar Percentages

	YV0.5P0.5O4
element	0.5% Eu3+ 10% Bi3+	1% Eu3+ 10% Bi3+	2% Eu3+ 10% Bi3+	5% Eu3+ 10% Bi3+	1% Eu3+ 1% Bi3+	1% Eu3+ 3% Bi3+	1% Eu3+ 5% Bi3+	1% Eu3+ 10% Bi3+	1% Eu3+ 15% Bi3+
Y	89.08	98.73	87.83	84.97	97.95	95.47	92.42	88.91	83.66
V	49.70	49.63	50.05	49.65	50.32	50.55	49.79	49.78	50.06
P	49.79	49.54	49.89	50.27	50.54	48.73	49.84	49.47	49.74
Eu	0.52	1.01	1.95	4.98	1.03	1.03	1.03	1.02	1.05
Bi	10.06		10.01	10.10	0.96	3.03	4.97	9.98	14.94

XRD results confirmed the crystal phase purity of YV0.5P0.5O4 doped derivatives (Figure ). The XRD peaks are broadened and further confirm the structural disorder, as observed earlier.[10,12,20] This broadening originates from the size difference between V5+ (0.36 Å, C.N. 4) and P5+ (0.17 Å, C.N. 4), when statistically distributed in YVP1–O4 solid solution. The lattice strains are influenced by changes in grain size. This is a result of point defects, vacancies,[34] and varying composition[35] as well as dislocations near the grain-boundaries[36] caused by the incompatibility of phosphorus and vanadium atoms. The observed changes in the fwhm of the XRD peaks may indicate the presence of lattice strains.[37]

Figure 2

Diffractograms (a, c) and fwhm analysis (b, d) obtained for x mol % Bi3+, 1 mol % Eu3+: YV0.5P0.5O4 and 10 mol % Bi3+, y mol % Eu3+: YV0.5P0.5O4.

Co-doping with Eu3+ does not impact the lattice and, by extension, the width of the XRD peaks. In contrast, it is found that codoping with Bi3+ contributes to narrowing of the XRD peaks (Figure c). This is clearly evidenced by Figure b,d. Additionally, increasing the Eu3+ amount in 10 mol % Bi3+ codoped samples does not affect the XRD peak widths significantly.

By considering Eu3+ as a local luminescent structural probe, we find three possible environments experienced by Y3+ ions in the compounds. This is depicted in Figure . In YPO4, the Eu3+ ions are surrounded by eight O atoms, thus forming a dodecahedron with D2 point symmetry. The first cation coordination consists of two P5+ at 3.01 Å and respectively four P5+ and four Y3+ at 3.76 Å in a second coordination. The exact charge (the BVS - Bond Valence Sum) carried by these cations is given in Figure . Incorporation of 50% V5+ creates two additional spheres at 3.14 and 3.89 Å (marked as red in Figure ) with statistical occupancy. Incorporation of Bi3+ results in a third sphere (marked as blue in Figure ) at 4.00 Å. The probability of finding a Bi3+ ion in this position is P(x) = 1–(1–x)4 wherein x is the molar percentage of Bi3+, e.g., P(0.1) = 35%. The bismuth ions (1.17 Å at C.N. 8) substitute yttrium ions (1.02 Å at C.N. 8) in a statistical manner. Since Bi3+ is about 15% larger than Y3+, its incorporation in the crystal lattice counterbalances (at least partly) the ionic radius mismatch between V5+ and P5+. This inverse relationship between V5+–P5+ and Bi3+–Y3+ ionic radii mismatches reduces the lattice strains in YV0.5P0.5O4 and thereby sharpens the XRD lines as the Bi3+ concentration is raised (Figure c).

Figure 3

Cation coordination spheres of Eu3+ in zircon-like YPO4, YP0.5V0.5O4, and (Y,Bi)P0.5V0.5O4. Oxygen atoms are not represented.

SEM images of YV0.5P0.5O4: 1 mol % Eu3+, x mol % Bi3+ materials are depicted in Figure . The powders look micrometric regardless of bismuth ion concentration. As the bismuth content is raised, the particles become smaller, and their surfaces become rougher. EDS maps (Figure ) obtained for 1 mol % Eu3+, 10 mol % Bi3+: YV0.5P0.5O4 confirm random distribution of the constituents.

Figure 4

SEM images obtained for YV0.5P0.5O4: 1 mol % Eu3+, xBi3+, where x = 5 mol % a), 10 mol % b), and 15 mol % c).

Figure 5

EDS maps obtained for 1 mol % Eu3+, 10 mol % Bi3+: YV0.5P0.5O4.

The TEM images in Figure demonstrate that the aggregates consist in fact of nanosized particles. Analysis of the SEM (Figure ) and TEM (Figure ) images reveals a wide distribution of particle sizes among the samples, although we note that the particles size increases as the Bi3+ content is raised (i.e., 19, 31, and 62 nm for doping rates of 1, 5, and 15 mol %, respectively). Differences in particles sizes contribute to the narrowing of the XRD peaks in addition to the reordering of the crystal structure. This work sheds a light into a complex structure of Eu3+, Bi3+: YV0.5P0.5O4. However, to individuate or quantify the role of each effect, further crystallographic research needs to be conducted. Additionally, d-spacing values were calculated by the means of FFT processing in ImageJ software. d-Spacing was calculated to be d101 = 0.45 nm for YV0.5P0.5O4 samples codoped with 1 mol % Eu3+ and 1 mol % Bi3+ and 1 mol % Eu3+ and 5 mol % Bi3+, as well as 1 mol % Eu3+ and 15 mol % Bi3+. Also d200 values were calculated. For samples codoped with 1 mol % Eu3+ and 1 mol % Bi3+, d200 was 0.35 nm. However, 5 mol % and 15 mol % Bi3+-doped samples had a d200 value of 0.33 nm. All d-spacing values coincide with d101 and d200 values from standard patterns of YVO4 (d101 = 0.47132 nm, d200 = 0.35591 nm) and YPO4 (d101 = 0.45379 nm, d200 = 0.34474 nm).

Figure 6

TEM images of YV0.5P0.5O4 doped with 1 mol % Eu3+ and 1 mol % Bi3+ a), 1 mol % Eu3+ and 5 mol % Bi3+ b), and 1 mol % Eu3+ and 15 mol % Bi3+ (c).

Spectroscopic Properties

The FT-IR spectra of YV0.5P0.5O4 codoped with Eu3+ and Bi3+ are shown in Figure . There are five strong absorption bands in the range of 1300–400 cm–1. The peaks at 524 cm–1 and at 639 cm–1 represent an antisymmetric bending vibration of ν4(PO4)3–. The antisymmetric stretching vibration of ν3(PO4)3– can be found at 1010 cm–1 and at 1110 cm–1.[38,39] The peak at 836 cm–1 is ascribed to the vibration mode of the (VO3)− group. The weak peak detected at 502 cm–1 is related to the Y–O vibration.[40] This mode is not observed for the samples with more than 10 mol % of codopant ions concentration. The Bi–O modes are on the verge of our measurement range.[41,42]

Figure 7

Fourier transformed infrared spectra of (a) codoped with Bi3+ and 1 mol % Eu3+ YV0.5P0.5O4, with varying Bi3+ concentrations and (b) codoped with 10 mol % Bi3+ and Eu3+, with varying Eu3+ concentrations.

Room temperature measurements revealed that all compounds exhibit red emission typical of Eu3+ upon direct 4f-4f excitation at 397 nm (Figure ). This emission increases in intensity with increasing Eu3+ and Bi3+ concentrations. The highest observed emission intensity is in samples containing 10 mol % Bi3+, 5 mol % Eu3+: YV0.5P0.5O4.

Figure 8

Emission spectra for x mol % Bi3+, y mol % Eu3+: YV0.5P0.5O4 under 397 nm excitation at room temperature.

The excitation spectra for x mol % Bi3+, y mol % Eu3+: YV0.5P0.5O4 materials for the 5D0 → 7F2 transition at 619 nm are shown in Figure . Three broad transitions are observed at 266, 300, and ≈340 nm (shoulder). They correspond to the O2– → Eu3+, O2– → V5+, and Bi3+ → V5+ charge transfers, respectively.[2,13,14,33] The intrinsic 4f-4f excitation lines of Eu3+ (namely 7F0 → 5L6, the 7F0 → 5D2, and 7F0 → 5D1 transitions) are comparatively much less intense.

Figure 9

Excitation spectra for x mol % Bi3+, y mol % Eu3+: YV0.5P0.5O4 materials measuring the intensity of the 5D0 → 7F2 transition at 619 nm at room temperature.

Figure depicts the emission spectra after excitation in the charge transfer bands. These intensities are normalized to the 5D0 → 7F1 transition of Eu3+. In addition to characteristic emission lines of Eu3+, broad emission signals are observed. Upon 340 nm excitation, the broad signal represents the emission of the Bi–V metal-to-metal CT,[15] whereas upon 300 nm excitation, the broad signal is more surely due to perturbed vanadate groups. Upon 266 nm excitation, these bandlike emissions possibly overlap. The presence of these emission bands indicates an incomplete sensitization of Eu3+ luminescence. Two possible sensitization paths are identified. They involve the (VO4)3- units or the Bi–V self-trapped excitons as energy donors and the Eu3+ ions as energy acceptors. Decay profiles were collected to quantify the efficiency of these energy transfers.

Figure 10

Emission spectra for x mol % Bi3+, y mol % Eu3+: YV0.5P0.5O4 under 266, 300, and 340 nm excitation normalized to the 5D0 → 7F1 transition of Eu3+.

Decay profiles are presented in Figure for 397 nm excitation (inner 7F0 → 5L6 Eu3+ transition) or 355 nm excitation (Bi–V MMCT). Corresponding average values of the luminescence lifetimes tav, were calculated as tav = ∫I(t)t dt/(∫I(t) dt). In this case, I(t) represents the emission intensity at time t. These values are provided in Table . A plot is proposed in Figure for discussion.

Figure 11

Decay time profiles measured for 5D0 → 7F2 transition monitored at 619 nm λexc = 397 nm (a), 5D0 → 7F2 transition, λem = 619 nm and λexc = 355 nm (b), and decay profiles measured for MMCT-Bi transition λem = 540 nm and λexc = 355 nm (c).

Table 2

Calculated Average Decay (tav) and Rise (trise) Times for x mol % Bi3+, y mol % Eu3+: YV0.5P0.5O4 Pumped with 397 and 355 nm Wavelengthsa

		5D0 → 7F2 (Eu3+) transition at 619 nm		Bi–V MMCT transition at 540 nm
		λexc = 397 nm	λexc = 355 nm	λexc = 355 nm
label	YV0.5P0.5O4	tav [ms]	tav [ms]	tav [μs]	η (%)
A	1 mol % Eu3+	1.5	1.1
B	1 mol % Bi3+			7.0
C	1 mol % Bi3+, 1 mol % Eu3+	1.6	1.2	6.9	1.1
D	3 mol % Bi3+, 1 mol % Eu3+	1.9	1.2	5.8	16.5
E	5 mol % Bi3+, 1 mol % Eu3+	1.6	1.2	5.8	17.2
F	10 mol % Bi3+, 1 mol % Eu3+	1.3	1.1	5.5	20.7
G	15 mol % Bi3+, 1 mol % Eu3+	1.1	0.9	5.6	19.5
H	10 mol % Bi3+, 0.5 mol % Eu3+	1.1	1.1	5.7	17.9
I	10 mol % Bi3+, 2 mol % Eu3+	1.0	1.0	5.9	16.2
J	10 mol % Bi3+, 5 mol % Eu3+	1.1	1.0	6.1	12.9

Each sample composition was assigned a label to facilitate the discussion below.

Figure 12

Integrated 5D0 → 7F2 emission intensity (●) and average emission lifetime of Eu3+ (□) for the different compounds listed in Table in correspondence with λexc = 397 nm (a) and λexc = 355 nm (b).

The parameter η in Table is related to the efficiency of the ET from Bi3+-to-Eu3+ in the codoped compounds. It is obtained from the equationwhere τdoped is the time constant in the presence of the Eu3+ acceptor, and τundoped is the time constant without Eu3+ (i.e., only Bi3+).

Figure (a) pertains to the inner 7F0 → 5L6 excitation of Eu3+. The left-hand side of the vertical dashed line corresponds to 1 mol % Eu3+ doped samples with increasing Bi3+ concentrations (compound A to compound H, excluding compound B, in Table ). Here, an increase in emission lifetime with increasing bismuth concentration is observed up to 3 mol % Bi3+ (compound D). These samples (A,C,D,E) also exhibit lower emission intensity relative to samples with higher Bi3+ concentrations. This suggests a lower radiative probability from the 5D0 state in these cases. Raising the Bi3+ amount further contributes to enhance the Eu3+ emission intensity, and it correlates with a shortening of the lifetime. This effect has already been observed in YPO4:Sm3+, Bi3+ and is ascribed to an increased refractive index of the host lattice.[43] In this case, the radiative probability is increased, contributing to a lower emission lifetime. A synergetic effect with the reordering of the crystal structure is not excluded. This requires further investigation. The right-hand side of the figure shows the effect of Eu3+ content for a fixed amount of Bi3+. In this case, the emission lifetime does not vary significantly because the medium’s refractive index is unchanged. Here, the increase in emission intensity is ascribed to the larger Eu3+ content that remains beyond the quenching concentration.[44,45]

Part (b) of Figure relates to an excitation in the Bi–V MMCT band, the intensity of which exceeds by far that of the 7F0 → 5L6 transition (Figure ). In this situation, the Eu3+ emission is produced after an energy transfer whose efficacy is given in Table . Efficacy is 20% in the Bi3+-rich compounds F and G but tends to fall off in compounds containing more than 1% Eu3+: η is for instance comparable in compound D (3 mol % Bi3+, 1 mol % Eu3+) and in compound I (10 mol % Bi3+, 2 mol % Eu3+). Nevertheless, the emission intensity of compound J amounts to 6.5 times that of compound D, which demonstrates a synergy between Bi3+ and Eu3+ contents in YV0.5P0.5O4. Furthermore, with respect to both 355 and 397 nm excitations, compound J exhibits the highest emission intensity related to direct Eu3+ excitation.

At sufficiently low temperatures, vibronic interactions can be frozen out. Thus, the luminescence spectra can give more detailed information regarding the electronic transitions of Eu3+ and can now be used as a structural probe. In 2008, Pan et. al[46] conducted investigations regarding the spectroscopic properties of Eu3+ in Y(V,P)O4 solid solution by laser-selective excitation. This work identified three symmetry sites in the yttrium orthovanadate-phosphate mixed compounds due to disorder generated by the distribution of (PO4) and (VO4) tetrahedra. The Judd–Ofelt intensity parameters further confirmed that significant changes in ligand polarizability contribute to differences in local environments experienced by Eu3+.[46]

Figure illustrates the excitation spectra collected at 5 K corresponding to the 619 nm emission which represents the Eu3+5D0 → 7F2 transition. Low temperature excitation spectra depict noticeable differences when compared to the room temperature spectra. These major differences are as follows:

Figure 13

Excitation spectra for x mol % Bi3+, 1 mol % Eu3+: YV0.5P0.5O4 materials monitoring the 619 nm emission corresponding to the 5D0 → 7F2 transition at T = 5 K.

differences in relative intensity of the intrinsic 7F0 → 5L6 transitions of Eu3+ to the CT excitation bands in compounds containing low amounts of Bi3+ (<5 mol %). This indicates a less efficient sensitization at 5 K. Furthermore, it demonstrates that in these conditions, the Bi-to-Eu energy transfer is efficiently phonon assisted. However, even at 5 K the Bi-to-Eu energy transfer regains efficacy when the Bi3+ concentration is 10 mol %. This coincides with the increased presence of a broad excitation band which peaks at ≈360 nm and extends up to ≈400 nm, currently attributed to Bi–V MMCT.

the presence of structures on the Bi–V MMCT excitation with maxima identified at ≈350, 330, and 307 nm. The relative intensity of these excitation maxima strongly depends on the Bi3+ content in the compound. There is a notable red shift in the excitation spectra as Bi3+ concentration increases. This shift of the Bi–V excitation edge with increasing the Bi content has been noted in previous studies, e.g., in ref[32].

the absence of excitation features pertaining to O2– → Eu3+ and O2– → V5+ charge transfers. This suggests these transitions are not involved in the sensitization process of Eu3+ at 5 K. Therefore, only the Bi–V MMCT operates as a sensitizing channel for Eu3+ at 5 K.

The emission spectra for 1, 3, 5, 10, 15 mol % Bi3+, 1 mol % Eu3+: YV0.5P0.5O4 were collected at 5 K. These samples were excited in the CT bands and at 395 nm (Figure ). The wavelengths of the Stark components observed in Y0.98Eu0.01Bi0.01V0.5P0.5O4 and Y0.89Eu0.01Bi0.10V0.5P0.5O4 are compiled in Table and compared to reference data on Eu3+ in YPO4 and YVO4.[47]

Figure 14

Emission spectra of 1 mol % Eu3+ in Y1–BiV0.5P0.5O4 (x = 0.01 to 0.15) upon various excitation wavelengths at 5 K. Spectra normalized to the 5D0 → 7F1 transition of Eu3+ .

Table 3

Stark Components (in nm) of Eu3+ in YVO4,[47] YPO4,[47] and Y1–BiV0.5P0.5O4 (x = 0.01, and 0.10)b

5D0 →	YPO4	YVO4	Y0.98Eu0.01Bi0.01V0.5P0.5O4	Y0.89Eu0.01Bi0.10V0.5P0.5O4
7F0	581.0	581.9
7F1	592.7	593.5
	596.1	595.0	594.7a	594.5a
7F2	613.4	615.5	610.5	609.9
	617.6	617.3	613.7	615.4
	619.3	619.4	615.3	618.8
	620.2	622.4	619.0	619.9
			619.9
7F4	691.6	690.5	696.7	698.4
	694.4	696.7	698.6	704.0
	696.2	698.5	703.4
	697.3	699.1
	699.3	701.2
	703.5	704.5
	704.7	708.2

Centroid position.

Data were taken at 5 K.

Data related to the 5D0 → 7F4 transition of Eu3+ are incomplete, when compared to the reference zircon compounds YPO4:Eu3+ and YVO4:Eu3+. The 5D0 → 7F1 transitions are also poorly resolved despite the low temperature. Thereby the results are discussed based on the 5D0 → 7F2 transitions. Two different signatures are noted for the Eu3+ transitions in Y0.98Eu0.01Bi0.01V0.5P0.5O4 depending on the excitation wavelength. This indicates the presence of more than one Eu3+ type site in this crystal structure, which agrees with the conclusions of Pan et al.[46] In Y0.89Eu0.01Bi0.10V0.5P0.5O4, however, only four Stark components (instead of five) corresponding to the 5D0 → 7F2 transition were observed. The spectrum in the 5D0 → 7F4 region looks also simpler. This simplification of the spectral signatures is consistent with the progressive crystal structure reordering of YP0.5V0.5O4 concomitant with an increase in Bi3+ concentration. This is previously observed in XRD results (Figure ). Compared to YPO4:Eu3+ and YVO4:Eu3+, the 5D0 → 7F2 spectrum of Y0.89Eu0.01Bi0.10V0.5P0.5O4 is more split (265 cm–1 against 178–180 cm–1), and its energy barycenter is upshifted. This is due to the presence of the Bi3+ ion in the second cationic neighborhood of Eu3+, as we have depicted in Figure . Owing to the BVS values given in Figure , it is concluded that the formal charge carried by bismuth is below that of yttrium, with the consequence that the formal charge carried by phosphorus and vanadium atoms in the Bi-doped compounds is enhanced with respect to the Bi-free compounds. This, in turn, affects the formal charge carried by the oxygen atoms in the first coordination of Eu3+ by reinforcing the crystal field and softening the nephelauxetic effect.

Conclusion

The pure crystal phase of x mol % Bi3+, y mol % Eu3+: YV0.5P0.5O4 was formed using a coprecipitation synthesis method. It is observed that codoping with Bi3+ is followed by narrowing of the XRD peaks, but codoping with Eu3+ does not significantly affect the widths of the peaks. The insufficient energy transfer to Eu3+, resulting from the Bi–V MMCT and the 3T1,2 → 1A1 (VO4)3– broad transition bands, is observed. Yet, Eu3+ ion emission is enhanced by increasing Bi3+ ion concentration. Further reaffirmed is the presence of more than one Eu3+ site due to (PO4)3– and (VO4)3– substitution. Additionally, an ordering of the crystal structure of YP0.5V0.5O4 with increasing the Bi3+ content can be observed. It is supposed that at low temperature the sensitizing pathway of Eu3+ is less efficient in comparison to the room temperature, and mainly the Bi–V MMCT contribution is noticeable. Based on luminescent decay times, the Bi–Eu ET efficiency was found to be the highest for bismuth-rich compounds.