Resonant energy transfer in (Eu³⁺, Bi³⁺)-codoped CaZrO₃ red-emitting phosphor

Abstract

(Eu³⁺, Bi³⁺)-codoped CaZrO₃ phosphor crystallizing in the orthorhombic structure was synthesized from a mixture of CaCO₃, ZrO₂, Eu₂O₃, and Bi₂O₃ using the solid-state reaction method. The structural and optical properties of this phosphor were investigated using X-ray diffraction analysis, photoluminescence (PL) analysis, PL excitation (PLE) spectroscopy, and PL decay measurements. The effects of Bi³⁺ codoping on the Eu³⁺ emission properties were discussed in detail. Temperature dependence of the PL intensity was also measured between T = 20 and 450 K and obtained a quenching energy of E_q ∼ 0.1 eV. Remarkably, the red emission intensity of Eu³⁺ in the (Eu³⁺, Bi³⁺)-codoped phosphor was enhanced more than 10 times compared to the Eu³⁺ singly doped phosphor, thereby suggesting an efficient resonant energy transfer from Bi³⁺ to Eu³⁺. The asymmetry ratio (⁵D₀ → ⁷F₂)/(⁵D₀ → ⁷F₁) for the Eu³⁺ emission had different values with different Eu³⁺ concentrations, indicating that its local site depends on the Eu³⁺/Bi³⁺ concentration. Comparative discussion was also given on the PL spectra for some red-emitting phosphors, CaZrO₃:Eu³⁺, Bi³⁺, SnO₂:Eu³⁺, and K₂SiF₆:Mn⁴⁺.

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1. Introduction

The introduction of lanthanide ions as luminescent centers in a wide variety of host lattices has been an improvement in the luminescent materials. Calcium zirconate (CaZrO₃) has good chemical and thermal stability as well as better mechanical resistance. It crystallizes in the perovskite structure and has been used as the host material of such phosphors. For example, there have been reports on trivalent lanthanide ions, such as Tm³⁺,¹ Eu³⁺,^2–7 Sm³⁺,⁸ Tb³⁺,⁹ Er³⁺,^10,11 Gd³⁺,¹² (Eu³⁺, Gd³⁺),⁴ (Eu³⁺, Mg²⁺),^6,13 (Eu³⁺, Sr²⁺),⁶ (Sm³⁺, Gd³⁺),⁸ (Er³⁺, Ce³⁺),¹⁰ (Tb³⁺, Mg²⁺),¹³ (Dy³⁺, Tm³⁺),¹⁴ (Er³⁺, Yb³⁺),¹⁵ and (Eu³⁺, Li⁺),¹⁶ in CaZrO₃. The Eu³⁺ ion is an efficient activator in a number of phosphor materials. There have been considerable studies on the luminescence properties of CaZrO₃:Eu³⁺ phosphors.^2–7 However, there have been a very few reports on the codoping effects of Eu³⁺ with other ions.^4,6,13,16 An enhancement in the Eu³⁺ emission intensity caused by resonant energy transfer (RET) has been observed by adding sensitizer ions (a factor of ∼3.2 (Gd³⁺),⁴ ∼1.4 (Mg²⁺),⁶ ∼1.2 (Mg²⁺),⁴ and ∼3.0 (Li⁺)).⁴ No detailed discussion on the RET phenomena has, however, been paid to these reports.

CaZrO₃ can be synthesized by the solid-state reaction method. The purpose of this paper is to study the relations between the solid-state reaction condition and material properties of CaZrO₃:Eu³⁺ and (Eu³⁺, Bi³⁺)-codoped CaZrO₃ phosphors. The structural properties were investigated using the X-ray diffraction (XRD) analysis. Temperature dependence of the photoluminescence (PL) properties was examined from T = 20 to 450 K in 10 K increments. Room-temperature PL excitation (PLE) and PL decay characteristics were also examined on these phosphors. Finally, a comparative discussion was given on the PL properties of some red-emitting phosphors.

2. Experimental procedure

The starting materials were trigonal CaCO₃ and monoclinic ZrO₂, together with dopant species of Eu₂O₃ and Bi₂O₃. They were mixed in a molar ratio of CaCO₃ : ZrO₂ : (X/2)Eu₂O₃ : (Y/2)Bi₂O₃ and ground in an agate mortar for about 30 min. The molar concentrations were varied for x (≡X/2) from 0 to 0.09 (Eu³⁺) and for y (≡Y/2) from 0 to 0.05 (Bi³⁺). For the solid-state reaction, the mixed power was placed in an alumina crucible and calcined in a muffle furnace from room temperature to a setting temperature in the range of T_c = 300–1200 °C. The calcination was performed for 1 h in air atmosphere. After calcination, the samples were ground down in an agate mortar.

The synthesized CaZrO₃:Eu_x, Bi_y phosphor powders were characterized by XRD analysis using a SmartLab X-ray diffractometer (Rigaku) provided with a Cu Kα radiation at λ = 0.1542 nm. The XRD measurements were performed in the θ–2θ scan mode at room temperature.

PL measurements were performed in a single monochromator equipped with a charge-coupled device (PIXIS 100B, Princeton Instruments) at T between 20 and 450 K in 10 K increments. A He–Cd laser at λ_ex = 325 nm (Kimmon IK3302R-E) was used as the excitation light source. PL and PLE measurements were performed using a 50 W xenon lamp as the excitation light source combined with a monochromator (JASCO CT-25C) and a Peltier-device-cooled photomultiplier tube (Hamamatsu R375). PL and PLE spectra were corrected for the excitation light intensity and the sensitivity of the detection system.

The PL decay times were measured by excitation at λ_ex = 266 and 355 nm (pulse duration: 5 ns) from a Nd:YAG laser (Teem Photonics STV-01E). The signal was detected at 300 K with a Peltier-element-cooled photomultiplier tube (Hamamatsu R375), a multichannel scaler (Stanford Research Systems, Inc. SR 430), and a preamplifier (Stanford Research Systems, Inc. SR 445A).

The luminescence quantum yields were measured by excitation at λ_ex = 320 and 396 nm using an absolute PL quantum yield spectrometer (Hamamatsu C9920-02). The measurements were carried out at 300 K.

3. Results

3.1 Effects of molar composition

Phosphors with x = 0–0.09 and y = 0.02 calcined at T_c = 1200 °C for 1 h. The diffraction pattern for orthorhombic CaZrO₃ (space group = Pnma–D_2h¹⁶) taken from the American Society for Testing and Materials (ASTM) card is also shown in the lower part of Fig. 1a.

Fig. 1 (a) XRD patterns and (b) PL spectra for (Eu³⁺, Bi³⁺)-codoped CaZrO₃ phosphors with x = 0–0.09 and y = 0.02 calcined at T_c = 1200 °C for 1 h. The experimental data were measured at 300 K. The diffraction pattern for orthorhombic CaZrO₃ (#01-076-2401), taken from the ASTM card, is shown in the lower part of (a).

The XRD data in Fig. 1a confirm that all the synthesized samples are orthorhombic CaZrO₃ without containing any residual crystals such as Eu₂O₃ and Bi₂O₃. One can also find that the diffraction peak intensity shows no or a very slight decrease with increasing x (Eu₂O₃) up to ∼0.07, indicating no or negligibly small deterioration in the crystalline quality for the samples with x up to ∼0.07.

The PL spectra in Fig. 1b show the typical Eu³⁺ intra-4f⁶-shell emission peaks at λ > 500 nm (x ≠ 0) and Bi³⁺-related emission peak at λ ∼ 400 nm (x = 0). As expected, the Eu³⁺ emission intensity increases with increasing x (Eu³⁺) and shows a saturated value at x ≥ 0.05. On the other hand, the Bi³⁺-related emission intensity at ∼400 nm gradually decreases with increasing x in spite of a fact of a fixed Bi₂O₃ dopant concentration (y = 0.02). Such trends can be easily understood from the plots of the Eu³⁺ and Bi³⁺ emission intensities against x in Fig. 2.

Fig. 2 Plots of the integrated Eu³⁺ and Bi³⁺ emission intensities (I_PL) against x obtained from the PL spectra in Fig. 1b.

The electronic dipole transitions between 4f levels of the rare earth ions, including Eu³⁺, are in principle strictly forbidden. The optical transitions between the ⁵D_I and ⁷F_J levels of the Eu³⁺ ion are also spin prohibition. However, this prohibition is not strict because the description of the ⁷F_J levels as states with six parallel spins is not entirely correct. The spin–orbit coupling makes ⁷F_J state as being composed of a pure ⁷F_J state with a slight “admixture” of the pure ⁵D_I state. Therefore, the spin prohibition no longer applies so strictly. The parity prohibition can be lifted only by the influence of the crystal lattice. If Eu³⁺ ion is located at a perfectly or a nearly perfectly centrosymmetric site, then the odd crystal field terms are absent and the parity prohibition can be lifted. In that case, only magnetic-dipole transition is possible [see, e.g., SnO₂:Eu³⁺ (ref. 17)]. The corresponding selection rule is ΔJ = J − I = 0, ±1 (except that J = 0 ↔ I = 0 is forbidden). In CaZrO₃:Eu³⁺ where Eu³⁺ is situated at a site far from perfectly centrosymmetric, and is brought into the ⁵D₀ state, the possible transitions accompanied by the emission of radiation are ⁵D₀ → ⁷F_J with J = 0–6 (see Fig. 13 below). The dominant PL peaks observed in Fig. 1b are at ∼595 nm (⁵D₀ → ⁷F₁), ∼615 nm (⁵D₀ → ⁷F₂), and ∼705 nm (⁵D₀ → ⁷F₄).

The strongest emission peak at ∼615 nm (⁵D₀ → ⁷F₂) in Fig. 1b corresponds to the electric-dipole transition. The fact suggests that the Eu³⁺ ions are mainly situated in the noncentrosymmetric sites. The asymmetry ratio, R_e–m ≡ (⁵D₀ → ⁷F₂)/(⁵D₀ → ⁷F₁), for the PL intensities is an important factor indicating the concentration ratio of Eu³⁺ in the noncentrosymmetric to the centrosymmetric site. From the large amount of tabulated data, its ratio was found to be lower than one for totally symmetric sites and increase to 10 for systems with very low symmetry sites.¹⁸

Fig. 3a plots R_e–m vs. x data obtained from the PL spectra in Fig. 1b. The asymmetry ratio R_e–m gradually increases from ∼1.2 at x = 0.01 to ∼2.7 at x = 0.07 and then tends to show a saturation with further increase of x. The inset of Fig. 3a shows the PL spectra, normalized at the magnetic-dipole (⁵D₀ → ⁷F₁) peak intensity, for the samples with x = 0.01 and 0.09. The results in Fig. 3a suggest that the Eu³⁺ ions are substituted for the local sites, which are more asymmetric at higher x (Eu³⁺) concentrations.

Fig. 3 (a) Plots of the asymmetry ratio, (⁵D₀ → ⁷F₂)/(⁵D₀ → ⁷F₁), vs. x data obtained from the PL spectra in Fig. 1b. (b) Plots of the asymmetry ratio, (⁵D₀ → ⁷F₂)/(⁵D₀ → ⁷F₁), vs. y data obtained from the PL spectra in Fig. 4b. The inset of (b) shows the PL spectra for the samples with x = 0.01 and 0.09 (y = 0.02). These spectra were normalized by the ⁵D₀ → ⁷F₁ emission intensity at ∼595 nm.

Fig. 4 shows (a) the XRD patterns and (b) PL spectra for the (Eu³⁺, Bi³⁺)-codoped CaZrO₃ phosphors with x = 0.05 and y = 0–0.05 calcined at T_c = 1200 °C for 1 h. The diffraction pattern for orthorhombic CaZrO₃ taken from the ASTM card is also shown in the lower part of Fig. 4a. It is worth noting that the crystalline quality deduced from the XRD intensities greatly improves by introducing Bi₂O₃ (Bi³⁺). No clear deterioration in the crystalline quality by the highly doped Bi³⁺ ions is also observed for y from 0.01 up to, at least, 0.05.

Fig. 4 (a) XRD patterns and (b) PL spectra for the (Eu³⁺, Bi³⁺)-codoped CaZrO₃ phosphors with x = 0.05 and y = 0–0.05 calcined at T_c = 1200 °C for 1 h. The experimental data were measured at 300 K. The diffraction pattern for orthorhombic CaZrO₃ (#01-076-2401), taken from the ASTM card, is shown in the lower part of (a).

The integrated Eu³⁺ emission intensities (I_PL) obtained from Fig. 4b are plotted against y in Fig. 5. An important fact found in Fig. 5 (Fig. 4b) is that the Eu³⁺ emission intensity greatly increases with the addition of Bi³⁺, as similar to the XRD intensities observed in Fig. 4a. This increase in the Eu³⁺ emission intensities at y ≠ 0 may come both from the improved crystalline quality and from an efficient energy transfer between Eu³⁺ and Bi³⁺ (see details below). It is also understood that the asymmetry ratio R_e–m does not show any large change by Bi³⁺ codoping from y = 0 to 0.05, as demonstrated in Fig. 3b.

Fig. 5 Plots of the integrated Eu³⁺ emission intensity (I_PL) against y obtained from the PL spectra in Fig. 4b.

3.2 Effects of calcination temperature

Fig. 6 shows (a) the XRD patterns and (b) PL spectra for the (Eu³⁺, Bi³⁺)-codoped CaZrO₃ phosphors with x = 0.05 and y = 0.02 calcined at T_c = 300–1200 °C for 1 h, together with those for the uncalcined sample. The ASTM card images for orthorhombic CaZrO₃, trigonal CaCO₃, and monoclinic ZrO₂ are also shown in the top and bottom of Fig. 6a. The synthesis of CaZrO₃ can now be written from the solid-state reaction as

CaCO₃ + ZrO₂ → CaZrO₃ + CO₂ (1)

Fig. 6 (a) XRD patterns and (b) PL spectra for the CaCO₃ : ZrO₂ : (X/2)Eu₂O₃ : (Y/2)Bi₂O₃ materials with x = 0.05 (=X/2) and y = 0.02 (=Y/2) calcined at T_c = 300–1200 °C for t = 1 h, together with those of the as-mixed raw powder (uc = uncalcined). The experimental data were measured at 300 K. The diffraction patterns for orthorhombic CaZrO₃ (#01-076-2401), rhombohedral CaCO₃ (#00-005-0586), and monoclinic ZrO₂ (#00-037-1484), taken from the ASTM cards, are shown on the top or bottom of (a).

Calcination treatment up to T_c ∼ 600 °C gives no large change in the XRD patterns from the uncalcined sample. The CaZrO₃ formation occurs at T_c ≥ 1100 °C and, as a result, the XRD peaks from single-phased orthorhombic CaZrO₃ are observed. In Fig. 6b, the characteristic Eu³⁺ emission peaks are observed regardless of T_c values, but their intensities are much weaker at T_c ≤ 900 °C. Such very weaker, but clear Eu³⁺ emission peaks may mainly come from unreacted and/or incompletely reacted Eu₂O₃ powders.

The integrated Eu³⁺ emission intensities (I_PL) as a function of T_c for the samples with x = 0.05 and y = 0.02 are plotted in Fig. 7. The I_PL data are obtained from Fig. 6b. A clear exponential dependence is inferred from Fig. 7:

where k is the Boltzmann constant. This equation yields E_a ∼ 2.5 eV as an activation energy of the Eu³⁺ ions in CaZrO₃. An activation of the Eu³⁺ ions was also observed to occur at E_a ∼ 0.65 eV in β-Ga₂O₃,¹⁹ ∼1.0 eV in SnO₂,¹⁷ ∼1.3 eV in Tb₃Ga₅O₁₂,²⁰ ∼0.4 eV in CaTiO₃,²¹ ∼0.8 eV in Al₂O₃,²² and ∼1.4 eV in Eu₂Ti₂O₇.²³ Such oxide phosphors were synthesized using metal–organic decomposition (ref. 19, 20 and 22) or chemical reaction,¹⁷ followed by high-temperature calcination treatment or solid-state reaction process.^21,23

Fig. 7 Integrated PL intensity I_PL vs. reciprocal calcination temperature 1/T_c for the (Eu³⁺, Bi³⁺)-codoped CaZrO₃ phosphors. The I_PL data were obtained from Fig. 6b. An exponential dependence shown by the solid line yields E_a ∼ 2.5 eV as the thermal activation of Eu³⁺ ions in the CaZrO₃ host.

3.3 Temperature dependence of PL properties

Temperature dependence of the luminescence intensity is not only of technological importance but also of scientific interest. We show in Fig. 8 the PL spectra of the (Eu³⁺, Bi³⁺)-codoped CaZrO₃ phosphor with x = 0.05 and y = 0.02 calcined at T_c = 1200 °C for 1 h. The PL spectra were measured at T = 20–450 K. The corresponding Eu³⁺ emission intensity (I_PL) vs. T⁻¹ plots are shown in Fig. 9.

Fig. 8 Temperature dependence of the PL spectra from T = 20 to 450 K in 30 K increments for the (Eu³⁺, Bi³⁺)-codoped CaZrO₃ phosphor with x = 0.05 and y = 0.02 calcined at T_c = 1200 °C for 1 h.

Fig. 9 Integrated PL intensity (I_PL) vs. T⁻¹ plots for the Eu³⁺ emission in the (Eu³⁺, Bi³⁺)-codoped CaZrO₃ phosphor (see Fig. 8). The solid lines show the results calculated using eqn (3), yielding quenching energies of E_q1 = 6 meV and E_q2 = 0.1 eV.

In Fig. 8, the Eu³⁺ emission intensity gradually decreases with increasing T. Because of the intra-f-shell transition nature of Eu³⁺, no remarkable change in the spectral shape can be observed between T = 20 and 450 K. The decrease in I_PL with increasing T observed in Fig. 9 can be explained by the thermal quenching popularly rationalized by

where E_qi is the quenching energy. The solid lines in Fig. 9 show the results calculated using eqn (3) with I₀ = 1.0, a₁ = 0.03, E_q1 = 6 meV, a₂ = 40, and E_q2 = 0.1 eV (i = 1, 2). The gradual decrease in I_PL at higher temperatures can be well explained by the thermal quenching of E_q2 = 0.1 eV.

4. Discussion

4.1 Effects of Eu³⁺ and Bi³⁺ doping on crystalline quality

It is known that perovskite-type ABO₃ oxides like CaZrO₃ have two different cation sites, A- and B-sites. The coordination numbers (CNs) of these sites are eight (A) and six (B). It has been reported that Eu³⁺ can be preferentially substituted for the B (Zr)-site in the CaZrO₃ host.^6,7 If so, a deficient positive charge derived from Eu³⁺ inclusion should be compensated by generating a charge-compensating lattice defect randomly distributed in the crystal volume. If the codopant ion Bi³⁺ is substituted for the A-site, it may cause an invariance of change in the manner

Ca²⁺ + Zr⁴⁺ ↔ Bi³⁺ + Eu³⁺ (4)

Thus, an improvement in the crystalline quality can be observed at least by doping Bi³⁺ into CaZrO₃ (cf. Fig. 1a and 4a). Note also that the ion radius of Bi³⁺ (0.117 nm; CN = 8) is nearly the same as that of Ca²⁺ (0.112 nm; CN = 8). One can thus expect easy replacement of Bi³⁺ with Ca²⁺ without introducing any undesirable lattice strain.

No remarkable decrease in the diffraction intensities has been observed in the (Eu³⁺, Bi³⁺)-codoped CaZrO₃ phosphors with x up to ∼0.7 (y = 0.02; Fig. 1a). This comes from the beneficial effect of the Bi³⁺ codoping represented by eqn (4). No clear shift in the diffraction angles, together with no appearance of any residual peaks, has also been observed at x up to 0.09 (Fig. 1a) and y up to 0.05 (Fig. 4a).

An improvement in the crystalline quality can be realized in the PL properties of the samples codoped with Bi³⁺ (see Fig. 4 and 5). The decrease in the Eu³⁺ emission intensities at x > 0.07 (Fig. 2) and y > 0.02 (Fig. 5) may be caused by the high dopant effects of activator/sensitizer, known as the concentration quenching. As mentioned before (Section 3.1), the relative intensity of the magnetic-dipole (⁵D₀ → ⁷F₁) and electric-dipole (⁵D₀ → ⁷F₂) peaks is very sensitive to the local site symmetry of Eu³⁺, promising R_e–m to be less than one in the centrosymmetric site and very large, in some cases, approaching 10, in the noncentrosymmetric site.¹⁸ The R_e–m values obtained in Fig. 3a suggest that at lower x with y = 0.02, the Eu³⁺ activator ions should be situated for the Ca²⁺ lattice but tends to be dominated for the Zr⁴⁺ lattice in the larger x phosphors. No large change in R_e–m against y in Fig. 3b supports that the Eu³⁺ ions can be preferentially substituted for the Zr⁴⁺ lattice,^6,7 regardless of Bi³⁺ dopant to be replaced for the Ca²⁺ lattice.

4.2 Resonant energy transfer from Bi³⁺ to Eu³⁺

Fig. 10 shows the PL and PLE spectra for (a) the CaZrO₃:Bi³⁺ (y = 0.02), (b) CaZrO₃:Eu³⁺ (x = 0.05), and (Eu³⁺, Bi³⁺)-codoped CaZrO₃ phosphors (x = 0.05, y = 0.02). The phosphors were synthesized by calcination at T_c = 1200 °C for 1 h. The PL spectra were obtained by excitation at λ_ex = 325 nm, whereas the PLE spectra were measured by monitoring at λ_em ∼ 400 nm (Bi³⁺ emission; Fig. 10a) or at λ_em ∼ 614 nm (Eu³⁺ emission; Fig. 10b and c).

Fig. 10 Room-temperature PL and PLE spectra for the CaZrO₃:Eu³⁺, Bi³⁺ phosphors with (a) x = 0.0 and y = 0.02; (b) x = 0.05 and y = 0; and (c) x = 0.05 and y = 0.02. Each phosphor was calcined at T_c = 1200 °C for 1 h.

An s² ion (Bi³⁺, In⁺, Sn²⁺, etc.) has a ¹S₀ ground state and sp excited states (³P₀, ³P₁, ³P₂, ¹P₁). The triplet states ³P₀, ³P₁, and ³P₂ can be split owing to the spin–orbit interaction. The ¹S₀ → ³P₀ transition is forbidden, and is therefore usually not observed in any optical spectra.²⁴ The large PLE peak at ∼315 nm in Fig. 10a can be characterized by the ¹S₀ → ³P₁ (Bi³⁺) absorption transition.

The PL peak observed at ∼400 nm can be attributed to the ³P₁ → ¹S₀ transition. The 6s² (Bi³⁺) electrons in various hosts are reported to show a single emission band in the UV,²⁵ blue,^26–32 blue-green,^33–35 or green region;³⁶ double emission bands in the blue and blue-green regions;³⁷ or triplet emission bands in the UV, blue, and blue-green (green) regions.^27,30–32 The multiple emission bands usually exhibit an excitation wavelength dependence. Like Eu³⁺, there are two possible sites for Bi³⁺ in the CaZrO₃ host, A- and B-sites. The first two higher-energy bands and last lower-energy band arise from the ³P₁ states of the Bi³⁺ ions in the centrosymmetric and noncentrosymmetric sites, respectively. Thus, our observed blue emission band at ∼400 nm can be attributed to the Bi³⁺ ions substituted for the Ca²⁺ lattice, which is more centrosymmetric than the Zr⁴⁺ lattice in this host,^25,27 in agreement with that mentioned in Section 4.1.

In Eu³⁺-doped phosphors, several sharp PLE peaks due to the 4f⁶ (⁷F₆) → 4f⁶ (⁵D_I, ⁵L₆, ⁵G₂) transitions are observed in the wavelength region of ∼350–500 nm (Fig. 10b). A large absorption peak at ∼290 nm in Fig. 10b is due to the charge transfer band (CTB) transition. The CTS (2p_oxide) levels for Eu³⁺, are usually lower in energy than its 4f⁵5d states (⁹E and ⁷E).³⁸ Thus, only the CTS band, but not any 4f⁵5d-related peaks, can be observed in the optical spectra of Eu³⁺-activated phosphors. The undoped CaZrO₃ sample also showed an intrinsic emission at ∼400 nm, i.e., lying in the same wavelength region as in the Bi³⁺ singly doped sample. The emission intensity for the undoped sample was weaker than that for the Bi³⁺ singly doped sample. The intrinsic light emission from the undoped CaZrO₃ sample showed a long lasting phosphorescence (LLP), as had also been observed in rare earth-doped CaZrO₃.³⁹ Interestingly, such LLP was not observed in our Eu³⁺ or Bi³⁺ singly doped and (Eu³⁺, Bi³⁺)-codoped samples.

It should be noted that the PL and PLE spectra for the Eu³⁺ singly doped sample (Fig. 10b) are almost the same as those for the (Eu³⁺, Bi³⁺)-codoped sample. The clear difference is that the Eu³⁺ emission intensity in the codoped sample is ∼13 times larger than that in the Eu singly doped sample (see Fig. 5 and 10). This is mainly due to the effects of an efficient energy transfer from Bi³⁺ to Eu³⁺ in the (Eu³⁺, Bi³⁺)-codoped sample. Doublet peaks at ∼300 nm and enhanced absorption peaks in the 400–550 nm region of the PLE spectrum in Fig. 10c support the occurrence of RET in the (Eu³⁺, Bi³⁺)-codoped sample. Indeed, we can clearly find an overlap of the Bi³⁺ emission (sensitizer) and Eu³⁺ excitation (activator) spectra in the 400–550 nm region.

To confirm the occurrence of Bi³⁺ → Eu³⁺ energy transfer process in CaZrO₃, we measured the PL decay curves for the (Eu³⁺, Bi³⁺)-codoped samples at x from 0 to 0.09 with y = 0.02 (Bi³⁺). Fig. 11a shows the results of this experiment. The PL decay curves were measured at λ_em ∼ 400 nm (Bi³⁺ emission). An evidence of the Bi³⁺–Eu³⁺ interaction can be understood from Fig. 11a. The experimental data are analyzed using the triple-exponential function:

withFor example, we obtained the decay parameters for the sample with x = 0.05 (y = 0.02) as a₁ = 0.5, τ₁ = 0.008 μs, a₂ = 0.4, τ₂ = 0.07 μs, a₃ = 0.1, and τ₃ = 0.35 μs. The analyzed data suggest that the larger the x value, the smaller the fast decay-component product a₁τ₁. One can thus understand that the higher doping of Eu³⁺ (x) produces the more striking non-radiative decay pathway for the Bi³⁺ emission as observed in the fast decay-time regime of Fig. 11a.

Fig. 11 (a) Room-temperature PL decay curves obtained by excitation at λ_ex = 355 nm and monitored at λ_em ∼ 400 nm (Bi³⁺ emission) in the (Eu³⁺, Bi³⁺)-codoped CaZrO₃ phosphor with x = 0–0.09 and y = 0.02 calcined at T_c = 1200 °C for 1 h. (b) Average decay time vs. x plots obtained from (a). The average decay times were calculated using eqn (7). The energy transfer efficiency η(x) from Bi³⁺ to Eu³⁺, calculated using eqn (8), vs. y plots are also shown in (b).

Average decay times τ_av are calculated using

The solid circles in Fig. 11b show the results calculated using eqn (7).

The energy transfer efficiency η(x) from Bi³⁺ to Eu³⁺ can be calculated by

where τ_av(0) and τ_av(x) represent the average decay times of the Bi³⁺ sensitizer in the absence and presence of the Eu³⁺ activator (x), respectively. The results of this calculation are shown by the open circles in Fig. 11b.

The decay curves for the Eu³⁺ emission in the same samples as in Fig. 11 are examined by monitoring at λ_em ∼ 614 nm. These results are shown in Fig. 12. At t > 0.5 ms, the decay curves can be fitted by the single-exponential function (i = 1 only) with good approximation. As demonstrated in the inset of Fig. 12, each decay curve has however an enhancement component at the early stage and then exhibits usual decay process. Simply, this enhancement component reflects a RET occurring in the (Eu³⁺, Bi³⁺)-codoped samples, i.e., the excessively supplied electrons from Bi³⁺ to Eu³⁺ give an enhanced luminescence intensity. Such luminescence enhancement never occurs in the sensitizer luminescence process of Fig. 11a.

Fig. 12 Room-temperature PL decay curves obtained by excitation at λ_ex = 266 nm and by monitoring at λ_em ∼ 614 nm (Eu³⁺) in the (Eu³⁺, Bi³⁺)-codoped CaZrO₃ phosphor with x = 0.01–0.09 and y = 0.02 calcined at T_c = 1200 °C for 1 h.

The most important feature taken from Fig. 5 (or Fig. 10) is that the Eu³⁺ emission intensity in the (Eu³⁺, Bi³⁺)-codoped sample was ∼13 times larger than that in the Eu³⁺ singly doped sample. This enhancement in the Eu³⁺ emission intensity is associated with an energy transfer from Bi³⁺ to Eu³⁺. There have been reported many studies on the effects of Eu³⁺ and Bi³⁺ codoping in various host materials. Majority of such studies gives an enhancement of the Eu³⁺ emission intensity of ≤4 (ref. 29, 35 and 40–57) and, in some case, less than unity.⁵⁸ An enhanced Eu³⁺ emission intensity was observed near ∼3% of the Bi concentration. A large enhancement in the Eu³⁺ emission intensity was also observed in various (Eu³⁺, Bi³⁺)-codoped phosphors: ∼6 in Ca₃SnSi₂O₉;²⁸ ∼7 in Sr₃P₄O₁₃ (ref. 59) and SrMoO₄;⁶⁰ and ∼10 in Y₂O₃ (ref. 33) and LuVO₄ (Lu = Y, La, Gd).⁶¹ Our observed value of ∼13 is the largest among them.

To obtain more detailed information on the radiation-induced process, we performed measurements on the luminescence quantum yields (Φ) of the Eu³⁺ singly doped phosphor (x = 0.05) and (Eu³⁺, Bi³⁺)-codoped phosphor (x = 0.05, y = 0.02). The results obtained here are: Φ = 4% (λ_ex = 320 nm) and 12% (λ_ex = 396 nm) for the Eu³⁺ singly doped CaZrO₃ phosphor; Φ = 14% (λ_ex = 320 nm) and 22% (λ_ex = 396 nm) for the (Eu³⁺, Bi³⁺)-codoped CaZrO₃ phosphor.

4.3 An energy level scheme in (Eu³⁺, Bi³⁺)-codoped CaZrO₃

Fig. 13 shows (a) the energy-level scheme and (b) PL and PLE spectra of the (Eu³⁺, Bi³⁺)-codoped phosphor synthesized with x = 0.05 and y = 0.02. The dominant ⁵D₀ (4f⁶) → ⁷F₂ (4f⁶) emission peaks appear in Fig. 13 at ∼614 nm. The next strongest emission peaks are at ∼595 nm due to the ⁵D₀ → ⁷F₁ transition. Similarly, the third strongest emission peaks are at ∼705 nm due to the ⁵D₀ → ⁷F₄ transition. The weak ⁵D₀ → ⁷F₀ emission peak appears at ∼582 nm. The ionic energy levels split by the action of the surrounding lattice ions (i.e., by the crystal-field interaction). However, levels with J = 0 (⁷F₀) and I = 0 (⁵D₀) are single, non-degenerate states and do not therefore split. Thus, the ⁵D₀ ↔ ⁷F₀ transition can be used to probe local crystallographic symmetry, number of states and site distribution of Eu³⁺ in the crystalline matrix. In the present case, we can expect two different symmetries of the cation sites, A- and B-sites. Therefore, we observed the asymmetry parameters (R_e–m) widely ranging from ∼1 to ∼3 (Fig. 3). Unfortunately, only one ⁵D₀ → ⁷F₀ peak was observed at ∼582 nm in our PL spectra due to its very weak nature (Fig. 13b). At T = 20 K, however, we can identify two clear ⁵D₀ → ⁷F₀ peaks at ∼581.5 and ∼586.0 nm (see vertical arrows in Fig. 8).

Fig. 13 (a) Electronic energy-level scheme for Eu³⁺ and Bi³⁺ in CaZrO₃:Eu³⁺, Bi³⁺ phosphor. (b) Room-temperature PL and PLE spectra for the (Eu³⁺, Bi³⁺)-codoped CaZrO₃ phosphor with x = 0.05 and y = 0.02 calcined at T_c = 1200 °C for 1 h. An efficient RET occurs from Bi³⁺ to Eu³⁺. CTB = charge transfer band; R-of-L = relaxation of lattice.

The PLE spectrum in Fig. 13b shows several sharp peaks above ∼350 nm and a double-peaked broad band at ∼300 nm. The sharp peaks at ∼535 nm can be assigned to the ⁷F₀ → ⁵D₁ transition, at ∼465 nm to the ⁷F₀ → ⁵D₂ transition, at ∼395 nm to the ⁷F₀ → ⁵L₆ transition, and at ∼365 nm to the ⁷F₀ → ⁵D₄ transition. Ofelt²⁴ gave the free-ion energy-level schemes for the 4f⁶ and 4f⁸ configurations of Eu³⁺, Sm²⁺, and Tb³⁺, which were calculated from energy matrices including interactions among the three highest multiplicities. According to Ofelt, an absorption peak at ∼400 nm in Fig. 13b is attributed to the ⁷F₀ → ⁵L₆ transition. Similarly, the absorption peaks at ∼385 and ∼365 nm are attributed to the ⁷F₀ → ⁵G₂ and ⁷F₀ → ⁵D₄ transitions, respectively.

The double-peaked broad band at ∼300 nm may consist of an absorption band from the ¹S₀ ground state to the first excited ³P₁ state in Bi³⁺ and a CTB (O₂ → Eu³). An observation of the ¹S₀ → ³P₁ (Bi³⁺) excitation peak monitored at λ_em ∼ 614 nm (Eu³⁺) further gives an evidence of RET in our (Eu³⁺, Bi³⁺)-codoped sample. The Bi³⁺-related optical absorption at ∼315 nm and emission at ∼400 nm involve the ordinary ³P₁ state and its state of “relaxation of lattice (R-of-L),” respectively.⁶² One can also understand that the PLE peak (Eu³⁺) shifted to long-wavelength side by codoping with Bi³⁺ (cf. Fig. 10b and c).

Finally, we compare in Fig. 14 the PL spectrum of the (Eu³⁺, Bi³⁺)-codoped CaZrO₃ phosphor to those obtained from SnO₂:Eu³⁺ (ref. 17) and K₂SiF₆:Mn⁴⁺.⁶³ These PL spectra were measured by excitation at λ_ex = 325 nm. The peak height is comparable between for the (Eu³⁺, Bi³⁺)-codoped CaZrO₃ and SnO₂:Eu³⁺ phosphors. An integration of each PL spectrum in the 550–750 nm spectral region also gives relative emission intensities of I_PL ∼ 1.00, ∼0.70, and ∼0.28 for the (Eu³⁺, Bi³⁺)-codoped CaZrO₃, SnO₂:Eu³, and K₂SiF₆:Mn⁴⁺ red-emitting phosphors, respectively. It should be noted that an efficient red emission can be expected from the K₂SiF₆:Mn⁴⁺ red-emitting phosphor by excitation at blue (∼460 nm) or UV light (∼360 nm).⁶³

Fig. 14 Room-temperature PL spectra for (a) the CaZrO₃:Eu³⁺, Bi³⁺ (x = 0.05, y = 0.02), (b) SnO₂:Eu³⁺,¹⁷ and (c) K₂SiF₆:Mn⁴⁺ red-emitting phosphors (ref. 63) measured by excitation at λ_ex = 325 nm.

5. Conclusions

(Eu³⁺, Bi³⁺)-codoped CaZrO₃ phosphor was synthesized using the solid-state reaction method and analyzed by means of the XPS, PL, PLE, and PL decay measurements. The relations between the solid-state reaction condition and synthesized material properties, especially their structural and luminescent properties, were investigated in more detail. The Eu³⁺ singly doped phosphor showed very poor crystalline quality and emitted light centered at ∼600 nm. The (Eu³⁺, Bi³⁺)-codoped sample also emitted red light, but its intensity was ∼13 times larger than that of the Eu³⁺ singly doped sample. This enhancement in the PL intensity was attributed to efficient energy transfer from Bi³⁺ to Eu³⁺. The highest PL enhancement ever-reported in the CaZrO₃ system was ∼3.2, obtained by codoping with Eu³⁺ and Gd³⁺. A comparison of the PL spectrum for CaZrO₃:Eu³⁺, Bi³⁺ with those for recently developed SnO₂:Eu³⁺ and K₂SiF₆:Mn⁴⁺ red-emitting phosphors was also performed.

Acknowledgements

This work was also supported by a Grant-in-Aid for Scientific Research (B) (26289085) and Young Scientists (B) (16K17506) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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