Synthesis and luminescent properties of BaGd₂O₄:Dy³⁺, an novel scintillating phosphor

Abstract

The BaGd_2−x O₄:xDy³⁺ (0 ≤ x ≤ 0.08) phosphors were synthesized at 1,300 °C in air by the solid-state reaction route. The as-synthesized phosphors were characterized by X-ray powder diffraction, photoluminescence excitation spectra, photoluminescence (PL) spectra, X-ray excited luminescence (XEL) spectra, and thermoluminescence (TL) spectra. It is found that the quenching concentration of Dy³⁺ ions in BaGd₂O₄ host is dependent on the selected excitation wavelength. The optimal PL intensity for the investigated BaGd_2−x O₄:xDy³⁺ phosphors is found to be x = 0.01, 0.02, and 0.04, upon excitation by 234, 277, and 350 nm ultraviolet light, respectively. The energy transfer among Dy³⁺ ions upon excitation by 350 nm is confirmed to be an electric dipole–dipole interaction mechanism based on the fitting of Huang’s rule. In addition, the intensive XEL from BaGd₂O₄:Dy³⁺ phosphor is observed by the naked eyes at room temperature, and TL properties of the investigated phosphors are analyzed and discussed. All the results imply that the investigated phosphors could be a promising scintillating phosphor.

1 Introduction

Potential and technological application in optical and optoelectric domains, such as scintillators and laser hosts, have received intensive research interest in rare-earth doped materials [1–5]. Scintillating materials, which convert ionizing radiation, such as X-rays, α-, β-, and γ-rays, or UV incident beams into visible or IR light, have attracted special attention. A good scintillator is characteristic with high-luminescence efficiency, high density, fast decay, high radiation damage threshold, desirable chemical and physical stability [2–4]. Up to date, many scintillating crystals and glasses, such as Bi₄Ge₃O₁₂, PbWO₄, and SCG-1 glass [4, 5], have been deeply explored and practically applied. Recently, some promising scintillating materials such as Lu₂Si₂O₅:Ce³⁺ (LSO:Ce), LaBr₃:Ce³⁺, and CaI₂:Ce³⁺ exhibit the potential application in the field of medical image [6–8]. However, the expensive cost of Lu₂O₃ and the hygroscopic nature of LaBr₃ or CaI₂ are challenging obstacles for their scale application. Therefore, the search for excellent scintillating materials is still going on.

Binary rare earth oxides AR₂O₄ (for A = Ca, Sr, Ba, and R = rare earth) within CaFe₂O₄ structure have been found many potential applications due to their special properties, especially in the luminescent and magnetic aspects. As a photoluminescence (PL) adopter, the structure characteristic and luminescent property of SrY₂O₄, SrGd₂O₄, SrLu₂O₄, BaY₂O₄, and BaGd₂O₄ have been investigated extensively [9–19]. In addition, the potential application of SrY₂O₄ and BaY₂O₄ as the thermal barrier coating material was proposed, and their thermophysical and mechanical properties were also explored [20, 21]. It is worthy to note that the scintillating properties for this kind of materials are rarely reported except for the vacuum ultraviolet investigation on SrGd₂O₄:Eu³⁺ phosphor by Wang et al. [22]. Compared with the SrGd₂O₄ host, rare-earth-doped BaGd₂O₄ host may be expected to have a better scintillating nature due to the following factors: (1) Both dense BaO and Gd₂O₃ are attributive to improving the density of the host. The reported density of BaGd₂O₄ is about 7.68 g/cm³ [23], which is slightly higher than of the promising LSO scintillating crystal with density of 7.4 g/cm³. (2) The BaGd₂O₄ host is very stable up to 1,860 °C, which is of significance for its practical application [24]. (3) The same valance state and similar ion radii of lanthanide ions have greatly facilitated for the incorporation of luminescent ions such as Ce³⁺, Tb³⁺, and Eu³⁺ into the Gd³⁺ site of the BaGd₂O₄ host. (4) And more importantly, the sensitizer ions of Gd³⁺ in BaGd₂O₄ host can efficiently transfer its absorbed energy to the incorporated luminescent centers, which results in the enhancement of light yield.

The first report on the luminescent properties of BaGd₂O₄:Eu³⁺ phosphors was focused on application as mercury lamp phosphors, and its quantum efficiency was found to be about 40 % by Blasse [25]. Later, this phosphor activated with both Tb³⁺ and Sm³⁺ ions was found to be highly effective in X-ray storage filed [23]. With the emergence of field emission displays and plasma plane display technologies, more attention have been paid on the synthesis and luminescent properties of rare-earth-doped BaGd₂O₄ in recent years [17–19]. To our knowledge, there is no report on scintillating properties of rare-earth-doped BaGd₂O₄ phosphors. Herein, in the present study, the structure and luminescent properties of the BaGd₂O₄:Dy³⁺ scintillating phosphors are investigated by X-ray diffraction (XRD), PL, X-ray excited luminescence (XEL) spectra, and thermoluminescence (TL) spectra in detail.

2 Materials and methods

Powder samples with stoichiometric composition of BaGd_2−x O₄:xDy³⁺ (x = 0.0, 0.005, 0.01, 0.02, 0.04, 0.06, and 0.08) phosphors were synthesized by the high temperature solid-state route. Raw materials of BaCO₃ (Shanghai Chemical Reagent, A.R. Grade), Gd₂O₃, and Dy₂O₃ (Ganzhou Qiandong Industrial Group Co. Ltd., 99.99 %) were mixed thoroughly in an agate mortar. The amount of BaCO₃ was excess in the range of 1–9 mol% to compensate its evaporation losses during the synthesis process. The well-ground mixtures were pre-fired at 900 °C for 180 min and subsequently calcined at 1,300 °C for 600 min in air, finally cooled down naturally to room temperature.

The phase purity of the as-synthesized phosphors was investigated by XRD with a X-ray diffractometer (Brucker D8) with Cu-Kα radiation at 40 kV and 40 mA. The XRD patterns were collected in the range of 25º ≤ 2θ ≤ 80º. The photoluminescence excitation (PLE) and PL spectra were recorded on a fluorescence spectrophotometer (Hitachi F-7000), equipped with a 150 W xenon lamp as the excitation source. The XEL of the investigated phosphors were examined using an X-ray excited spectrometer (Fluormain) in which an F-30 X-ray tube (W anticathode target) was used as the X-ray source operating at 80 kV and 4 mA. All the investigated phosphors were irradiated for 5 min by an 254 nm ultraviolet lamp with 5 W before TL measurement, and the TL curves were collected at a linear rate of 1 °C/s in the temperature 25–350 °C using a TL system (FJ427A, Beijing Nuclear Instrument Factory). All the measurements were carried out at room temperature except for the TL spectra.

3 Results and discussion

3.1 Crystal structure and XRD analysis

It is reported that the crystal structure of BaGd₂O₄ is isostructural to CaFe₂O₄-related structure, which is composed of a double octahedral Gd₂O₄ ²⁻ framework and alkaline earth-ion reside within the framework. As shown in Fig. 1, Gd³⁺ ions have two non-equivalent Gd sites [10, 12, 15, 22], and both Gd sites are coordinated by six oxygen atoms and have C_s symmetry. The Gd(I) site is nearly octahedral, while the Gd(II) site is in a more distorted coordination environment. The Ba-site in this host is eight-coordinates and also exhibits C_s symmetry. It is well-known that an acceptable percentage difference in ion radii between the doped and substituted ions must not exceed 30 % [26], which suggests that Dy³⁺ ions (0.912 Å, CN = 6) may prefer to substitute Gd³⁺ (0.938 Å, CN = 6) cation ions rather than Ba²⁺ (1.42 Å, CN = 8) ions in the BaGd₂O₄ host. Therefore, the general formula of the investigated phosphors is described as Ba₂Gd_2−x O₄:xDy³⁺, where x denoting the substitution ratio of Dy³⁺ for Gd³⁺ ions.

Fig. 1

Crystal structure of BaGd₂O₄

The effect of excess BaO on the structure evolution of BaGd₂O₄ host is illustrated by XRD patterns shown in Fig. 2, and the standard PDF files of both Gd₂O₃ (JCPDS: 86-2477) and BaGd₂O₄ (JCPDS: 82-2320) are also displayed for comparison. It is obvious that the BaGd₂O₄ phase begins to form, when the excess BaO is 3 %. However, minor Gd₂O₃ impurity phase labeled as blue arrow can also coexist in the investigated BaGd₂O₄ host. This impurity can be reduced or deleted by adding more BaO, and the purity phase BaGd₂O₄ host is finally obtained in the case of excess 9 % BaO.

Fig. 2

XRD patterns of BaGd₂O₄ host, standard PDF files of Gd₂O₃ (86-2477) and BaGd₂O₄ (82-2320). The effect of excess BaO on the structure evolution of BaGd₂O₄ host refers to the text

3.2 Luminescence properties

3.2.1 PL spectra

The PLE spectra of Ba₂Gd_2−x O₄:xDy³⁺ phosphors are shown in Fig. 3a, b when monitoring Dy³⁺ 580 nm and Eu³⁺ 614 nm emission, respectively. As shown in Fig. 3b, there exhibits an additional broad excitation band located at 235 nm when the 614 nm emission of Eu³⁺ ions is monitored, which is tentatively identified as the charge-transfer state of Eu³⁺ ions being a trace element of the starting Gd₂O₃ raw materials [27, 28]. The charge transfer excitation of Eu³⁺ ions is an electronic transition from the ground-state to the excited state of the 4f shell, and the corresponding band position in excitation spectrum is mainly determined by the covalency of Eu–O bond and the coordination environment of Eu³⁺ ions as well. In the Eu–O couple, the excitation energy for charge transfer can be estimated by the following Jorgensen’s equation [29],

Fig. 3

PLE spectra of BaGd_2−x O₄:xDy³⁺ phosphors monitoring the emission peak at a λ_em = 580 nm and b λ_em = 614 nm excitation, respectively

$$ v = 3[\chi ({\text{O}}) - \chi ({\text{Eu}})] \times 10^{4} $$

(1)

where v in cm⁻¹ denotes the position of the charge transfer band, χ(O) and χ(Eu) are the optical electronegativity of the oxygen and europium ions, respectively. Taking χ(O) = 3.2 and χ(Eu) = 1.75 [30], the band position for Eu–O charge transfer is calculated to be at 238 nm, which is in line with our results. In addition, the typical excitation peaks at 277 and 313 nm are assigned to optical transition of Gd^{3+ 8}S_7/2 → ⁶I_J, and ⁶I_J, respectively. All the PLE intensity of these peaks decrease with the increasing substitution ratios of Dy³⁺ for Gd³⁺ ions. The appearance of Gd³⁺ excitation peaks indicates that energy transfer from Gd³⁺ to Eu³⁺ ions may occur.

To study the luminescent property of Dy³⁺ in BaGd₂O₄ host, the excitation spectra of Ba₂Gd_2−x O₄:xDy³⁺ phosphors monitoring at 580 nm of Dy³⁺ ions emission is shown in Fig. 3a. Except for the characteristic excitation peaks of Gd³⁺ ions in the vicinity of 276 and 313 nm, there exists a strong broad band situated at 230 nm and a weak line shape peak at 350 nm. The former superposed bands may be associated with the charge transfer states of both O²⁻−Dy³⁺ and O²⁻−Eu³⁺ ions [23], while the latter one is assigned to the ⁶H_15/2 → ⁴M_15/2, ⁶P_7/2 transitions of Dy³⁺ ions. It is also interesting that the strongest excitation intensity of 234, 277, and 350 nm is x = 0.01, 0.02, and 0.04, respectively.

Figure 4 shows the PL spectra of BaGd_2−x O₄:xDy³⁺ phosphors under 234, 277, and 350 nm excitation, respectively. As shown in Fig. 4c, the PL spectrum of Dy³⁺ ions consists of three emission regions: blue, yellow, and weak red emission regions. For Dy³⁺ ions, the blue emission between 435 and 500 nm is assigned to the optical transition of ⁴F_9/2 → ⁶H_13/2, the yellow emission in the wavelength region 555–595 nm corresponds to the ⁴F_9/2 → ⁶H_13/2 transition, while the weak red emission (640–680 nm) is associated with the ⁴F_9/2 → ⁶H_11/2 transition. As shown in Fig. 4a, b, upon BaGd_2−x O₄:xDy³⁺ phosphors under both 234 and 277 nm ultraviolet light excitation, the characteristic emissions of Eu³⁺ ions are also observed at 595 and 613 nm, which belong to the ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions of Eu³⁺ ions, respectively. The characteristic emission of Gd³⁺ 313 nm only exists in the case of 277 nm excitation. With the elevated Dy³⁺ content, the characteristic PL intensity of both Gd³⁺and Eu³⁺ ions reduce remarkably, which imply the energy transfer may happen from Gd³⁺ and Eu³⁺ to Dy³⁺.

Fig. 4

PL spectra of BaGd_2−x O₄:xDy³⁺ phosphors under a λ_ex = 234 nm, b λ_ex = 277 nm and c λ_ex = 350 nm excitation, respectively

The emission intensity of Dy³⁺ 580 nm as a function of both Dy³⁺-doping concentration and the selected excitation wavelength are comparably presented in Fig. 5. The results suggest that the optimal content of Dy³⁺ ions emission in BaGd_2−x O₄:xDy³⁺ phosphors depends on the selected excitation wavelength. The quenching concentration of Dy³⁺ in the investigated phosphors is x = 0.01, 0.02, and 0.04 under 234, 277, and 350 nm, respectively. They are in accordance with the PLE spectra in Fig. 3a.

Fig. 5

PL intensity of Dy³⁺ 580 nm emission as function of both Dy³⁺-codoping concentration and different excitation light under a λ_ex = 234 nm, b λ_ex = 277 nm, and c λ_ex = 350 nm in BaGd_2−x O₄:xDy³⁺ phosphors

The concentration quenching of the luminescence is due to the energy transfer from one activator to another until all the energy is consumed. For this reason, it is necessary to obtain the critical distance (R _c) that is the critical separation between donor (activators) and acceptors (quenching site). The critical distance R _c of the energy transfer between the same activators Dy³⁺ in BaGd_2−x O₄:xDy³⁺ could be estimated according to the following equation [31]:

$$ R_{\text{c}} = 2\left(\frac{3V}{{4\pi x_{\text{c}} N}}\right)^{\frac{1}{3}} $$

(2)

where x _c is the critical concentration, N is the number of Gd³⁺ ions in the unit cell, and V is the volume of the unit cell. By taking the experimental and analytic values of x _c, N, and V (0.04, 8 and 451.804 Å³, respectively), the critical transfer distance of Dy³⁺ in BaGd_2−x O₄:xDy³⁺ phosphors is estimated to be about 13.92 Å.

According to Huang’s rule [32, 33], the relationship between the integral luminescent intensity I and doping concentration x can be expressed as follows:

$$ I \propto \alpha^{{\left( {1 - \frac{s}{d}} \right)}} \Upgamma \left( {1 + \frac{s}{d}} \right) $$

(3)

$$ \alpha = x\Upgamma \left( {1 - \frac{d}{s}} \right)\left[ {X_{0} \frac{{\left( {1 + A} \right)}}{\gamma }} \right]^{\frac{d}{s}} $$

(4)

where γ is the intrinsic transition probability of sensitizer, s is index of electric multipole, which is 6, 8, and 10 for electric dipole–dipole, electric dipole–quadrupole, and electric quadrupole–quadrupole interaction, respectively. If s = 3, the interaction type is an exchange interaction. d is the dimension of the sample, here d = 3. A and X ₀ are the constants and Γ(1 + s/d) is a Γ function. From Eqs. (3) and (4), it can be derived that

$$ \log \left( \frac{I}{x} \right) = - \frac{s}{d}\log x + \log f $$

(5)

where f is independent of the doping concentration.

Figure 6 shows the log(I/x)–logx plot for the ⁴F_9/2 → ⁶H_15/2 transitions of Dy³⁺ ions in BaGd_2−x O₄:xDy³⁺ phosphors. According to Eq. (5), using the linear fitting to deal with the experimental data in the region of high concentrations, the value of the slope parameter −s/d is obtained to be −1.65 for the ⁴F_9/2 → ⁶H_15/2 transition. The slope parameter is approximately to −2. Therefore, the index of the electric multipole energy transfer is 6. The result means that the electric dipole–dipole interaction mechanism is dominant by the energy transfer of Dy³⁺ ions in the investigated phosphors.

Fig. 6

The relation of the concentration of Dy³⁺ ions log(x) and the log(I/x) for the ⁴F_9/2 → ⁶H_15/2 transition under 350 nm excitation

The energy levels of Gd³⁺, Dy³⁺, and Eu³⁺ ions, and visible PL emission and energy transfer of BaGd₂O₄:Dy³⁺ phosphors are depicted in Fig. 7 according to the calculated data of energy levels by Carnall et al. [34–36]. When the 234 nm UV light is used to excite the BaGd₂O₄:Dy³⁺ phosphors, the electron is excited into the ⁶D_J level of Gd³⁺ and the charge transfer states of both Dy³⁺ and Eu³⁺ ions. The initial excited electrons relax quickly to their lower energy levels until it arrives at the ⁶P_J of Gd³⁺ ions, ⁴F_9/2 of Dy³⁺ ions, and ⁵D₀ of Eu³⁺ ions by phonon-assisted process, respectively. Then they give rise to the responding characteristic emissions. However, the energy transfer occur for Gd³⁺−Dy³⁺ and Gd³⁺−Eu³⁺ ion pairs due to the matched energy levels between Gd^{3+ 6}P_J levels and Dy^{3+ 4}K_15/2 or Eu^{3+ 5}H_J levels. In additional, the energy transfer may happens from ⁵H_J of Eu³⁺ ions to ⁶P_7/2 of Dy³⁺ ions due to the matched energy levels, and the population of Dy^{3+ 4}F_9/2 level may also be enhanced by the so-called cross-relaxation process of ⁵H_J (Eu³⁺) + ⁴H_9/2 (Dy³⁺) → ⁶H_9/2 (Dy³⁺) + ⁵D₀ (Eu³⁺) [37, 38]. So both characteristic emissions of Gd³⁺ 313 nm and Eu³⁺ 614 (593) nm are observed to be decreased remarkably, especially under excitation at 277 nm shown in Fig. 4b. It is interesting that the BaGd₂O₄:Dy³⁺ phosphor is excited by 350 nm UV light, there only exists Dy³⁺ characteristic emission without any Eu³⁺ emission. It is this phenomenon that energy transfer cannot happen from Dy³⁺ to Eu³⁺ ions. For Dy³⁺ ions, the energy gap between ⁴F_9/2 and ⁶H_9/2 levels matches well with that of the ⁶F_1/2 and ⁶H_15/2 levels indicating that the energy transfer occurs via ⁴F_9/2 (Dy³⁺) + ⁶F_1/2 (Dy³⁺) → ⁶H_9/2 (Dy³⁺) + ⁶H_15/2 (Dy³⁺) cross-relaxation process by electric dipole–dipole interaction in nature. The probability of the energy transfer gets higher with the increasing Dy³⁺ ions content, particularly in the BaGd_2−x O₄:xDy³⁺ phosphors with high concentration of Dy³⁺ ions, which results in the depopulation of the ⁴F_9/2 level and the decrease of the blue and yellow emissions.

Fig. 7

Energy level diagrams and characteristic emissions of Gd³⁺, Dy³⁺, and Eu³⁺ ions, energy transfer (solid red lines with arrow) and cross relaxation (dashed blue lines) are also labeled, the non-radioactive transition is also described by dashed magenta lines with arrow

3.2.2 XEL spectra

The representative XEL spectra of BaGd_2−x O₄:xDy³⁺ (x = 0.0, 0.02, and 0.06) phosphors under X-ray excitation are shown in Fig. 8. These spectra show similar characteristics as those under ultraviolet light excitation. Namely, the undoped BaGd₂O₄ phosphors is observed with red emission of Eu³⁺ ions, which originates from the purity of Gd₂O₃ raw materials because the impurity Eu³⁺ ions is frequently present in Gd₂O₃ raw material [27, 28]. While the strongest XEL intensity of Dy³⁺ ions in the blue and yellow regions is observed when the content of Dy³⁺ is 0.02 mol by the naked eyes at room temperature. The XEL intensity reduce dramatically when the content of Dy³⁺ is 0.06 mol due to concentration quenching. The results indicate that the BaGd₂O₄:Dy³⁺ phosphor is of significance for scintillating filed.

Fig. 8

XEL spectra of BaGd_2−x O₄:xDy³⁺ (x = 0.0, 0.02 and 0.06) phosphors

3.3 TL glow curves

The TL glow curves of BaGd_2−x O₄:xDy³⁺ phosphors after 254 nm UV light irradiation for 5 min are shown in Fig. 9. They are fitted to a good approximation by a set of three Gaussian TL glow curves assuring the lowest χ ² value [39, 40]. The fitted peaks 1, 2, and 3 locate about 55, 85, and 120 °C, respectively. The inset of Fig. 9 shows the representative three Gaussian fitting for the x = 0.0 and 0.01 phosphors. With the elevated Dy³⁺ content, the TL intensity of both peaks 2 and 3 are always reduced, while the peak 1 increases until x = 0.01 and decreases dramatically due to concentration quenching of Dy³⁺ ions. There are no additional peaks in glow curves indicates the introduction of Dy³⁺ into BaGd₂O₄ host cannot create any new kinds of traps, but resulting in the change of concentration of traps.

Fig. 9

TL curves for BaGd_2−x O₄:xDy³⁺ phosphors. The inset shows the x = 0.0 and 0.01 TL curves deconvolved by three Gaussian fitting

The TL glow-peaks indicate the existence of shallow as well as deep traps for the investigated phosphors which are responsible for the trapping of charge carries produced by the irradiation process. The Gaussian curve fitting procedure suggests the possibility of having a second order mechanism for TL process [39]. In this case, the kinetic parameters can be obtained by using the following equations.

First, the depth traps E _t (eV) or activation energy can be calculated by the equation [41]:

$$ E_{\text{t}} = 3.5\left( {\frac{{kT_{\text{m}}^{2} }}{w}} \right) - 2kT_{\text{m}} $$

(6)

where k is the Boltzman constant, T _m (K) is the thermal peak temperature, and w is the full width at half maximum of the thermal glow-peak. The lifetime τ(s), is the time to empty the traps, at temperature T _m is given by [41]:

$$ \tau = {\text{s}}^{ - 1} \exp \left( {\frac{{E_{\text{t}} }}{{kT_{\text{m}} }}} \right) $$

(7)

where s (s⁻¹) correspond to the frequency factor. This factor is associated to the freed of the trapped electrons from each trap and increase or decrease depending of the trap. It is related with the activation energy through the general equation valid to any order [42],

$$ \frac{{\beta E_{\text{t}} }}{{kT_{\text{m}}^{2} }} = s\left[ {1 + \frac{{2(b - 1)kT_{\text{m}} }}{{E_{\text{t}} }}} \right]\exp \left( { - \frac{{E_{\text{t}} }}{{kT_{\text{m}} }}} \right) $$

(8)

where β is the heating rate, and b determines the kinetic order process, being 1 and 2 for the first and second order, respectively.

The calculated parameters, such as peak position T, depth trap E _t, frequency factors s, and lifetime τ are listed in Table 1. The analyzed results imply the peak positions of both peaks 2 and 3 are little changed with the introduction of Dy³⁺ ions, while the corresponding peak 1 is shifted towards the lower temperature side. It is noteworthy that the lifetimes to empty the traps are similar in the vicinity of 10 s. However, the frequency factor is changed over a wide range in more than nine orders of magnitude.

Table 1 Peak position T and depth traps or activation energy E _t of each component obtained by the deconvolution of the TL glow curves for BaGd_2−x O₄:xDy³⁺ phosphors

4 Conclusions

In summary, novel BaGd_2−x O₄:xDy³⁺ scintillating phosphors have been successfully synthesized and their luminescent properties have been investigated. The purity phase BaGd₂O₄ host is obtained by adding excess 9 mol% BaO. The optimal quenching concentration for strongest Dy³⁺ emission depends on the selected excitation wavelength, and it is found to be x = 0.01, 0.02, and 0.04 under 234, 277, and 350 nm, respectively. The energy transfer from Gd³⁺ to both dopant Dy³⁺ and impurity Eu³⁺ ions, and energy transfer from Eu³⁺ to Dy³⁺ ions are simultaneously observed by the PL spectra. The deconvolution of TL spectra by Gaussian fitting suggests any new traps cannot create with the introduction of Dy³⁺ ions. The bright XEL intensity for x = 0.02 samples is observed by the naked eyes at room temperature. All the results imply that the investigated phosphor may be of significance for scintillating application.