Photoluminescence features of new Eu3+ -doped Gd4Mo7O27 phosphors synthesized using glass crystallization technique

ABSTRACT New Eu3+-doped Gd4Mo7O27 crystals (the molar ratio of Gd2O3/MoO3 = 1/3.5) with a monoclinic structure C2/c (an inversion symmetry) were synthesized through the crystallization of xEu2O3-(18.89-x)Gd2O3-66.11MoO3-15B2O3-1Al2O3 glasses (x = 0.0472 and 1.889) and photoluminescence (PL) emissions of Eu3+ ions were measured for the first time. The crystallized glass with no Eu2O3 addition (x = 0) showed a blue color under the irradiation of ultra-violet light with a wavelength of λ = 254 nm, the emitting color of the crystallized glass with x = 0.0472 was pink, and that of the crystallized glass with x = 1.889 was orange. The charge transfer (CT) of O2-→Mo6+ providing broad peaks centered at around 325 nm was observed in the crystallized glasses. The peak intensity at 591 nm for the 5D0→7F1 transition of Eu3+ ions in the crystallized glasses with x = 0.0472 and 1.889 was very close to that at 615 nm for the 5D0→7F2 transition for the excitation of λex = 394.5 nm. We propose potential of Gd4Mo7O27 as a new host crystal for rare-earth-doped phosphors.


Introduction
Excellent photoluminescence (PL) performances of rareearth (RE) ions in glasses and crystals have been achieved through the design of the coordination and bonding states of RE ions. RE 3+ -doped molybdenum oxide (MoO 3 )-based materials such as Eu 3+ -doped CaMoO 4 and Gd 2-x Eu x (MoO 4 ) 3 have been regarded as excellent phosphors emitting intense red light [1,2]. Very recently, the present authors [3] succeeded in synthesizing new crystallized glasses consisting of Gd 4 Mo 7 O 27 crystals (the molar ratio Gd 2 O 3 /MoO 3 = 1/3.5) using the composition designed Gd 2 O 3 -MoO 3 -B 2 O 3 glasses with Gd 2 O 3 / MoO 3 = 1/3.5. The Gd 4 Mo 7 O 27 crystalline phase has not been reported in the phase diagram for the binary Gd 2 O 3 -MoO 3 system [4]. Naruke and Yamase [5] [6]. From the perspectives of PL performances of RE 3+ ions, Gd 4 Mo 7 O 27 crystal is very attractive as host medium, because it has an inversion symmetry in the crystal structure [5] and the maximum phonon energy of this crystal based on MoO 3 is expected to be small [3]. RE 3+ -doped Gd 4 Mo 7 O 27 crystals are, therefore, expected to be new phosphors emitting intense PL light.
The purpose of this study is to synthesize Eu 3 + -doped Gd 4

Experimental
The nominal compositions (molar ratio) of glasses synthesized in this study were 18 The glass transition (T g ) and crystallization peak (T p ) temperatures were determined using differential thermal analysis (DTA) at a heating rate of 10 K/ min. The as-quenched glasses were annealed at~T g for 30 min to release internal stress and then polished mechanically to a mirror finish with CeO 2 powders. Optical absorption spectra at room temperature for the glasses were measured with a spectrometer (SHIMADZU: UV-3150). The glasses were heat-treated at different temperatures in an electric furnace, and the crystalline phases present in the crystallized samples were identified from X-ray diffraction (XRD) analysis (CuKα radiation). PL spectra at room temperature for the glasses and crystallized samples were measured with a PL spectrometer (JASCO FP-6500).  [7]. The DTA patterns for bulk and powdered samples of Glass #3 (Eu 2 O 3 = 1.889) are shown in Figure 1, and the values of T g = 494°C and T p = 570°C for the bulk sample and T g = 496°C and T p = 524°C for the powdered sample are obtained. A large difference in the T p values between the bulk and powder samples suggests that the surface crystallization takes place predominantly in Glass #3 (Eu 2 O 3 = 1.889). The XRD patterns at room temperature for the bulk samples heat-treated at 476, 490, 590, and 640°C for 3 h in Glass #3 (Eu 2 O 3 = 1.889) are shown in Figure 2, suggesting the formation of the Gd 4 Mo 7 O 27 crystalline phase with the monoclinic structure C2/c (ICDD: 01-070-9838) [5] at the surface of the samples heat-treated at 490, 590, and 640°C. The color of the sample crystallized at 640°C was white, suggesting that Mo 5+ ions present in the base glass were oxidized largely to Mo 6+ ions during the heat treatment at 640°C for 3 h in air.

Formation of Eu
The XRD pattern at room temperature for the powdered (the heat-treated bulk sample was pulverized) sample heat-treated at 640°C for 3 h is shown in Figure 3 Figure 1. DTA patterns for bulk and powdered glasses in Glass #3(Eu 2 O 3 = 1.889). Heating rate was 10 K/min. T g and T p are the glass transition and crystallization peak temperatures, respectively. The optical photograph of the glass is included.   As seen in Figure 3, the XRD peaks for the fully crystallized (640°C, 3 h) sample are still broad. At this moment, the composition design for the crystallization of RE 4 Mo 7 O 27 in Gd 2 O 3 /Eu 2 O 3 -MoO 3 -B 2 O 3 -Al 2 O 3 glasses might not be optimal. In order to draw the decisive conclusion for the incorporation of Eu 3+ ions into Gd 4 Mo 7 O 27 crystals from the lattice constant, further XRD analyses would be required. The formation of Eu 3+ -doped Gd 4 Mo 7 O 27 crystals in the crystallized samples of both Glass #2 (Eu 2 O 3 = 0.0472) and Glass #3(Eu 2 O 3 = 1.889) will be discussed more clearly from PL spectra of the crystallized samples, which will be described in the following sections.

PL properties of Gd 4 Mo 7 O 27 crystals
Glass #1(Eu 2 O 3 = 0) with no Eu 2 O 3 addition has the values of T g = 507°C and T p = 572°C for the bulk glass sample and was heat-treated at 580°C for 3 h in air in order to form Gd 4 Mo 7 O 27 crystals [3]. Glass #1 (Eu 2 O 3 = 0) shows a brown color due to the presence of Mo 5+ ions, providing not steep absorption edges. The absorption edges of λ = 468 nm were estimated from the optical absorption spectra at room temperature for the bulk glass and heat-treated (510°C for 3 and 10 h) samples. The PL excitation spectra at room temperature for the glass and crystallized (580°C, 3 h) sample of Glass #1(Eu 2 O 3 = 0) are shown in Figure 4 (a), in which the wavelength of the monitoring emission light (λ em ) was 420 nm. The PL emission spectra at room temperature for these samples are shown in Figure 4(b), in which the wavelength of the excitation light (λ ex ) was 280 nm. The optical photograph for the crystallized sample during the irradiation of ultraviolet (UV) light with a wavelength of 254 nm is included in Figure 4(b), showing a blue color. In the PL emission spectrum for the crystallized sample, a broad emission peak centered at around 420 nm is observed. Furthermore, several sharp peaks are observed at around 420 and 470 nm. It is obvious that these emissions are related to the generation of blue color in the crystallized sample as shown in Figure 4(b).
The PL emission spectra under the excitation of λ ex = 280 nm for the base glass and the crystallized samples are shown in Figure 6. Any clear intense PL peak is not observed in the base glass. For the crystallized samples, however, the PL peaks typical to the 4f-4f transitions of Eu 3+ ions are observed, indicating that Eu 3+ ions are incorporated into Gd 4 Mo 7 O 27 crystals, i.e. the formation of Eu 3 + -doped Gd 4 Mo 7 O 27 crystals. The PL emission peak at 591 nm corresponds to the 5 D 0 → 7 F 1 transition of Eu 3+ , i.e. the magnetic dipole (MD) transition, and that at 615 nm corresponds to the 5 D 0 → 7 F 2 transition of Eu 3+ , i.e. the electric dipole (ED) transition. Furthermore, the broad bands together with several sharp peaks are also observed clearly in the range of 350-550 nm. Since these broad bands are not observed in the base glass ( Figure 6), the origin of the PL emissions at 350-550 nm would be related to the Eu 3+ -doped Gd 4 Mo 7 O 27 crystals. The optical photograph for the crystallized (630°C, 3 h) sample under the UV light (λ = 254 nm) irradiation is included in Figure 6, showing a pink color. It is considered that the appearance of the pink color is the results of the combination of the PL emissions at 350-550 nm and due to the 4f-4f transitions of Eu 3+ ions. The PL emission spectra under the excitation of λ ex = 394.5 nm for the crystallized samples are shown in Figure 7. The emission peaks corresponding to the 5 D 0 → 7 F 1 and 5 D 0 → 7 F 2 transitions of Eu 3+ ions are clearly detected. It should be pointed that the peak intensity of the 5 D 0 → 7 F 1 transition is very close to that of the 5 D 0 → 7 F 2 transition for the excitation of λ ex = 394.5 nm. That is, the ratio (R) of the peak intensity (615 nm) of the 5 D 0 → 7 F 2 transition to that

Discussion
Prior to the discussion of the PL features of the crystallized glasses consisting of Gd 4 Mo 7 O 27 crystals observed in the previous section, the structural features of Gd 4 Mo 7 O 27 crystal (a monoclinic structure C2/c) are described. Gd 4 Mo 7 O 27 crystal has a structure consisting of MoO 4 , Mo 3 O 11 , and Gd-containing layers (these layers are stacked along the a-direction) [5,6]. Looking the structural units in more detail [5,6] crystals formed in the crystallized samples has not been determined, and thus, the following discussion on the PL performance obtained in the present study is qualitative. Again, we emphasize that the present study is the first report on PL properties of Eu 3+ -doped Gd 4 Mo 7 O 27 crystals.

PL features of Gd 4 Mo 7 O 27 crystals
The generation of blue color observed in the crystallized sample under the UV light (λ = 254 nm) (Figure 4) in Glass #1(Eu 2 O 3 = 0) is interest. The wavelength (λ = 254 nm) of UV light is much shorter than that (λ = 468 nm) of the optical absorption edge in the heat-treated sample, suggesting the excitation of electrons in the valence band to the conduction band in Gd 4 Mo 7 O 27 crystals. The blue emission would be, therefore, closely related to the CT model [8]. That is, the CT from 2p orbitals of O 2ions to 4d orbital of Mo 6+ ions with the 4d°electronic configuration in MoO 4 and/or MoO 5 units in Gd 4 Mo 7 O 27 is induced, and the excited electrons relax to O2p orbitals through the radiative transition, generating the broad peak centered at around 420 nm and consequently radiating blue light.
At this moment, the origin of the sharp peaks at around 420 and 470 nm (Figure 4) is not clear. However, it should be pointed out that Gd 4 Mo 7 O 27 crystal has a unique double-layer structure consisting of seven different MoO 4 , MoO 5 , GdO 7 and GdO 8 units. Furthermore, it is known that Gd 3+ ions show the f-f transitions of 8 S 7/2 → 6 I 7/2 at around 270 nm and 8 S 7/2 → 6 P 7/2 at around 310 nm [9]. It is, therefore, expected that the excitation of 4f electrons in Gd 3+ ions in Gd 4 Mo 7 O 27 crystal would be induced under the UV light (λ = 254 nm) irradiation. One possible mechanism for the origin of the sharp peaks at around 420 and 470 nm shown in Figure 4

PL features of Eu 3+ -doped Gd 4 Mo 7 O 27 crystals
Many studies on the excitation spectra of Eu 3+ -doped molybdates suggest that the broad peak at about 240-260 nm are assigned to the CT transition from O 2ions to Eu 3+ ions, i.e. O 2-→Eu 3+ CT, and the broad peaks at about 260-400 nm are due to the CT transition from O 2ions to Mo 6+ ions in (MoO 4 ) 2-, i.e. O 2-→Mo 6+ CT [10][11][12]. Considering the previous assignments of PL peaks of Eu 3+ ions [8][9][10], the broad bands at around 260 and 325 nm in the excitation spectra ( Figure 5) obtained for the crystallized samples of Glass #2(Eu 2 O 3 = 0.0472) would be assigned to mainly the O 2-→Eu 3+ CT and O 2-→Mo 6+ CT transitions, respectively. It is noted that the intensity of the peak at around 260 nm is stronger than that at around 325 nm as shown in Figure 5, suggesting that the O 2-→Eu 3+ CT transition is taking place largely in Eu 3+ -doped Gd 4 Mo 7 O 27 crystals with a small amount of Eu 3+ ions. On the other hand, as shown in Figure 8, [3], the Raman scattering spectra at room temperature for the crystallized samples of the base glass, and it was found that the Raman band peaks of Gd 4 Mo 7 O 27 crystals appear below 1000 cm −1 and the peak at 947 cm −1 shows the strongest intensity. It is suggested that the maximum phonon energy of Gd 4 Mo 7 O 27 crystal would be around 950 cm −1 , generating clear PL emissions of Eu 3+ ions.
As one of the PL features of Eu 3+ ions in Eu 3 + -doped Gd 4 Mo 7 O 27 crystals, it is noted that the peak intensity of the 5 D 0 → 7 F 2 transition is very close to the intensity of the 5 D 0 → 7 F 1 transition for the excitation of λ ex = 394.5 nm (Figures 7 and 10), i. e, R = 1.06 for Glass #2(Eu 2 O 3 = 0.0472) and R = 1.06 for Glass #3(Eu 2 O 3 = 1.889). Furthermore, it should be also pointed out that the increase in the Eu 3+ content, i.e. the Eu 2 O 3 contents of 0.0472 mol% and 1.889 mol%, does not change the shape and peak positions of the PL emission spectra of Eu 3+ ions in the crystallized samples. Since the 5 D 0 → 7 F 2 transition of Eu 3+ ions observed at around 615 nm is the ED transition, its peak intensity is sensitive for the local symmetry of coordination environments around Eu 3 + . That is, in the coordination environments with the non-inversion site symmetry (i.e. large electric field gradients sites) for Eu 3+ sites, the intensity of the 5 D 0 → 7 F 2 transition would be strong, and contrary, Eu 3+ ions present in the inversion symmetry sites would have weak intensities for the 5 D 0 → 7 F 2 transition. On the other hand, the 5 D 0 → 7 F 1 transition observed at around 591 nm is the MD transition, being not sensitive for the site symmetry of Eu 3+ ions. The crystal structure of Eu 3+ -doped Gd 4 Mo 7 O 27 has an inversion symmetry and this structural feature would be an origin for the weak intensity of the 5 D 0 → 7 F 2 transition, consequently providing the peak intensity ratio close to one (R~1.0) in Eu 3+ -doped Gd 4 Mo 7 O 27 crystals formed in the crystallized samples of Glass #2 (Eu 2 O 3 = 0.0472) and Glass #3(Eu 2 O 3 = 1.889).
It is known that Gd 3+ ion has a very high absorption coefficient in the UV region due to the excitations to the excited levels such as 6 P J (usually centered at about λ = 275 nm) and 6 I J (at about λ = 310 nm) from the ground state 8 S 7/2 and can transfer energy to other RE 3+ ions such as Eu 3+ ions [9,18,19]

Conclusions
The Eu 3+ -doped Gd 4 Mo 7 O 27 crystals were synthesized through the crystallization of xEu 2 O 3 -(18.89-x) Gd 2 O 3 -66.11MoO 3 -15B 2 O 3 -1Al 2 O 3 glasses, i.e. Glass #1(Eu 2 O 3 = 0), Glass #2(Eu 2 O 3 = 0.0472), and Glass #3(Eu 2 O 3 = 1.889), and PL emissions of Eu 3+ ions were measured for the first time. The crystallized glasses showed the following colors under the irradiation of UV light with λ = 254 nm; blue for the base glass, pink for the Glass #2(Eu 2 O 3 = 0.0472), and orange for Glass #3(Eu 2 O 3 = 1.889). The CT of O 2-→Mo 6+ providing broad peaks centered at around 325 nm was observed in the crystallized glasses. The peak intensity of the 5 D 0 → 7 F 1 transition of Eu 3+ ions in the crystallized glasses with Eu 3+ -doped Gd 4 Mo 7 O 27 crystals was very close to that of the 5 D 0 → 7 F 2 transition for the excitation of λ ex = 394.5 nm, resulting from the structural features of Gd 4 Mo 7 O 27 crystals with an inversion symmetry. We propose potential of Gd 4 Mo 7 O 27 as a new host crystal for RE 3+ -doped phosphors.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
This work was supported by JSPS KAKENHI Grant No. 17H03387.