Significance of the SrGd204 and BaGd204Host

The AGd204 (A = Ba or Sr) system exhibits excellent thermal stability, high density of about 7.13 g/cm3, high melting point (around 2400 °C) and good charge stability, which is significant for practical applications (Raju et al. 2014a, b, c; Zhou et al. 2007; Mari et al. 2011). Moreover, the AGd204 host can effectively transfer its absorbed energy to the incorporated RE activators via the sensitizing effect of Gd3+ ions resulting in the enhancement of emission intensity (Carnall et al. 1968 a, b; Som et al. 2015; Zhang et al. 2012; Tian et al. 2012; Das et al. 2014; Reddy et al. 2012). Additionally, in the AGd204 host, the Gd3+ ions have the same valence state and similar ionic radius as other lanthanide ions. These are beneficial for the introduction of luminescent centers of various rare-earth ions such as Eu3*, Tb3* and Dy3t since they can easily substitute the identical ionic sites of the Gd3+ ions.

The AGd204 system crystallizes in the form of a calcium ferrite (CaFe204)- related structure having space group Pnain, which is composed of a double octahedral Gd2042' framework with alkaline earth ions residing within the framework. The full structure of AGd204is shown in Fig. 9.1 (a) and the Gd06 octahedra are shown in polyhedral representation. It is worth mentioning that the Gd3+ ions occupy two crystallographically inequivalent Gd sites; both sites are coordinated by six oxygen atoms with Cs point symmetry (Lakskminarasimhan and Varadaraju 2008; Sun et al. 2014). Figure 9.1 (a) indicates that the Gd (1) site is nearly octahedral while, the Gd (2) site is in a more distorted coordination environment (Fig. 9.1(b)). In the a-b plane, Gd3* ions are linked in a network of hexagons and also linked by triangles along c-axis. Two crystallographically inequivalent Gd3+ ions (yellow and green in color) form two different triangular ladders along the c-direction (Singh et al. 2016). The Sr site in this host is composed of eight nearest neighbors exhibiting C, symmetry. It is well manifested, that an acceptable percentage

The crystal structure of AGd,0. Two crystallographically inequivalent Gd' ions, Gd (1) is shown as green color, whereas Gd (2) is shown as yellow color

Fig. 9.1. The crystal structure of AGd,04. Two crystallographically inequivalent Gd3' ions, Gd (1) is shown as green color, whereas Gd (2) is shown as yellow color.

difference in ionic radii between doped and substituted host ions should not exceed ±30% (Singh et al. 2016), which suggests that Eu3* ions (0.947 A, CN = 6) prefer to substitute Gd3* (0.938 A, CN = 6) ions rather than Sr2* (1.18 A, CN = 8) ions or Ba2* (1.35 A, CN = 8) ions (Zhang et al. 2012; Maekawa et al. 2007).

Synthesis Routes and Characterization Techniques

Generally, there are many types of synthesis routes to prepare the different types of the oxide samples for various applications. Among them, one of the key routes to prepare the microparticles is the homogeneous precipitation method followed by subsequent combustion process.

Homogeneous Precipitation Method Followed by Combustion Process

The powder samples of AGd204: Eu (A = Sr or Ba) were prepared via the homogeneous precipitation method followed by a subsequent combustion process with controlled heating at 1200 °C for 3 hours in air ambiance. The method of Singh et al. 2016 is followed to prepare the phosphors. The procedure is as follows:

  • (a) First, the stoichiometric amounts of starting materials Sr(N03)2 or Ba(N03)2 (Otto Chemie Pvt. Limited 99%), Gd(N03)3.6H20 (Otto Chemie Pvt. Limited, 99.9%) arid Eu(N03)3.xH20 (Sigma-Aldrich, 99.9%) were dissolved in the excess of ethanol.
  • (b) The above mixture taken in a glass beaker was stirred well for 30 minutes under heating at 80 °C to get a homogeneous clear solution; after that, the quantity of urea (H2NCONH2) was added to the nitrate solution. The molar ratio of urea to metal nitrate is fixed to 2.5.
  • (c) Later the obtained solution was continuously stirred at 100 °C for at least 3 hours until the liquid totally evaporated and white powders were left.
  • (d) In the next step, the powders were ground properly and kept at 1200 °C for 3 hours in order to investigate the systematic studies.
  • (e) Interestingly, the combustion and the annealing effect simultaneously occurred during this process. During the combustion process, the material goes through a rapid dehydration and foaming followed by decomposition and generating combustible gases. Thereby, a voluminous solid is yielded when these volatile combustible gases ignite and burn with a flame. The combustion process utilizes the enthalpy of combustion and the whole process is completed in a short duration of time. Along with the combustion synthesis the concurrent annealing process leads to the increase in the crystallinity of the solid obtained.

Characterization Techniques

The X-Ray Diffraction (XRD) patterns of the as-prepared samples were recorded on Bruker-D8 Focus X-ray powder diffractometer with Cu Ka = 1.5406 A in the range 20°< 29 < 70° at a scan rate 1° min1 with 0.02° step size, since all the prominent peaks are available in the concerned range. The morphology of the prepared samples was examined by using a Field- Emission Scanning Electron Microscope (FESEM Supra-55, Germany) images and a Philips CM12 transmission electron microscope. The room- temperature photoluminescence (PL) spectra and life time measurement were recorded on Horiba FL3-21 fluorescence spectrophotometer.

Results and Discussion

Structural and Luminescence Characteristics of SrGd204 and BaGd204 Phosphors

Phase Identification and Morphological Characterizations

The XRD studies were carried out to investigate the structural information and influence of dopants in the crystal lattice of the prepared samples. XRD patterns of the as-prepared strontium and barium digadolinium oxide phosphors were recorded and are shown in Fig. 9.2. The detailed structural information of SrGd204 phosphors was reported in our previous work (Singh et al. 2016). The diffraction pattern of phosphors show the dominance of the orthorhombic SrGd204 and BaGd,C>4 phase (JCPDS Card No. 82-2320) with space group Pnam (62) along with the negligible presence of Gd2Os phase. No diffraction peaks were observed due to the europium doping which indicates the successful incorporation of Eu3+ ions into Gd3+ lattice sites of the SrGd204 host matrix.

The microstructure of as-synthesized BaGd204:Eu3* (4 mol%) phosphors were studied and are shown in Fig. 9.3. The FESEM images show agglomerated spherical morphology with particle sizes in the range 0.01-0.1 pm. However, the FESEM images of Eu3+ doped SrGd204 samples exhibit agglomerated rod-like structures with particle sizes in the range 0.3-3 pm

XRD pattern of BaGd.Eu0 (x = 1-8 mol%) phosphors

Fig. 9.2. XRD pattern of BaGd2(1.x)Eu2x04 (x = 1-8 mol%) phosphors.

(a) FESEM images and (b) corresponding ТЕМ images of BaGd0

Fig. 9.3. (a) FESEM images and (b) corresponding ТЕМ images of BaGd204: 4 mol% Eu3' samples.

(Singh et al. 2016). The doping concentration did not show any impact on the morphology of the samples.

In order to investigate the effect of doping of Eu3+ ions in AGd204 (A = Ba or Sr) crystals more precisely, Transmission Electron Microscopy (ТЕМ) of the BaGd204: Eu3+ (4 mol%) samples was also carried out and shown in Fig. 9.3b. ТЕМ images reveal spherical agglomerates having average sizes

~0.01-0.1 pm. However, ТЕМ images of SrGd204: Eu3* (4 mol%) samples are presented in Figure [SI (supplementary file)]. The ТЕМ image of SrGd204: Eu3+ exhibits irregular agglomerates having average sizes -0.3-3 pm. The ТЕМ results of both the phosphors are consistent with the corresponding SEM data.

Luminescence Characterizations

Figure 9.4 shows the photoluminescence excitation (PLE) and emission spectra of Eu3+ doped BaGd192Eu008O4 and SrGd1Q2Eu00SO4 phosphors. The excitation spectra of prepared phosphors were recorded keeping the emission wavelength fixed at 615 run. In general, the excitation spectrum of Eu3+ doped phosphor materials contains two different regions:

  • 1. Charge Transfer (CT) transition.
  • 2. Intra 4f transitions.
Room temperature photoluminescence excitation spectra of SrGd,EuO and BaGdEuO sample

Fig. 9.4. Room temperature photoluminescence excitation spectra of SrGd4 9,Eu0 0SO4 and BaGd192Eu00SO4 sample.

The broad band extending from 200-330 nm with band maximum at 276 nm in the higher energy region is associated with a Charge Transfer State (CTS), owing to an electron that is transferred from the oxygen 2p orbital to the empty 4f orbital of the europium.

It is worth noting that the f-f transition sS7/2^“Iq/2 at 276 nm of Gd3+ ions was overlapped with this strong CTS band in SrGd4 92Eu0 0SO4 samples. However, the f-f transition arid CTS band can be distinctly seen in the excitation spectra of BaGd4 ^Euq 0SO4 samples. The sharp lines in the lower energy region correspond to direct excitation of the intra-4f forbidden transitions of Eu3* ions: 7F0->5H3 at 323 nm, 7F0->5D4 at 362 nm, 7F05G2 at 381 nm, 7F05L„ at 394 nm, 7F05D3 at 414 nm and 7F05D2 at 465 nm respectively. Additionally, the typical excitation peak at 313 nm is assigned to f-f transitions of Gd3+ (^/г—>6Р7/2) ions (Raju et al. 2014b, c; Zhou et al. 2007; Mari et al. 2011; Singh et al. 2016). The assignment of all the excitation and emission transitions is shown in Figure [S2 supplementary file)].

The effects of the Eu3+ contents on the emission behaviors in BaGd2 (1. x) 04:2xEu3+ and SrGd2 (1.x) 04:2xEu3+ phosphors are illustrated in Fig. 9.5 and Figure [S3 supplementary file)]. The intensity of the emission from the 5D3дд to the 7Fj level decreases with increasing Eu3+ concentrations and the intensity of the emission from the 5D0h>7Fj transitions gradually increases. These results are owing to the higher energy level 5D3 21 being quenched by a cross-relaxation mechanism (Raju et al. 2014b, c). This is a non-radiative process in which the excitation energy of the ion decaying from a higher excited state (5D321) of Eu3+ promotes a neighboring ion from the ground state to a metastable state by the following equations: Room temperature photoluminescence emission spectra of various BaGd,Eu,0 (x = 1-8 mol%) samples (/. = 276 nm)

Fig. 9.5. Room temperature photoluminescence emission spectra of various BaGd,(lx)Eu,x04 (x = 1-8 mol%) samples (/.ex = 276 nm).

As the concentration of Eu3+ increases, a condition is met where the distance between two nearest Eu3* ions becomes small enough to allow resonant energy transfer. Therefore, the energy can be easily transferred from one luminescent center to another. As a consequence, when the Eu3+ concentration is sufficiently high, the emission from the higher level (5D3 21) can be easily quenched via cross-relaxation and the 5D0 emission becomes dominant. The dominant emission peaks of europium at 594 and 615 nm are attributed to 5D07Fa and 5D0->7F2 transitions respectively.

All the emission bands are narrow in nature due to the characteristic electron shielding effect in trivalent rare-earth ions (Eu3*) (Singh et al. 2016a, b). The intense emission peak observed at 615 nm is attributed to the electric dipole transition, which is hypersensitive in the host structure symmetry. The transitions, which are highly sensitive to the environment of the host matrix and become more intense, are generally called hypersensitive transitions. The parity allowed transitions at 594 nm is ascribed to the magnetic dipole transition of 5D07Fa and is insensitive to the site symmetry It is worth noting that if the Eu3+ ions occupy an inversion symmetry center, only the magnetic dipole transition 5D07Fa is expected in the emission spectrum rather than electric dipole transition 5D07F2 (Raju et al. 2014b, c; Zhou et al. 2007; Mari et al. 2011). In the present case, the dominance of the 5D0^7F2 transitions indicates that the location of the Eu3* ions deviates from the inversion symmetry i.e. at low symmetry sites (Chaker et al. 2003; Li et al. 2009; Karunadasa et al. 2005; Mari et al. 2011).

The photoluminescence intensity increases with increasing Eu3* doping concentration. However, the PL emission intensity tends to decrease beyond a critical doping concentration (x = 4 mol%) in both the samples because of non-radiative energy transfer among Eu3* ions (Fig. 9.6). This non-radiative energy transfer can occur generally due to exchange interaction, radiation reabsorption or multipole-multipole interaction.

Initially, the PL intensity increases monotonically with the increase in rare earth concentration. Then, a condition is encountered where the distance between two Eu3+ ions is such that the resonant energy transfer dominates,

Concentration quenching phenomenon of both the phosphors as a function of Eu* concentrations

Fig. 9.6. Concentration quenching phenomenon of both the phosphors as a function of Eu3* concentrations.

which will increase the non-radiative relaxation effectively resulting in the decreased PL intensity (Fig. 9.6). This effect is known as concentration quenching (Singh et al. 2016).

 
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