# Sol-Gel Derived Eu3+-Doped Gd2Ti2O7 Pyrochlore Nanopowders

Authors: Sanja Ćulubrk; Željka Antić; Vesna Lojpur; Milena Marinović-Cincović; Miroslav D. Dramićanin
Journal: Journal of Nanomaterials
 (2015)

---

## Abstract

Herein we presented hydrolytic sol-gel synthesis and photoluminescent properties of Eu3+-doped Gd2Ti2O7 pyrochlore nanopowders. According to Gd2Ti2O7 precursor gel thermal analysis a temperature of 840°C is identified for the formation of the crystalline pyrochlore phase. Obtained samples were systematically characterized by powder X-ray diffraction, scanning and transmission electron microscopy, and photoluminescence spectroscopy. The powders consist of well-crystalline cubic nanocrystallites of approximately 20 nm in size as evidenced from X-ray diffraction. The scanning and transmission electron microscopy shows that investigated Eu3+-doped Gd2Ti2O7 nanopowders consist of compact, dense aggregates composed entirely of nanoparticles with variable both shape and dimension. The influence of Eu3+ ions concentration on the optical properties, namely, photoluminescence emission and decay time, is measured and discussed. Emission intensity as a function of Eu3+ ions concentration shows that Gd2Ti2O7 host can accept Eu3+ ions in concentrations up to 10 at.%. On the other hand, lifetime values are similar up to 3 at.% (~2.7 ms) and experience decrease at higher concentrations (2.4 ms for 10 at.% Eu3+). Moreover, photoluminescent spectra and lifetime values clearly revealed presence of structural defects in sol-gel derived materials proposing photoluminescent spectroscopy as a sensitive tool for monitoring structural changes in both steady state and lifetime domains.


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## Body

## 1. Introduction

The insulating rare-earth titanate pyrochlore oxides described by the general formula RE2Ti2O7 (RE = Ln3+, Sc3+, or Y3+) have the face-centered cubic crystal structure with the space group Fd3
¯m. In the structure RE3+ and Ti4+ ions occupy 16d and 16c sites, respectively, with the D
3
d symmetry. Due to the ability to accept rare-earth elements in high concentrations, chemical stability, and the fact that they do not show electronic conductivity at ordinary temperatures, rare-earth titanate pyrochlores could be proposed as a potential phosphor and laser host materials. However, due to their unusual structural, magnetic, and electrical conductivity properties most of the research in this field is done on the structural trends [1], thermodynamic and dynamic magnetic properties [2–4], disorders [5, 6], and nonstoichiometry [7].Many synthesis approaches for rare-earth pyrochlores such as solid state [8–10], sol-gel [11, 12], molten salt [13], hydrothermal [14], stearic acid method [15], and Pechini process [16] have been developed. Among them sol-gel method has been considered as a low cost approach for the preparation of novel nanostructured materials. It is a wet-chemical technique widely used in the fields of materials science and engineering for the fabrication of materials starting from a chemical solution—sol which acts as the precursor for an integrated network of discrete particles—gel. It refers broadly to the room temperature solution routes and enables synthesis of materials with desirable hardness, optical transparency, chemical durability, tailored porosity, and thermal resistance.In this paper, Eu3+-doped Gd2Ti2O7 nanopowders with different dopant concentrations (0.5; 1; 3; 5; 7; 10; and 15 at.% with respect to Gd) were fabricated using sol-gel technique. There are two reasons for using europium. Firstly, it provides an intense red emission and therefore Gd2Ti2O7 powders are new red nanophosphors. Secondly, due to nondegenerated ground (7F0) and excited (5D0) states together with nonoverlapping 2S+1LJ multiplets that give emission spectra with clear dependence on the host material structure and site symmetry, Eu3+ is well-known structural probe ion [17, 18]. In this way photoluminescent spectroscopy could be used as a structural probe technique. The thermal decomposition behavior and proper sintering temperature of the Gd2Ti2O7 samples are determined using thermogravimetric/differential thermal analysis (TG-DTA). Fourier transform infrared spectroscopy (FTIR) is used to identify the changes taking place within the precursor gel during drying and calcination. Detailed structural and morphological characterization of the nanocrystalline powders was performed using X-ray diffraction (XRD), scanning (SEM), and transmission electron microscopy (TEM). Optical properties were determined by photoluminescence (PL) excitation, emission, and lifetime measurements. The doping concentrations were systematically investigated in order to obtain the pure phase pyrochlore structure, strongest photoluminescent intensity, and the longest lifetime.So far there is a lack in reports on photoluminescent properties of rare-earth doped Gd2Ti2O7 powders. To date, photoluminescent properties of Eu3+-doped Gd2Ti2O7 have been investigated by others in thin film [19], core-shell [20], and nanocomposite [21] form or codoped with V4+ ions in powder form [11, 22, 23]. Our previous article reported photoluminescent properties of Sm3+ and Eu3+-doped Gd2Ti2O7 [16] obtained by mixed metal-citric acid synthesis and this study is the continuation of our research on the rare-earth doped Gd2Ti2O7 powders. Moreover, aim of this report is to introduce luminescent spectroscopy as a sensitive tool for monitoring structural changes.


## 2. Materials and Methods

### 2.1. Synthesis of Gd2Ti2O7:Eu3+ Nanoparticles

To produce undoped and Eu3+-doped pyrochlore Gd2Ti2O7 in the form of nanopowders the hydrolytic sol-gel route has been adopted, starting from rare-earth nitrates and titanium(IV)-isopropoxide. For the synthesis titanium(IV)-isopropoxide (Alfa Aesar, 97%), water, ethanol, and nitric acid were mixed in molar ratio of 1 : 3 : 20 : 0.08 [24]. In the first step, stoichiometric quantities of Gd
NO
3
3
*
6
H
2
O (Alfa Aesar, 99.9%) and Eu2O3 (Alfa Aesar, 99.9%) were dissolved in appropriate amount of water and HNO3. Then titanium(IV)-isopropoxide was dissolved in ethanol under constant magnetic stirring and added to the rare-earth nitrates mixture. For undoped sample only appropriate amount of Gd(NO3)3 was used. Transparent gels were obtained within few minutes and dried at 70°C for 5 h under atmospheric pressure. In order to obtain pure phase Gd2Ti2O7 nanoparticles the undoped and Eu+3-doped dried gels were calcinated at 840°C for 4 hours followed by natural furnace cooling to room temperature. The calcination temperature is chosen according to TG/DTA analysis, the results of which are presented in Section 3.1. Finally, undoped Gd2Ti2O7 and 7 Gd2Ti2O7 samples doped with x at.% Eu3+ (x = 0.5; 1; 3; 5; 7; 10; and 15 with respect to Gd3+) were synthesized.


### 2.2. Instruments and Measurements

TG/DTA analysis was performed on the SETARAM SETSYS Evolution-1750 instrument. The Gd2Ti2O7 dried gel (~10–15 mg) was heated at 10°C min−1 heating rate, in air atmosphere (air flow 16 mL min−1), from 30 to 1000°C. Fourier transmission infrared (FTIR) measurements were carried out on the Thermo Nicolet 380 FT-IR instrument, in a reflection mode with a resolution of 4 cm−1. X-ray diffraction (XRD) measurements were performed using Rigaku SmartLab diffractometer. Diffraction data were recorded in a 2θ range from 10° to 90°, counting 0.7°/minute in 0.02° steps. Relevant results of structural analysis (unit cell parameter, crystal coherence size, microstrain values, and data fit parameters) were obtained using built-in package software. Microstructural characterization was done using a JEOL JSM–6610LV scanning electron microscope (SEM) with an acceleration voltage of 20 kV. Microstructure at a local level was analyzed by transmission electron microscopy (TEM) using JEOL-JEM 2100 LaB6 operated at 200 kV.Photoluminescence (PL) measurements were performed under room temperature conditions. Measurements were done on Fluorolog-3 Model FL3-221 spectrofluorometer system (Horiba Jobin-Yvon), utilizing 450W Xenon lamp as excitation source for emission measurements and Xenon-Mercury pulsed lamp for lifetime measurements. TBX-04-D PMT detector is used for both lifetime and steady state acquisitions. The line intensities and positions of the measured spectra were calibrated with a standard mercury-argon lamp. All photoluminescence measurements were performed on pellets prepared from the powders under a load of 5 tons and without any additives.


## 2.1. Synthesis of Gd2Ti2O7:Eu3+ Nanoparticles

To produce undoped and Eu3+-doped pyrochlore Gd2Ti2O7 in the form of nanopowders the hydrolytic sol-gel route has been adopted, starting from rare-earth nitrates and titanium(IV)-isopropoxide. For the synthesis titanium(IV)-isopropoxide (Alfa Aesar, 97%), water, ethanol, and nitric acid were mixed in molar ratio of 1 : 3 : 20 : 0.08 [24]. In the first step, stoichiometric quantities of Gd
NO
3
3
*
6
H
2
O (Alfa Aesar, 99.9%) and Eu2O3 (Alfa Aesar, 99.9%) were dissolved in appropriate amount of water and HNO3. Then titanium(IV)-isopropoxide was dissolved in ethanol under constant magnetic stirring and added to the rare-earth nitrates mixture. For undoped sample only appropriate amount of Gd(NO3)3 was used. Transparent gels were obtained within few minutes and dried at 70°C for 5 h under atmospheric pressure. In order to obtain pure phase Gd2Ti2O7 nanoparticles the undoped and Eu+3-doped dried gels were calcinated at 840°C for 4 hours followed by natural furnace cooling to room temperature. The calcination temperature is chosen according to TG/DTA analysis, the results of which are presented in Section 3.1. Finally, undoped Gd2Ti2O7 and 7 Gd2Ti2O7 samples doped with x at.% Eu3+ (x = 0.5; 1; 3; 5; 7; 10; and 15 with respect to Gd3+) were synthesized.


## 2.2. Instruments and Measurements

TG/DTA analysis was performed on the SETARAM SETSYS Evolution-1750 instrument. The Gd2Ti2O7 dried gel (~10–15 mg) was heated at 10°C min−1 heating rate, in air atmosphere (air flow 16 mL min−1), from 30 to 1000°C. Fourier transmission infrared (FTIR) measurements were carried out on the Thermo Nicolet 380 FT-IR instrument, in a reflection mode with a resolution of 4 cm−1. X-ray diffraction (XRD) measurements were performed using Rigaku SmartLab diffractometer. Diffraction data were recorded in a 2θ range from 10° to 90°, counting 0.7°/minute in 0.02° steps. Relevant results of structural analysis (unit cell parameter, crystal coherence size, microstrain values, and data fit parameters) were obtained using built-in package software. Microstructural characterization was done using a JEOL JSM–6610LV scanning electron microscope (SEM) with an acceleration voltage of 20 kV. Microstructure at a local level was analyzed by transmission electron microscopy (TEM) using JEOL-JEM 2100 LaB6 operated at 200 kV.Photoluminescence (PL) measurements were performed under room temperature conditions. Measurements were done on Fluorolog-3 Model FL3-221 spectrofluorometer system (Horiba Jobin-Yvon), utilizing 450W Xenon lamp as excitation source for emission measurements and Xenon-Mercury pulsed lamp for lifetime measurements. TBX-04-D PMT detector is used for both lifetime and steady state acquisitions. The line intensities and positions of the measured spectra were calibrated with a standard mercury-argon lamp. All photoluminescence measurements were performed on pellets prepared from the powders under a load of 5 tons and without any additives.


## 3. Results and Discussion

### 3.1. Thermal Analysis (TG/DTA)

The Gd2Ti2O7 gels prepared with hydrolytic sol-gel route were of amorphous nature and under appropriate sintering they transformed to crystalline Gd2Ti2O7. Thermal analysis of dried gel was performed in order to determine proper sintering temperature for the transformation to pyrochlore phase. Results of TG/DTA analysis clarified the existence of three temperature regions (see Figure 1). Between room temperature and 275°C strong endothermic peak attributed to the vaporization of adsorbed organic molecules occurs in the DTA curve. In the same temperature range the TG curve shows a marked weight loss (~30%). In the second temperature range between 275°C and 500°C two endothermic peaks are indicated by DTA. At the same time weight loss (~20%) indicated by TG could be related to the elimination of the residual organic compounds. In the third region a DTA exothermic peak at ~815°C could be attributed to the amorphous to crystalline transformation of Gd2Ti2O7, while after 500°C the mass remains constant. According to these results calcination temperature of 840°C was chosen. Comparing with our previous results on rare-earth doped Gd2Ti2O7 obtained with mixed metal-citric acid synthesis (appropriate sintering temperature was ~880°C, [16]) this temperature is somewhat lower. Additionally, more structured shape of TG/DTA curves shows that thermal behavior of sol-gel derived materials is more complex compared to mixed metal-citric acid ones.Figure 1
Thermogravimetry (TG, solid line) and differential thermal analysis (DTA, dotted line) curves for undoped Gd2Ti2O7 dried gel (70°C for 5 h).According to our previous study on the crystallization of phosphor materials prepared by sol-gel method [25], one should expect that materials crystallize in a two-step process. In the primary crystallization stage nonconstant radial growth rate is present, while in the second stage two-dimensional crystal growth occurs.


### 3.2. Fourier Transform Infrared Spectroscopy (FTIR)

Figure2 presents the FTIR spectra of undoped Gd2Ti2O7 gel, dried gel, and powder obtained after calcination at 840°C for 4 hours. Broad band with a maximum at ~3310 cm−1 arises from the stretching vibrations of OH− groups [26] and is especially visible in the Gd2Ti2O7 gel. Many peaks in the gel’s and precursor’s spectra could be attributed to the organic and nitrate presence (see Table 1, [26]). One can notice that after heat treatment peaks from H2O, NO
3
- groups and organic disappear completely. Due to formation of new lattice, the main feature of the titanate pyrochlores, the strong band in the range 600–400 cm−1 [27], is only visible after thermal treatment at 840°C.Table 1
Characteristic vibrational modes obtained from FTIR measurements.
Functional group
Region (cm−1)
Comments
-OH stretching vibration
3600–3000
From water
Aliphatic C-H stretching vibration
3000–2800
From isopropoxide
O-N=O asymmetrical stretching
~1650
From nitrates
Asymmetrical and symmetrical CH3 deformation vibrations
1450–1350
From isopropoxide
CH(CH3)2 skeletal vibrations
1170–880
From isopropoxide
N-O stretching vibrations
~800
From nitrates
CO2
2400–2300
Inevitable in the atmosphereFigure 2
FTIR spectra of undoped Gd2Ti2O7: (a) gel, (b) dried gel at 70°C for 5 h, and (c) powder obtained after calcination at 840°C for 4 h.


### 3.3. X-Ray Diffraction (XRD)

Gd2Ti2O7 pyrochlore crystallizes in the face-centered cubic lattice (space group Fd3
¯m, number 227) with the unit cell containing eight molecules (Z
=
8) and four crystallographically nonequivalent sites (Ti at 16c, Gd at 16d, O at 48f, and O′ at 8b). In the studied structure, Ti4+ and Gd3+ cations (and consequently rare-earth dopants) are situated in a centrosymmetric site of D
3
d symmetry.XRD patterns of undoped and seven Eu+3-doped Gd2Ti2O7 powders together with ICDD card number 01-074-9640 are presented in Figures 3(a) and 3(b). No other phase peaks or traces of impurities were detected up to 10 at.% Eu3+ indicating that, up to this concentration, dopant Eu3+ ions have been effectively incorporated into the Gd2Ti2O7 host lattice. However, at 15 at.% Eu3+, barely visible traces of impurities are present. In order to investigate presence of trace impurities in more detail diffractogram of 15 at.% Eu3+-doped sample is recorded under longer acquisition time and presented in Figure 4. Additional diffractions could be assigned to the traces of Ti5O9 (ICDD card number 01-075-1815) and Ti6O11 (ICDD card number 01-085-1058). According to the Ti–O phase diagram [28] large number of oxides and suboxides, TiO2, Ti6O, Ti3O, Ti2O, TiO, Ti2O3, and Ti3O5, as well as Magnelli phases Ti
n
O
2
n
-
1 (4
<
n
<
10), could be present at 298 K. However, for given stoichiometry, besides TiO2, most probable is formation of Magnelli phase impurities. Even though amounts of all constituting ions are precisely calculated and weighted, presence of bigger in size dopant Eu3+ in high concentration leads to structural misbalance and formation of stoichiometrically close Ti5O9 and Ti6O11 impurity phases traces.XRD patterns of (a) undoped and Eu+3-doped Gd2Ti2O7 powder samples (0.5–7 at.% Eu3+) and (b) 10 at.% and 15 at.% Eu3+-doped Gd2Ti2O7 powder samples. The diffraction peaks are indexed according to the presented ICDD card number 01-074-9640.
(a)
(b)Figure 4
XRD diffractogram of 15 at.% Eu3+-doped sample taken under longer acquisition time. Traces of impurities could be denoted by both Ti5O9 (ICDD card number 01-075-1815) and Ti6O11 (ICDD card number 01-085-1058) Magnelli phases.Compared to our previous results [16], one can conclude that sol-gel route allows incorporation of Eu+3 ions into Gd2Ti2O7 host lattice in lower concentrations (up to 10 at.%) compared with mixed metal-citric acid synthesis (up to 15 at.%). However, more detailed investigation (between the range of 10 and 15 at.% Eu3+) is necessary to precisely determine the extent to which the Eu3+ ion can be accommodated into sol-gel obtained Gd2Ti2O7 pyrochlore matrix.Relevant results of structural analysis (unit cell parameter, crystal coherence size, microstrain values, and data fit parameters) are presented in Table2. The starting parameters for the structural analysis were taken according to [9]. Unit cell parameter, a, increases with rare-earth doping due to replacement of smaller Gd3+ (r
VI
⁡
3
+ = 0.938 Å) with bigger in size Eu3+ (r
VI
⁡
3
+ = 0.947 Å) ion. The largest unit cell parameter was found for the sample with the highest Eu3+ dopant concentration (10 at.%). Similar crystallite size of around 20 nm found for all samples shows that dopant concentration does not affect crystallite size. Data fit parameters, the regression sum of relative errors (Rp), the regression sum of weighted squared errors (Rwp), and the goodness of fit (GOF) are small indicating a highly satisfactory reliability. Microstrain values are low suggesting good ion ordering in the nanocrystals.Table 2
Selected structural parameters obtained from XRD measurements.
Gd2Ti2O7
a (Å)
Crystallite size (nm)
Microstrain (%)
Rwp (%)
Rp (%)
GOF
0.5 at.% Eu3+
10.1624(4)
17.9(3)
0.23(10)
3.02
2.42
1.0467
1 at.% Eu3+
10.1692(6)
23.8(3)
0.04(8)
3.56
2.75
1.4959
3 at.% Eu3+
10.1714(6)
26.4(3)
0.07(6)
3.76
2.89
1.5241
5 at.% Eu3+
10.1722(4)
25.0(3)
0.12(8)
2.69
2.14
1.1209
7 at.% Eu3+
10.1724(5)
24.7(2)
0.07(6)
2.79
2.21
1.1396
10 at.% Eu3+
10.1731(6)
21.6(4)
0.05(10)
2.86
2.26
1.1435


### 3.4. Scanning and Transmission Electron Microscopy (SEM/TEM)

A SEM study was conducted to investigate the surface morphology of the powders and representative sample images are given in Figures5(a) and 5(b). The micrographs show that representative 3 at.% Eu3+-doped Gd2Ti2O7 consists of compact, dense aggregates composed entirely of nanoparticles. Microstructure at a local level investigated by TEM (Figures 5(c) and 5(d)) revealed that sol-gel synthesis produces powders with irregular both particles size and shape. The nanoparticles are organized in agglomerates, some of which formed the bigger crystals. This was visible for all Eu3+-dopant concentrations as well. The local crystal structure was investigated with the selected area electron diffraction technique (SAED) and one representative ring electron diffraction pattern is presented as Figure 5(c) inset. The presence of rings is the evidence of Gd2Ti2O7 polycrystalline nature, while their grainy appearance is connected to the fact that the constituent crystallites have a size of ~20 nm.SEM image of 3 at.% Eu3+-doped Gd2Ti2O7 nanopowder under (a) low (×10000) and (b) high magnification (×50000). The bright-field TEM images show that (c) most particles have irregular particles size and shape and that (d) nanoparticles are organized in agglomerates, some of which formed the bigger crystals. The SAED diffraction pattern of Gd2Ti2O7 presented as an inset in Figure 5(c) shows good crystallinity of the material.
(a)
(b)
(c)
(d)


### 3.5. Photoluminescence (PL)

In Figure6(a) excitation spectrum of representative Gd2Ti2O7:10 at.% Eu3+ sample with marked transitions, measured in the spectral region between 325 and 560 nm at room temperature, is presented. Photoluminescence emission spectra of samples with different Eu3+ concentration are presented in Figure 6(b). Emission intensity as a function of Eu3+ ions concentration (see inset in Figure 6(b)) shows that emission increases with Eu3+ doping and that Gd2Ti2O7 host can accept Eu3+ ions in concentrations up to 10 at.% without concentration quenching. Figures 6(c)-6(d) show more clearly emission spectra of Gd2Ti2O7:10 at.% Eu3+ and 15 at.% Eu3+ samples recorded at room temperature. Excitation into the 5L6 level (λ
ex
=
393 nm) produces distinct luminescence bands in the visible spectral region. Both spectra show five characteristic bands placed around 579 nm, 589 nm, 612 nm, 652 nm, and 712 nm associated with 5D0 → 7FJ (J
=
0,1
,
2,3
,
4) spin forbiddenf-f transitions, respectively. However, in the 10 at.% Eu3+ spectrum the most intense sharp orange-reddish line placed around 589 nm (magnetic-dipole 5D0 → 7F1 transition) is followed by quite broad red emission placed around 612 nm (electric-dipole 5D0 → 7F2 transition). On the other hand, in the 15 at.% Eu3+ sample spectrum this ratio is inverted. It is generally acknowledged that when a rare-earth ion is situated in a centrosymmetric site then, according to the Laporte rule, only magnetic-dipole transitions are possible. According to Tanner [29] when Eu3+ ion is placed in crystallographic site of D
3
d point symmetry (with a strict center of symmetry) only doublet corresponding to 5D0 → 7F1 transition should be visible. Therefore, presence of dominant sharp orange-reddish 5D0 → 7F1 doublet in 10 at.% sample suggests a crystalline environment of Eu3+ ions and their incorporation into nanoparticles. On the other hand, observation of broad forbidden 5D0 → 7FJ (J
=
0,2
,
3,4) electric-dipole transitions could be related to the distorted sites near nanoparticle’s surface or other defects. Observed most intense 5D0 → 7F2 transition in 15 at.% sample clearly indicates presence of additional defects in the structure. Even though XRD spectra showed hardly noticeable difference, photoluminescence spectroscopy clearly revealed presence of additional impurities. This observation indicates that luminescence spectroscopy is an effective method for investigating the presence of defects and could be used as extremely sensitive structural probe technique.(a) Excitation spectrum (λ
em = 589 nm) of representative Gd2Ti2O7:10 at.% Eu3+ sample with marked transitions recorded at room temperature and (b) emission spectra of Gd2Ti2O7:x at.% Eu3+ (x = 0.5; 1; 3; 5; 7; 10) samples (λ
exc = 393 nm) with dependence of the emission intensity versus Eu3+ concentration as inset. All spectra were recorded at room temperature and under the same experimental conditions. Photoluminescent emission spectra recorded in the spectral range 525–725 nm at room temperature of (c) Gd2Ti2O7:10 at.% Eu3+ and (d) Gd2Ti2O7:15 at.% Eu3+ sample.
(a)
(b)
(c)
(d)The emission decay curves of the5D0 emitting level are obtained under excitation at 393 nm (λ
em
=
588 nm) (see Figure 7(a)) and indicate a complex luminescence decay process. As can be seen, all curves could be fitted with at least two exponential functions. For this reason we found it more appropriate to calculate an average lifetime using the following equation [30]:
(1)
τ
avg
=
∫
0
∞
t
I
(
t
)
d
t
∫
0
∞
I
(
t
)
d
t
,
where I
(
t
) represents the luminescence intensity at time t, corrected for the background, and the integrals are evaluated on the range 0
<
t
<
t
m, where t
m
≫
τ
avg.(a) Luminescence decay curves of5D0 level (λ
em = 588 nm, λ
ex = 393 nm) for Gd2Ti2O7:x at.% Eu3+ (x = 0.5; 1; 3; 5; 7; 10) samples and (b) change of 5D0 level lifetime with doping concentration.
(a)
(b)Figure7(b) shows how the dopant concentration influences lifetime values. Up to 3 at.% Eu3+ values are similar experiencing decrease at higher concentrations. Generally, these lifetime values are almost two times shorter than our previous results on the same material obtained by mixed metal-citric acid method [16]. This clearly indicates that sol-gel method produces materials with more defects in the structure and confirms that photoluminescent spectroscopy could be used as a sensitive tool for monitoring structural changes in both steady state and lifetime domains.


## 3.1. Thermal Analysis (TG/DTA)

The Gd2Ti2O7 gels prepared with hydrolytic sol-gel route were of amorphous nature and under appropriate sintering they transformed to crystalline Gd2Ti2O7. Thermal analysis of dried gel was performed in order to determine proper sintering temperature for the transformation to pyrochlore phase. Results of TG/DTA analysis clarified the existence of three temperature regions (see Figure 1). Between room temperature and 275°C strong endothermic peak attributed to the vaporization of adsorbed organic molecules occurs in the DTA curve. In the same temperature range the TG curve shows a marked weight loss (~30%). In the second temperature range between 275°C and 500°C two endothermic peaks are indicated by DTA. At the same time weight loss (~20%) indicated by TG could be related to the elimination of the residual organic compounds. In the third region a DTA exothermic peak at ~815°C could be attributed to the amorphous to crystalline transformation of Gd2Ti2O7, while after 500°C the mass remains constant. According to these results calcination temperature of 840°C was chosen. Comparing with our previous results on rare-earth doped Gd2Ti2O7 obtained with mixed metal-citric acid synthesis (appropriate sintering temperature was ~880°C, [16]) this temperature is somewhat lower. Additionally, more structured shape of TG/DTA curves shows that thermal behavior of sol-gel derived materials is more complex compared to mixed metal-citric acid ones.Figure 1
Thermogravimetry (TG, solid line) and differential thermal analysis (DTA, dotted line) curves for undoped Gd2Ti2O7 dried gel (70°C for 5 h).According to our previous study on the crystallization of phosphor materials prepared by sol-gel method [25], one should expect that materials crystallize in a two-step process. In the primary crystallization stage nonconstant radial growth rate is present, while in the second stage two-dimensional crystal growth occurs.


## 3.2. Fourier Transform Infrared Spectroscopy (FTIR)

Figure2 presents the FTIR spectra of undoped Gd2Ti2O7 gel, dried gel, and powder obtained after calcination at 840°C for 4 hours. Broad band with a maximum at ~3310 cm−1 arises from the stretching vibrations of OH− groups [26] and is especially visible in the Gd2Ti2O7 gel. Many peaks in the gel’s and precursor’s spectra could be attributed to the organic and nitrate presence (see Table 1, [26]). One can notice that after heat treatment peaks from H2O, NO
3
- groups and organic disappear completely. Due to formation of new lattice, the main feature of the titanate pyrochlores, the strong band in the range 600–400 cm−1 [27], is only visible after thermal treatment at 840°C.Table 1
Characteristic vibrational modes obtained from FTIR measurements.
Functional group
Region (cm−1)
Comments
-OH stretching vibration
3600–3000
From water
Aliphatic C-H stretching vibration
3000–2800
From isopropoxide
O-N=O asymmetrical stretching
~1650
From nitrates
Asymmetrical and symmetrical CH3 deformation vibrations
1450–1350
From isopropoxide
CH(CH3)2 skeletal vibrations
1170–880
From isopropoxide
N-O stretching vibrations
~800
From nitrates
CO2
2400–2300
Inevitable in the atmosphereFigure 2
FTIR spectra of undoped Gd2Ti2O7: (a) gel, (b) dried gel at 70°C for 5 h, and (c) powder obtained after calcination at 840°C for 4 h.


## 3.3. X-Ray Diffraction (XRD)

Gd2Ti2O7 pyrochlore crystallizes in the face-centered cubic lattice (space group Fd3
¯m, number 227) with the unit cell containing eight molecules (Z
=
8) and four crystallographically nonequivalent sites (Ti at 16c, Gd at 16d, O at 48f, and O′ at 8b). In the studied structure, Ti4+ and Gd3+ cations (and consequently rare-earth dopants) are situated in a centrosymmetric site of D
3
d symmetry.XRD patterns of undoped and seven Eu+3-doped Gd2Ti2O7 powders together with ICDD card number 01-074-9640 are presented in Figures 3(a) and 3(b). No other phase peaks or traces of impurities were detected up to 10 at.% Eu3+ indicating that, up to this concentration, dopant Eu3+ ions have been effectively incorporated into the Gd2Ti2O7 host lattice. However, at 15 at.% Eu3+, barely visible traces of impurities are present. In order to investigate presence of trace impurities in more detail diffractogram of 15 at.% Eu3+-doped sample is recorded under longer acquisition time and presented in Figure 4. Additional diffractions could be assigned to the traces of Ti5O9 (ICDD card number 01-075-1815) and Ti6O11 (ICDD card number 01-085-1058). According to the Ti–O phase diagram [28] large number of oxides and suboxides, TiO2, Ti6O, Ti3O, Ti2O, TiO, Ti2O3, and Ti3O5, as well as Magnelli phases Ti
n
O
2
n
-
1 (4
<
n
<
10), could be present at 298 K. However, for given stoichiometry, besides TiO2, most probable is formation of Magnelli phase impurities. Even though amounts of all constituting ions are precisely calculated and weighted, presence of bigger in size dopant Eu3+ in high concentration leads to structural misbalance and formation of stoichiometrically close Ti5O9 and Ti6O11 impurity phases traces.XRD patterns of (a) undoped and Eu+3-doped Gd2Ti2O7 powder samples (0.5–7 at.% Eu3+) and (b) 10 at.% and 15 at.% Eu3+-doped Gd2Ti2O7 powder samples. The diffraction peaks are indexed according to the presented ICDD card number 01-074-9640.
(a)
(b)Figure 4
XRD diffractogram of 15 at.% Eu3+-doped sample taken under longer acquisition time. Traces of impurities could be denoted by both Ti5O9 (ICDD card number 01-075-1815) and Ti6O11 (ICDD card number 01-085-1058) Magnelli phases.Compared to our previous results [16], one can conclude that sol-gel route allows incorporation of Eu+3 ions into Gd2Ti2O7 host lattice in lower concentrations (up to 10 at.%) compared with mixed metal-citric acid synthesis (up to 15 at.%). However, more detailed investigation (between the range of 10 and 15 at.% Eu3+) is necessary to precisely determine the extent to which the Eu3+ ion can be accommodated into sol-gel obtained Gd2Ti2O7 pyrochlore matrix.Relevant results of structural analysis (unit cell parameter, crystal coherence size, microstrain values, and data fit parameters) are presented in Table2. The starting parameters for the structural analysis were taken according to [9]. Unit cell parameter, a, increases with rare-earth doping due to replacement of smaller Gd3+ (r
VI
⁡
3
+ = 0.938 Å) with bigger in size Eu3+ (r
VI
⁡
3
+ = 0.947 Å) ion. The largest unit cell parameter was found for the sample with the highest Eu3+ dopant concentration (10 at.%). Similar crystallite size of around 20 nm found for all samples shows that dopant concentration does not affect crystallite size. Data fit parameters, the regression sum of relative errors (Rp), the regression sum of weighted squared errors (Rwp), and the goodness of fit (GOF) are small indicating a highly satisfactory reliability. Microstrain values are low suggesting good ion ordering in the nanocrystals.Table 2
Selected structural parameters obtained from XRD measurements.
Gd2Ti2O7
a (Å)
Crystallite size (nm)
Microstrain (%)
Rwp (%)
Rp (%)
GOF
0.5 at.% Eu3+
10.1624(4)
17.9(3)
0.23(10)
3.02
2.42
1.0467
1 at.% Eu3+
10.1692(6)
23.8(3)
0.04(8)
3.56
2.75
1.4959
3 at.% Eu3+
10.1714(6)
26.4(3)
0.07(6)
3.76
2.89
1.5241
5 at.% Eu3+
10.1722(4)
25.0(3)
0.12(8)
2.69
2.14
1.1209
7 at.% Eu3+
10.1724(5)
24.7(2)
0.07(6)
2.79
2.21
1.1396
10 at.% Eu3+
10.1731(6)
21.6(4)
0.05(10)
2.86
2.26
1.1435


## 3.4. Scanning and Transmission Electron Microscopy (SEM/TEM)

A SEM study was conducted to investigate the surface morphology of the powders and representative sample images are given in Figures5(a) and 5(b). The micrographs show that representative 3 at.% Eu3+-doped Gd2Ti2O7 consists of compact, dense aggregates composed entirely of nanoparticles. Microstructure at a local level investigated by TEM (Figures 5(c) and 5(d)) revealed that sol-gel synthesis produces powders with irregular both particles size and shape. The nanoparticles are organized in agglomerates, some of which formed the bigger crystals. This was visible for all Eu3+-dopant concentrations as well. The local crystal structure was investigated with the selected area electron diffraction technique (SAED) and one representative ring electron diffraction pattern is presented as Figure 5(c) inset. The presence of rings is the evidence of Gd2Ti2O7 polycrystalline nature, while their grainy appearance is connected to the fact that the constituent crystallites have a size of ~20 nm.SEM image of 3 at.% Eu3+-doped Gd2Ti2O7 nanopowder under (a) low (×10000) and (b) high magnification (×50000). The bright-field TEM images show that (c) most particles have irregular particles size and shape and that (d) nanoparticles are organized in agglomerates, some of which formed the bigger crystals. The SAED diffraction pattern of Gd2Ti2O7 presented as an inset in Figure 5(c) shows good crystallinity of the material.
(a)
(b)
(c)
(d)


## 3.5. Photoluminescence (PL)

In Figure6(a) excitation spectrum of representative Gd2Ti2O7:10 at.% Eu3+ sample with marked transitions, measured in the spectral region between 325 and 560 nm at room temperature, is presented. Photoluminescence emission spectra of samples with different Eu3+ concentration are presented in Figure 6(b). Emission intensity as a function of Eu3+ ions concentration (see inset in Figure 6(b)) shows that emission increases with Eu3+ doping and that Gd2Ti2O7 host can accept Eu3+ ions in concentrations up to 10 at.% without concentration quenching. Figures 6(c)-6(d) show more clearly emission spectra of Gd2Ti2O7:10 at.% Eu3+ and 15 at.% Eu3+ samples recorded at room temperature. Excitation into the 5L6 level (λ
ex
=
393 nm) produces distinct luminescence bands in the visible spectral region. Both spectra show five characteristic bands placed around 579 nm, 589 nm, 612 nm, 652 nm, and 712 nm associated with 5D0 → 7FJ (J
=
0,1
,
2,3
,
4) spin forbiddenf-f transitions, respectively. However, in the 10 at.% Eu3+ spectrum the most intense sharp orange-reddish line placed around 589 nm (magnetic-dipole 5D0 → 7F1 transition) is followed by quite broad red emission placed around 612 nm (electric-dipole 5D0 → 7F2 transition). On the other hand, in the 15 at.% Eu3+ sample spectrum this ratio is inverted. It is generally acknowledged that when a rare-earth ion is situated in a centrosymmetric site then, according to the Laporte rule, only magnetic-dipole transitions are possible. According to Tanner [29] when Eu3+ ion is placed in crystallographic site of D
3
d point symmetry (with a strict center of symmetry) only doublet corresponding to 5D0 → 7F1 transition should be visible. Therefore, presence of dominant sharp orange-reddish 5D0 → 7F1 doublet in 10 at.% sample suggests a crystalline environment of Eu3+ ions and their incorporation into nanoparticles. On the other hand, observation of broad forbidden 5D0 → 7FJ (J
=
0,2
,
3,4) electric-dipole transitions could be related to the distorted sites near nanoparticle’s surface or other defects. Observed most intense 5D0 → 7F2 transition in 15 at.% sample clearly indicates presence of additional defects in the structure. Even though XRD spectra showed hardly noticeable difference, photoluminescence spectroscopy clearly revealed presence of additional impurities. This observation indicates that luminescence spectroscopy is an effective method for investigating the presence of defects and could be used as extremely sensitive structural probe technique.(a) Excitation spectrum (λ
em = 589 nm) of representative Gd2Ti2O7:10 at.% Eu3+ sample with marked transitions recorded at room temperature and (b) emission spectra of Gd2Ti2O7:x at.% Eu3+ (x = 0.5; 1; 3; 5; 7; 10) samples (λ
exc = 393 nm) with dependence of the emission intensity versus Eu3+ concentration as inset. All spectra were recorded at room temperature and under the same experimental conditions. Photoluminescent emission spectra recorded in the spectral range 525–725 nm at room temperature of (c) Gd2Ti2O7:10 at.% Eu3+ and (d) Gd2Ti2O7:15 at.% Eu3+ sample.
(a)
(b)
(c)
(d)The emission decay curves of the5D0 emitting level are obtained under excitation at 393 nm (λ
em
=
588 nm) (see Figure 7(a)) and indicate a complex luminescence decay process. As can be seen, all curves could be fitted with at least two exponential functions. For this reason we found it more appropriate to calculate an average lifetime using the following equation [30]:
(1)
τ
avg
=
∫
0
∞
t
I
(
t
)
d
t
∫
0
∞
I
(
t
)
d
t
,
where I
(
t
) represents the luminescence intensity at time t, corrected for the background, and the integrals are evaluated on the range 0
<
t
<
t
m, where t
m
≫
τ
avg.(a) Luminescence decay curves of5D0 level (λ
em = 588 nm, λ
ex = 393 nm) for Gd2Ti2O7:x at.% Eu3+ (x = 0.5; 1; 3; 5; 7; 10) samples and (b) change of 5D0 level lifetime with doping concentration.
(a)
(b)Figure7(b) shows how the dopant concentration influences lifetime values. Up to 3 at.% Eu3+ values are similar experiencing decrease at higher concentrations. Generally, these lifetime values are almost two times shorter than our previous results on the same material obtained by mixed metal-citric acid method [16]. This clearly indicates that sol-gel method produces materials with more defects in the structure and confirms that photoluminescent spectroscopy could be used as a sensitive tool for monitoring structural changes in both steady state and lifetime domains.


## 4. Conclusion

Eu3+-doped Gd2Ti2O7 samples produced via sol-gel synthesis and annealed at 840°C for 4 h were studied in order to observe the dopant’s influence on the structural and optical properties. XRD measurements revealed that up to 10 at.% dopant Eu3+ ions have been effectively incorporated into the Gd2Ti2O7 host lattice while at higher dopant concentrations barely visible traces of impurities are present. Microstructure at a local level revealed that sol-gel synthesis produces particles with well-defined crystallinity and irregular size and shape, organized in agglomerates, some of which formed the big crystals. The dependence of luminescence emission intensity on Eu3+ concentration shows that the strongest emission is found for sample containing 10 at.%. On the other hand, with an increase in Eu3+ concentration the lifetime of the 5D0 level decreases from ~2.8 ms (for 1 at.% Eu3+) to ~2.4 ms (for 10 at.% Eu3+). Correlation between structural and photoluminescent properties reveals that photoluminescent spectroscopy of rare-earths (e.g., Eu3+ ions) could be proposed as an effective method for investigating the presence of defects and structural changes in optically active materials.



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*Source: 514173-2015-03-02.xml*