Structural, FT-IR, XRD and Raman scattering of new rare-earth-titanate pyrochlore-type oxides LnEuTi2O7 (Ln = Gd, Dy)

Journal of Alloys and Compounds 573 (2013) 43–52

Contents lists available at SciVerse ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom

Structural, FT-IR, XRD and Raman scattering of new rare-earth-titanate pyrochlore-type oxides LnEuTi2O7 (Ln = Gd, Dy) A. Garbout ⇑, I. Ben Taazayet-Belgacem, M. Férid Laboratoire de Physico-Chimie des Matériaux Minéraux et leurs Applications, CNRSM, technopole de Borj Cedria, B.P. 95, Hammam-Lif 2050, Tunisia

a r t i c l e

i n f o

Article history: Received 7 January 2013 Accepted 29 March 2013 Available online 8 April 2013 Keywords: A1. Solid solutions A1. X-ray diffraction A1. Characterization B1. Rare-earth compounds

a b s t r a c t We report the synthesis and structural study of mixed oxides in LnEuTi2O7 series. We are presently investigating new phases with Ln = Gd and Dy, including the pyrochlore family Re2Ti2O7 (Re = Eu, Gd and Dy). Starting from stoichiometric mixtures of elemental oxides TiO2, Eu2O3 and Ln2O3 (Ln = Gd and Dy), singlephase samples were obtained using a conventional ceramic method. The presence of crystalline phases after heat treatment from 1000 to 1200 °C was studied by combining XRD and Raman spectroscopy. Xray diffraction combined to the Raman analysis confirmed the powder’s single-phase nature at 1200 °C. The expected compositions GdEuTi2O7 and DyEuTi2O7 were confirmed by energy dispersive spectroscopy (EDAX) and X-ray diffraction (XRD). FT-IR spectra of the obtained compounds also show the absorption bands corresponding to the pyrochlore structure A2B2O7 and give information about the distribution of ions between the A and B sites. X-ray powder diffraction results showed that the resulting phases GdEuTi2O7 and DyEuTi2O7 crystallize with the pyrochlore type structure, cubic Fd3m space group. The structures are determined by Rietveld refinement. The evolution of cell parameters and interatomic distances as a function of lanthanide cation size is discussed. The refined cell parameter a, and the positional coordinate, x, for O, determinated from Rietveld refinements, were related to the Ln3+ ions (Ln3+ = Gd3+, Dy3+) occupying the Re site. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Rare earth titanates with the chemical formula Re2Ti2O7 belong to a family of compounds, A2B2O7 pyrochlores, with a crystal structure which can be considered as a superstructure of the defect fluorite lattice. Titanate pyrochlore, materials are important because of their potential use as solid electrolytes and mixed ionic/electronic conducting electrodes [1–9], catalysts [10], and ferroelectric/ dielectric device components [11–14]. Most of the pyrochlore-type compounds known and reported in the literature were prepared by solid state reactions, at elevated temperature (1300 °C) [15–18]. The unit cell exhibits a parameter a close to 10 Å and contains eight formula units. Pyrochlore oxides, with the general formula A2B2O6O0 , have a cubic structure, crystallizing in space group Fd3m, with all atoms occupying special positions. There is only one variable positional parameter, x, which describes the position of the O atoms on the 48(f) site (x, 0.125, 0.125). Despite this apparent simplicity, the properties of various pyrochlore oxides are sensitive to the precise structural distortions, and these in turn are given by the values of the crystallographic parameters a and x [19].

⇑ Corresponding author. Tel.: +216 23298716. E-mail address: [email protected] (A. Garbout). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.03.279

Mostly A is a trivalent rare-earth ion, but can also be a mono, divalent cation, and B may be 3d, 4d or 5d transition element having an appropriate oxidation state required for charge balance to give rise to the composition A2B2O7. The eight coordinated A-site (16c) located at the center of a scalenohedron, is normally occupied by the larger cation whereas the six-coordinated B-site (16d) located at the center of a trigonal antiprism, is usually occupied by the smaller cation. The O atoms occupy the 48f site coordinated to two B4+ and two A3+ cations while the O0 anions occupy the 8a site being tetrahedrally coordinated to four A3+ cations. Additionally, there is another anionic tetrahedral site (8b) coordinated to four B4+ ions, which is systematically vacant in ordered pyrochlores [20]. The pyrochlore structure is described as a network of octahedra linked corner to corner with the A cations filling the interstices [21]. It can also be thought as a fluorite derivative [22], with the A and B cations ordered into rows in the h1 1 0i directions and the oxide vacancies into unoccupied 8a oxygen sites (see Fig. 1). The degree of cation disordering is related to the ratio of the ionic radii of the cations at the A and B sites [23], and cation disordering greatly enhances the formation of anion Frenkel defects [24]. Interesting enough, different degrees of disorder can be reached in systems of solid solutions by using the appropriate substitutions on the A and B sites. A decrease of the cation ionic radius ratio gradually results in a structural transformation from a fully

44

A. Garbout et al. / Journal of Alloys and Compounds 573 (2013) 43–52 The samples were also examined by scanning electron microscopy (SEM) using FEI Quanta 200 environmental scanning electron microscope equipped with an EDAX Inc. energy-dispersive X-ray detector for microanalysis. Energy-dispersive X-ray spectroscopy (EDS or EDX) is an analytical technique used for the elemental analysis or chemical characterization of a sample. Infrared spectra of the synthesized samples are recorded on a Perkin Elmer (FTIR 2000) spectrometer in the range of 400–4000 cm1 using KBr pellets. X-ray powder diffraction (XRD) data were obtained using a PANalytical Pro X’Pert MPD (40 kV, 30 mA) diffractometer using Cu Ka radiation at room temperature with a scan step of 0.017°. Preliminary, the existing phases in the calcined (1/2Gd2O3 + 1/2Eu2O3 + 2TiO2) and (1/2Dy2O3 + 1/2Eu2O3 + 2TiO2) mixtures at 1000 °C and 1200 °C and the presence of the expected cubic were confirmed by analyzing the X-ray powder diffraction patterns of the products collected in the 2h range from 10° to 70° with a 0.017° step and a fixed time of 1 h. Phase’s presents were identified by comparison with the PDF2 database. For the structural refinements, the XRD pattern of monophasic samples Eu2Ti2O7, Gd2Ti2O7, Dy2Ti2O7, GdEuTi2O7 and DyEuTi2O7 treated at 1200 °C were recorded from 2h = 10° to 100° with a fixed time of 3 h. The structural characterization by the Rietveld method [27] was performed using the FULLPROF program [28].

3. Results and discussion Fig. 1. The A2B2O6O0 pyrochlore structure. Purple BO6 octahedra and green O0 A4 tetrahedra are shown. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

ordered pyrochlore structure to a completely disordered aniondeficient defect fluorite structure [25]. Rare earth elements (Re = La, Ce, etc.) have recently been involved in a wide range of advanced technologies (microelectronics, gas sensor technologies), and in industrial chemistry (e.g., membranes for catalytic methane conversion) [26]. Since solid solutions can easily be formed with Re3+ ions, these materials can be adapted to a large field of applications. In the case of thermal barriers, the aim of mixing Re3+ cations is to lower thermal conduction and increase thermal expansion. In the case of electrical properties, solid solutions might offer a wide range of ionic and electronic behaviors. In the present work, we deal with the low cost creation of new phases based on rare earth elements that could be used for their thermal behavior, ionic conduction, and catalytic properties in miniaturized devices. We are presently investigating new phases belonging to the Ln2O3–TiO2–Eu2O3 system (Ln = Gd and Dy), including the pyrochlore family Re2Ti2O7 (Re = Eu, Gd and Dy) with two different objectives:  Develop low cost materials for industrial applications based on rare earth elements.  Test new materials for electronic and catalytic applications associated with high temperature uses (thermal barriers, catalysis, and ionic conduction).

3.1. Synthesis and caracterisations of pyrochlore-type phases Fig. 2a and b shows the evolution of the X-ray diffraction patterns of the (1/2Gd2O3 + 1/2Eu2O3 + 2TiO2) and (1/2Dy2O3 + 1/ P

(a)

Starting mixture:1/2Gd2O3+1/2Eu2O3+2TiO2

P

P : Pyrochlore r : rutile : Gd2O3

P P

P

P

P P

P P

P

P r 20

30

rP

r 40

P 12 h at 1200°C

P

P

r

10

P

50

60

12 h at 1000°C 70

2θ(°)

(b)

Starting mixture:1/2Dy2O3+1/2Eu2O3+2TiO2 P P : Pyrochlore r : rutile

2. Experimental

: Dy2O3

2.1. Samples preparation The starting materials, i.e., Gd2O3, Eu2O3, Dy2O3 and TiO2, are heated at 700 °C to remove moisture. Stoichiometric amounts of reactants were weighed to prepare compositions corresponding to Re2Ti2O7 (Re = Eu, Gd, Dy, GdEu and DyEu). The well-ground mixtures were heated in pellet form initially at 1000 °C for 12 h. In order to attain better homogeneity, the products obtained after the second heating were again ground, pelletized and heated at 1200 °C for 12 h, which was the final annealing temperature. The heating and cooling was done at 2 °C min1 and the atmosphere was static air.

P P

P P P

P

P P

P

P P 12 h at 1200°C

P

Pr

P

P r

P 12 h at 1000°C

2.2. Characterizations 10

The two compositions GdEuTi2O7 and DyEuTi2O7 were selected to study the evolution of powder mixtures with temperature (1000–1200 °C), characterizing them by using X-ray powder diffraction and Raman spectroscopy. Raman scattering experiments were performed using a micro Raman spectrometer (LabRam HR 800), working in a backscattering configuration, equipped with a He+ ion (k = 633 nm) laser. The spectral resolution of the system was 3 cm1.

20

30

40

50

60

70

2θ (°) Fig. 2. Evolution of the studied compounds with annealing temperature. (a) X-ray diffraction pattern of the (1/2Gd2O3 + 1/2Eu2O3 + 2TiO2) mixture calcined 12 h at 1000 and 1200 °C; (b) X-ray diffraction pattern of the (1/2Dy2O3 + 1/2Eu2O3 + 2TiO2) mixture calcined 12 h at 1000 and 1200 °C.

45

A. Garbout et al. / Journal of Alloys and Compounds 573 (2013) 43–52

2Eu2O3 + 2TiO2) mixtures, respectively with the temperature of calcination. The diffractograms recorded at 1000 °C, indicates a mixture of cubic pyrochlore, TiO2 and starting oxides, with pyrochlore as the predominant phase. There are some peaks from unreacted starting powders in the two compositions, which reveal that sintering at 1000 °C for 12 h is insufficient for a full reaction. To obtain only one pyrochlore phase, we must increase the temperature of the annealing. As shows the diffractograms of Fig. 2a and b at 1200 °C, we note that all the diffraction peaks can be indexed to the pyrochlore structure without any impurity phase. A common feature observed in all powder patterns after calcination at 1200 °C is a disapeared of the characteristic diffraction peaks of the starting oxides. Interestingly, with the increase of the calcination temperature, the intensity of each peak grew and the position of 2h indicating a pyrochlore structure did not change for the (1/2Gd2O3 + 1/2Eu2O3 + 2TiO2) mixture. Whereas, for the (1/2Dy2O3 + 1/2Eu2O3 + 2TiO2) mixture, the diffraction peaks of the pyrochlore structure shifted to a higher angle with the increase of the temperature from 1000 to 1200 °C due to the variation of composition. Raman spectroscopy is more surface sensitive than XRD because an excitation energy in the near IR region is less penetrating than X-rays. A comparison of Raman spectroscopy with X-ray diffraction can be used to estimate the difference between the surface and bulk compositions of pyrochlore powders [29]. The evolution of Raman spectrum with annealing temperature of the (1/2Gd2O3 + 1/2Eu2O3 + 2TiO2) and the (1/2Dy2O3 + 1/2Eu2O3 + 2TiO2) mixtures is given in Fig. 3a and b, respectively. The bands in the Raman spectra are tentatively assigned to symmetry species by comparing with previously published Raman spectra of pyrochlore oxides Re2Ti2O7 [30] including Gd2Ti2O7, Dy2Ti2O7 [31–33]. The lowest frequency lines, between 205 and 216 cm1 are assigned to the F2g mode. The strongest bands centered between 304 and 308 cm1 is attributed to O–Re–O bending mode and, in fact, it consists of two modes (Eg, F2g) with very similar frequencies. The band around 445 cm1 is assigned to the Ti–O stretching vibration. Another intense band is observed between 515 and 519 cm1 is the A1g mode, which is attributed to Re–O stretching [31,34]. Although, heating the samples from 1000 to 1200 °C, results in changes in the vibrational spectra. The curves of the two mixtures treated at 1000 °C, show mainly the pyrochlore compound mixed with a few rutile TiO2, which are detected by the 143 cm1 line and the increase in the intensity of bands at around 447 and 610 cm1 [35]. This result corroborates with the XRD analysis, where TiO2 was also observed in the samples treated at 1000 °C. This bands which are confidently attributed to the rutile TiO2 decrease in intensity or vanish in the compounds heated at 1200 °C. This result corroborates with the XRD analysis, where TiO2 was also observed at 1000 °C and disapeared from the samples treated at 1200 °C. For the Raman spectra of the (1/2Gd2O3 + 1/2Eu2O3 + 2TiO2) mixture heated at 1000 °C (Fig. 3a), we notice the appearance of band around 363 cm1 assigned to the starting reactant Gd2O3 which disappeared when sample is treated at 1200 °C. At this temperature, the complete reaction between the reactifs is achieved. The Raman results confirmed the powder’s single-phase nature observed by XRD even when treated at 1200 °C.

(a)

O-Re-O

Starting mixture:1/2Gd2O3+1/2Eu2O3+2TiO2

Gd2O3 340 350 360 370 Re-O Ti-O Ti-O

rutile

12h at 1200°C Gd2O3

12h at 1000°C 100

200

300

400

500

600

700

800

900

1000

-1

Wavenumber (cm )

(b)

O-Re-O

Starting mixture:1/2Dy2O3+1/2Eu2O3+2TiO2

Re-O

rutile

Ti-O

12h at 1200°C

Ti-O

12h at 1000°C 100

200

300

400

500

600

700

800

900

1000

-1

Wavenumber (cm ) Fig. 3. Evolution of the synthesised oxides with annealing temperature. (a) Raman spectra of the (1/2Gd2O3 + 1/2Eu2O3 + 2TiO2) mixture heat treated for 12 h, calcined temperature 1000 and 1200 °C; (b) Raman spectra of the (1/2Dy2O3 + 1/2Eu2O3 + 2TiO2) mixture heated 12 h at 1000 and 1200 °C.

It could be seen that the particle distribution was homogeneous and the increase in grain size is observed with the rare earth cationic radius. EDX micrograph of LnEuTi2O7 (Ln = Gd and Dy) powders calcinated at 1200 °C are given in Fig. 5a and b. EDAX microanalysis confirms the chemical homogeneity of the powders prepared by solid state route. Chemical composition revealing atomic ratios very close to those of the nominal compositions. From EDAX analyses all the studied crystals present a Re/Ti molar ratio close to 1 and no other element was detected. The expected LnEuTi2O7 (Ln = Gd and Dy) composition was confirmed by EDAX.

3.3. Raman study 3.2. Scanning electronic microscopy SEM micrographs obtained for LnEuTi2O7 (Ln = Gd and Dy) are displayed in Fig. 4a and b. The phase texture consists in small particles agglomerated in porous blocks. Samples consist basically of sub-micron size agglomerates of irregular shape.

The Fig. 6 shows the Raman spectra of the studied powders after calcination 12 h at 1200 °C. The whole wavenumber values observed in our compounds synthesised by solid state reaction are given in Table 1. We note that the Raman spectra collected for the different powders agreed with the pyrochlore-like phase [36,30],

46

A. Garbout et al. / Journal of Alloys and Compounds 573 (2013) 43–52

Fig. 4. Scanning electron micrographs of the studied powders after calcination 12 h at 1200 °C. (a) GdEuTi2O7; (b) DyEuTi2O7.

Fig. 5. EDX micrograph of the samples calcinated at 1200 °C. (a) GdEuTi2O7; (b) DyEuTi2O7.

even though it is not quite exact for the mode counting due to peak intensity and overlap of vibrations modes. The major spectral difference between LnEuTi2O7 (Ln = Gd and Dy) and Re2Ti2O7 (Re = Eu, Gd, Dy) is the shift observed for the wavenumber value of the more intense band. It is clear that this mode, characteristic of the O–Re–O bending vibration, is sensitive to the nature of the cation associated at the Ti2 O6 7 anion [37]. By comparison with Gd2Ti2O7 and Dy2Ti2O7, a shift of the most intense band towards lower wavenumber is to be noticed when europium is substituted in the mixed pyrochlore LnEuTi2O7 (Ln = Gd and Dy) lattice. Indeed, this conclusion is also corroborated by the observation that the ionic radius of Eu3+ (1.066 Å) is slightly larger compared to Gd3+ (1.053 Å) and appreciably larger than that of Dy3+ (1.027 Å) [38]. Frequencies assigned to Ti–O stretching are also very close for these titanates Re2Ti2O7 (Re = Eu, Gd, Dy, GdEu and DyEu). Therefore, even an important perturbation of the Re4O0 network has a weak influence on the TiO6 network and on the rigidity of the structure. From a vibrational point of view, the two networks are practically energetically independent.

3.4. Infrared spectral studies Fourier transform infrared spectroscopy has been used for studying the nature of metal-oxygen bonds in the pyrochlore

oxides since infrared active optic modes originate from the bending and stretching vibrations of the metal–oxygen bonds [39]. It is chosen as a complementary method to provide further evidence for the site preference of Ln (Gd and Dy) atoms in Eu2Ti2O7 because it is sensitive to the change of local structure resulting from atomic substitutions and distortion of polyhedra. The infrared lattice vibration frequencies of some pyrochlore compounds with a formula A2B2O7 have been studied. There are seven IR-active optic modes in the infrared spectra of the pyrochlore compound and the band (t1) at about 600 cm1 is from the B–O stretching vibration in the BO6 octahedron. The band (t2) at about 500 cm1 is from the A–O0 stretching vibration, and the band (t3) at about 400 cm1 is from the A–O stretching vibration in the AO6 O02 polyhedron of A2B2O7, respectively. Besides, the band (t4) at about 300 cm1 is from the O–B–O bend vibration, the band (t5) at about 200 cm1 is from the A–BO6 stretching vibration, the band (t6) at about 150 cm1 is from the O–A–O bend vibration and the band (t7) at about 100 cml is from the O0 –A–O0 bend vibration, respectively [23]. In our infrared spectra experiments, the spectra only in the region 3000–400 cm1 are measured. FTIR spectra of Re2Ti2O7 (Re = Eu, Gd, Dy, GdEu and DyEu) powders recorded in the 400– 1000 cm1 range are given in Fig. 7a and b. The region of interest in IR absorption bands of inorganic compounds are usually in the range of 100–1000 cm1 which attributed to the vibrations of ions in the crystal lattice [40–42]. Infrared-active phonon modes have

47

A. Garbout et al. / Journal of Alloys and Compounds 573 (2013) 43–52

(a)

O-Re-O

(a)

Gd2Ti2O7 Transmittance (arb.units)

Gd2Ti2O7

Re-O

GdEuTi2O7

Eu2Ti2O7

Eu2Ti2O7 GdEuTi2O7

400

500

600

700

800

900

1000

-1

Wavenumbers (cm ) 100

200

300

400

500

600

700

800

900

1000

(b)

-1

Wavenumber (cm )

(b)

DyEuTi2O7

Transmittance (arb.units)

O-Re-O

Dy2Ti2O7

Eu2Ti2O7

Re-O

Eu2Ti2O7 400

500

Dy2Ti2O7 100

200

300

400

500

600

700

800

900

1000

600

700

800

900

1000

-1

DyEuTi2O7

Wavenumbers (cm ) Fig. 7. IR spectra of the synthesised oxides after calcination 12 h at 1200 °C. (a) Gd2Ti2O7, Eu2Ti2O7 and GdEuTi2O7 pyrochlores; (b) Dy2Ti2O7, Eu2Ti2O7 and DyEuTi2O7 powders.

-1

Wavenumber (cm ) Fig. 6. Raman spectra of the studied powders after calcination 12 h at 1200 °C. (a) Gd2Ti2O7, Eu2Ti2O7 and GdEuTi2O7 pyrochlores; (b) Dy2Ti2O7, Eu2Ti2O7 and DyEuTi2O7 oxides.

Table 1 Observed Raman wavenumbers (in cm1) in Re2Ti2O7. Refs. [31,37]

A1g F2g Eg F2g

Observed Raman wavenumbers for compounds annealed at 1200 °C Eu2Ti2O7

Gd2Ti2O7

Dy2Ti2O7

GdEuTi2O7

DyEuTi2O7

678 524 440 314 212 107

688 519 450 314 210 97

700 526 315 215 106

673 515 308 205 101

685 519 442 305 207 95

been assigned to specific bending and stretching vibrational modes, which originated from vibration and bending of metal– oxygen bond. The differences in the spectral shapes between the two phases LnEuTi2O7 (Ln = Gd and Dy) and Eu2Ti2O7 reflected the different structural frameworks. All the FTIR results are summarized in Table 2. It can be observed in the two spectra of the mixed oxides GdEuTi2O7 and DyEuTi2O7 that there are four prominent absorption

Table 2 Peak position and types of the corresponding vibrational modes (cm1) in different titanates pyrochlores Re2Ti2O7. Assignment

Eu2Ti2O7

Gd2Ti2O7

Dy2Ti2O7

GdEuTi2O7

DyEuTi2O7

m(Re–O) m(Re–O0 ) m(Ti–O)

408 464 600

416 485 600

421 490 600

409, 416 460, 485 600

408, 420 460, 506 600

bands with slight variation in the position and intensity of the bands depending on the nature of the rare earth ion. The two spectra (Fig. 7a and b) show a sharp absorption at 408 cm1 followed by a weak absorption around 416 cm1 for the GdEuTi2O7 and 420 cm1 for the DyEuTi2O7 compounds. The two weak bands are just in the region of the A–O stretching vibration frequency. It is reasonable to assign the first absorption (408 cm1) to the Eu–O stretching vibration and the second weak band to the Gd–O (416 cm1) stretching vibration for the GdEuTi2O7 oxide and to the Dy–O (420 cm1) stretching vibration for the DyEuTi2O7 material. This is reminiscent of results reported earlier [43,44]. The stretching vibration frequency (t) of the bond relates to the mass of the bonding atoms. The lower the stretching vibration frequency of the bond, the more the mass of the bonding atoms. The atomic weight of the substituted rare earth Eu is much weaker than the atomic weight of Dy, and Gd, therefore, Eu–O frequency is more lower than Dy–O frequency and less lower than Gd–O frequency.

48

A. Garbout et al. / Journal of Alloys and Compounds 573 (2013) 43–52

The next absorption is observed around 460 cm1 followed by the absorption bands around 485 cm1 for GdEuTi2O7 and 506 cm1 for DyEuTi2O7, which is near the A–O0 stretching vibration. Similar to the above explanation, these can be assigned to both Eu–O0 and Ln–O0 bonds, (Ln = Gd and Dy), respectively. Generally, the intensity of a band is in direct proportion to the amount of bond in the compound [44]. In LnEuTi2O7, the number of the Ln2O0 tetrahedra is equal to the Eu2O0 tetrahedra. Thus, the intensity of the Eu–O stretching vibration may be equal to the Ln–O stretching vibration in the infrared spectra of LnEuTi2O7. It is likely that by the above type of reasoning, the broad absorption band in the region 530–700 cm1 is due to the Ti–O stretching vibration in TiO6 octahedron of the pyrochlore. It can be concluded from the above absorption bands that the phase formed is pyrochlore and Re–O stretching frequency is shifted slightly to higher region with the decrease in mass of the rare earth cation.

(a)

Eu2Ti2O7 GdEuTi2O7 Gd2Ti2O7

3.5. X-ray diffraction study 10

The additional collections data with a greater integration time for the samples GdEuTi2O7 and DyEuTi2O7 calcined 12 h at 1200 °C are compared to the spectra of Gd2Ti2O7, Eu2Ti2O7 and to the spectra of Dy2Ti2O7, Eu2Ti2O7 synthesised by solid-state route at 1200 °C, respectively (Fig. 8a and b). All the reflections in the diffractograms of the samples treated at 1200 °C are indexed with respect to the cubic phase of space group Fd3m without any impurity phase. Substitution of Eu3+ sites in the pyrochlore oxide LnEuTi2O7 (Ln = Gd and Dy) produces very similar patterns, with variations in peak positions. These variations result from changes in unit cell dimensions and composition. When investigating the X-ray diffraction patterns, it was found that every diffraction peak of the same Re2Ti2O7 (Re = Eu, Gd, Dy, GdEu and DyEu) shifted to a slightly higher angle with the increase of atomic number of Re. 3.5.1. Structure refinements The structures of Re2Ti2O7 (Re = Eu, Gd, Dy, GdEu and DyEu) compounds were refined from powder XRD data starting from a pyrochlore structure model: space group Fd3m, Re atoms on 16c site (0 0 0), and Ti cations randomly distributed on 16d site (0.5 0.5 0.5), O on 48f site (x 1/8 1/8), and O0 on 8a site (1/8 1/8 1/8). Structural analysis has been done by using Rietveld refinement program Fullprof-2006 [28]. First of all, the background parameters and scale factor were adjusted. The background was fitted with sixth order polynomial. The diffraction peak profile was fitted with

20

30

40

50

60

70

80

90

100

2θ(°)

(b)

Eu2Ti2O7

DyEuTi2O7 Dy2Ti2O7

10

20

30

40

50

60

70

80

90

100

2θ(°) Fig. 8. X-ray powder diffraction patterns of the prepared pyrochlores treated at 1200 °C. (a) Gd2Ti2O7, Eu2Ti2O7 and GdEuTi2O7 powders; (b) Dy2Ti2O7, Eu2Ti2O7 and DyEuTi2O7 samples.

Table 3 Crystallographic data for GdEuTi2O7 and DyEuTi2O7 in the space group Fd3 m (No.227). (Biso = isotropic displacement parameter; Occ = occupancy)

GdEuTi2O7

Atom

Wyckoff position

x

y

z

Biso (Å2)

Occ

Eu Gd Ti O O0

16 c 16 c 16 d 48 f 8a

0 0 0.500 0.500 0.425(2)

0 0 0.500 0.125 0.125

0 0 0.500 0.125 0.125

1.218(2) 1.218(2) 1.044(3) 1.086(7) 1.086(7)

0.500 0.500 1 1 1

16 c 16 c 16 d 48 f 8a

0 0 0.500 0.500 0.431(6)

0 0 0.500 0.125 0.125

0 0 0.500 0.125 0.125

0.242(3) 0.242(3) 0.216(3) 0.394(1) 0.394(1)

0.500 0.500 1 1 1

0

a (Å A) = 10.197(6) v2 = 2.97; Rf = 6.37; Rwp = 12.0; RBragg = 6.61 DyEuTi2O7 Eu Dy Ti O O0 a (A) = 10.165(3) X2 = 2.52; Rf = 6.19; Rwp = 12.3; RBragg = 6.60

49

A. Garbout et al. / Journal of Alloys and Compounds 573 (2013) 43–52

pseudo-Voigt profile function and then the FWHM parameters were adjusted. No absorption parameter was considered during refinement. Subsequently, individual thermal parameters were refined. All atoms occupy special positions. There is only one variable positional parameter, x, which describes the position of the O atoms on the 48f site. Therefore, for the structural refinement of LnEuTi2O7 (Ln = Gd and Dy) compounds, Ln was introduced in the 16c site. Finally, positional parameters were refined. Satisfactory reliability factors R were obtained after a few cycles of refinement of zero-point correction, scale factors, background polynomial coefficients, peak width, O (48f) variable position x, isotropic atomic displacement parameters, and the usual pseudo-Voigt profile parameters. The refinement suggests that the site 16c is the only insertion position for Ln3+ ions in LnEuTi2O7 oxides. Indeed, this conclusion is also corroborated by the observation that the ionic radii of Gd3+ (1.053 Å) and Dy3+ (1.027 Å) is slightly different compared to

Eu3+ (1.066 Å) and appreciably larger than that of Ti4+ (0.605 Å) [38]. The output of the refinement of the LnEuTi2O7 oxides obtained are in Table 3. Structural parameters for single-phase Re2Ti2O7 compounds from Rietveld refinements of powder X-ray data are summarized in Table 4. Fig. 9a and b shows the graphic result of the fitting of the X-ray diffraction patterns corresponding to Table 4 Structural parameters for Re2Ti2O7 (Re = Eu, Gd, Dy GdEu and DyEu) compounds from Rietveld refinements of powder X-ray data. Numbers inside the table are cell parameters a of the cubic pyrochlores synthesised. ri = ionic radii of trivalent cations [38]. x(O) = only atomic coordinate variable.

ri (Å) a (Å) x(O)

Eu

Gd

Dy

GdEu

DyEu

1.066 10.207(3) 0.424(5)

1.053 10.186(4) 0.427(4)

1.027 10.131(5) 0.434(7)

– 10.197(6) 0.425(2)

– 10.165(3) 0.431(6)

Fig. 9. (a) Rietveld refinement plot showing the observed and calculated diffraction data and their difference for GdEuTi2O7 mixed oxide obtained by solid state route at 1200 °C. (b) Rietveld refinement plot showing the observed and calculated diffraction data and their difference for DyEuTi2O7 mixed pyrochlore obtained by solid state route at 1200 °C.

50

A. Garbout et al. / Journal of Alloys and Compounds 573 (2013) 43–52

Eu

10,21

Dy GdEu

0,432

cell parameter (Å)

10,19

Gd

DyEu

10,18

0,430 10,17

DyEu

10,16

0,428

Gd

10,15

0,426

10,14 10,13 10,12 1,02

positional parameter x (O)

10,20

0,434

GdEu

Dy 1,03

1,04

1,05

1,06

Eu 0,424 1,07

Re-site cationic radius (Å) Fig. 10. Evolution of cell parameter a (triangles) and of O positional parameter x(O) (diamonds) as a function of lanthanide ionic radius ri [38] throughout the Re2Ti2O7 series.

GdEuTi2O7 and DyEuTi2O7 obtained by solid-state route after a thermal treatment at 1200 °C for 12 h, respectively. We note that refined cubic lattice parameters of 10.197(6) Å and 10.165(3) Å are obtained for pyrochlore structure in GdEuTi2O7 and DyEuTi2O7 powders, respectively. These values

Fig. 11. The two interpenetrating network structure types that together constitute LnEuTi2O7 in projection close to h1 1 0i orientation. The first network (see Fig. 11a) is a (Ti4+)2O6 array of corner-connected, cation-centered ‘octahedra’. The second network (see Fig. 11b) is an O0 ((Ln/Eu)3+)2 array of corner-connected, oxygencentered tetrahedra (of ‘ideal’anti-b-cristobalite structure type).

are intermediate between the observed ones in the compounds (Gd2Ti2O7 (10.186(4) Å), Eu2Ti2O7 (10.207(3) Å)) and (Dy2Ti2O7 (10.131(5) Å) Eu2Ti2O7 (10.207(3) Å)), respectively. A distortion of the cell parameter was clearly evidenced. This confirms the formation of a solid solution, in which various Ln cations (Ln = Gd and Dy) are present in the pyrochlore structure LnEuTi2O7. Table 4 shows an increase of the lattice parameter whith the increase of the relative size of ionic radii of Ln3+ (Gd3+ and Dy3+), that substitute an equal number of Eu3 (1.066 Å) ones in the structure of LnEuTi2O7. Also, the lattice constant of Re2Ti2O7 decreased along with the decrease of ionic radius of Re3+; this was quite consistent with the XRD results. A possible reason for this phenomenon is that the atomic structures of Re affect the crystal structures of Re2Ti2O7 and that when the atomic number of Re increases [45–47]. The corresponding ionic radius decreases from Eu3+, Gd3+, to Dy3+; thus, the lattice parameters of Re2Ti2O7 decrease in the sequence of Eu2Ti2O7 > (EuGd)Ti2O7 > Gd2Ti2O7 > (EuDy)Ti2O7 > Dy2Ti2O7. The evolution of cell parameter a and x(O) (the only atomic coordinate variable) in Re2Ti2O7 pyrochlores as a function of Re3+ ionic radius using average ionic radii for GdEu and DyEu pairs on the Re-site is shown in Fig. 10. The cell parameters a of the new LnEuTi2O7 pyrochlores are compared to those of the corresponding Re2Ti2O7 pyrochlores in Fig. 10. As expected, a increases with

Fig. 12. Cation-pyrochlore coordinations in the crystal structure of LnEuTi2O7 (Ln = Gd and Dy). (a) View of TiO6 trigonal antiprism illustrating the coordination around the smaller cation. (b) (Ln/Eu)3+ coordination spheres in the mixed oxides.

51

A. Garbout et al. / Journal of Alloys and Compounds 573 (2013) 43–52 Table 5 Selected interatomic distances (Å) in Re2Ti2O7 pyrochlores. Eu2Ti2O7

Gd2Ti2O7

Dy2Ti2O7

GdEuTi2O7

DyEuTi2O7

Eu–O = 2.532(4) Eu–O0 = 2.210(4) Ti–O = 1.964(4)

Gd–O = 2.548(9) Gd–O0 = 2.205(9) Ti–O = 1.948(9)

Dy–O = 2.585(6) Dy–O0 = 2.193(6) Ti–O = 1.912(6)

Gd/Eu–O = 2.536(7) Gd/Eu–O0 = 2.208(7) Ti–O = 1.958(7)

Dy/Eu–O = 2.572(3) Dy/Eu–O0 = 2.201(3) Ti–O = 1.929(3)

increasing ri. There is an almost linear relation between lattice constant and ri. That is, lattice constant increases linearly with the increasing of A3+ ionic radius (Table 4). The most important observation of the present investigation is that the 48f oxygen ‘x’ parameter systematically decreased with the increasing of the lattice parameter a. Table 4 indeed shows that x(O) increases in a given series from Eu to Dy, thus compensating the cell parameter decrease.

3.5.2. Structure description-bonding trends X-ray diffraction refinement study of LnEuTi2O7 (Ln = Gd and Dy) provides new insight into the detailed crystal structure, and allows us to examine in detail the local coordination environment of Ln/Eu. The pyrochlore structure of LnEuTi2O7 can be viewed as containing two interpenetrating networks, the first a corner shared Ti2O6 octahedra network and the second an (Ln/Eu)2O0 network similar to the anticristobalite Cu2O network, Fig. 11 [48]. The Ti cations are six coordinated and located within trigonal antiprisms (distorted octahedra) with all six anions equidistant from the central cations (Fig. 12a). Selected interatomic distances in Re2Ti2O7 pyrochlores are listed in Table 5. The Ti–O distance in the mixed oxide LnEuTi2O7 is lower than the observed one in Eu2Ti2O7. The decreasing volume associated with the substitution of larger ion Eu3+ by smaller ion Gd3+or Dy3+ at the A-site has an appreciable influence on the Ti– O distances. The Ti–O values increased with the increasing of the lattice parameter a. The TiO6 octahedra sharing corners to give a Ti2O6 sublattice, which can be considered as the back-bone of the structure. The cage-like holes of this network contain a second sublattice (Ln/ Eu)2O0 , not essential for the stability of the structure. The larger (Ln/Eu) cations in the pyrochlore oxide LnEuTi2O7 (Ln = Gd and Dy) are eight coordinated and located within scalenohedra (distorted cubes) that contain six equally spaced anions (the O anions) and two additional axial anions (the O0 anions) at a slightly shorter distance from the central cations (Fig. 12b) [49]. For the Re2Ti2O7 pyrochlores studied, the cell parameter and both Re–O distances decrease with increasing lanthanide atomic number, in agreement with the decrease in Re ionic radius. The (Gd/Eu)–O distance, 2.536(7) Å, in the mixed oxide GdEuTi2O7 is intermediate between the observed ones in the two compounds Gd2Ti2O7 (2.548(9) Å) and Eu2Ti2O7 (2.532 (4) Å) (Table 5). This value is slightly higher than the observed one in Gd2Ti2O7, in good agreement with the slightly larger ionic radius of Eu3+ (1.066 Å) compared to Gd3+ (1.053 Å). It is clear that in the mixed pyrochlore DyEuTi2O7, the (Dy/Eu)–O distance 2.572(3) Å is more lower than the Eu–O distance 2.532 (4) Å observed in the Eu2Ti2O7 pyrochlore. The O anions occupy the 48f site coordinated to two Ti4+ and two (Ln3+/Eu3+) cations while the O0 anions occupy the 8a site being tetrahedrally coordinated to four (Ln3+/Eu3+) cations. Additionally, there is another anionic tetrahedral site (8b) coordinated to four Ti4+ ions, which is systematically vacant in ordered pyrochlores [20]. Thus, there is a possibility that the oxide-ion can be

transported via a hopping mechanism through the vacancy sites, namely the 8(b) sites [50]. Finally, note that the lattice parameter a increases with increasing r (Re). A similar trend is observed on both Ti–O and Re–O0 distances throughout this series Re2Ti2O7 (see Table 5). This is consistent with the influence of the O atomic position, which varies in the direction opposite to that of the cell parameter [51]. It has been shown that a decrease in x(O) induces an increase in the Ti–O distance, and thus compensates for the cell volume increase with lanthanide cationic radius. 4. Conclusion Solid solutions in the Ln2O3–TiO2–Eu2O3 system (Ln = Gd and Dy) with pyrochlore structure type, were synthesized using a conventional ceramic method. Powder X-ray diffraction (XRD) and Raman spectroscopy were used to determine the crystalline phases present after heat treatment from 1000 to 1200 °C, and it was found that pure pyrochlore-type phases are formed at 1200 °C. It can be observed in the two FT-IR spectra of the mixed oxides GdEuTi2O7 andDyEuTi2O7 that there are four prominent absorption bands with slight variation in the position and intensity of the bands depending on the nature of the rare earth ion. Raman spectroscopy measurements revealed that by comparison with Gd2Ti2O7 and Dy2Ti2O7, a shift of the most intense band towards lower wavenumber is to be noticed when europium is substituted in the mixed pyrochlore LnEuTi2O7 (Ln = Gd and Dy) lattice. The expected compositions GdEuTi2O7 et DyEuTi2O7 were confirmed by energy dispersive spectroscopy (EDAX) and Rietveld refinement. X-ray diffraction reveals a linear relationship between the pyrochlore lattice parameter and the ionic radii of the rare earth. Acknowledgements Prof. Annick Rubbens and Rose-Noëlle Vannier from the Unit of Catalysis and Solid State Chemistry, UMR CNRS 8181, France, are acknowledged for Raman scattering and X-ray diffraction data. References [1] S. Kramer, M. Spears, H.L. Tuller, Solid Oxide Fuel Cells Sympos. Proc. 119 (1993) 93. [2] V.F. Zinchenko, V.D. Kozlov, G.A. Teterin, I.M. Minaev, Inorg. Mater. 25 (3) (1989) 391. [3] T.H. Yu, H.L. Tuller, Ceram. Trans. 65 (1995) 3. [4] S.A. Kramer, H.L. Tuller, Solid State Ion. 82 (1995) 15. [5] B.J. Wuensch, K.W. Eberman, JOM—J. Min. Met. Mater. Soc. 52 (2000) 19. [6] P.K. Moon, H.L. Tuller, Solid State Ion. 470 (1988) 28. [7] R.E. Williford, W.J. Weber, R. Devanathan, J.D. Gale, J. Electroceram. 3 (1999) 409. [8] P.J. Wilde, C.R.A. Catlow, Solid State Ion. 112 (1998) 173. [9] P.J. Wilde, C.R.A. Catlow, Solid State Ion. 112 (1998) 185. [10] J. Christopher, C.S. Swamy, J. Mater. Sci. 26 (18) (1991) 4966. [11] A.M. Sych, S.U. Stefanovich, U.A. Titov, T.N. Bondarenko, V.M. Mel’nik, Inorg. Mater. 27 (12) (1991) 2229. [12] J.H. Lee, Y.M. Chiang, J. Electroceram. 6 (1) (2001) 7. [13] R.J. Cava, J. Mater. Chem. 11 (1) (2001) 54. [14] J.P. Mercurio, A. Lambachri, B. Frit, Sci. Ceram. Sympos. Proc. 14 (1988) 967. [15] O. Ait Sidi Ahmed, A. Tairi, A. Chagraoui, S. Khairoun, J.C. ChamparnaudMesjard, B. Frit, Annl. Chim. Sci. Mater. 25 (2000) 201.

52 [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]

[29] [30] [31] [32] [33] [34]

A. Garbout et al. / Journal of Alloys and Compounds 573 (2013) 43–52 M.K. Ehlert, J.E. Greedan, M. Subramanian, J. Solid State Chem. 75 (1988) 188. Y. Piffard, M. Dion, M. Tournoux, Acta Crystallogr. B 34 (1978) 366. B.J. Ismunandar, B.A. Kennedy, J. Hunter, Solid State Chem. 130 (1997) 81. D. Ismunandar, T. Kamiyama, K. Oikawa, A. Hoshikawa, B.J. Kennedy, Y. Kubota, K. Kato, Mater. Res. Bull. 39 (2004) 553. K.J. Moreno, A.F. Fuentes, M. Maczka, J. Hanuza, U. Amador, J. Solid State Chem. 179 (2006) 3805. A. Bystrom, Ark. Kemi, Mineral. Geol. 18A (1945) 1. W.R. Cook, H. Jaffe, Phys. Rev. 89 (1953) 1297. M.A. Subramanian, G. Aravamudan, G.V.S. Rao, Prog. Solid State Chem. 15 (1983) 55. L. Minervini, R.W. Grimes, J. Am. Ceram. Soc. 83 (2000) 1873. A. Zhang, M. Lu, Z. Yang, G. Zhou, Y. Zhou, Solid State Sci. 10 (2008) 74. F.W.B. Lopes, C.P. Souza, A.M.V. Morais, J.-P. Dallas, J.-R. Gavarri, Hydrometallurgy 97 (2009) 167. H.M. Rietveld, J. Appl. Crystallogr. 2 (1969) 65. J. Rodriguez-Carvajal, FULLPROF program for Rietveld refinement and pattern matching analysis, Version 3.5d, LLB-JRC, Laboratoire Leon Brillouin (CEACNRS), October 1998. G. Busca, G. Ramis, J.M.G. Amores, V.S. Escribano, P. Piaggio, J. Chem. Soc., Faraday Trans. 90 (1994) 3181. F.X. Zhang, S.K. Saxena, Chem. Phys. Lett. 413 (2005) 248. A.F. Fuentes, K. Boulahya, M. Maczka, J. Hanuza, U. Amador, Solid State Sci. 7 (2005) 343. Z. Ying, D. Linghong, P. Xinling, Z. Weifeng, J. Rare Earths 27 (2009) 900. M. Saif, J. Photochem. Photobiol. A: Chem. 205 (2009) 145. G. Jose, K.A. Amrutha, T.F. Toney, V. Thomas, C. Joseph, M.A. Ittyachen, N.V. Unnikrishnan, Mater. Chem. Phys. 96 (2006) 381.

[35] L.V. Hong, N.T.H. Le, N.C. Thuan, N.D. Thanh, N.X. Nghia, N.X. Phuc, J. Raman Spectrosc. 36 (2005) 946. [36] Y.I. Kim, S.H. Nahm, D.J. Yoon, D.H. Gregory, J. Electroceram. 14 (2005) 265. [37] A. Garbout, A. Rubbens, R.N. Vannier, S. Bouattour, A.W. Kolsi, J. Raman Spectrosc. 39 (2008) 1469. [38] R.D. Shannon, Acta Crystallogr. A 32 (1976) 751. [39] J.A. Alonso, E. Mzayek, I. Rasines, M. Ventanilla, Inorg. Chim. Acta 140 (1987) 145. [40] H. Du, X. Yao, J. Electroceram. 9 (2002) 117. [41] H. Du, X. Yao, L. Zhang, Ceram. Int. 28 (2002) 231. [42] M. Chen, D.B. Tanner, J.C. Nino, Phys. Rev. B 72 (2005) 054303. [43] P. Prabhakar Rao, S.J. Liji, K. Ravindaran Nair, Peter Koshy, Mater. Lett. 58 (2004) 1924. [44] Y. Xuan, R. Liu, Y.Q. Jia, Mater. Chem. Phys. 53 (1998) 256. [45] V.V. Nemoshkalenko, V.N. Uvarov, S.V. Borisenko, A.I. Senkevich, T.N. Bondarenko, J. Electron Spectrosc. Relat. Phenom. 385 (1998) 88. [46] L.L. Zhang, J. Yang, W.G. Zhang, L.D. Lu, X.J. Yang, X. Wang, J. Solid State Chem. 178 (2005) 761. [47] W. Zhang, L. Zhang, H. Zhong, L. Lu, X. Yang, X. Wang, Mater. Carac. 61 (2010) 154. [48] Y. Liu, R.L. Withers, L. Norén, J. Solid State Chem. 177 (2004) 4404. [49] A. Mergen, W.E. Lee, Mater. Res. Bull. 32 (1997) 175. [50] M. Mori, G.M. Tompsett, N.M. Sammes, E. Suda, Y. Takeda, Solid State Ion. 158 (2003) 79. [51] S. Zouari, R. Ballou, A. Cheikh-Rouhou, P. Strobel, Mater. Lett. 62 (2008) 3767.