Materials Chemistry and Physics 117 (2009) 192–198

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Photoluminescence behavior in MgTiO3 powders with vacancy/distorted clusters and octahedral tilting E.A.V. Ferri a , J.C. Sczancoski a , L.S. Cavalcante a,∗ , E.C. Paris a , J.W.M. Espinosa b , A.T. de Figueiredo b , P.S. Pizani a , V.R. Mastelaro c , J.A. Varela b , E. Longo b a b c

Universidade Federal de São Carlos, C.P. 676, 13565-905, São Carlos, SP, Brazil LIEC-IQ, Universidade Estadual Paulista, C.P. 355, 14801-970, Araraquara, SP, Brazil IFSC-USP, P.O. Box 369, 13560-970, São Carlos, SP, Brazil

a r t i c l e

i n f o

Article history: Received 4 January 2009 Received in revised form 14 May 2009 Accepted 18 May 2009 Keywords: A. Optical materials B. Chemical synthesis C. XAFS D. Luminescence

a b s t r a c t MgTiO3 powders were prepared by the complex polymerization method and heat treated at different temperatures for 2 h. These powders were analyzed by X-ray diffraction (XRD), Rietveld refinements, micro-Raman (MR) spectroscopy, X-ray absorption near edge spectroscopy (XANES), ultraviolet–visible (UV–vis) absorption spectroscopy and photoluminescence (PL) measurements. XRD patterns and MR spectra showed that the crystalline powders have a rhombohedral structure. XANES spectra indicated that the local structure of crystalline MgTiO3 powders is composed only by [TiO6 ] clusters, while the disordered exhibit the simultaneous presence of both [TiO5 ] and [TiO6 ] complex clusters into the lattice. UV–vis spectra revealed different optical band gap values as a function of heat treatment temperature. This result can be an indicative of intermediary energy levels within the band gap of this material because of structural defects into the perovskite-type structure. PL behavior was attributed to the structural order–disorder and/or distortions on the [TiO6 ]–[TiO6 ] complex clusters. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The magnesium titanate (MgTiO3 ) is a ceramic oxide characterized by an ilmenite structure with space group R3¯ [1,2]. In the last years, this material has been investigated and employed as ceramic capacitors and resonators because of its low dielectric loss and high thermal stability at high frequencies [3,4]. Moreover, the MgTiO3 has a technological potential for applications in filters, antennas for communication, radar, direct broadcasting satellite and global positioning system operating at microwave frequencies [5,6]. Different synthesis methods have been employed for the formation of this material, such as: solid-state reaction [7–10], mechanochemical activation [11–13] and thermal decomposition [14]. However, these methods present some drawbacks, such as: high heat treatment temperatures, long processing times, contamination by impurities, nonuniform particle size distribution and irregular morphologies. These problems can be minimized by means of wet chemical methods, mainly including: co-precipitation [15,16], metalorganic chemical vapor [17,18],

∗ Corresponding author. Tel: +55 16 3361 5215/9176 49 43 (mobile). E-mail address: [email protected] (L.S. Cavalcante). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.05.042

sol–gel [19–25] and semi-alkoxide [26]. In particular, the complex polymerization method has become a versatile synthesis method due to use low heat treatment temperatures and to reduce the phase segregation. These advantages allow a better cation distribution into the polymeric resin, ensuring a complete chemical homogeneity at molecular scale into this system [27–29]. Currently, it is well-known that the pure or doped MgTiO3 phase presents interesting microwave dielectric properties [30–32]. On the other hand, the literature has reported few works on the photoluminescence (PL) properties of this ceramic oxide. Recently, Kang et al. [33] observed three sharp emission peaks (440, 416 and 461 nm) on the PL spectra of nanocrystalline MgTiO3 powders synthesized by the stearic acid gel method. However, these authors reported a limited information on the origin of this optical property in this material. In this work, MgTiO3 powders were prepared by the complex polymerization method and heat treated at different temperatures for 2 h under air atmosphere. These powders were structurally characterized by X-ray diffraction (XRD), Rietveld refinements, micro-Raman (MR) spectroscopy and X-ray absorption near edge spectroscopy (XANES). The optical properties were analyzed by ultraviolet–visible (UV–vis) absorption spectroscopy and photoluminescence (PL) measurements.

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2. Experimental details 2.1. Synthesis of MgTiO3 powders MgTiO3 powders were synthesized by the complex polymerization method. In this synthesis, magnesium acetate (CH3 CO2 )2 Mg (98% Aldrich), titanium butoxide Ti[O(CH2 )3 CH3 ]4 (99% Aldrich), ethylene glycol C2 H6 O2 (99.5% Synth) and citric acid C6 H8 O7 (99.5% Synth) were used as raw materials. Initially, C6 H8 O7 was dissolved in deionized water heated at 75 ◦ C under constant stirring. Afterwards, Ti[O(CH2 )3 CH3 ]4 was quickly added into this citric acid aqueous solution to avoid hydrolysis reaction between alkoxide and air environment. This system was kept at 90 ◦ C under constant stirring up to the formation of a clear and homogenous titanium citrate solution. The gravimetric procedure was performed for the correction and determination of the stoichiometric value correspondent to the mass (g) of titanium oxide contained into this citrate. In the sequence, stoichiometric quantities of (CH3 CO2 )2 Mg were dissolved into the titanium citrate solution. After homogenization, C2 H6 O2 was added into this solution heated at 120 ◦ C in order to promote the citrate polymerization by the polyesterification reaction. The formed polymeric resin was then placed into a conventional furnace and heat treated at 350 ◦ C for 4 h to promote the organic compounds decomposition of arising from C6 H8 O7 and C2 H6 O2 . Finally, the obtained precursors were heat treated at different temperatures in the range from 450 ◦ C to 700 ◦ C for 2 h under air atmosphere. 2.2. Characterizations of MgTiO3 powders MgTiO3 powders were structurally characterized by XRD using a DMax/2500PC diffractometer (Rigaku, Japan) with Cu K ␣ radiation ( = 1.5406 Å) in the 2 range −1 from 20 ◦ to 75◦ with angular step of 0.02◦ min . The Rietveld routine was performed −1 in the 2  range from 10 ◦ to 110◦ through an angular step of 0.02◦ min and exposure time of 2 s. MR measurements were performed using a T64000 spectrometer (JobinYvon, France) triple monochromator coupled to a CCD detector. The spectra were obtained using a 514.5 nm wavelength of an argon ion laser, keeping its maximum output power at 8 mW. A 100 ␮m lens was used to prevent sample overheating. The titanium XANES spectra were recorded on the D04B-XAS1 line, using ring energy of 1.36 GeV and storage current of 160 mA. These spectra were carried out on the Ti Kedge (4966 eV) in transmission mode, using a Si(1 1 1) channel-cut monochromator. UV–vis spectra were taken with a Cary 5G (Varian, USA) spectrometer in total reflection mode. PL measurements were performed through a U1000 (Jobin-Yvon, France) double monochromator coupled to a cooled GaAs photomultiplier with a conventional photon counting system. The 488 nm excitation wavelength of an argon ion laser was used, keeping its maximum output power at 30 mW. Cylindrical lenses were used to avoid sample overheating. The slit width utilized was 300 ␮m. All measurements were performed at room temperature.

3. Results and discussion 3.1. X-ray diffraction and Rietveld refinement analyses Fig. 1 shows the XRD patterns of MgTiO3 powders heat treated at different temperatures for 2 h under air atmosphere. According to the literature [29], the XRD patterns are able to predict the structural order–disorder at long-range of a material.

Fig. 2. Rietveld refinement of MgTiO3 powders heat treated at 700 ◦ C for 2 h.

On this assumption, our results indicate that the powders heat treated from 450 ◦ C to 550 ◦ C present structural order–disorder at long-range caused by phase transformation (Fig. 1(a)–(c)). It is due to these powders exhibit an incomplete structural organization under these temperature conditions. However, the strong and sharp diffraction peaks indicate that the MgTiO3 powders heat treated at 700 ◦ C are highly crystallized and structurally ordered at long-range. In these crystalline powders, all diffraction peaks can be indexed to the rhombohedral structure in agreement with the respective joint committee on powder diffraction standards card No. 06-0494 and with those previously reported in the literature [34,35](Fig. 1(d)). Fig. 2 shows the Rietveld refinement of MgTiO3 powders heat treated at 700 ◦ C for 2 h under air atmosphere. The Rietveld refinement [36] was performed through the GSAS program [37]. The diffraction peak profiles were better adjusted by the Thompson-Cox-Hastings pseudo-Voigt (pV-TCH) function and by an asymmetry function as described by Finger et al. [38]. The strain anisotropy broadening was corrected by the phenomenological model described by Stephens [39]. The obtained results from Rietveld refinement are displayed in Table 1. In this table, the fitting parameters (RBragg , Rp , Rwp and 2 ) indicate a good agreement between the calculated and observed X-ray patterns for the MgTiO3 phase. Moreover, the lattice parameters (a = b = 5.05831 Å and c = 13.90858 Å) and bond angles with medium values (˛ = ˇ = 90◦ and  = 120◦ ) estimated from the refinement confirmed the rhombohedral structure for this material. These Rietveld refinement results are in agreement with the reported by Kim et al. [40]. 3.2. Representation of MgTiO3 unit cell Fig. 3 shows the schematic representation of a MgTiO3 unit cell ¯ This unit cell was modeled using the 1×1×1 with space group R3. Java Structure Viewer Program (Version 1.08lite for Windows) and VRML-View (Version 3.0 for Windows) [41,42] by means of the atomic coordinates listed in Table 1. Table 1 Rietveld refinement results and atomic coordinates employed in order to model the MgTiO3 unit cell.

Fig. 1. XRD patterns of MgTiO3 powders heat treated at different temperatures: (a) 450 ◦ C, (b) 500 ◦ C, (c) 550 ◦ C and (d) 700 ◦ C for 2 h under air atmosphere.

Atoms

Site

x

y

z

Magnesium Titanium Oxygen

6c 6c 18f

0.0 0.0 0.31669

0.0 0.0 0.01764

0.35553 0.14467 0.24761

RBragg = 3.54, Rp = 6.95, Rwp = 9.28, and 2 = 2.63.

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Fig. 3. Schematic representation of a MgTiO3 1 × 1 × 1 unit cell with rhombohedral structure. Table 2 Atomic coordinates (x, y and z), bond distances and bond angles (ε, , , ı, ˛ and ˇ) between O–Ti–O and O–Mg–O. Atomic coordinates titanium and magnesium (x; y; z)

(0.0; 0.0; 0.14467)

(0.0; 0.0; 0.35553)

Bonds (Ti–O) (Mg–O)

Bond distance (Å)

Bonds [O–Ti–O] [O–Mg–O]

Bond angles (◦ )

Ti–O1 Ti–O2 Ti–O3 Ti–O4 Ti–O5 Ti–O6

1.8786(1) 1.8786(1) 2.1168(4) 2.1168(4) 1.8786(1) 2.1168(4)

O1 –Ti–O2 O2 –Ti–O3 O3 –Ti–O4 O1 –Ti–O4 O5 –Ti–O6 O6 –Ti–O5

ε = 102  = 93  = 79 ı = 86 ˛ = 162 ˇ = 198

Mg–O1 Mg–O2 Mg–O3 Mg–O4 Mg–O5 Mg–O6

2.0221(1) 2.0221(1) 2.1643(1) 2.1643(1) 2.0221(1) 2.1643(1)

O1 –Mg–O2 O2 –Mg–O3 O3 –Mg–O4 O1 –Mg–O4 O5 –Mg–O6 O6 –Mg–O5

ε = 103  = 89  = 79 ı = 89 ˛ = 162 ˇ = 198

In MgTiO3 unit cell, the Mg and Ti atoms are coordinated to six oxygens ([MgO6 ] and [TiO6 ] complex clusters), resulting in a polyhedron-type with octahedral configuration. Both complex clusters are slightly distorted into the unit cell, presenting up to six different bond angles (ε, , , ı, ˛ and ˇ) between O–X–O (X = Mg, Ti) bonds. On the other hand, these [MgO6 ] and [TiO6 ] complex clusters are bonded each other, forming tilt angles of approximately 128.42◦ , 120.13◦ , 86.46◦ and 134.51◦ (Fig. 3). In materials with perovskite-type structure, the octahedral tilting distortion is associated with the Goldschmidt tolerance factor. In principle, a low value is an indicative of available spaces for tilt into the structure [43]. In addition, the MgTiO3 structure is characterized by a character ionic between the Mg–O bonds, while the Ti–O bonds present a covalent nature. As shown in Fig. 3, just two coordinations (MgO6 and TiO6 ) were highlighted into the unit cell (Fig. 3). The bond distances and angles between O–X–O (X = Mg, Ti) are listed in Table 2.

gesting an ordered structure at short-range. The Ag modes situated at 229.9 cm−1 and 310 cm−1 are arising from the vibrations of Mg and Ti atoms along the z-axis. The others Ag modes observed at 400.2 cm−1 , 501.2 cm−1 and 716.4 cm−1 are ascribed to the vibra-

3.3. Micro-Raman analyses Fig. 4 shows the MR spectra of MgTiO3 powders heat treated at different temperatures for 2 h under air atmosphere. As it can be seen in this figure, the MgTiO3 powders heat treated at 700 ◦ C for 2 h presented all ten Raman-active modes (5Ag + 5Bg ) as theoretically predicted in the literature [44–46], sug-

Fig. 4. MR spectra of MgTiO3 powders heat treated at different temperatures: (a) 450 ◦ C, (b) 500 ◦ C, (c) 550 ◦ C and (d) 700 ◦ C for 2 h under air atmosphere. Inset shows the distorted [TiO6 ] cluster.

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tions of O atoms. Particularly, for the modes at 501.2 cm−1 and 716.4 cm−1 the six O atoms present breathing-like vibrations, but each with different vibration directions into the octahedral configuration. The intense Eg mode verified at 285.8 cm−1 is related to the anti-symmetric breathing vibration of the O octahedron. The Eg modes located at 330.9 cm−1 and 356.5 cm−1 can be described as the twisting of the O octahedron with the vibrations of Mg and Ti atoms parallel to the xy-plane. The Eg modes at 488.9 cm−1 and 643.9 cm−1 are due to the anti-symmetric breathing and twisting vibrations of the O octahedra with the cationic vibrations parallel to the xy-plane. For the mode at 488.9 cm−1 , both Mg and Ti atoms are involved in the vibration, while that the mode at 643.9 cm−1 is associated to the Ti–O stretch [46]. MgTiO3 powder heat treated at 450 ◦ C for 2 h has not active Raman modes (Fig. 4(a)). Well-defined Raman-active modes for the powders heat treated at 500 ◦ C and 550 ◦ C were not observed, indicating the presence of structural order–disorder at short-range (Fig. 4(b) and (c)). The horizontal arrows in Fig. 4(b) indicate the overlapping of two small bands, suggesting the formation of [TiO6 ]–V•• O –[TiO5 ] complex clusters into the structure. This behavior can be caused by an incomplete structural organization of MgTiO3 phase due to the presence of defects (oxygen vacancies = V•• O ) into the lattice [29]. The heat treatment performed at 550 ◦ C resulted in the presence of only a band on the Raman spectrum, as shown by the horizontal arrows in Fig. 4(c). It is an indicative that this temperature decreased the concentration of oxygen vacancies into the lattice, resulting in a more ordered structure than at 450 ◦ C. 3.4. X-ray absorption near edge structure analyses Fig. 5 (a) and (b) shows the Ti K-edge XANES spectra of MgTiO3 powders heat treated at different temperatures for 2 h under air atmosphere. In these spectra, it was observed a small peak situated at around 4971 eV (), which is ascribed to the 1s–3d electronic transition [47,48]. This forbidden electronic transition is normally allowed because the mixture of oxygen 2p states and empty titanium 3d states [49]. An increase in the intensity of this peak () indicates that the local environment of Ti is noncentrosymmetric or off-center displaced into the octahedral configuration [50]. Farges et al. [51] reported that the intensity and energy of this peak in titanates can be classified into three distinct groups, which are dependent of the coordination number of Ti atoms (4, 5 or 6) with oxygens. We believe that all interpretations reported in the literature for the XANES spectra are well consistent; however, our results were explained by another way. In Fig. 5(a) and (b), it was observed that the XANES spectrum of MgTiO3 powders heat treated at 700 ◦ C exhibited a significant difference (small shoulder) in the absorption edge in relation to the other powders. Frenkel et al. [52] calculated the area of this peak in amorphous and quasi-amorphous titanates and showed its shifting toward lower energies in the XANES spectra. According to these authors, the existence of this displacement and the decreasing in the intensity of this peak can be associated to the bond length disorder due to the Ti off-center displacement. In our work, we attributed to the amorphous titanates with the disordered MgTiO3 phase, the quasi-amorphous titanates to the powders with structural order–disorder and crystalline titanates with the structurally ordered MgTiO3 phase. Thus, it was evaluated the area on each of the peaks (located at around 4971 eV) of MgTiO3 powders heat treated from 450 ◦ C to 700 ◦ C for 2 h, in order to estimate the percentage of [TiO5 ] and [TiO6 ] complex clusters into the lattice (Fig. 5(b)). In Fig. 5(b), it was verified that the area of this peak increases as a function of heat treatment temperature, i.e., this behavior suggests an increase in the concentration of sixfold coordinations (distorted TiO6 octahedrons) into the MgTiO3

195

structure. Fig. 5(c) shows the calculated area as well as the percentage of [TiO5 ] and [TiO6 ] complex clusters obtained on each of the peaks also the respective ordered and ordered–disordered MgTiO3 structures. As observed in the XRD and MR results (Figs. 1 and 4), the powders heat treated at 700 ◦ C for 2 h presented a high structural organization level at long and short-range. Therefore, we consider that these powders have the highest percentage of [TiO6 ] clusters into the lattice. This interpretation was based on the previous theoretical studies of titanates structurally disordered reported by our research group [53,54]. As it can be seen in Fig. 5(d), the increase of heat treatment temperature is able to increase the degree of structural order into the lattice. Consequently, this behavior is accompanied by a progressive reduction in the percentage of [TiO5 ] clusters on these titanates due to a transformation from [TiO5 ]–[TiO6 ] to [TiO6 ]–[TiO6 ] complex clusters into this system. The proposed equations in order to explain these cluster models were based on the Kröger–Vink notation by means of complex clusters [55,56]. For the network former: [TiO6 ]x + [TiO5 · VxO ] → [TiO6 ] + [TiO5 · VO• ] [TiO6 ]

x

+ [TiO5 · VO•

[TiO5 · V•• O]+

] → [TiO6 ]



+ [TiO5 · V•• O]

1 O2 → [TiO6 ] 2

(1) (2) (3)

Also, this model can be extended for the network modifiers: [MgO6 ]x + [MgO5 · VxO ] → [MgO6 ] + [MgO5 · VO• ] x



[MgO6 ] + [MgO5 · VO• ] → [MgO6 ] + [MgO5 · V•• O] [MgO5 · V•• O]+

1 O2 → [MgO6 ] 2

(4) (5) (6)

The local defects into the rhombohedral structure caused by the [TiO5 ·VO z ] complex clusters are associated to the oxygen vacancies, which can present up to three different charge states: (I) neutral (VO x )—capture up to two electrons, (II) singly ionized (VO • )—capture only one electron and (III) double ionized (V•• O )—it is not able to trap electrons. These species play an important role in  the formation of hole–electron (h • –e ) pairs, resulting in a charge gradient in the lattice. 3.5. Ultraviolet–visible absorption spectroscopy analyses Fig. 6 show the UV–vis absorbance spectra of MgTiO3 powders heat treated at different temperatures for 2 h under air atmosphere. The optical band gap energy [(Egap )] was estimated by the method proposed by Wood and Tauc [57]. According to these authors the optical band gap is associated with absorbance and photon energy by the following equation: n

h ˛ ∝ (h − Egap ) ,

(7)

where ˛ is the absorbance, h is the Planck constant, is the frequency, Egap is the optical band gap and n is a constant associated to the different types of electronic transitions (n = 1/2, 2, 3/2 or 3 for direct allowed, indirect allowed, direct forbidden and indirect forbidden transitions, respectively). According to literature [53,58], the disordered titanates are characterized by an indirect allowed electronic transition and hence, the n = 2 value it was adopted as standard in Eq. (7). Thus, the Egap values were evaluated extrapolating the linear portion of the curve or tail when [y = 0] in the UV–vis absorbance spectra. In disordered MgTiO3 powders heat treated from 450 ◦ C to 550 ◦ C, the absorbance measurements suggest an optical band gap behavior of materials with structural defects (Fig. 6(a)–(c)), while the powders heat treated at 700 ◦ C present a typical absorption spectra of ordered or crystalline materials (Fig. 6(d)). The obtained

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Fig. 5. (a) XANES spectra in the range from 4960 eV to 5100 eV of MgTiO3 powders heat treated at different temperatures for 2 h under air atmosphere; (b) Ti K-edge XANES spectra in the range from 4965 eV to 4975 eV. Inset shows the area of pre-edge peaks as a function of heat treatment temperature; (c) calculated area by means of integration of the XANES spectra in the range from 4967 eV to 4974 eV for of MgTiO3 powders heat treated at different temperatures for 2 h and (d) integration of XANES spectra in percentage for [TiO5 ] and [TiO6 ] clusters and the insets show the distorted [TiO6 ]–[TiO6 ] octahedral cluster for the ordered structure and distorted [TiO5 ·VOz ]–[TiO6 ] octahedral cluster with order–disorder structural.

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Fig. 6. UV–vis absorbance spectra of MgTiO3 powders heat treated at different temperatures: (a) 450 ◦ C, (b) 500 ◦ C, (c) 550 ◦ C and (d) 700 ◦ C for 2 h under air atmosphere.

results indicate that the increase in Egap values can be associated to a reduction of intermediary energy levels within the optical band gap. Probably, these reductions are caused by the increase of structural organization into the lattice due to the increase of heat treatment temperature.

XANES spectrum showed that this structural order–disorder is caused by a high concentration of [TiO5 ] complex clusters into the lattice (Fig. 5(d)). In the last years, our group has used first principle calculations to show that in disordered materials with perovskite-

3.6. Photoluminescence behavior analyses According to the literature [59], PL is a powerful tool for investigating the energy levels of materials that are invisible to absorption measurements. Due to its sensitivity of this technique, can be combining with UV–vis absorbance measurements to understand the intermediary energy level distribution within the band gap and evolution the degree of structural order–disorder in the lattice. Fig. 7 illustrates the PL spectra at room temperature of MgTiO3 powders heat treated at different temperatures for 2 h under air atmosphere. An analysis of the PL spectra indicated that the MgTiO3 powders heat treated at 450 ◦ C for 2 h exhibited the highest intensity with a maximum situated at around 585 nm (yellow emission) (Fig. 7(a)). As observed through the UV–vis measurements, these powders presented the lowest Egap values (3.07 eV), suggesting the presence of intermediary energy levels within the band gap. In this case, these energy levels are arising from the structural order–disorder at long and short-range, in agreement with the XRD and MR results (Figs. 1(a) and 4(a)). As previously reported in the literature, the

Fig. 7. PL spectra of MgTiO3 powders heat treated at different temperature: (a) 450 ◦ C, (b) 500 ◦ C, (c) 550 ◦ C and (d) 700 ◦ C for 2 h under air atmosphere. Inset shows the [TiO5 ]–[TiO6 ] and [TiO6 ]–[TiO6 ] complex clusters.

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type structure occur local symmetry breaks favoring the trapping of e and h• , resulting in the polarization and/or charge gradient into the lattice [60,61]. On the basis of this assumption, we believe that this same mechanism can be the key factor on the PL response of disordered MgTiO3 powders. Also, the intrinsic distortion between the O–X–O (X = Mg, Ti) bonds of [MgO6 ] or [TiO6 ] clusters as well as the octahedral tilting observed in the MgTiO3 structure can be two fundamental variables for the understanding the PL emission process of these titanates. Fig. 7(b) and (c) shows that the increase of heat treatment temperature resulted in a considerable reduction on the PL emission intensity of these powders. This behavior can be associated to the decrease of intermediary energy levels within the band gap, since the Egap values increase with the structural organization level at long and short-range (Figs. 1 and 4). Hence, the quantity of [TiO5 ] clusters is reduced significantly into the lattice, in agreement with the XANES results (Fig. 5). In Fig. 6(d), it was not verified PL emission for the MgTiO3 powders heat treated at 700 ◦ C for 2 h. As shown by the XRD and MR measurements (Figs. 1 and 4), these crystalline powders are highly ordered at long and short-range. This behavior suggests that the maxima structural organization leads to the formation of only [TiO6 ] clusters into the lattice, drastically reducing the presence of intermediary energy levels within the band gap. Thus, the wavelength’s energy (2.54 eV) is not enough to promote a band to band electronic transition in these powders and consequently, the PL emission process is not observed (Fig. 7(d)). 4. Conclusions In summary, MgTiO3 powders were prepared by the complex polymerization method and heat treated at different temperatures for 2 h under air atmosphere. XRD patterns, Rietveld refinements and MR spectra confirmed that the powders heat treated at 700 ◦ C crystallize in a rhombohedral structure without the presence of secondary phases. Also, the XRD patterns and MR spectra showed that the powders heat treated from 450 ◦ C to 550 ◦ C present a high degree of structural order–disorder at long and short-range. XANES spectra indicated that the increase of heat treatment temperature favors a transformation from [TiO5 ]–[TiO6 ] to [TiO6 ]–[TiO6 ] complex clusters into the structure. UV–vis absorption spectra showed that the increase of optical band gap values is caused by a reduction of intermediary energy levels within the band gap. PL spectra indicated that the MgTiO3 powders heat treated at 450 ◦ C for 2 h exhibited the highest intensity, which was mainly attributed to the high degree of structural order–disorder into the lattice. The reduction of PL emission with the increase of heat treatment temperature was related with the decrease of intermediary energy levels within the band gap, as consequence of a high concentration of [TiO6 ]–[TiO6 ] clusters into the lattice. Acknowledgements The authors thank the financial support of the Brazilian research financing institutions: CAPES, CNPq, FAPESP and LNLS (Projeto No. D04B - XAFS1 - 8050). References [1] T. Okada, T. Narita, T. Nagai, T. Yamanaka, Am. Mineral. 93 (2008) 39. [2] J.A. Linton, Y. Fei, A. Navrotsky, Am. Mineral. 84 (1999) 1595. [3] D.C. Woo, H.Y. Lee, J.J. Kim, T.H. Kim, S.J. Lee, J.R. Park, T.G. Choy, IEEE International Symposium on Applications of Ferroelectrics, vol. 8, 1996, p. 863. [4] X. Zhou, Y. Yuan, L. Xiang, Y. Huang, J. Mater. Sci. 42 (2007) 6628. [5] V.M. Ferreira, J.L. Baptista, J. Petzelt, G.A. Komandin, V.V. Voitsekhovskii, J. Mater. Res. 10 (1995) 2301. [6] H. Jantunen, R. Rautioaho, A. Uusimki, S. Leppvuori, J. Eur. Ceram. Soc. 20 (2000) 2331. [7] H.T. Kimy, Y.H. Kim, J.D. Byun, J. Korean Phys. Soc. 32 (1998) 159.

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