1 Chemical Physics Letters 398 (2004) A DFT rationalization of the room temperature photoluminescence of Li 2 TiSiO 5 E. Orhan a,b, *, V.C. Albarici b, M.T. Escote b, M.A.C. Machado c, P.S. Pizani c, E.R. Leite b, J.R. Sambrano d, J.A. Varela a, E. Longo b a Instituto de Química, Universidade Estadual Paulista, Araraquara, SP, Brazil b Departamento de Química, Universidade Federal de São Carlos, Via Washington Luiz, Km. 235, Caixa Postal 676, São Carlos, SP, Brazil c Departamento de Física, Universidade Federal de São Carlos, P.O. Box 676, São Carlos, SP, Brazil d Laboratório de Simulação Molecular, DM, Universidade Estadual Paulista, P.O. Box 473, Bauru, SP, Brazil Received 31 July 2004; in final form 16 September 2004 Available online 7 October 2004 Abstract Li 2 TiSiO 5 powders were synthesized by the polymeric precursor method. The calcination temperatures were progressively increased until the complete crystallization of the phase occurring at 870 C. For the first time, a strong photoluminescence was measured at room temperature with a 488 nm excitation wavelength for the non-crystalline samples. This photoluminescence in disordered phases has been interpreted by means of high-level quantum mechanical calculations based on density functional theory. Two periodic models have been used to represent the crystalline and disordered powders. They allowed to calculate electronic properties consistent with experimental data and to explain the relations between photoluminescence and structural disorder. Ó 2004 Elsevier B.V. All rights reserved. 1. Introduction In the last years, the interest in development of new photoluminescent (PL) materials with several potential applications has rapidly grown. Our group has reported that the PL phenomena are related to the structural disorder [1 3]. In particular, Leite and co-workers [2,4,5] have found an intense visible photoluminescence at room temperature in amorphous ATiO 3 compounds prepared by sol gel method. It has been shown by XANES [6,7] studies that the peculiarity of those compounds is the coexistence of two types of Ti coordination, fivefold and sixfold. The higher the heat treatment temperature, the rarer the pentacoordination. Bouma and Blasse  presented the dependence of luminescence of A 2 TiOBO 4 (A = Li, Na; B = Si, Ge) * Corresponding author. address: (E. Orhan). titanates on their crystal structure. These authors have reported that titanates where exist TiO 6 clusters containing a short Ti O bond, show efficient PL at room temperature if these octahedra are isolated from each other, and ascribed this effect to a broadening of energy bands. In particular, the compounds with A = Li (Li 2 Ti- BO 5 ) are strongly luminescent when excited by a 250 nm wavelength. In this Letter, we present outstanding measurements of a broad PL band at room temperature as a function of heat treatment of tetragonal lithium-titanosilicate, Li 2 TiSiO 5  and ab initio periodic quantum-mechanical calculations based on density functional theory (DFT) to investigate the origin of this PL in function of structural parameters and electronic properties. Li 2 TiSiO 5 has never been the object of a theoretical investigation up to now, but we insist in the necessity of confronting it with experimental results for achieving a far more complete investigation. The aim of our syn /$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi: /j.cplett
2 E. Orhan et al. / Chemical Physics Letters 398 (2004) ergetic strategy between experimental results and electronic structure is not to explain how PL occurs, as many valid hypothesis already exist [10,11], but to explain why it occurs at room temperature of PL in the non-crystalline powders. 2. Experimental procedure The soft chemical processing used to synthesize the Li 2 TiSiO 5 powders, polymeric precursor method [12,13], is based on the formation of soluble coordination compounds followed by a polymerization. Titanium isopropoxide, tetraethylortosilicate (TEOS) and lithium nitrate were dissolved in an alcoholic solution of citric acid (CA). After homogenization of the metallic precursors, ethylene glycol (EG) was added to promote the polymerization by polyesterification reaction between heterometallic CA complex and EG. The CA/ EG ratio was fixed at 60/40 (mass ratio) and the molar ratio between the lithium, titanium and silicon cations was 2:1:1. After polymerization at 90 C/1 h, the formed polymeric precursor was calcinated at 350 C/1 h to promote pre-pyrolysis. The heat treatment was carried out at 500, 600 and 700 C for 4 h, and at 870 C for 24 h at a heating rate of 5 C/min. The powders obtained were structurally characterized by X-ray powder diffraction (XRD) (Cu Ka radiation). The diffraction patterns were performed on a Rigaku D/Max-2400 diffractometer using the Cu Ka radiation. Typical 2h angular scans ranging from 20 to 60 in steps varying of 0.02 were used. The room temperature PL spectra of Li 2 TiSiO 5 powders were taken with a U1000 Jobin Yvon double monochromator coupled to a cooled GaAs photomultiplier and a conventional photon counting system. The exciting wavelength of an argon ion laser was used, with the laserõs maximum output power kept at 20 MW. A cylindrical lens was used to prevent the sample from overheating. The slit width used was 100 lm. 3. Computational method and models Periodic ab initio quantum-mechanical calculations performed with the B3LYP hybrid functional [14,15] have been carried out by means of the CRYSTAL98 computer code . This functional has already been successfully employed in studies on the electronic and structural properties of the bulk and surfaces of TiO 2 , PbTiO 3  and Ba 0.5 Sr 0.5 TiO 3  systems. The standard 6-31G* basis set was selected for oxygen atom. For the Ti, Li and Si atoms we selected, respectively, d4-1G*, 6-1G*, 8-41G* . The k-points sampling was chosen to be 40 points within the irreducible part of the Brillouin zone. The crystal structure is composed by TiSiO 5 layers bound by Li ions planes. In the layers, there are SiO 4 tetrahedra connecting TiO 5 square pyramids. However, it is also possible to consider the titanate group as distorted octahedra in which the apical bonds are alternatively short and long (cf. Fig. 1). The ground-state equilibrium geometry was obtained by minimizing the total energy per unit cell with respect to the lattice parameters and atomic positions. From this optimized structure, we have built two periodic models to represent the crystalline and disordered phases of Li 2 TiSiO 5. The first one, which will be designated as 5c, is simply the unit cell using the optimized parameters. All titanium atoms are fivefold-coordinated and the bonds between titanium and the apical oxygen, Ti O2, are alternatively 1.70 Å (O2 inside the unit cell) and 2.70 Å (O2 out of the unit cell). For the second model, 6c, the apical oxygen, O2, has been dislocated by 0.5 Å in the z direction, turning the Ti O2 bonds regular, 2.20 Å (see bottom of Fig. 1). Therefore, the titanium atom of 6c model is sixfold-coordinated. It has to be underlined here that the 6c model does not exactly represent the phases before crystallization, as they do not have a long-range organization, but is disordered as compared to the crystalline model. 4. Results and discussion Fig. 2 shows the X-ray diffraction patterns of three Li 2 TiSiO 5 powders heat treated at 500 and 600 C for 4 h and at 870 C for 24 h. The patterns obtained for the samples annealed at temperatures below 600 C revealed the possible presence of the Li 2 TiO 3 phase. However, due to the rather broad and low intense Bragg reflections, it is possible to have small amounts of other Li Ti O phases, as for example LiTi 2 O 4. In addition, the very noisy background attests for the presence of a disordered phase, an inorganic precursor which crystallization has not yet occurred. The beginning of the crystallization of the Li 2 TiSiO 5 phase occurs at temperatures close to 800 C, and it is very well crystallized after being heat treated at 870 C for 24 h. It has to be underlined that the powder melts at 900 C, and thus that 870 C is the highest temperature that has been reached without achieving the melting point. However the presence of a small amount of TiO 2 (rutile) was remarked and quantified by Rietveld refinement as being 5% . The additional phases in the disordered powders as in the crystalline ones were checked to have no influence on the PL properties by individual measurements. Fig. 3 shows the PL spectra of three Li 2 TiSiO 5 powders annealed at 500 and 600 C for 4 h and at 870 C for 24 h. The highest PL intensity is observed for the 500 C annealed powder. This large band spectrum can be deconvoluted into two Gaussian-type functions,
3 332 E. Orhan et al. / Chemical Physics Letters 398 (2004) Fig. 1. Representation of two Li 2 TiSiO 5 unit cells (space group P 4/nmm ). At bottom the Ti coordinations in 5c and 6c periodic models are represented left and right, respectively. The vertical arrows illustrate the displacement of O2. Fig. 2. X-ray diffraction patterns of Li 2 TiSiO 5 powders heat treated at: (a) 500 C, (b) 600 C for 4 h and at (c) 870 C for 24 h. Fig. 3. Photoluminescence spectra excited by the 488 nm line of an argon ion laser of three Li 2 TiSiO 5 powders annealed at: (a) 500 C, (b) 600 C, for 4 h and at (c) 870 C for 24 h. Curves (1) and (2) are the plots of the deconvolution of the spectrum (a) in two Gaussian-type functions.
4 E. Orhan et al. / Chemical Physics Letters 398 (2004) one peaking at 2.08 ev (orange-red band) and the other one at 2.28 ev (green band). The powder heat treated at 600 C also exhibits a strong PL signal, however, it is shifted towards higher energies as compared to the 500 C spectrum. The green component of the 600 C spectrum is much bigger than its orange-red component, which almost vanished with the partial structural organization. This is in accordance with our previous investigations on titanates where the higher the heat-treatment temperature, the rarer the pentacoordination of titanium and the weaker the PL emission. Following the example of Macke , it is possible to relate the green and orange-red components of the PL bands to one particular type of titanium coordination existing in the structure, as observed by Ponader et al.  that detected the presence of hexa and pentacoordinated titaniums in titanosilicate glasses. The orange-red component must somehow be linked to the pentacoordinated titanium and the green component to the hexacoordinated titanium. The crystalline, fully ordered phase (870 C) does not present any PL at all when excited with the 488 nm line of the argon ion laser, only the Raman signal is visible. However, Bouma and Blasse  already noticed that Li 2 TiSiO 5 in its crystalline form do present PL at room temperature when excited by a shorter wavelength line (250 nm). They related this PL to the existence of isolated TiO 5 square pyramid in the structure. In order to rationalize the links between PL and structural disorder, we performed a detailed theoretical study of the electronic structure in two models, a crystalline and a disordered one. Fig. 4 represents the band structure of 5c (a) and 6c (b) models. For the 5c model, the top of the valence band (VB), coincident with the Fermi Energy (E F )is located at C point. The minimal indirect gap between C and M is 5.47 ev. The minimal direct gap at C is 5.73 ev. The top of the VB for the 6c model is at R point and bottom of conduction band (CB) is located at M, as in the 5c model. The indirect minimal gap between R and M is 3.65 ev while the minimal direct gap at C is 4.11 ev. The calculated band gap energies cannot be compared with the experimental ones as the results have been turned untrustable by the presence of the additional phases. The fact that the band gap energy decreases from the ordered to disordered models shows that it is directly linked to the degree of structural arrangement, in accordance with the energy levels linked to structural defects described by Montoncello et al. . The atom-projected and total DOS of the 5c (a) and 6c (b) models are depicted in Fig. 5. In the 5c model, the VB consists mainly of 2p orbitals of O atoms, with a higher contribution of O1 atom in the upper part. In the 6c model, although the valence band is also made of the 2p orbitals of O atoms, the upper part presents a strong O2 character, the oxygen atom that was moved away from the titanium. The CB is clearly preponderantly made of the Ti (3d) states in both cases, with a weak Si contribution. In 5c model, the Ti O2 bond is shorter (and thus tighter) than the Ti O1 bonds (1.70 and 1.97 Å, respectively), this is why the upper part of VB is made of 2p O1 states: the Ti O2 interaction leads to greater splitting of the crystal orbitals. In the case of 6c model, this is the Ti O1 bond that is shorter and thus 2p O2 states fill the upper VB. We have calculated the Mulliken charge distributions of SiO 4, TiO 5 and TiO 6 clusters. In the 5c model, SiO 4 has a formal charge of 0.38 and TiO 5 of 1.47 jej. In the 6c, the charges are 0.36 and 1.49 jej for SiO 4 and TiO 6, respectively. The individual atomic charges only suffer weak variations with the structural deformation. The hypothesis we emitted throughout our studies of the origin of PL in disordered titanates  is that to exhibit room temperature PL, a system must fulfill two conditions: (i) to possess at least two types of differently charged clusters creating a polarization of the structure Fig. 4. Band structure for the 5c (a) and 6c (b) periodic models.
5 334 E. Orhan et al. / Chemical Physics Letters 398 (2004) Fig. 5. Total and projected DOS for the 5c (a) and 6c (b) periodic models. and (ii) to present some localized levels inside its band gap, levels that are mostly resulting of some structural disorder. The junction of those two conditions allows an easy trapping of electrons and holes during excitations and thus favors the radiative decay causing PL emission. In disordered Li 2 TiSiO 5, the two conditions are encountered and this is why the non-crystalline powders present a strong PL. In the crystalline powder, no PL emission is observed with our 488 nm excitation wavelength because it corresponds to an energy (2.54 ev) far lower than the calculated energy gap (5.47 ev) that must be very close to the experimental one as showed in previous studies [18,19,25]. Although this phase possesses two types of clusters, it presents very little disorder, and no localized levels allowing electrons and holes to be trapped. The fact that Bouma and Blasse detected PL at room temperature with an excitation band of about 250 nm (4.96 ev) is probably not only due to the presence of isolated square TiO 5 pyramids like they suggested, but to the coexistence of the SiO 4 TiO 5 clusters and the fact that the excitation energy is very close to the band gap energy, so that there is no need of intermediate levels to assist the excitation. 5. Conclusions In this work, we presented a theoretical and experimental study based on DFT calculations and polymeric precursor method to understand the existence of room temperature PL emission in Li 2 TiSiO 5. The main conclusions are listed as follows: (i) The existence of PL emission is strongly related to the crystal structural disorder. (ii) To be PL, a compound must possess at least two types of differently charged clusters, creating a polarization, and some structural disorder leading to the presence of localized levels able to trap electrons and holes and favoring the radiative return to the ground state. (iii) The CB is clearly preponderantly made of the Ti (3d) states in both cases, with a weak Si contribution. (iv) In the crystalline powder, no PL emission is observed with our 488 nm excitation wavelength because it corresponds to an energy (2.54 ev) far lower than the calculated energy gap (5.47 ev). If it were sufficient to excite the system with an energy close to the band gap, every semiconducting compound would present PL behavior. Acknowledgements This work is supported by Brazilian Funding Agencies FAPESP, CAPES and CNPq. Computer facilities of the ÔLaboratório de Simulação MolecularÕ, Unesp, Bauru, Brazil, are also acknowledged. References  E.R. Leite, F.M. Pontes, E.J.H. Lee, R. Aguiar, E. Longo, D.S.L. Pontes, M.S.J. Nunes, H.R. Macedo, P.S. Pizani, F. Lanciotti, T.M. Boschi, J.A. Varela, C.A. Paskocimas, Appl. Phys. A Mater. Sci. Process. 74 (2002) 529.  P.S. Pizani, H.C. Basso, F. Lanciotti, T.M. Boschi, F.M. Pontes, E. Longo, E.R. Leite, Appl. Phys. Lett. 81 (2002) 253.  F.M. Pontes, E.R. Leite, E. Longo, J.A. Varela, P.S. Pizani, C.E.M. Campos, F. Lanciotti, Adv. Mater. Opt. Electron. 10 (2000) 81.  F. Lanciotti, P.S. Pizani, C.E.M. Campos, E.R. Leite, L.P.S. Santos, N.L.V. Carreño, E. Longo, Appl. Phys. A Mater. Sci. Process. 74 (2002) 787.  L.P.S. Santos, E.R. Leite, N.L.V. Carreño, E. Longo, C.A. Paskocimas, J.A. Varela, F. Lanciotti, C.E.M. Campos, P.S. Pizani, Appl. Phys. Lett. 78 (2001) 2148.
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