Elsevier

Chemical Physics

Volume 322, Issue 3, 20 March 2006, Pages 343-348
Chemical Physics

Theoretical and experimental study of disordered Ba0.45Sr0.55 TiO3 photoluminescence at room temperature

https://doi.org/10.1016/j.chemphys.2005.09.024Get rights and content

Abstract

Disordered and crystalline Ba0.45Sr0.55TiO3 (BST) powder processed at low temperature was synthesized by the polymeric precursor method. The single-phase perovskite structure of the ceramics was identified by the Raman and X-ray diffraction techniques. Photoluminescence at room temperature was observed only in a disordered BST sample. Increasing the calcination time intensified the photoluminescence (PL), which reached its maximum value in the sample heat treated at 300 °C for 30 h. This emission may be correlated with the structural disorder. Periodic ab initio quantum-mechanical calculations using the CRYSTAL98 program can yield important information regarding the electronic and structural properties of crystalline and disordered solids. The experimental and theoretical results indicate the presence of intermediary energy levels in the band gap. This is ascribed to the break in symmetry, which is responsible for visible photoluminescence in the material’s disordered state at room temperature.

Introduction

Much interest has focused on the photoluminescence (PL) of disordered or nanostructured material since this phenomenon was first observed in porous silicon at room temperature [1]. The luminescence of different kinds of compounds has been extensively studied in doped crystalline samples or single crystals, due to their potential optoelectronic applications [2], [3], [4], [5], [6]. Semiconductors of titanate-type compounds have presented many luminescence phenomena [7], [8], [9]. The optical properties of disordered semiconductor compounds are characterized by the presence of a tail in the plot of photon energy versus optical absorption. The optical absorption in this so-called tail falls almost asymptotically to zero in a region that is normally transparent in crystalline solids [10]. The Urbach edge is attributed to the presence of localized electronic states near the band edges of disordered semiconductors [11]. Several interesting properties of these disordered materials have been reported and it has been emphasized that the PL emission wavelength is related to the exciting wavelength and to the disordered state [12], [13], [14]. There are many hypotheses to explain the photoluminescence phenomenon in crystalline or disordered titanate compounds [15], [16], [17]. Leite et al. discuss the nature of visible photoluminescence at room temperature in disordered lead titanate in the light of the results of recent experimental and theoretical calculations. According to them, there is a possible formation of a fivefold coordination of the disordered system to introduce the localized electronic levels between valence and the conduction band. They also suggest the possibility of a radioactive recombination (electron–hole pairs), which may be responsible for the emission of photoluminescence [18]. Another study recently reported by our group involved the photoluminescent properties and structural disorder of BaSrTiO3 films and SrTiO3 powder, in which it was assumed that the presence of localized electronic levels in the band gap resulting from the break in symmetry is responsible for the visible photoluminescence of the disordered state at room temperature [19].

In this article we present, for the first time, measurements of broad PL as a function of heat treatment in BST samples prepared by the polymeric precursor method and it was also investigated the electronic structure and optical properties of perovskites using the first-principles with the density functional theory (DFT). The theoretical results are confronted with experimental data and both are coherent.

Section snippets

Experimental procedure

Ba0.45Sr0.55TiO3 (BST) powders were prepared by the polymeric precursor method (PPM), which is based on the chelation of the metal cations by citric acid in a solution of water and ethylene glycol. The precursor solution for BST coating was prepared from a titanium citrate formed by dissolving titanium isopropoxide in an aqueous solution of citric acid heated to about 70 °C. A stoichiometric amount of BaCO3 was added to the titanium citrate solution, which was stirred slowly until the reactional

Results and discussion

Fig. 2 shows the Raman spectra of the barium titanate (BT) (a) and barium strontium titanate (BST) (b) powders. The modes further split into longitudinal (LO) and transverse (TO) components the long electrostatic forces associated with lattice ionicity. The spectrum in Fig. 2(a) shows the stretching mode of A1(TO1), A1(TO2) and A1(TO3) at around 155, 260 and 518 cm−1, respectively [29], [30], [31]. The stretching mode of E(TO2) associated with the tetragonal–pseudocubic phase transition appeared

Conclusions

Low cost soft-chemical processing was used to prepare the Ba0.45Sr0.55TiO3 powders, which displayed an intense broad photoluminescence at room temperature in the visible range in the structurally disordered material. The experimental results revealed by Raman, XRD, Rietveld refinements, UV–vis and photoluminescence measurements, combined with the periodic ab initio quantum-mechanical method, indicated the presence of intermediary energy levels in the band gap. These intermediary levels led to a

Acknowledgements

This work was supported by the Brazilian research-financing institutions: FAPESP-CEPID, CNPq-PRONEX and CAPES. The authors thank the institutions CEFET-MA and UEMA.

References (37)

  • J. Ballato et al.

    J. Lumin.

    (2000)
  • H. Liu et al.

    J. Lumin.

    (1999)
  • Z. Bryknar et al.

    J. Lumin.

    (2000)
  • S. Murakami et al.

    J. Lumin.

    (2000)
  • J. Meng et al.

    Phys. Lett. A

    (1995)
  • B. Bouma et al.

    J. Phys. Chem. Solids

    (1995)
  • E. Orhan et al.

    J. Solid State Chem.

    (2004)
  • F.M. Pontes et al.

    Thin Solid Films

    (2001)
  • L.T. Canham

    Appl. Phys. Lett.

    (1990)
  • V. Trapakov et al.

    Phys. State Solidi

    (1994)
  • W.F. Zhang et al.

    Appl. Phys. A

    (2000)
  • D.L. Wood et al.

    Phys. Rev. B

    (1972)
  • M. Capizzi et al.

    Phys. Rev. Lett.

    (1970)
  • P.S. Pizani et al.

    Appl. Phys. Lett.

    (2000)
  • R. Leoneli et al.

    Phys. Rev. B

    (1986)
  • R.I. Eglitis et al.

    Eur. Phys. J. B

    (2002)
  • M.S. Zang et al.

    J. Mater. Process. Technol.

    (2003)
  • E.R. Leite et al.

    J. Mater. Sci.

    (2003)
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