Theoretical and experimental study of disordered Ba 0.45 Sr 0.55 TiO 3 photoluminescence at room temperature I.A. Souza a, * , M.F.C. Gurgel b , L.P.S. Santos b , M.S. Go ´es a , S. Cava c , M. Cilense a , I.L.V. Rosa b , C.O. Paiva-Santos a , E. Longo a a Laborato ´ rio Interdisciplinar em Cera ˆ mica, Departamento de Fı ´sico-Quı ´mica, Instituto de Quı ´mica, Universidade Estadual Paulista, R. Francisco Degni, s/n, Bairro Quitandinha, CEP 14800-900, Araraquara, SP, Brazil b Laborato ´ rio Interdisciplinar de Eletroquı ´mica e Cera ˆmica, Departamento de Quı ´mica, Universidade Federal de Sa ˜o Carlos, Rod. Washington Luiz, km 235, C.P. 676, CEP 13565-905, Sa ˜o Carlos, SP, Brazil c Laborato ´ rio Interdisciplinar de Materiais Cera ˆmicos, Centro Interdisciplinar de Pesquisa e Po ´ s-Graduac ¸a ˜o, Universidade Estadual de Ponta Grossa, Av. Gal. Carlos Cavalcanti, 4748, Campus – Uvaranas, CEP 84035-900, Ponta Grossa, Pr, Brazil Received 24 June 2005; accepted 12 September 2005 Available online 19 October 2005 Abstract Disordered and crystalline Ba 0.45 Sr 0.55 TiO 3 (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. Photolu- minescence at room temperature was observed only in a disordered BST sample. Increasing the calcination time intensified the photo- luminescence (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 infor- mation 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 materialÕs disordered state at room temperature. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Titanate; Photoluminescence; BST; Disordered 1. 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 crystal- line samples or single crystals, due to their potential optoelectronic applications [2–6]. Semiconductors of tita- nate-type compounds have presented many luminescence phenomena [7–9]. The optical properties of disordered semiconductor compounds are characterized by the pres- ence 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 nor- mally 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 [12–14]. There are many hypotheses to explain the photoluminescence phe- nomenon in crystalline or disordered titanate compounds [15–17]. Leite et al. discuss the nature of visible photolumi- nescence at room temperature in disordered lead titanate in the light of the results of recent experimental and theoret- 0301-0104/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2005.09.024 * Corresponding author. Tel.: +55 16 3361 5215. E-mail address: iedo@posgrad.iq.unesp.br (I.A. Souza). www.elsevier.com/locate/chemphys Chemical Physics 322 (2006) 343–348