Local electronic structure, optical bandgap and photoluminescence (PL) properties of Ba(Zr_0.75Ti_0.25)O₃ powders

Abstract

Ba(Zr_0.75Ti_0.25)O₃ (BZT-75/25) powders were synthesized by the polymeric precursor method. Samples were structurally characterized by X-ray diffraction (XRD), Rietveld refinement, X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) techniques. Their electronic structures were evaluated by first-principle quantum mechanical calculations based on density functional theory at the B3LYP level. Their optical properties were investigated by ultraviolet-visible (UV-Vis) spectroscopy and photoluminescence (PL) measurements at room temperature. XRD patterns and Rietveld refinement data indicate that the samples have a cubic structure. XANES spectra confirm the presence of pyramidal [TiO₅] clusters and octahedral [TiO₆] clusters in the disordered BZT-75/25 powders. EXAFS spectra indicate distortion of Ti–O and Ti–O–Ti bonds the first and second coordination shells, respectively. UV-Vis absorption spectra confirm the presence of different optical bandgap values and the band structure indicates an indirect bandgap for this material. The density of states demonstrates that intermediate energy levels occur between the valence band (VB) and the conduction band (CB). These electronic levels are due to the predominance of 4d orbitals of Zr atoms in relation to 3d orbitals of Ti atoms in the CB, while the VB is dominated by 2p orbitals related to O atoms. There was good correlation between the experimental and theoretical optical bandgap values. When excited at 482 nm at room temperature, BZT-75/25 powder treated at 500 °C for 2 h exhibited broad and intense PL emission with a maximum at 578 nm in the yellow region.

Introduction

Solid solutions known as barium zirconate titanate [Ba(Zr_xTi_1−x)O₃ ceramics, BZT] are prepared by merging barium zirconate (BaZrO₃) and barium titanate (BaTiO₃) ceramics. BZT ceramics have been used as alternative dielectric materials to replace barium strontium titanate [(Ba_xSr_1−x)TiO₃, BST] ceramics [1], [2], [3], [4], [5], [6], [7]. BZT compounds have two main advantages over BST ceramics, low dielectric loss and a high dielectric constant [8]. Moreover, BZT ceramics exhibit excellent microwave dielectric properties at gigahertz frequencies [9], [10]. The Zr/Ti ratio is a very important parameter that tailors the type of ferroelectric–paraelectric phase transition and its characteristic Curie temperature [11]. Several studies on ferroelectric–relaxor properties and diffuse transition of BZT ceramics with different compositions and dopants have been published [12], [13], [14], [15], [16], [17].

In general, this material can easily be synthesized because Zr⁴⁺ ions are chemically more stable than Ti⁴⁺ ions [18]. BZT ceramics can be prepared by substitution of Ti atoms (atomic weight 47.9 g/mol, ionic radius 74.5 pm) by Zr atoms (atomic weight 91.2 g/mol, atomic radius 86 pm) in the B-sites of this perovskite [19]. Replacement of Ti by Zr depresses the conduction by small polarons hopping between Ti⁴⁺ and Ti³⁺ ions and decreases the leakage current [20]. A few studies on the optical properties of crystalline and non-crystalline BZT powders or thin films have been reported, including their infrared [21], [22], refractive index [23], [24], [25], and photoluminescence (PL) properties [26], [27], [28].

The electronic structure of BZT ceramic powders has only been addressed in some of the relevant studies. Lauhé et al. [29] used density functional theory (DFT) as implemented in the Vienna ab initio simulation package and projection augmented plane waves with the Perdew–Wang exchange correlation potential to determine deformations induced by substitution of octahedral [ZrO₆]/[TiO₆] clusters in the B-sites of perovskite BZT ceramics. Chibisov [30] used quantum mechanical calculations based on electronic DFT and pseudopotential theory to verify the effect of Zr on the atomic and local distortions of BaTiO₃. Yin et al. [31] used first-principle calculations based on the pseudopotential plane wave method with the generalized gradient approximation to calculate the optical bandgap and static dielectric constants for cubic and tetragonal structures of Ba(Zr_xTi_1−x)O₃ (x=0, 0.25, 0.5, and 0.75). However, there are few reports on ab initio theoretical and experimental investigations of the electronic structure of BZT perovskite [32], [33], [34]. In these studies, the electronic structure was evaluated by first-principle quantum mechanical calculations based on DFT at the B3LYP level [35], [36]. In recent years, huge computational advances have been possible because of technological progress in storage capacity and computer processing. The new generation of computers with multiple processors can drastically reduce the processing time required to investigate the electronic structure of complex solids. This study provides information on the local structure at long and short range by means of X-ray diffraction (XRD), X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) techniques. Moreover, first-principle quantum mechanical calculations of the electronic structure [band structure and density of state (DOS)] were performed to determine the optical bandgap values and electronic transitions responsible for the room-temperature PL properties of Ba(Zr_0.75Ti_0.25)O₃ (BZT-75/25) powders synthesized by the polymeric precursor method (PPM).