Elsevier

Optik

Volume 211, June 2020, 164611
Optik

Original research article
Electronic, structural and optical properties of BaTiO3 doped with lanthanum (La): Insight from DFT calculation

https://doi.org/10.1016/j.ijleo.2020.164611Get rights and content

Highlights

  • A detailed theoretical study of Lanthanum doped BaTiO3 is presented.

  • Emergence of new states at Gamma point lead to the reduction of bandgap in doped system.

  • Slight shifting of absorption edge to lower energy is also observed upon doping.

  • Refractive index of the doped system is smaller than the pure system.

  • Emerging physical properties are strongly correlated with electronic structural studies.

Abstract

We have studied the barium titanate (BaTiO3) by using the CASTEP which based on density functional theory (DFT) under the correlation function of GGA and PBE. Material under study has been doped by Lanthanum (La) at the sites of Barium (Ba). Before and after doping we have calculated the different optical, structural and electronic properties and have been discussed in detail. After doping, reduction of band gap is noticed and as well as nature of band gap is also changed from indirect band gap (IBG) to direct band gap (DBG). This variation in band gap has been explained clearly on the behalf of partially and total density of states. Refractive index (n) is 2.598 for pure and 2.482 for doped system. Absorption peak also effected by inclusion of La.

Introduction

A compound having the structure like calcium titanate known as perovskites. It’s general formula which used is ABX3 [1]. In this formula A and B are positive ions and X is negative charged atom which makes bond with cations. It also can be any halogen atom. Perovskite is first discovered by Russian mineralogist Gustave Rose from Ural mountain in Russia. It was named after Russian mineralogist Lev Perovski. SrTiO3 is considered as an ideal perovskite and its structure is cubic [2]. In this cubic isometric structure, Ti ions are located at corners and Sr ion is body centered (BC). The oxygen is face centered (FC) at twelve edges of cube which give us corner shared strings of TiO6 octahedra and it extends in three dimensions.

Perovskite family has many oxides like BaTiO3, CaTiO3 exhibit as an insulator, some behaves as metals and some shows magnetic behavior and these are all transition metal oxide [3]. Perovskite of the form ABO3 are considered as the most interesting materials for investing and exploring the properties due to number of application in electronics and optoelectronic devices [[4], [5], [6]].

BaTiO3 is a synthetic perovskite that crystalizes itself in ideal perovskite structure or in hexagonal modification [7]. Goldschmidt firstly studied and prepared synthetic perovskites on varying composition together with Barium titanate. However, the past of Barium Titanate is meticulously related with mica which was only insulator material (with dielectric constant of 10) used in capacitors. Scientists were finding material with high dielectric constant (starting from 80). In 1940−41 Germany reported different alkaline earth titanate in production. Wainer and Solomon identified BaTiO3 has possibly high dielectric constant [8]. After this many scientists in different labs declared it as a most suitable insulator for dielectric with high dielectric constant. It is a perovskite which exist in different phases at different temperatures e. g cubic structure exist at 120 °C, and it is most stable state and as temperature is decreased it undergoes three ferroelectric phases e. g below 120 °C it shifts into tetragonal structure which exist down to 5 °C [2].

Barium titanate exists in five different polymorphic crystalline forms; there are three (tetragonal, orthorhombic, and rhombohedral) which are ferroelectric, and two paraelectric (cubic as well as hexagonal). The paraelectric cubic form of BaTiO3 belongs to pm3m space group. BaTiO3 cubic unit cell consists of five atoms. In its structure Ba is placed at origin (0.0,0.0,0.0) a, Ti sitting at body center and oxygen is face centered [8] (Fig. 1).

It is utilized in several application like capacitors, piezoelectric sensors, transducer, thermistors, mover of energy etc. [[10], [11], [12], [13]]. In multilayer capacitor, the BaTiO3 is used as dielectric material because of high capacitance [14]. BaTiO3 also offers better electrical property and mechanical properties at room temperature [15].

Pure BaTiO3 shows a great variation in dielectric constant. For the modification of the properties and for broaden the number of applications the BaTiO3 is doped with impurities. Adding of PbTiO3 rises the transformation (Curie) temperature [[16], [17], [18], [19]]. The Excess of TiO2 enhances the solubility of CeO2 in BaTiO3 [20,21]. The niobium shows a notable influence on the dielectric properties of BaTiO3 [22]. Benlahrache et al. [[22], [23], [24]] found that by adding NaNbO3 exhibits a significant variation of the microstructure and also increases dielectric constant. We have noticed that lanthanum with symbol La with atomic number 57 and most abundant rare earth elements [25]. Its physical appearance is white silvery metallic. Lanthanum has 57 electrons revolving around the nucleus and its configuration is 1s22s22p2 3s23p63d104s24p64d10 5s25p65d16 s2 and its more compact form as [Xe] 5d16 s2. Three electrons in valence shell. One electron in 5d and two electrons in 6 s subshell are responsible for chemical properties and form +3 oxidation state. As we want to dope lanthanum in the place of barium, and it has radius close to the Ba. It may result in significance change in structure and microstructure. This will change the band structure and ultimately change in dielectric constant [26,27].

Section snippets

Computational details

CASTEP program was used in material studio package to study the structural, electronic and optical properties of BaTiO3, which follows quantum mechanical codes and specially developed for band gap and bond lengths investigation. This programme was first developed by Payne and his companions in the beginning of 1990’s [28]. With the help of DFT, this Programme is used for designing different materials and for doping. These materials can be crystalline solids, amorphous solids, molecules etc. The

Geometry optimization

From Birch-Murnaghan equation of state [29] the lattice parameters of BaTiO3 have been optimized i.e a = b = c =4.034 Å. this calculated value of lattice constant in our work is approximately same as experimentally reported 4.00 Å [30]. The difference between experimental and our calculated value is just 0.034 Å (less than 1%). This shows the cogency of this work. Earlier theoretically conveyed lattice constants are a = b = c = 4.03 Å [31,32] which are in best agreement with our calculated

Conclusion

Structural electronic and optical properties of pure BaTiO3 and doped with La have been investigated by first principles calculations. The properties studied with regarding of La doped system shows good agreement with earlier reported results. The BG of pure and doped BaTiO3 have been calculated. The band structure has been elaborated in regarding with TDOS and PDOS. Band gap has been decreased with La-doping and conduction band has been partially shifted towards valance band with appearance of

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The corresponding author would like to thank Higher Education Commission (HEC) of Pakistan for financial support under the grant number 6934/NRPU/HEC.

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