Band gap engineering and transport properties of Ba2In2O5: effect of fluorine doping and hydration

Abstract

Two types of fluorine doped barium indate solid solutions were prepared by a solid state method: Ba_2−0.5xIn₂O_5−xF_x (x = 0, 0.1, 0.2) and Ba₂In₂O_5−0.5yF_y (y = 0, 0.1, 0.25). Good agreement between the theoretical and experimental values of the pycnometric densities for the studied solid solutions confirms these two distinctly different models of solid solution formation. According to analysis of the XRD data introduction of fluorine ions into the oxygen sublattice of barium indate leads to a decrease in the a lattice parameter and unit cell volume for both types of solid solutions. The effect of fluorine doping and hydration on the band gap of barium indate was studied by means of diffuse reflectance spectroscopy. The estimated value of the band gap width E_g for undoped Ba₂In₂O₅ is 2.94 eV and is in good agreement with literature data. Introduction of fluorine results in a slight increase of E_g and emergence of an additional absorption band in the region of 2.56–2.65 eV, near the fundamental absorption edge, which can be attributed to the -defects appearing upon fluorine doping. The increase of E_g with fluorine introduction correlates well with the decrease of the electronic transport numbers and can be explained by two competing effects: (1) lowering of the top of the valence band due to replacement of O²⁻ ions by F⁻; and (2) changes in the local structure, i.e., lattice contraction and tilting of the InO(F)_x framework. Hydration of the barium indate also leads to an increase of E_g, which was attributed to structural transformation from orthorhombic symmetry to tetragonal. However, with an increase in the fluorine concentration such changes of E_g become less pronounced because of the decrease of the hydration degree due to formation of -defects.