Effect of structural disorder on the electronic and phononic properties of Hf doped BaTiO ₃

The classical ferroelectric material BaTiO ₃ has been doped with large cation i.e., Hf at Ti site in order to understand the effect of structural disorder in electronic and phononic state. The Raman spectroscopy measurement on these samples indicates increase in the structural disorder with Hf doping. The detail analysis of the Raman spectroscopy data clearly suggests the appearance of new Raman mode at ~ 780 cm ⁻¹ and the presence of an asymmetry in almost all Raman modes. The appearance of new Raman mode has been attributed due to the structural disorder induced phonon modes; and this is further confirmed using laser irradiation studies. Additionally, it has been observed that the intensity of this new phonon mode increases systematically with Hf doping; indicating increase in the phononic disorder. It has been observed that the line shape Raman phonon modes show significant asymmetry and this asymmetry along with the full peak width at half maxima (FWHM) of Raman phonon mode shows systematic variation with Hf doping. The observed asymmetric Raman line profile has been analysed through Fano model of electron–phonon coupling which suggests an increase in the electron–phonon coupling with Hf doping. In order to get further insight on increase in the electron–phonon coupling near band edge optical absorption spectroscopy measurements has been carried out and value of electronic disorder in the form of Urbach energy has been estimated and the same is observed to scale with Hf doping. Thus, the systematic increase in the intensity of disorder phonon mode and that of electronic disorder has been observed. This suggests that structural disorder not only affects phonons but electronic state of the system as well. Thus, it appears that the increase in the width of electronic and phononic disorder may overlap in energy scale and may be responsible for the observed increase in the electron–phonon coupling parameter as estimated through Fano equation.