Investigating the structural, vibrational, optical, and dielectric properties in Mg-substituted LaAlO₃

Abstract

The structural, vibrational, optical, and dielectric properties have been probed in Mg-substituted LaAlO₃. For this purpose, a series of LaAl_1−xMg_xO_3−δ (0 ≤ x ≤ 0.20) polycrystalline samples has been prepared using the sol–gel synthesis route. Changes in structural properties due to Mg substitution have been analyzed with the X-ray diffraction technique. The correlation of the degree of distortion of AlO₆ octahedra with the E_g and A_1g modes in Raman spectra has been discussed. Mg substitution leads to decrease the optical bandgap and increases in the electronic disorder in terms of Urbach energy ${(E}_{\mathrm{u}})$, which has been investigated using optical absorption spectroscopy. An asymmetry in E_g mode (36 cm⁻¹) has been observed in Raman spectra, which could be a signature of electron–phonon interaction induced due to disorder via Mg-substitution. Thus, the role of disorder in the electron–phonon interaction (1/|q|) has been understood with the relation between asymmetry parameter (q) of Raman phonon mode and ${E}_{\mathrm{u}}$ in accordance with an earlier model: $\frac{{I}}{{{q}}^{2}}\propto{ \xi}{{E}}_{\text{u}}{+\lambda}$. Further, Impedance spectroscopy has been used to study dielectric response with Mg substitution in LAO. Decrement in the dielectric constant ($\kappa$) has been correlated with the presence of oxygen vacancies and electronic disorder with a relationship ${{E}}_{\text{u}}\propto 1/\kappa^{2}$.