Unraveling the role of Mn doping in transforming SrLaLiTeO6 perovskites: structural, optical, and dielectric insights

Abstract

This study investigates the consequence of Mn substitution in SrLa_1-xMn_xLiTeO₆ (0 ≤ x ≤ 0.3) perovskites to tune their optical, structural, and dielectric properties for potential optoelectronic applications. The presence of a P2₁/n monoclinic structure in every composition was determined by analyzing the diffraction patterns at room temperature. The consistency in the peak positions in all the compositions indicates that Mn³⁺ is successfully substituted for La³⁺. Rietveld refinement revealed changes in the unit cell’s volume upon Mn doping. The x = 0 (parent compound) exhibited a double perovskite structure of the ordered type. Variations in the bond angles and lengths are observed across all compositions. Octahedral tilting is observed in all compositions brought about by variations in the tolerance factor ‘t’ and the tilting angle ‘ɸ.’ The FTIR spectra confirm the perovskite structure with characteristic vibrational features indicative of perovskite-type bonding and Mn substitution effects. The 472–488 cm⁻¹ peaks can be attributed to Mn–O and Li–O stretching vibrations, observed peak shifts with increasing x indicating local structure modification, and possible Mn-induced strain in the lattice. Strong absorption bands observed at 662–686 cm⁻¹ correspond to Te–O stretching vibrations within the TeO₆ octahedra, confirming the presence of Te in the perovskite structure. UV–visible studies revealed decreased band gap energy from 4.3 eV in the undoped composition to 2.17 eV in the doped composition (x = 0.3) with increasing Mn content, suggesting enhanced conductivity. This also indicates that Mn doping introduces new electronic states within the band gap, thereby reducing the energy for the required electronic transitions. The observed transitions in the UV region, involve d-d transitions within the Mn³⁺ ions combined with the charge transfer from the oxygen ligands to Mn³⁺. Raman spectra recorded for all compositions indicated the symmetry is the same for all compositions and the successful doping of Mn into the parent compound. SEM and EDX analysis verified the elemental composition and highlighted the presence of particle clustering. Impedance spectroscopy analysis indicated a hike in AC conductivity with a hike in frequency and a drop in grain resistance for all compositions, pointing to a single dielectric relaxation mechanism. Cole–Cole plots depicted a non-Debye-type behavior, attributed to inherent defects within the materials. Dielectric studies demonstrated a frequency-dependent drop in both dielectric constant and tangent loss, suggesting a reduction in net polarization at higher frequencies. Hence, the work carried out here addresses the lack of detailed understanding of how Mn doping affects the structural, optical, and dielectric properties of SrLaLiTeO₆, a material with the potential for photovoltaic and optoelectronic applications, and specifically explores the impact of Mn doping bandgap reduction and dielectric enhancements, which are vital for improving material performance in applications like semiconductors and LEDs.