Effects of Manganese (Mn) doping at b-site on the structural, optical, and dielectric properties of La₂CuTiO₆, La₂CuRuO₆, and La₂NiRuO₆ double perovskites

Double perovskites offer broad compositional flexibility and tunable structure, band gap, and dielectric response, making them attractive candidates for high-performance dielectric and energy storage applications. This thesis investigates the effects of B site manganese (Mn) doping modifies structure, band gap, and dielectric response in La₂CuTiO₆, La₂CuRuO₆, and La₂NiRuO₆ with a focus on capacitor energy storage. The gap addressed is the lack of a coherent cross system understanding that connects crystal structure and microstructure to frequency dependent dielectric behavior under identical processing. All compositions were synthesized by the same solid state route and characterized using one toolkit that includes Rietveld X–ray diffraction, Field emission scanning electron microscopy (FESEM) with energy dispersive X–ray (EDX) analysis, Fourier transform infrared spectroscopy (FTIR), UV–Vis diffuse reflectance, impedance and modulus spectroscopy. X–ray diffraction shows that La₂CuTi₁̵-xMnxO₆ remains single phase with orthorhombic Pnma symmetry, La2NiRu₁-xMnxO₆ remains single phase with monoclinic P2₁/n symmetry, and La₂CuRu₁-xMnxO₆ contains two closely related perovskite phases with a dominant P2₁/n symmetry. In La₂CuTi₁-xMnxO₆ and La₂NiRu₁-xMnxO₆, manganese increases the unit cell volume and shifts BO₆ stretching modes, and elemental maps show uniform chemistry. For the La₂CuRu₁- xMnxO₆ series, which is two phases, a monotonic lattice trend and chemical uniformity cannot be stated from our data, so structure property discussion is made with reference to the dominant phase only. The optical band gap narrows with Mn doping, for example from 2.06 eV to 1.62 eV in La₂CuTiO₆ and to about 1.64 eV in La₂CuRuO₆ at x = 0.10, with a similar decrease in La₂NiRuO₆. Dielectric measurements from 1 kHz to 1 MHz link directly to these structural changes. Manganese mainly tunes the bulk pathway by lowering grain resistance and modestly adjusting grain capacitance at the best compositions, which shifts the dominant relaxation to higher frequency, increases the real permittivity, lowers the dielectric loss, and reduces the Nyquist arc diameter, consistent with faster intra grain transport. The optimal compositions identified are x = 0.05 for La₂CuTiO₆ and x = 0.04 for both La₂CuRuO₆ and La₂NiRuO₆, defining practical operating ranges for capacitors in the upper kHz to MHz region. The novelty of this work is a direct cross system comparison under identical processing with one analysis framework that delivers general design rules connecting crystal structure and local distortion to relaxation kinetics and dielectric loss. In summary, manganese doping preserves the host frameworks, moves dielectric relaxation to higher frequency, increases permittivity, and reduces loss at the stated compositions, thereby improving the suitability of these La based double perovskites for capacitor grade energy storage.