In the field of optoelectronics and other sustainable energy conversion applications, vacancy-ordered double perovskite materials are thought to be the most promising resources nowadays. In this thesis we represent the structural, mechanical, electronic, optical and thermoelectric properties of vacancy-ordered double perovskite K₂SeCl₆ studied by first principle calculations under ambient and different hydrostatic pressure ranging up to 80 GPa. The structural stability of the compound is ensured by the Goldsmiths tolerance factor (0.98) and negative formation energy (−0.53 eV). It exhibits good mechanical stability when pressure is applied revealing that induced pressure has a substantial influence on the mechanical stability. The band structure shows p-type semiconducting nature with an indirect band gap of 2.502 eV at ambient condition and band gap decreases gradually with induced pressure. The optical absorbance and conductivity increases with increasing pressure making them more appropriate for optoelectronic applications in Ultra-violet region. The optical analysis has been conducted by computing the dielectric function, absorption coefficient, conductivity, optical reflection, and refractive index. Thermoelectric characteristics are investigated using BoltzTraP to estimate the electrical and thermal conductivities, Seebeck coefficient, power factor and figure of merit. The computed value of figure of merit at room temperature (0.78) reveals the promising potential for renewable energy applications of K₂SeCl₆ in both ambient and pressurized conditions.

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