Halide perovskites have been in the spotlight for their intriguing optoelectronic properties and foreseen as a game changer in the next generation energy applications. However, the major bottleneck which stalled their wide acceptance is the material instability and lead-toxicity. In this talk the novel double perovskite materials which are stable and non-toxic while retaining the cubic perovskite structure and optoelectronic properties will be discussed. Heterovalent substitution of metal cations in the ABX3 structure result in a cubic double perovskite A₂B'B"X₆ structure. These compounds are exceptionally stable towards light, heat, and atmospheric ambience. However, significant knowledge should be gained on the structure property correlations specifically related to tailoring the optical and electrical properties. In this work, we explored the idea of sublattice mixing in halide double perovskites and studied its influence on the structural and optical properties. Specifically, the choice of B' and B" cations and halide anions (X= Cl, Br) in structure have been explored. Bandgap engineering has been demonstrated in cationic substituted double perovskites. Interestingly bandgap bowing effect has been observed in these double perovskite for the first time. Sublattice distortion resulting from alloying compositionally distinct double perovskites is shown to influence photoluminescence emissions. These emissions are attributed to self-trapped excitons (STE). A strong synergy between STE emissions, electron−phonon coupling, bandgap, and phonon vibrations in double perovskites with sublattice distortions is demonstrated. Further, anionic substitution in double perovskites has been explored and demonstrated the bandgap engineering with a lowest achieved bandgap of ~ 2 eV for Cs2AgBiBr6. The conventional wisdom suggests that a linear variation in the lattice parameter and/or the bandgap, as per the Vegard’s law, implies a true alloying with homogenous mixing of halide anions (X). Our work demonstrates that such a generalization can be significantly misleading, and we show that metal-halide octahedra are either Cl or Br rich in alloyed compositions. A detailed Raman spectroscopic analyses on various compositions of double perovskites clearly disseminates the individual octahedral contributions to the evolution of lattice phonons in the structure. These vibrational attributes play an important role in tuning the electronic and optical properties of double perovskites.