Antimony trioxide is a white, odorless crystalline powder primarily used as a flame retardant synergist and opacifying agent. It is obtained either by oxidation of antimony metal or as a by-product from smelting operations. The compound exhibits excellent stability, high refractive index, and strong opacity in glass, ceramics, and plastics. Due to its versatile chemical nature, it is widely applied in polymer formulations, pigments, and fire protection systems.