Low-temperature electrolytic coloration and spectral property of ammonium alum crystals

► We expanded the traditional electrolysis method. ► Low-temperature electrolysis is realized for the first time. ► Ammonium alum crystals are colored electrolytically for the first time. ► SO3−, SO2− and O3− hole-trapped centers are produced in colored crystals. ► Color centers are from thermal decomposition of SO42− radicals at high voltage.