On a biphononic origin of the 1125 cm^(-1) absorption band in cuprous   oxide
  We report on the IR spectroscopic studies in both reflection (50-900 cm^{-1}) and transmission (900-3000 cm^{-1}) mode of the vibration spectrum of the cuprous oxide. A detailed analysis based on a comparison of the temperature dependences of the absorption band at 1125 cm^{-1} and of IR and Raman active fundamental vibrations results in assignment of the former to a biphonon. 
V.M.Burlakov, M.Goeppert, A.Jolk, A.Dinger, R.Becker, and   C.F.Klingshirn