Anharmonic self energy shift of a librational mode

A sizable anharmonic self energy shift of the Raman active librational modes in KN₃ has been found and shown to be a linear function of temperature above the Debye characteristic temperature of the solid (ca. 351°K). The Grüneisen parameter for the librational mode is estimated from the data to be 1.40. The Raman active mode involving the translation of the K⁺ ion shows a negligibly small anharmonic shift.