The mechanism responsible for the lambda-phase transitions in ammonium
halides is investigated within the framework of an extended theory of
the disorder-induced Raman scattering. The Raman spectra of ammonium
halides have been predicted for both the disordered beta phase (NH4Cl
and NH4Br) and the ordered gamma phase (NH4Br). It is postulated that
those terms which dominate the dynamic distortions of the lattices and
thereby cause the bulk of the disorder-allowed Raman scattering, will
also dominate the quasi-static distortion which is the lambda-phase t
ransition in these crystals. From this we find that the four-spin corr
elation function for the M-point boundary phonons in the disordered ph
ase (beta), and the two-spin correlation function for the zone centre
phonons in the ordered phases (gamma and delta) dominate the mechanism
of the lambda-phase transitions in ammonium halides. Predictions obta
ined for the Raman scattering agree in most cases with our Raman obser
vations on NH4Cl and NH4Br.