Evidence of strongly coupled modes in a Rb_1-x(NH₄)_xH₂PO₄glass by inelastic coherent neutron scattering

Abstract

Transverse acoustic (TA) phonons propagating in the basal plane (3.5') of R~~-X ( N H~)~H~ PO4 with x4.25 have been measured by means of coherent inelastic neutron scattering over a temperature range including the dipolar glass transition. These modes seem to be mom and more damped as temperature is decreased towardsTg, the width of energy scans reachs a maximum at about 80 K then decreases to a normal value at very low temperatures.During the decrease of temperature no softening of TA phonons is observed. Although measured at higher frequencies, the TA phonons determined by neutron scattering behave exactly as those observed in Brillouin scattering.One has applied the model of piezoelectrically coupled modes which accounts for Brillouin spectra. The agreement with neutron data is good. So the picture of freezing characterized by a wide distribution of relaxation times is valid on a very large scale of frequency.