Brillouin scattering study of ferroelectric transition mechanism in multiferroic metal-organic frameworks of [NH4][Mn(HCOO)3] and [NH4][Zn(HCOO)3]

Temperature dependence of acoustic properties of [NH4][Mn(HCOO)(3)] and [NH4][Zn(HCOO)(3)] metal-organic frameworks was investigated by high-resolution micro-Brillouin scattering. Clear anomalies in the Brillouin shift and damping were observed near the transition temperature T-c upon cooling for the acoustic phonon corresponding to the c(11) elastic constant. Analysis of the acoustic anomalies showed that the order parameter exhibits critical slowing down near T-c with the relaxation time of similar order of magnitude as for other order-disorder ferroelectrics. (C) 2014 AIP Publishing LLC.