Brillouin-scattering study of KAl(MoO4)(2) and NaAl(MoO4)(2)

Polarized Brillouin spectra have been measured at 293 K for trigonal KAI(Mo O4)(2) and monoclinic NaAl(MoO4)(2). These crystals belong to the large fam ily of layered double molybdates and tungstates in which transitions to fer roelastic phases have been found. KAI(MoO4)(2) and NaAl(MoO4)(2) crystals m ay be regarded as model crystals for the paraelastic D-3d(3) = P3ml and fer roelastic C-2h(6) = C2/c phase, respectively. The observed differences in t he Brillouin spectra, recorded for the both crystals, are discussed in term s of the differences in the crystal structures. The measured frequencies of acoustic modes propagating perpendicular to the layers are used to provide information on the origin of some lattice modes. (C) 2001 Elsevier Science B.V. All rights reserved.