Effect of a Rare-Earth Ion on the Structural Instability in RFe3(BO3)(4) Crystals

摘要

The dynamics of the crystal lattice of RFe3(BO3)(4) (R = Pr, Nd, Sm, Gd, Tb, Dy, and Ho) compounds in the high-symmetry R32 phase has been calculated. Significant changes in spectra of compounds with various rare-earth ions have been obtained only near the edge point of the Brillouin zone (q = 1/3(-2b(1) + b(2) + b(3), where b(1), b(2), and b(3) are the reciprocal lattice vectors) for acoustic oscillation branches. A decrease in the frequency of an acoustic mode at the point has been revealed in all studied compounds. This frequency depends on the type of rare-earth ion and decreases from a compound with Pr to a compound with Ho down to imaginary values. Such a behavior of the frequency of the unstable acoustic mode is in good agreement with experimental data on the dependence of the temperature of the R32 P3(1)21 structural phase transition on the type of rare-earth ion in ferroborates.