Infrared phonon spectrum of the tetragonal helimagnet Ba2CuGe2O7

The lattice dynamics of Ba2CuGe2O7, a compound which develops Dzyaloshinsky-Moriya helical magnetism below T-N = 3.2 K, has been studied by measuring the infrared reflectivity of a single crystal with the radiation polarized both in the ab plane and along the c axis of its tetragonal cell, from 7 to 300 K. In this compound, where the unit cell has no inversion symmetry, 14 E phonon modes of the ab plane, out of the 18 predicted, and nine B-2 modes of the c axis, out of the ten predicted, have been observed. They have been assigned to the atomic motions by a comparison with shell-model calculations, which provided vibrational frequencies in good agreement with the experiment, while most calculated intensities turned out to be much lower than the experimental values. This discrepancy has been tentatively explained by assuming strong electron-phonon interactions, a hypothesis supported by the failure of the f-sum rule if restricted to the phonon region. Indeed, we observe a remarkable increase in the oscillator strengths between 300 and 200 K for the phonons of the ab plane (between 300 and 100 K for the c axis), which implies an increase in the dielectric constant of Ba2CuGe2O7 below room temperature.