Infrared spectroscopy of KTa(1-x)Nb(x)O(3) crystals.

Fourier-transform infrared and time-domain THz dielectric spectroscopy measurements on KTa(1-x)Nb(x)O(3) crystals for x = 0.018 and 0.022 were performed. The temperature behaviour of the soft mode frequency and damping have been determined above the ferroelectric phase transition. No dielectric dispersion in the frequency range below the polar phonon absorption is observed down to kHz frequencies at temperatures from room temperature down to 60 K. At lower temperatures the KTa(1-x)Nb(x)O(3) samples become opaque due to additional losses, which persist down to liquid helium temperatures. This supports the nanoscopic dipolar glassy state rather than macroscopic ferroelectricity at low temperatures.