Broad-band dielectric spectroscopy of Ba2NaNb5O15 single crystal

Barium sodium niobate (BNN) single crystals are studied by IR spectroscopy, time-domain THz transmission spectroscopy, HF coaxial wave-guide technique and LF dielectric spectroscopy to cover the frequency range 10(2)-10(14) Hz in a wide temperature interval. The dielectric response parallel and perpendicular to the polar c-axis is discussed. The ferroelectric transition at T-c = 830 K is driven by a relaxational soft mode coupled with another central-mode type relaxation which both gradually disappear on cooling in the ferroelectric phase. Below T-i the parameters of the expected IR active amplitudon were estimated. The low-temperature permittivity increase on cooling for the Eperpendicular toc field direction has been explained by an incipient proper ferroelectric-ferroelastic transition driven by an IR and Raman active B-2-symmetry soft mode.