Fast polarization mechanisms in uniaxial tungsten-bronze SBN-81

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The high-frequency dielectric response of the uniaxial strontium barium niobate (SrxBa1−xNb2O6) crystals with 81% of Sr (x = 0.81) was studied from 1 kHz to 30 THz along the polar axis in a wide temperature interval [E. Buixaderas et al, Sci. Rep. 7, 18034 (2017)]. Relaxor properties were observed in the complex dielectric response and four main excitations were ascertained below the phonon frequencies. These fast polarization mechanisms take place at THz, GHz and MHz ranges and show different temperature evolution.

A central mode excitation in the THz range, due to anharmonic dynamics of cations, shows critical behaviour towards T~400 K. At lower frequencies, around 10 GHz, another excitation (νDW) appears below Tm~330K, which is related to the development of ferroelectric microdomains.

In addition, several relaxations below the phonon frequencies, play an essential role in the dielectric response of the crystal. The main contribution to the permittivity comes from a strong relaxation (ν01) present in the GHz range at high temperatures which slows down on cooling following the Arrhenius law. A second relaxation with lower frequency (ν02) slows down, as well, contributing to the permittivity mainly near Tm. Both these relaxations can be assigned to polar fluctuations, probably flipping and breathing of polar nanodomains.

Altogether, the four mechanisms explain, above the kHz range, the ferroelectric transition in SBN-81 as well as its relaxor character, which differs from the behaviour displayed by SBN-61 and lead-based relaxors.

Excitations in SBN
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Temperature dependence of the frequencies of main excitations in the tetragonal tungsten-bronze structure SBN-81, related to the microstructure of the crystal.