Coexisting polarization mechanisms in ferroelectric uniaxial tetragonal tungsten bronze Ca_0.3Ba_0.7Nb_2O_6

TL;DR

This study reveals that Ca0.3Ba0.7Nb2O6 exhibits a complex ferroelectric phase transition driven by multiple polarization mechanisms, including phonons, anharmonic vibrations, and nanometric polarization fluctuations, without relaxor behavior.

Contribution

It demonstrates the coexistence of multiple polarization mechanisms in Ca0.3Ba0.7Nb2O6 and characterizes their roles in the phase transition using broad band dielectric spectroscopy and vibrational analysis.

Findings

Identification of mixed displacive and order-disorder phase transition mechanisms.

Detection of a soft central mode and nanometric polarization fluctuations.

Comparison showing similar mechanisms in related tungsten bronze compounds.

Abstract

Using a broad band dielectric spectroscopy approach (1 to 10^14 Hz) we prove that the tungsten bronze Ca0.3Ba0.7Nb2O6 (CBN-30) displays a ferroelectric phase transition of mixed displacive and order-disorder character, and its paraelectric phase does not show traces of relaxor behaviour but precursor effects as polar fluctuations below about 550 K. The analysis of the sub-MHz dielectric response together with infrared and Raman spectroscopy reveals that simultaneous polarization mechanisms are responsible for the phase transition. The comparison of the excitations found in CBN-30 with those of (Sr,Ba)Nb2O6 reveals that these mechanisms are congruous, although in CBN-30 the main relaxation process behaves differently due to the different domain structure. The excitations are phenomenologically assigned to phonons, to an anharmonic vibration of cationic origin which plays the role of a soft central mode, and to a relaxation in the GHz range probably due to polarization fluctuations of nanometric size which carries the main part of the permittivity and splits below TC into several weaker excitations with different polarization correlation lengths. The overall dielectric response is therefore explained by the coexistence of several excitations with different thermal behaviors, corroborating the complexity of the tetragonal tungsten bronze structures.