Coexisting polarization mechanisms in tetragonal tungsten bronze Ca_0.3Ba_0.7Nb₂O₆

Tetragonal tungsten bronzes are the second most important ferroelectric family after the perovskite one, and they give more versatility to play with multiferroic properties, as it has 5 different crystallographic sites to fill with different elements (see Figure 1). In this work we have used a broad band dielectric spectroscopy approach to study the dielectric response of one of these interesting compounds Ca_0.3Ba_0.7Nb₂O₆ (CBN3-30) within an impressive frequency range of 14 decades: from 1 Hz to 10¹⁴ Hz. We have proven that CBN-30 displays a ferroelectric phase transition of mixed displacive and order-disorder character, and that its paraelectric phase does not show traces of relaxor behaviour but precursor effects as polar fluctuations below about 550 K. This is partially attributed to the presence of Ca in the lattice, and its effect on maintaining the long-range polarization due to the Nb displacements along the main axis and suppressing the perpendicular displacements.

Description

Figure 1: Structure of CBN-30 in two orientations. Nb(1) are inside the linking octahedra in dark colour, and Nb(2) inside the perovskite-like octahedra in light colour.

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 main excitations have been phenomenologically assigned to phonons, to a soft anharmonic vibration of cationic origin, and to a relaxation in the GHz range related to polarization fluctuations of nanometric size. This GHz relaxation carries the main part of the permittivity at high temperatures in the paraelectric phase and on cooling it splits below T_C into several weaker excitations with different polarization correlation lengths. The comparison of the excitations found in CBN-30 with those of the famous (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 and the presence of more distorted oxygen octahedra network. The overall dielectric response was therefore explained by coexistence of several excitations with different thermal behaviors, corroborating the complexity of the tetragonal tungsten bronze structures.

Description

Figure 2: Temperature dependence of the frequencies of the main excitations in CBN-30 (a), and their dielectric contribution to the permittivity (b), together with selected experimental data.

Acknowledgement 
This work was partially supported by the Czech Academy of Sciences and the Lithuanian Academy of Sciences through the bilateral Project No. LAS-21–02. E.B. acknowledges support from the Ministry of Education, Youth, and Sports of the Czech Republic by the EU cofunded grant “Ferroic Multifunctionalities”, Project No. CZ.02.01.01/00/22_008/0004591. 

Reference 
[1] E. Buixaderas, Š. Svirskas, C. Kadlec, M. Savinov, P. Lapienytė, Anirudh K.R., C. Milesi-Brault, D. Nuzhnyy, and J. Dec Coexisting polarization mechanisms in ferroelectric uniaxial tetragonal tungsten bronze Ca_0.3Ba_0.7Nb₂O₆ (CBN-30) Phys. Rev. B 110, 104302 (2024).

Contact person: Elena Buixaderas