Disorder in Strontium Barium Niobate: Local Structure, Phonons, and Polarization Dynamics

The main representative of the tetragonal tungsten bronze family, strontium barium niobate, has been recently studied by the scattering and spectroscopic techniques [diffuse scattering (DS), Raman, infrared, and high-frequency dielectric spectroscopies] to decipher the nature of its ferroelectric-relaxor transformation tuned by composition. DS revealed that mesoscopic range ordered correlations along the polar axis are presented in the crystal, being less organized in the relaxor compositions with higher Sr content. For these compositions, transverse polar correlations are found to be anisotropic. Pair distribution function experiments and first-principle calculations in several compositions showed a concomitant new peak around
_~
6.5 Å, which develops on cooling and cannot be explained by the current structural models with long-range incommensurate oxygen tilts but with a short-range ordered tilt arrangement. This relates with the development of the macroscopic polarization, tightly linked to the structure, and shows two different contributions related to the two different crystallographic sites for the ferroactive Nb atom, recalling a picture of two independent polarization networks showing disparate dynamics, temperature behavior, and correlation lengths. Cationic substitution in different channels affects mainly the relaxations. Above the MHz range, all members of the solid solutions show three main relaxations, whose temperature dependence is unalike, due to the distinct correlation lengths and nature of the processes. On increasing Sr content, the frequencies are higher, and the contribution to the permittivity of excitations at GHz-THz ranges gets stronger. A soft anharmonic central mode at THz range is the characteristic for all the compositions, together with the slowing down of a relaxation at GHz-MHz ranges, revealing the coexistence of displacive and order-disorder mechanisms of the ferroelectric phase transition in these solid solutions.