Abstract
Full sets of elastic constants ${c}_{ij}$ of tetragonal tungsten bronze relaxor ferroelectric Ca${}_{0.28}$Ba${}_{0.72}$Nb${}_{2}$O${}_{6}$ (CBN-28) single crystals are measured above Curie temperature up to 1503 K employing resonant ultrasound spectroscopy. Thermal expansion measurements on as-grown unpoled CBN-28 reveal the existence of a characteristic temperature $T$ * (\ensuremath{\sim}800 K) for CBN-28 between the Burns temperature ${T}_{b}$ (\ensuremath{\sim}1100 K) and the temperature of maximum dielectric permittivity ${T}_{m}$ (\ensuremath{\sim}600 K). The influence of polar nanoregions (PNRs) on the elastic properties of CBN-28 is studied in detail. The temperature evolution of ${c}_{ij}$ shows pronounced anomalies. All independent elastic constants evolved differently, with temperature reflecting their coupling to different types of the reorientational motion of PNRs through their interaction with acoustic waves. The anisotropy of longitudinal elastic stiffness coefficients and the deviation from Cauchy relations for CBN-28 are also studied, showing the evolution of material anisotropy and the nature of bonding interactions with temperature, respectively.
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