We report high-resolution neutron powder diffraction on ${\mathrm{Sr}}_{0.61}{\mathrm{Ba}}_{0.39}{\mathrm{Nb}}_2{\mathrm{O}}_6$ in the temperature range $15–500\mathrm{K}$. The results indicate that the low-temperature anomalies $(T⩽100\mathrm{K})$ observed in the dielectric dispersion are due to small changes in the incommensurate modulation of the $\mathrm{Nb}{\mathrm{O}}_6$ octahedra, as no structural phase transition of the average structure was observed. This interpretation is supported by specific heat measurements, which show no latent heat, but a glass-like behavior at low temperatures. Furthermore, we find that the structural changes connected with the ferroelectric phase transition at $T_c\simeq 350\mathrm{K}$ start already at $200\mathrm{K}$, explaining the anisotropic thermal expansion in the temperature range $200–300\mathrm{K}$ observed in a recent x-ray diffraction study.

• Received 1 June 2006

DOI:https://doi.org/10.1103/PhysRevB.74.134103