Structural properties in Sr0.61a0.39Nb2O6 in the temperature range 10 K   to 500 K investigated by high-resolution neutron powder diffraction and   specific heat measurements
  We report high-resolution neutron powder diffraction on Sr0.61Ba0.39Nb2O6, SBN61, in the temperature range 15-500 K. The results indicate that the low-temperature anomalies (T<100K) observed in the dielectric dispersion are due to small changes in the incommensurate modulation of the NbO6-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 Tc approx. 350K start already at 200K, explaining the anisotropic thermal expansion in the temperature range 200-300K observed in a recent x-ray diffraction study. 
J. Schefer, D. Schaniel, V. Pomjakushin, U. Stuhr, V. Petricek, Th.   Woike, M. Wohlecke and M. Imlau