Electron diffraction study of crystal structures of (Sr1-xBax)2Nb2 O7

Selected-area electron diffraction and convergent-beam electron diffraction (CBED) techniques are used to reveal the Ba doping effect on the crystal structure of (Sr1-xBax)2Nb2O7 [SBN(x)] in the temperature range from 293 to 693 K. Ba doping of Sr2Nb2O7 (SN) causes weakening of the incommensurate modulation and deviation from the C-centered lattice. On the other hand, symmetries of fundamental reflections in CBED patterns of SBN(x) are seen to be the same as those of SN, showing a small structural deviation from Cmc21. Crystal symmetries above and below the temperature of a relaxorlike dielectric anomaly in SBN (x=0.32) at 465 K remain unchanged. CBED experiments with a nanometer-sized electron probe do not show any indication of the presence of a nanodomain around this temperature. However, the presence of a few-micron-sized polar-inversion domains is found at around 573 K in SBN (x=0.32). The origin of the reported relaxorlike dielectric anomaly in SBN (x=0.32) is discussed on the basis of the experimental results of CBED.