Two high-temperature paraelectric phases in Sr_0.85Bi_2.1Ta₂O₉

Sr0.85Bi2.1Ta2O9 is shown, by Rietveld refinement of powder neutron-diffraction data, to undergo two phase transitions at elevated temperature. The low-temperature ferroelectric phase (space group A2(1)am) transforms to a paraelectric phase (Amam) at T(c)similar to 375 degreesC. This phase does not transform to the expected tetragonal phase (I4/mmm) until T similar to 550 degreesC. The transition from A2(1)am to Amam involves not only loss of the displacive mode along the polar a axis, but also loss of a TaO6 octahedral tilt mode around the c axis.