Synchrotron powder diffraction methods have been used to determine the structure of the layered ferroelectric oxide ${\mathrm{BaBi}}_2{\mathrm{Nb}}_2{\mathrm{O}}_9$ (BBN) above and below its Curie temperature. Although BBN is found to be tetragonal above and below the ferroelectric-paraelectric transition, there is clear evidence for a structural phase transition. The low-temperature ferroelectric structure is found to be well described in space group $I4mm,$ while the high-temperature structure is in the more common $I4/mmm$ space group. The identification of the low-temperature tetragonal structure resolves a long-standing question as to the origin of ferroelectric behavior in BBN and related oxides.

• Received 2 April 2002

DOI:https://doi.org/10.1103/PhysRevB.66.212102