Two high-temperature paraelectric phases in ${\mathrm{Sr}}_{0.85}{\mathrm{Bi}}_{2.1}{\mathrm{Ta}}_2{\mathrm{O}}_9$

${\mathrm{Sr}}_{0.85}{\mathrm{Bi}}_{2.1}{\mathrm{Ta}}_2{\mathrm{O}}_9$ 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 ${A2\mathrm{}}_{1}am)$ transforms to a paraelectric phase (Amam) at $T_c\sim 375°\mathrm{C}.$ This phase does not transform to the expected tetragonal phase $\mathrm{}(I4/mmm)\mathrm{}$ until $T\sim 550°\mathrm{C}.$ The transition from ${A2\mathrm{}}_1\mathrm{am}$ to Amam involves not only loss of the displacive mode along the polar a axis, but also loss of a ${\mathrm{TaO}}_6$ octahedral tilt mode around the c axis.