A structural study of the (Na _{1− <i>x</i>} K _<i>x</i> ) _0.5 Bi _0.5 TiO ₃ perovskite series as a function of substitution ( <i>x</i> ) and temperature

Abstract

Rietveld neutron powder profile analysis of the (Na 1− x K x ) 0.5 Bi 0.5 TiO 3 (NKBT) series ( x =0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0) is reported over the temperature range 293–993 K. A detailed characterization of the structures and phase transitions occurring across this series as a function of temperature has been made. Room-temperature refinements have revealed a rhombohedral phase, space group R 3 c for x =0, 0.2, and 0.4, which exhibits an antiphase, a − a − a − oxygen tilt system with parallel cation displacements along [111] p . An intermediate zero-tilt rhombohedral phase, space group R 3 m possessing cation displacements along [111] p , has been established for x =0.5 and 0.6. At the potassium-rich end of the series at x =0.8 and 1.0, a tetragonal phase, space group P 4 mm is observed possessing cation displacements along [001]. At the sodium-rich end of the series for x =0.2, the unusual tetragonal structure with space group P 4 bm is seen for Na 0.5 Bi 0.5 TiO 3 which possesses a combination of in-phase a 0 a 0 c + tilts and antiparallel cation displacements along the polar axis. Temperature-induced phase transitions are reported and structural modifications are discussed.