Investigation of the structure of the perovskite series (Na1-xKx)1/2Bi1/2TiO3
O'Brien, A. (Aoife)
The structures of the lead-free perovskites in the solid solution range\ud (KxNa1-x)0.5Bi0.5TiO3 (KNBT100x) have been investigated using high-resolution x-ray\ud diffraction (XRD) on polycrystalline samples. The room-temperature structure of\ud Na0.5Bi0.5TiO3 (NBT) has been studied, and identified as having monoclinic Cc symmetry,\ud with distortion very close to that of rhombohedral R3c. High-temperature XRD has been\ud used to identify a phase transition between the pseudo-rhombohedral phase and a\ud tetragonal P4bm phase at 300 °C. This transition has been found to occur via a pseudocubic\ud structure, although local variations in transition temperatures lead to the coexistence\ud of both phases, with a hysteresis of ~ 30 °C. The Curie temperature, TC, has been found to\ud lie in the range 450 °C – 530 °C. The room-temperature structures of compositions from\ud NBT to (K0.65Na0.35)0.5Bi0.5TiO3 have been studied, and a phase boundary identified at x ~ 0.2.\ud Compositions from NBT – KNBT20 refine as monoclinic Cc, with pseudo-rhombohedral\ud symmetry, whilst KNBT25-65 were found to have equilibrium phase P4mm, with a stressinduced\ud R3m phase also present. A relationship was observed between the spontaneous\ud tetragonal strain and the proportion of tetragonal phase present in these samples. KNBT10\ud and KNBT40 did not show evidence of the high-temperature P4bm phase; instead, TC values\ud were found to be ~ 225 °C and ~ 300 °C respectively. Experimental data have been\ud combined with data from published literature to construct a phase diagram of the KNBT\ud system.\ud KNBT synthesised via a hydrothermal method has been investigated. Hydrothermal\ud NBT, once heat-treated, has been found to show similar structure to solid state NBT and\ud undergoes the pseudo-rhombohedral to tetragonal transition at 305 °C, with a hysteresis of\ud ~ 20 °C between heating and cooling. TC was found to be higher, in the range 530 °C –\ud 700 °C. The XRD patterns of non-heat-treated hydrothermal KNBT have broader peaks and\ud a more undulating background, indicative of smaller crystallite size and diffuse scattering\ud from a more disordered material. The proposed source of this disorder is the presence of\ud oxygen vacancies, OH- groups, or A-site clustering. Significant peak splitting was observed\ud in the XRD patterns of hydrothermal K0.5Bi0.5TiO3 (KBT) and tetragonal KNBT. This was not\ud the case for solid state KNBT, showing a greater tetragonal distortion in materials prepared\ud via the hydrothermal method. Heat-treating this material was found to effect a significant\ud structural change, with single peaks remaining after cooling. It is proposed that this could\ud be caused by the release of OH- groups, recrystallisation, and mixing of the A-site species.
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