DOPANT SEGREGATION IN BISMUTH-SODIUM-TITANATE SINGLE CRYSTALS INVESTIGATED BY ELECTRON PROBE MICRO ANALYSIS

摘要

Bismuth sodium titanate (BNT) and its solid solutions, e.g., with bismuth potassium titanate or barium titanate have evoked an outbreak of research activities because of the existence of an extended strain phenomenon under electric field that make such materials promising candidates for replacing lead zirconate titanate based ceramics in piezoelectric actuator applications. The investigation of structure-property relationships is hampered by a complex succession of phase transitions that are visible in the dielectric response but a difficult access to structural changes by powder diffraction methods due to a more or less pseudo-cubic system with minimal distortions to rhombohedral or tetragonal symmetries. In order to facilitate the investigation and determination of structural features and lattice parameters, single crystals of BNT derived compositions have been grown by a flux method. The BNT based compound was prepared as a powder by a mixed oxide process and subsequently re-crystallized in a Bi_2O_3-K_2CO_3 flux. The single crystals were grown at temperatures between 1,300 and 1,350 ℃. In a multicomponent system the re-crystallisation of the elements depends on solubility and chemical affinity. Therefore, electron probe microanalysis was performed to check the stoichiometry and homogeneity of the single crystals obtained. Measurements were done on a Jeol JXA8200 "Superprobe" electron probe microanalyzer equipped with five wavelength-dispersive X-ray spectrometers for simultaneous elemental mapping. Concentration gradients and zoning could be shown for the main elements bismuth, sodium and potassium, but also for dopants like neodymium on the level of 2 mol%. With this investigation the capability of this tool to monitor single crystal quality is demonstrated.