Bi_0.5Na_0.5TiO₃ (BNT) and (Bi_0.5Na_0.5)_1-xBa_xTiO₃ (BNBT) belong to the group of actively investigated materials, which crystallize in a perovskite-type structure and show a number of interesting physical properties. 

One of the most interesting features of BNBT is the morphotropic phase boundary (MPB) in the range of about x=0.055 where both ferroelectric and piezoelectric properties are strongly enhanced. Therefore BNBT is a potential candidate to replace lead containing perovskites such as PZT, PZN-PT, PMN-PT, etc.

BNT crystallizes at T=1290°C. The high-temperature phase BNT is described by the structure of a cubic symmetry (space group ). Upon cooling BNT undergoes two phase transitions at ~580°C to the tetragonal (P4bm) phase and at ~320°C to the rhombohedral R3c (now discussed to be monoclinic Cc [1]).

Inside the binary systems Bi₂O₃-TiO₂ and Na₂O-TiO₂ there are a lot of different compounds, so that a very complex three-component BNT system can be expected. The knowledge about the phase diagram of BNT is essential for the synthesis of large and defect reduced crystals. The phase diagram is largely unexplored in BNT and BNBT, therefore first thermoanalytical data was gathered with DSC/TG-measurements. Both BNT and BNBT show a congruent melting behavior according to DSC-data, so a decreasing liquidus curve in the direction of higher Bi₂O₃ and Na₂O content can be expected.

BNBT single crystals of different compositions (x=0.055, x=0.07 and x=0.1) were grown by Top Seeded Solution Growth (TSSG) method in dimensions up to 16x16x4 mm³ using congruent melt composition. The growth process was initiated by supercooling of approximately 5K. This near equilibrium crystal growth led to crystals with a {100} habitus. The crystals chemical compositions were determined by electron microprobe analysis showing significantly lower Ba-content than melt content.

Contrary to BNBT, BNT single crystals were obtained using a flux composition with an excess of 10 wt%. of Na₂O and Bi₂O₃, lowering the melt temperature about 15K.

Both BNT and BNBT crystals were also obtained using µ-PD-method for the first time.

The phase transition sequence was investigated using heating cooling microscopy and optical birefringence microscopy. The deviations of the results from the literature data will be discussed in the poster.

[1] Gorfman, S., Glazer, A. M., Noguchi, Y., Miyayama, M., Luo, H., & Thomas, P. (2012). Observation of a low-symmetry phase in Na0.5Bi0.5TiO3 crystals by optical birefringence microscopy. J. Appl. Cryst. 45 , pp. 444-445.

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