Crystal Growth of Boron Sillenite Bi24B2O39

The Sillenite type Bi 24 B 2 O 39 is an incongruently melting compound at T p = 650 °C. Single crystals have been grown from non-stoichiometric melts as well as from high temperature solutions by the Czochralski method and by a top seeded solution growth technique (TSSG), respectively. The main difficulty in the crystal growth of Bi 24 B 2 O 39 arises from the very small field of crystallization in the binary system Bi 2 O 3 -B 2 O 3 . Further problems are caused by the nearly simultaneous formation of the 2:1 compound Bi 4 B 2 O 9 and the 12:1 compound Bi 24 B 2 O 39 . Therefore, a precise thermal reinvestigation of the phase diagram was carried out using DTA-technique on the Bi 2 O 3 -rich side. Additionally, crystal growth runs have been started in the ternary system Bi 2 O 3 -B 2 O 3 -Li 2 O in order to extend the crystallization field. Homogeneous melts were more difficult to prepare because of the high density difference between Bi 2 O 3 (ρ = 9.3 g/cm 3 ) and B 2 O 3 (ρ = 2.46 g/cm 3 ). The homogeneity of the melts were improved, using Bi 2 O 3 and synthesized Bi 4 B 2 O 9 (ρ = 8.25 g/cm 3 ) as starting materials. As a result of this procedure, small crystals of Bi 24 B 2 O 39 were grown from these starting materials and the lattice parameter were determined.