Peculiar properties of barium diborate polycrystals

By crystallisation of stoichiometric composition glass two different crystal forms of BaO.2B 2 O 3 , the well known α-modification and a new β-one, can be obtained under the same temperature-time conditions depending on the pressure of dissolved gases and the state of the glass surface. The result depends on the presence or absence of dissolved gases in the glass and the state of its surface. Thermal expansion, density, and electric properties have been investigated in the range of 20-700°C for the glass and its crystal modifications. α-BaO.2B 2 O 3 , the known crystal modification, exhibits some peculiar properties. Thermal expansion anisotropy in the textured compact samples is so significant that the samples suffer contraction (the linear thermal expansion coefficient, TEC, is -49×10 -7 K -1 ) on heating, in a plane perpendicular to the crystal growth direction. The isotropic polycrystalline samples show dilatometric hysteresis, the range for the TEC lying from +110×10 -7 to -210×10 -7 K -1 . By proper heat treatment, compact isotropic samples of α-BaO.2B 2 O 3 can be prepared with predetermined TEC, down to a uniform contraction under heating of -34×10 -7 K -1 .