Thermal expansion property of sintered polycrystals and unit cell of celsian solid solution (Ba,Sr)Al<sub>2</sub>Si<sub>2</sub>O<sub>8</sub>

The purpose of this work is to clarify the transformation mechanism from hexacelsian to celsian and the thermal expansion properties of celsian solid solutions (Ba 1-x Sr x Al 2 Si 2 O 8 , x = 0-1.0)using high-temperature powder X-ray diffraction and dilatometry of sintered polycrystals.Hexacelsian polycrystals were prepared from BaCO 3 , SrCO 3 and finely purified kaolin powder (Al 2 O 3 2SiO 2 2H 2 O) by heat-treatment at 950-1400C.Sr-hexacelsian polycrystals transformed to celsian via nucleation and growth from inner to surface to develop cracks at surface at 1100 -1200C.Single phase of celsian polycrystals in composition of Ba 1-x Sr x Al 2 Si 2 O 8 , x = 0 -0.8) were prepared by heat-treatment at 1400C for 1 hour and their dilatometric thermal expansion coefficient (TEC) were almost 4.0 -4.2x10 -6 /C from 25 to 900C.On the other hand, average TEC of unit cell of celsian measured by high temperature X-ray diffraction were lower than the values of dilatometry and increased from 3.0x10 -6 /C to 3.8x10 -6 /C with the Sr content.