Phase Transition and Thermal Expansion of Ba₃RB₃O₉ (R = Sm-Yb, and Y)

High temperature powder X-ray diffraction measurements of Ba₃RB₃O₉ (R=Sm-Yb, and Y) were carried out at temperatures ranging from room temperature to just below the corresponding melting temperatures (1,200-1,300 °C). No phase transition was found for the H-type phase (R3) with R=Sm-Tb and the L-type phase (P6₃ cm) with R=Tm-Yb. On the other hand, phase transition from the L phase to the H phase was observed for R=Dy-Er, and Y at around 1,100-1,200 °C. The obtained axial thermal expansion coefficient (ATEC) of the a-axis was larger than that of the c-axis for the H phase, and the ATEC of the c-axis was larger than that of the a-axis for the L phase. The observed anisotropic nature of ATEC is attributed to the distribution of the BO₃ anionic group with rigid boron-oxygen bonding in the structures of the H and L phases.