Temperature dependence of lattice parameters and anisotropic thermal expansion of bismuth oxide

Temperature dependence of lattice parameters of bismuth se-squioxide (Bi₂O₃) has been obtained between 26° and 778°C by the Rietveld method using neutron powder diffraction data. Lattice parameters a*_α, b*_α, c *_α, a_δ and unit-cell volume of α and δ phases increased, while the β angle of α phase decreased with an increase of temperature. Here the a*_α denotes the lattice parameter a of the α phase on the basis of pseudo-fluorite lattice. The thermal expansion coefficients of α phase were 26.7, 6.6, and 9.0 ( × 10^-6°C^-1) for a *_α, b*_α, and c *_α axes, respectively, indicating a large anisotropy. The α- to -δ phase transition of Bi₂O₃ was confirmed between 721° and 760°C on heating. At the α-δ transition point, the lattice parameters and unit-cell volume discontinuously changed, indicating that the transition is of the first order.