Phase equilibria in the Bi2TeO5Bi2SeO5 system and a high temperature neutron powder diffraction study of Bi2SeO5

Phase equilibria in the Bi2TeO5Bi2SeO5 system were studied by X-ray, DTA and second harmonic generation (SHG). The samples were synthesized by solid state reactions of the Bi, Te and Se oxides. The phase diagram is interpreted as a quasibinary peritectic one with wide ranges of solid solutions on the basis of both compounds. The SHG study showed Bi2SeO5 to undergo a phase transition at about 250 °C. Neutron diffraction (25–650 °C) showed no major changes in the structure of Bi2SeO5 at high temperatures. However, the analysis of the oxygen atom thermal factors and site occupancies suggested that the mechanism of the phase transformation is an order–disorder transition involving reorientation of the SeO3 group.