Bi17Yb7O36 and BiYbO3: two new compounds from the Bi2O3–Yb2O3 equilibrium phase diagram determination

Synthesis and thermal behavior investigation of Bi1−xYbxO1.5 materials (0.166 ≤ x ≤ 0.666) have been realized, in order to obtain the low temperature equilibrium phases. A plot of the Bi2O3–Yb2O3 equilibrium phase diagram is proposed. Two new compounds, structurally related to the δ-Bi2O3 variety, are evidenced. Bi17Yb7O36 (x = 0.292) crystallizes in an orthorhombic cell with a = 16.259(2), b = 10.705(1), c = 5.487(1) Å. It transforms into the cubic δ-Bi2O3 type variety over 855 °C. Only this cubic form, easily quenchable in a metastable form, was reported up to now as a composition of a δ solid solution domain (δ). A narrow domain of orthorhombic solid solution (ε) could exist (0.275 ≤ x ≤ 0.305). A second compound BiYbO3 crystallizes in a triclinic cell with a = 8.487(1), b = 8.419(1), c = 9.928(1) Å, α = 86.03(1), β = 111.29(1), γ = 74.25(1)°. At 970 °C, a peritectoı̈d transformation into a mixture of δ and Yb2O3-based solid solution (Yb2O3-s.s.) occurs; this latter disappears at about 1200 °C. This last event is easily reversible on cooling, whereas the first one is very sluggish. Consequently, for the composition domain x > 0.35, the samples cooled from the high temperature are generally constituted of a mixture of δ-type phase and Yb2O3-s.s.