Bi 17Yb 7O 36 and BiYbO 3: two new compounds from the Bi 2O 3–Yb 2O 3 equilibrium phase diagram determination

Abstract

Synthesis and thermal behavior investigation of Bi 1− x Yb x O 1.5 materials (0.166 ≤ x ≤ 0.666) have been realized, in order to obtain the low temperature equilibrium phases. A plot of the Bi 2O 3–Yb 2O 3 equilibrium phase diagram is proposed. Two new compounds, structurally related to the δ-Bi 2O 3 variety, are evidenced. Bi 17Yb 7O 36 ( 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-Bi 2O 3 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 BiYbO 3 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 Yb 2O 3-based solid solution (Yb 2O 3-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 Yb 2O 3-s.s.