Polymorphism of Bi1−xLnxO1.5 phases (0<x<0.40): Characterization of a new compound Bi4Ln2O9 (x=0.33; Ln=La, Pr, Nd)

The Bi1−xLnxO1.5 solid solutions (Ln=La, Pr, Nd), of the β2/β1/ε (Bi–Sr–O) structural type, have been investigated in their Ln-rich domains. For Ln=La, Pr, and Nd, the upper limits are 0.35, 0.35 and 0.33, respectively. The Bi4Ln2O9 ε phase (x=0.33) appears to be the single definite compound. For Bi4La2O9, Bi4Pr2O9 and Bi4Nd2O9, the ε-type cells are respectively: a=9.484(4) Å, b=3.982(2) Å, c=7.030(3) Å, β=104.75(3)°; a=9.470(5) Å, b=3.945(2) Å, c=6.968(4) Å, β=104.73(3)° and a=9.439(3) Å, b=3.944(2) Å, c=6.923(2) Å, β=105.03(3)°. Upon heating, each monoclinic (ε) compound transforms successively into rhombohedral phases (β2/β1) and finally into a cubic fluorite-type phase. For La- and Pr-based compounds, all transitions are reversible; for Nd, depending on the thermal treatment, the reversibility of ε→β2 can be incomplete. These transformations are characterized using X-ray thermodiffractometry, differential thermal analysis, dilatometry and impedance spectroscopy versus temperature. Examination of Bi4(Ln, Ln′)2O9 samples allows to correlate the evolution of the thermal behavior and of the unit cell parameters, to the lanthanide size. A partial plot of the (Bi2O3)1−x–(La2O3)x phase diagram (0≤x≤0.40) is proposed.