The Bi(1-x)Ln(x)O(1.5) solid solutions (Ln = La, Pr, Nd), of the beta(2)/beta(1)/epsilon (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 Bi(4)Ln(2)O(9) epsilon phase (x = 0.33) appears to be the single definite compound. For Bi4La2O9, Bi4Pr2O9 and Bi4Nd2O9, the epsilon-type cells are respectively: a = 9.484(4) Angstrom, b = 3.982(2) Angstrom, c = 7.030(3) Angstrom, beta = 104.75(3)degrees; a = 9.470(5) Angstrom, b = 3.945(2) Angstrom, c = 6.968(4) Angstrom, beta = 104.73(3)degrees and a = 9.439(3) Angstrom, b = 3.944(2) Angstrom, c = 6.923(2) Angstrom, beta = 105.03(3)degrees. Upon heating, each monoclinic (epsilon) compound transforms successively into rhombohedral phases (beta(2)/beta(1)) 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 epsilon --> beta(2) can be incomplete. These transformations are characterized using X-ray thermodiffractometry, differential thermal analysis, dilatometry and impedance spectroscopy versus temperature. Examination of Bi-4(Ln, Ln')(2)O-9 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 less than or equal to x less than or equal to 0.40) is proposed. (C) 2002 Elsevier Science Ltd. All rights reserved.