Evidence for a new monoclinic Bi-La oxide solid solution: Identification and structural relationship to the rhombohedral Bi-Sr-O type

As a term of the Bi-Sr-O rhombohedral type Bi1-yLayO1.5 solid solution, the Bi17La4O31.5 mixed oxide (y no, 0.19) has been investigated, Air-quenching from the high temperature domain of the beta(1) disordered form allows one to preserve at room temperature a metastable form of beta(1). This form tr ansforms exothermally on heating near 200 degrees C to a new monoclinic pha se. This transformation and the transition monoclinic phase --> stable orde red rhombohedral beta(2), which occurs at 445 degrees C, are both irreversi ble on cooling for this composition. The beta(2) --> beta(1) transition occ urs reversibly near 660 degrees C, under usual dynamic conditions (300 degr ees h(-1)). The monoclinic cell lattice constants, a = 9.492(2), b = 7.951( 2), c = 7.028(2) Angstrom, beta = 104.74(1)degrees, lead to Z = 12 for the unit formula Bi0.81La0.19O1.5 and imply a disordered cationic sublattice. T his mixed oxide appears as a term of a new variety of a aide solid solution (that will name epsilon hereafter). (C) 2000 Academic Press.