A new high-temperature cubic fluorite-type phase Mo0.16Bi0.84O1.74 with a rare three-dimensional incommensurate modulation

A new high-temperature phase Mo0.16Bi0.84O0.17 was obtained by quenching from 800 degrees C, Its average structure is of the cubic Fm (3) over bar m flourite type with a = 5.6478(1) Angstrom and was refined from single-crystal Mo K alpha X-ray diffraction (XRD) data to a weighted R-value of 3.7% for 16 unique reflections and isotropic thermal displacement parameters refined for the metal atoms, U-iso = 0.0564(4) Angstrom(2), and the O atoms, U-iso = 0.186(6) Angstrom(2). The structure exhibits in addition a novel 3-D incommensurate modulation that was studied by electron diffraction and both powder and single crystal XRD, Extinction conditions indicate the symmetry P:Fm (3) over bar m:Fm (3) over bar m, which has previously only been reported once. The three q-vectors are along the unit cell axes and have lengths of 0.29256(6) unit cells in reciprocal space. The modulated structure is compared with the previously reported and similarly modulated structure of the solid solution NbxBi1-xO1.5+x, with 0.12 less than or equal to x less than or equal to 0.46 and symmetry P:Fm (3) over bar m: Fd (3) over bar m. (C) 2000 Academic Press.