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 fr om 800 degrees C, Its average structure is of the cubic Fm (3) over bar m f lourite type with a = 5.6478(1) Angstrom and was refined from single-crysta l 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 refine d for the metal atoms, U-iso = 0.0564(4) Angstrom(2), and the O atoms, U-is o = 0.186(6) Angstrom(2). The structure exhibits in addition a novel 3-D in commensurate modulation that was studied by electron diffraction and both p owder 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 report ed 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 co mpared with the previously reported and similarly modulated structure of th e solid solution NbxBi1-xO1.5+x, with 0.12 less than or equal to x less tha n or equal to 0.46 and symmetry P:Fm (3) over bar m: Fd (3) over bar m. (C) 2000 Academic Press.