Ag1/8Pr5/8MoO4: An incommensurately modulated scheelite-type structure

The crystal structure of a complex molybdenum oxide Ag1/8Pr5/8MoO4 is reported. The Ag1/8Pr5/8MoO4 sub-structure can be described on the base of the scheelite (CaWO4) structure. Transmission electron microscopy reveals that the real structure is better described in a (3+1)D formalism. According to electron diffraction study the new scheelite-type complex oxide Ag1/8Pr5/8MoO4 crystallizes in the B2/b(αβ0)00 (3+1)D superspace group with unit cell parameters image, image, image and image (image) and modulation vector image. The structure of Ag1/8Pr5/8MoO4 is refined from X-ray powder data in the scheelite setting I2/b(αβ0) with image, image, image with fixed image angle (image, image, image, image, image). Displacement modulations apply for all atoms. The occupancy modulation shows that one-fourth of the Ag/Pr atoms are absent. The structure can be considered as a crystallographic shear structure with incommensurate ordering of vacancies and displacement modulations for all atoms. The arrangement of Ag/Pr atoms and vacancies is at the origin of the incommensurate modulation in the cation-deficient Ag1/8Pr5/8MoO4 phase.