Ag 1/8Pr 5/8MoO 4: An incommensurately modulated scheelite-type structure

Abstract

The crystal structure of a complex molybdenum oxide Ag 1/8Pr 5/8MoO 4 is reported. The Ag 1/8Pr 5/8MoO 4 sub-structure can be described on the base of the scheelite (CaWO 4) 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 Ag 1/8Pr 5/8MoO 4 crystallizes in the B2/ b( αβ0)00 (3+1)D superspace group with unit cell parameters a ≈ 7.5 0 Å , b ≈ 5.3 0 Å , c ≈ 1 1.7 4 Å and γ ≈ 1 3 5 ° ( Z = 4 ) and modulation vector q = 0.5 6 a * + 0.5 9 b * . The structure of Ag 1/8Pr 5/8MoO 4 is refined from X-ray powder data in the scheelite setting I2/ b( αβ0) with a = b = 5.3 0 1 3 ( 4 ) Å , c = 1 1.7 4 0 7 ( 1 1 ) Å , q = 1.1 4 6 9 0 ( 1 4 ) a * + 0.5 8 9 2 1 ( 1 2 ) b * with fixed γ = 9 0 ° angle ( R p = 0.0 3 3 , R = 0.0 3 3 , R m = 0.0 2 9 , R 1 = 0.0 4 7 , S = 1.3 6 ). 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 Ag 1/8Pr 5/8MoO 4 phase.