Incommensurately modulated structures of the M 1/8 Pr 5/8 MoO 4 ( M = Li, Na, K) scheelites

The effect of cation substitutions in the scheelite-type framework is investigated to understand the ordering of M+/Pr3+ cations and vacancies in the structure. The cation deficient M1/8Pr5/8□1/4MoO4 (M = Li, Na, K) phases were synthesized using solid state reactions. The K:Pr ratio in the K1/8Pr5/8MoO4 sample was determined by inductively coupled plasma mass spectroscopy, inductively coupled plasma optical emission spectrometry and total reflection X-ray fluorescence spectroscopy. The local element distributions in K1/8Pr5/8□1/4MoO4 were determined by energy-dispersive X-ray spectrometry. The incommensurately modulated crystal structures of the scheelite-based M1/8Pr5/8MoO4 (M = Li, Na, K) phases were refined from synchrotron powder X-ray diffraction data. The analysis revealed that cation ordering in the M1/8Pr5/8MoO4 (M = Li, Na) structures is incomplete and is more accurately described by continuous, rather than discontinuous (step-like), occupational modulation functions. The occupancy modulation waves for Li/Na and Pr atoms demonstrate an antiphase relation. Compared with M = Li, Na, the order of K and Pr cations in the A position in the K1/8Pr5/8MoO4 structure is best approximated by crenel functions. Refining the coordinates and lengths of three atomic domains yields the composition of K0.145Pr0.618MoO4. In all cases, the modulation arises from ordering of M/Pr cations and cation vacancies at the A-sublattice of the parent scheelite ABO4 structure. The distortion of PrO8 and MO8 polyhedra is practically independent of the radius of the M+ cations. The refinements of the M1/8Pr5/8MoO4 (M = Li, Na, K) structures reveal that MoO42- tetrahedra in these scheelite-type compounds demonstrate a flexible geometry. Both Mo-O bond distances and O-Mo-O bond angles vary significantly with changing the population of the A site by cations with different sizes.