Structures of three polymorphs of the complex oxide K 5Yb(MoO 4) 4

Abstract

Polymorphous modifications ( γ-, β- and α-) of the double potassium ytterbium molybdenum oxide K 5Yb(MoO 4) 4 were synthesized by the solid-state method and their structures were studied by X-ray powder diffraction, electron diffraction and high-resolution electron microscopy. DSC analysis shows that the γ→β↔α phase transitions are not accompanied with a significant reconstruction of the palmierite-type structure. All modifications of K 5Yb(MoO 4) 4 are related to the mineral palmierite—K 2Pb(SO 4) 2. The palmierite-type structure is made up of isolated AO 4 tetrahedra, which connect the MO n polyhedra into a 3-D framework via common vertices. Cations occupy two crystallographic positions M1 and M2. The γ-phase crystallizes in a monoclinic system (space group C2/ c) with unit-cell parameters: a=14.8236(1) Å, b=12.1293(1) Å, c=10.5151(1) Å, β=114.559(1)°, Z=4. The α-phase has space group R 3 ̄ m with unit-cell parameters: a=6.0372(1) Å, c=20.4045(2) Å. The structures of the γ- and α-modification were refined by the Rietveld method ( R wp=6.25%, R I=2.16% and R wp=9.09%, R I=5.80% for γ- and α-, respectively). In K 5Yb(MoO 4) 4 ytterbium cations occupy M1 while K + cations occupy M2 and M1 positions of the palmierite-type structure. In the high-temperature ( α-) modification the Yb 3+ and K + occupy the M1 site in a statistical manner ( M1=0.5Yb 3++0.5K +) while in the low-temperature ( γ-) modification these cations occupy this site in an ordered way. The intermediate β-phase shows an incommensurate modulated structure.