Layer-Structural R₄Mo₇O₂₇ (R=Ce and Pr)Prepared by Thermal Degradation of R-Polyoxomolybdate Precursor

Abstract

Well-formed single crystals of R₄Mo₇O₂₇ (R=Ce and Pr), novel phases in the R₂O₃-MoO₃ system, have been obtained by thermal degradation of R-containing polyoxomolybdate precursor, [R₂(H₂O)₁₂Mo₈O₂₇]•nH₂O, in air at 800°C 12 h. Both compounds crystallize in a flux of the dehydrated precursor as a result of loss of excess MoO₃. The two species are isomorphous with space group C2/c, Z=8, a=4.6354(2) and 4.6046(2) nm, b=0.74853(3) and 0.74712(2) nm, c=1.43536(9) and 1.42640(5) nm, and β=100.897(2) and 101.006(2)°, for R₄Mo₇O₂₇ (R=Ce and Pr), respectively. The structure is built up with two kinds of {R(MoO₄)}₂ double-layers and one {Mo₃O₁₁} mono-layer, which are stacked alternately parallel to the (110) plane. An arrangement of [R(MoO₄)]⁺ unit is different between the two {R(MoO₄)}₂ double-layers. The Mo₃O₁₁ group in the mono-layer consists of one MoO₅ trigonal bipyramid corner-shared with two MoO₄ tetrahedra. In comparison with other rare earth molybdates, we found that several materials commonly have similar {R(MoO₄)}₂ double-layers. Details of their layer structures are discussed.