Well-formed single crystals of
R4Mo
7O
27 (
R=Ce and Pr), novel phases in the
R2O
3-MoO
3 system, have been obtained by thermal degradation of
R-containing polyoxomolybdate precursor, [
R2(H
2O)
12Mo
8O
27]•
nH
2O, in air at 800°C 12 h. Both compounds crystallize in a flux of the dehydrated precursor as a result of loss of excess MoO
3. 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
R4Mo
7O
27 (
R=Ce and Pr), respectively. The structure is built up with two kinds of {
R(MoO
4)}
2 double-layers and one {Mo
3O
11} mono-layer, which are stacked alternately parallel to the (110) plane. An arrangement of [
R(MoO
4)]
+ unit is different between the two {
R(MoO
4)}
2 double-layers. The Mo
3O
11 group in the mono-layer consists of one MoO
5 trigonal bipyramid corner-shared with two MoO
4 tetrahedra. In comparison with other rare earth molybdates, we found that several materials commonly have similar {
R(MoO
4)}
2 double-layers. Details of their layer structures are discussed.
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