Phase formation features in the systems M2MoO4–Fe2(MoO4)3 (M=Rb, Cs) and crystal structures of new double polymolybdates M3FeMo4O15

The systems M2MoO4–Fe2(MoO4)3 (M=Rb, Cs) were shown to be non-quasibinary joins of the systems M2O–Fe2O3–MoO3. New compounds M3FeMo4O15 were revealed along with the known MFe(MoO4)2 and M5Fe(MoO4)4. The unit cell parameters of the new compounds are a=11.6192(2), b=13.6801(3), c=9.7773(2) Å, β=92.964(1)°, space group P21/c, Z=4 (M=Rb) and a=11.5500(9), b=9.9929(7), c=14.513(1) Å, β=90.676(2)°, space group P21/n, Z=4 (M=Cs). In the structures of M3FeMo4O15 (M=Rb, Cs), a half of the FeO6 octahedra share two opposite edges with two MoO6 octahedra linked to other FeO6 octahedra through the bridged MoO4 tetrahedra by means of the common oxygen vertices to form the chains along the a axis. The difference between the structures is caused by diverse mutual arrangements of the adjacent polyhedral chains.