Ab initio determination of the novel perovskite-related structure of La7Mo7O30 from powder diffraction

A new mixed valence molybdate, La7Mo7O30, first prepared by high energy bal l milling, has been successfully synthesized by controlled hydrogen reducti on of La2Mo2O9. Its original crystal structure was determined from X-ray an d neutron powder diffraction (space group R-(3 over bar); a = b = 17.0051(2 ) Angstrom, 6.8607(1) Angstrom; Z = 3;; reliability factors: R-p = 0.081, R -wp = 0.091, chi(2) = 3.1, R-Bragg = 0.049, R-F = 0.033). It consists in th e hexagonal stacking of individual cylinders of perovskite-type arrangement . These cylinders are built up from perovskite cages sharing corners in tra ns-position along their diagonal axis. Two different mixed-valence molybden um sites coexist, with more (Mo+5.75) Or less (Mo+4.5) distorted octahedral environments. Lanthanum atoms are located within the perovskite cages and around them, very close to their regular positions in the perovskite struct ure. Lanthanum and molybdenum atoms thus form two rows of almost perfect cu bes, shifted from each other by c/2. An electron microscopy study revealed the defect-free cationic and octahedral arrangements in the (a,b) plane. (C ) 1999 Academic Press.