PREPARATION AND CRYSTAL-STRUCTURE OF RUTHENIUM METAPHOSPHATE RU(PO3)(3) WITH AN 8-FOLD SUPERSTRUCTURE - ANALYSIS OF STRUCTURAL FRUSTRATION WITH A SIMPLE-MODEL

Monoclinic Ru(PO3)(3) was Prepared as the third form of RuP3O9, and th e structure was determined by the single-crystal X-ray technique. It c ontains infinite spiral chains of polyphosphate ions and RuO6 octahedr a as found in other C-form metaphosphates, While other C-form metaphos phates have 3-fold superstructures, monoclinic Ru-(PO3)(3) has a super structure with an 8-fold b-axis. The space group is P2(1)/a, and the c ell parameters are a=10.578(5) Angstrom, b=51.154(5) Angstrom, c=9.394 (4) Angstrom, and beta=97.74(4)degrees. The comparison between the rea l structure and the parent subcell structure with the space group 12/a shows that the deviations of the O1 atoms, which bridge two phosphoru s atoms, are larger than those of other atoms, The displacements of th e O1 atoms exhibit a static longitudinal sine wave along the b-axis wi th a wavelength of b/3. The bond angle P-OI-P is 180 degrees in the pa rent subcell structure, and the shifts of the O1 atoms to attain more stable bond angles cause mechanical frustration, which leads to the su perstructure. The observed superstructures of the monoclinic RU(PO3)(3 ) and C-form metaphosphates can be explained by a simple analytical mo del, in which neighboring O1 atoms are mechanically coupled through po lyhedral chains to cause opposite displacements.