STRUCTURE AND OXYGEN STOICHIOMETRY IN SR3CO2O7-Y ESS-THAN-OR-EQUAL-TO-Y-LESS-THAN-OR-EQUAL-TO-1.22)

A new, oxygen-deficient Ruddlesden-Popper phase, Sr3Co2 0(7-y) (0.94 l ess than or equal to y less than or equal to 1.22) has been prepared a nd its structural characterization has been achieved by the Rietveld r efinement of powder X-ray and powder neutron diffraction data. At low y the system is tetragonal but as y increases a phase change to orthor hombic occurs. Sr3Co2O6.06 crystallizes in the space group I4/mmm, ado pting the Sr3Ti2O7 type structure, but with the oxygen vacancies lying exclusively on the site linking the CoO6 octahedra. This leads to an unusual square-pyramidal coordination for Co(III). The orthorhombic st ructures of the samples y = 1.22 and y = 1.06 have been determined usi ng powder neutron diffraction. Crystallizing in the space group Immm, the ordering of oxygen vacancies in the Ruddlesden-Popper phase, leads to a threefold tripling along the b-axis direction. (C) 1995 Academic Press, Inc.