NEW OXIDES OF THE FILLED-TI2NI TYPE-STRUCTURE

The metal atom ordering and oxygen occupancy of four compounds of the filled-Ti2Ni type structure have been determined by single-crystal X-r ay diffraction studies. Three additional phases have been identified a nd indexed from powder X-ray diffraction data. All the phases crystall ize in space group Fd3mBAR with the following compositions: Zr4Ni2O (a = 12.1970(9) angstrom), Zr6Ni4Ti2O0.6 (a = 12.0299(9) angstrom), Nb6N i6O (a = 11.2117(5) angstrom), and Nb6Ni4Ta2O2 (a = 11.5813(3) angstro m). Phases identified using powder data are Nb4Ni2O (a = 11.5933(3) an gstrom), Zr4Cu2O (a = 12.2659(7) angstrom), and Zr6Co4Ti2O (a=11.8649( 9) angstrom). A comparison of the structures and oxygen occupancies is presented, as well as tentative explanations based on results of exte nded Huckel calculations. The role of interstitial oxygen as an electr on acceptor and its effect on metal-metal bonding are considered.