New oxides of the filled-Ti₂Ni type structure

Abstract

The metal atom ordering and oxygen occupancy of four compounds of the filled-Ti₂Ni type structure have been determined by single-crystal X-ray diffraction studies. Three additional phases have been identified and indexed from powder X-ray diffraction data. All the phases crystallize in space group $\text{Fd}\text{3}\text{m}$ with the following compositions: Zr₄Ni₂O ($\text{a = 12.1970(9)}\text{A}\text{̊}$), ZΓ₆Ni₄Ti₂O_0.6 ($\text{a = 12.0299(9)}\text{A}\text{̊}$), Nb₆Ni₆O ($\text{a = 11.2117(5)}\text{A}\text{̊}$), and Nb₆Ni₄Ta₂O₂ ($\text{a = 11.5813(3)}\text{A}\text{̊}$). Phases identified using powder data are Nb₄Ni₂O ($\text{a = 11.5933(3)}\text{A}\text{̊}$), Zr₄Cu₂O ($\text{a = 12.2659(7)}\text{A}\text{̊}$), and Zr₆Co₄Ti₂O ($\text{a = 11.8649(9)}\text{A}\text{̊}$). A comparison of the structures and oxygen occupancies is presented, as well as tentative explanations based on results of extended Hückel calculations. The role of interstitial oxygen as an electron acceptor and its effect on metal-metal bonding are considered.