Study of the ferrodistorsive orbital ordering in NaNiO2 by neutron diffraction and submillimeter wave ESR

NaNiO2 has been studied by neutron-powder diffraction, magnetic susceptibil ity and submillimeter wave ESR. The monoclinic structure at room temperatur e is characterised by a ferrodistorsive orbital ordering due to the Jahn-Te ller (JT) effect of the Ni3+ ions in title low spin state. NaNiO2 undergoes a structural transition at around 480 EC, above which the orbital ordering disappears. The high temperature phase is rhombohedral with the layered al pha -NaFeO2 structure (R (3) over barm space group). The magnetic susceptib ility exhibits hysteresis and we observe a change of the Curie-Weiss law pa rameters above the JT transition. The anisotropy of the g-factor at 200 K c an be attributed to the JT effect which favours the \3z(2) - r(2)> orbital occupation. Finally, the interplay between the magnetic and structural prop erties of NaNiO2 and Li1-xNi1+xO2 is discussed.