A chemical contribution to the search for novel electronic properties in transition metal oxides: LiNiO2

ANiO2 (A=Li, Na; the layered α-NaFeO2 structure) contains Ni3+ (or Ni2+ L) with S=1/2 and a 2-fold orbital degeneracy in the two-dimensional (2D) triangular (/O/Ni/O/)− unit. Though the unit seems to be ferromagnetic (FM) and hence free from spin frustration, rhombohedral LiNiO2 does not show any well-defined long-range magnetic ordering. In contrast, NaNiO2 is known to undergo a cooperative Jahn–Teller distortion from rhombohedral to monoclinic symmetry at such a high temperature of 490 K and then allows an antiferromagnetic (AF) inter-unit ordering at TN=20 K. These structural and magnetic dissimilarities suggest a dimensional crossover controlled by the spacer, A+: It seems to be strengthened in the more compact Li-oxide enough to switch the orbital- and spin-ordered ground state realized in the Na-oxide (the isolated type) to another one (the coupled type) that remains somehow free from these instabilities.