Orbital and Spin Order in the Triangular S=1/2 Layered Compound (LI,NA)NIO2

Magnetic phase diagram of theS= 1/2 triangular layered compound NaNiO2: a single crystal study

“…Macroscopic effects linked to twinning are then probably responsible, but difficult to evaluate due to the low symmetry of the system. Note that the critical field at which this regime disappears has the following angular dependence: H c1 (θ ) = H c1 / cos(θ − θ 01 ) with μ 0 H c1 = 1.7 T, in agreement with previous measurements [1,6]. The free spin direction is at θ 01 = −10 • , in agreement with the neutron diffraction data [7].…”

“…We have plotted, in figure 6, the critical fields obtained in this work, as well as in previous studies mainly on powder samples [9,11], except for the first magnetization measurements, which were taken on a twinned crystal up to 3 T [6]. At low temperature, five phases are present: phase (1) where the spin lies at 10 • from the Ni plane, phases (2) and (3) with different spin orientations, and finally phases (4) and (5), which are close to full saturation. Phases (1)-(3) disappear at T N = 20 K , but a crossover line persists above.…”

“…In magnetic systems with a two dimensional triangular lattice, magnetic frustration may occur depending on the nature of the magnetic interactions and the orbitals involved. The case of LiNiO 2 , where Ni 3+ has half filled e g orbitals, has been studied for a long time both experimentally and theoretically [1]. Recent progress on the understanding of the orbital and magnetic properties of this compound has been achieved thanks to an appropriate control of the non stoichiometry or doping effects since no pure samples are available.…”

And Yet It Moves: LiNiO2, a Dynamic Jahn–Teller System

“…The correlation between JT-active centers can of course be disrupted by lattice vibrations and temperature, so that there will always exist a critical temperature above which the centers become dynamically independent and the lattice will exhibit a more symmetric structure. For example, the parent compound NaNiO 2 exhibits a distorted monoclinic ( C 2/ m ) structure up to 480 K, when a rhombohedral ( R 3̅ m ) structure prevails . Moreover, real samples of LiNiO 2 are always reported to contain some amount of residual off-stoichiometry (Ni excess) from the synthesis, that is, extra Ni occupying the Li site as in Li 1– x Ni 1+ x O 2 , with x = 1–2% in the best cases .…”

“…This structure is universally observed by bulk sensitive experimental methods such as X-ray powder diffraction and neutron powder diffraction . However, this picture conflicts with the renowned Jahn–Teller (JT) activity of low-spin trivalent Ni, commonly observed in isostructural layered oxides such as NaNiO 2 . The existence of local JT distortions has been experimentally proven by means of X-ray absorption techniques and pair distribution functions. − Extended X-ray absorption fine structure data (EXAFS) clearly indicate the presence of four short (1.91 Å) and two long Ni–O bonds (2.09 Å) and support the occurrence of noncooperative JT distortions .…”