Abstract: | High and Low Spin Behaviour of Ni3+?Ions in Octahedral Coordination The compounds Cs2NaNiF6, Cs2KNiF6, Rb2KNiF6, K3NiF6, and Na3NiF6 were investigated by ligand field and EPR spectroscopy between 298 and 4,2 K. These fluorides ? with the exception of the first one – crystallise in the cubic elpasolite lattice or in distorted modifications of this structure type and contain the Ni3+ ions in the low spin configuration te. This configuration is stabilised versus the high spin alternative te by an appreciable Jahn-Teller splitting of the 2Eg-state of about 7000 cm?1. The NiF6-octahedra are tetragonally elongated, the distortion being dynamical at 298 K. In case of the cubic compound Rb2KNiF6 a transition to a tetragonal structure with c/a > 1 as a consequence of a ferrodistortive Jahn-Teller ordering is observed at lower temperatures. It is calculated from the anisotropic g-parameters, that the first excited quartet level 4A2g(4T1g ? te) has an energy which is about 1000 cm?1 higher than that of the 2A1g(2Eg ? te) groundstate. Spin-orbit interactions between the energetically neighboured 2A1g(2Eg) and 4A2g, 4Eg(4T1g) states lead to third order contributions to the g-factors, which are very sensitive with respect to the doublet-quartet separation. In the hexagonal compound Cs2NaNiF6 finally, in which half of the Ni3+ ions occupy octahedral sites connected by common corners as in the other fluorides, while the other half is located in octahedral sites with common faces, high and low spin Ni3+ ions are found side by side. Obviously the latter half of these Ni3+ ions is geometrically restricted with respect to a Jahn-Teller distortion and hence the high spin configuration energetically favoured. |