Transition metals, rare earths and actinides share the occurrence of local spin and orbital moments, due to strong intra-atomic electrostatic interactions. In the 3d transition metal ions these local interactions are similar in magnitude to the interatomic interactions, which gives rise to a range of ground states with different spin and orbital moments and affected by spin-orbit coupling and charge transfer. In transition metal oxide systems, these states act as the local components of the band structure and the resulting orbital and magnetic ordering. The determination of the spin moment implies (in most cases) the determination of the ground state symmetry. Recent experiments on mixed valent Mn, Fe and Co oxides show that their basic understanding is still far from clear. Because these systems play a crucial role in, for example, devices, sensors, catalysts and batteries, a more unified knowledge of their fundamental properties would be helpful.

The spin moment of transition metals can be determined from various synchrotron X-ray methods. Vanko et al. use Kß X-ray emission to probe the high-spin and low-spin state of FeII. They study switchable spin-crossover complexes that can be trapped in a long-lived metastable state that has a different spin moment than the ground state. The interesting finding was that the X-rays themselves can be used to bring the FeII ions into their excited state. It is proposed that irradiation with hard X-rays can be exploited to populate metastable states when optical excitation is limited by the characteristics of the sample or by its environment, such as for nontransparent samples or under extreme conditions.

Sikora et al. use the Kb X-ray emission spectra to study the spin state evolution of the transition metals in the LaMn1-xCoxO3 perovskite series. They show that the spin moment can be derived from Kß X-ray emission spectra by calculations and with reference systems. The average spin states of Mn and Co can be quantified using the integrals of the absolute values of the difference spectra (IAD), as developed by Vanko. The Kß X-ray emission spectra reveal that the spin state of Mn changes from S=2 MnIII in LaMnO3 to S=3/2 MnIV in LaCo0.98Mn0.02O3. Concurrently, the spin state of Co at room temperature changes from S=3/2 CoII in LaCo0.02Mn0.98O3 to S=1/2 CoIII in LaCoO3, with a further decrease to S=0 CoIII at 10K. A CoIII ion is highly covalent and has a charge transfer energy close to zero. This suggests that the S=0 low-spin state with a t2g6eg0 configuration mixes with an S=1 intermediate state t2g5eg1 configuration via the charge transfer state with a t2g6eg1 L configuration. This S=1/2 CoIII state can be seen as a mixed spin state between the low-spin and intermediate-spin state. Sikora et al. also use K edge X-ray absorption to determine the spin state, using the determination of the average valence from the edge position and also the assumption that the average valence can be transferred to the average spin state. This method works fine for manganese but yields results different from Kß X-ray emission in the case of cobalt. This indicates that the room temperature spin configuration of the CoIII is altered upon doping.

An alternative to probing the spin state is to use nuclear resonance scattering. McCammon et al. show the use of Mössbauer and nuclear forward scattering (NFS) at high temperature and pressure to study materials that exist in the lower mantle, i.e. Mg0.88Fe0.12SiO3 and Mg0.86Fe0.14Si0.98Al0.02O3 oxides. The main valence of iron in these 8-fold (distorted) cubic systems is FeII and a transition is visible from S=2 high-spin FeII with an eg3 t2g3 configuration to a S=1 intermediate spin FeII with an eg4 t2g2 configuration between 40 and 80 GPa. Because of charge transfer both FeII states mix with a eg4 t2g3 L configuration and can generate a mixed spin state, where the mixing of the components depends on pressure. The experimental NFS data show that elevated temperatures stabilise the intermediate-spin state, implying that FeII in silicate perovskite is predominantly in this state throughout most of the lower mantle.

F.M.F. de Groot, Department of Inorganic Chemistry and Catalysis, Utrecht University, The Netherlands.