Abstract: The coordination, the electronic structures and the spin of the ground state of Ni3+ (3d7) and Co3+ (3d6) introduced as impurities in LaAlO3 are investigated through optical spectroscopy and magnetic measurements. The unusual trivalent valence state in both transition-metal ions was stabilised via a sol?gel process followed by high oxygen pressure treatments. We show that the crystal-field strength at the nearly Oh transition-metal site in LaAlO3 locates Ni3+ and Co3+ near the spin state crossover, yielding a low-spin ground state in both cases. We analyse how the interplay between the Jahn?Teller (JT) effect and the spin state affects the magnetic moment of the ion and its temperature dependence. The optical spectra reveal a JT effect associated with a low-spin ground state in Ni3+ and with a thermally populated high-spin low-lying first excited state in Co3+. The corresponding JT distortions are derived from structural correlations. We conclude that the JT effect is unable to stabilise the intermediate spin state in Co3+. A low-spin ground state in thermal equilibrium with a high-spin low-lying first excited state is detected in diluted Co3+-doped LaAlO3. These results are compared with those obtained in the parent pure compounds LaNiO3 and LaCoO3.

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