title: Optical and magnetic characterisation of Co3+ and Ni3+ in LaAlO3: interplay between the spin state and Jahn-Teller effect.
abstract: The coordination, the electronic structures and the spin of the ground state of Ni(3+) (3d(7)) and Co(3+) (3d(6)) introduced as impurities in LaAlO(3) 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 O(h) transition-metal site in LaAlO(3) locates Ni(3+) and Co(3+) 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 Ni(3+) and with a thermally populated high-spin low-lying first excited state in Co(3+). The corresponding JT distortions are derived from structural correlations. We conclude that the JT effect is unable to stabilise the intermediate spin state in Co(3+). A low-spin ground state in thermal equilibrium with a high-spin low-lying first excited state is detected in diluted Co(3+)-doped LaAlO(3). These results are compared with those obtained in the parent pure compounds LaNiO(3) and LaCoO(3).
authors.affiliation: MALTA Consolider Team, Unidad Asociada ICMA-CSIC, DCITIMAC, Facultad de Ciencias, Universidad de Cantabria, E-39005 Santander, Spain. sanzmn@unican.es
authors.forename: Marta N
authors.initials: MN
authors.lastname: Sanz-Ortiz
authors.forename: Fernando
authors.initials: F
authors.lastname: Rodríguez
authors.forename: Jesús
authors.initials: J
authors.lastname: Rodríguez
authors.forename: Gérard
authors.initials: G
authors.lastname: Demazeau