Optical and magnetic characterisation of Co³⁺ and Ni³⁺ in LaAlO₃: interplay between the spin state and Jahn-Teller effect

Marta N. Sanz-Ortiz (1) , Fernando Rodríguez (1) , Jesús Rodríguez (1) , Gérard Demazeau (2)

1 MALTA Consolider Team, Facultad de Ciencias
2 ICMCB - Institut de Chimie de la Matière Condensée de Bordeaux

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 O_h 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.

Mots clés: Inorganic compounds; Jahn-Teller effect; Crystal impurities; Spin; Lanthanum; Oxides

Domaines: Matériaux

Marta N. Sanz-Ortiz, Fernando Rodríguez, Jesús Rodríguez, Gérard Demazeau. Optical and magnetic characterisation of Co³⁺ and Ni³⁺ in LaAlO₃: interplay between the spin state and Jahn-Teller effect. Journal of Physics: Condensed Matter, 2011, 23 (41), 415501 (6 p.). ⟨10.1088/0953-8984/23/41/415501⟩. ⟨hal-00636362⟩