Examination of temperature dependent spin state transitions of Ca3Co4O9 compound

In this study, spin state transitions of Co ions in Ca3Co4O9 compound at high temperature region have been theoretically studied. Experimental data show that this compound has a magnetic anomaly in the interval of 700-1000 K. It is believed that this anomaly is due to spin state transitions of Co2+, Co3+ and Co4+ ions which may cause a long-range spin-spin interaction. In order to explain this anomaly, a detail in study is carried out for Co ions: for example symmetry properties, free ion energies and crystal field splitting energies (10 Dq) in an octahedral field. In order to determine which Co ion promote from low spin (LS) configuration to high spin (HS) configuration earlier than other in anomaly region, ions symmetry properties and crystal field splitting energies according to oxidation states have been modeled theoretically. The results show that, according to free ion energy terms of Co ions, Co4+ ion, ( ), has minimum energy. In addition, according to ground state energies of the LS and HS configurations of Co ions in an octahedral field, a HS-LS transition of Co4+ ions are required less energy. According to the symmetry properties, a LS-HS transition brings ¹A_1g→⁵T_2g for Co3+ ion and ²T_2g→⁶A_1g for Co4+ ion. By the symmetry it is predictable that Co4+ ion promote to HS configuration earlier because spherical symmetrical case more preferably. On the other hand according to the oxidation state for the crystal field splitting energy, LS Co4+ ions require more energy than LS Co3+ ions to promote HS configuration. Namely in terms of 10 Dq, to transition to HS configuration for Co4+ ions need more energy compared with Co3+ ions. All of the studies indicated that in this compound Co3+ ions promote to HS state earlier than Co4+ ions which need an experimental clarification.