Spin and valence state equilibria of cobalt and magnetic properties of LaCoO 3

Abstract

On the basis of the spin and valence state equilibria and superexchange interaction of the various cobalt ions in LaCoO 3, an approximate semiempirical formula has been proposed and used to calculate magnetic susceptibilities of LaCoO 3 over a wide temperature range (100–1200 K). The results indicate that there are thermodynamic equilibria between the low spin state Co III (t 2g 6e g 0) ion, the high spin state Co 3+ (t 2g 4e g 2) ion, the Co II (t 2g 6e g 1) ion and the Co IV (t 2g 5e g 0) ion in LaCoO 3. The energy difference between the low spin state Co III and the high spin state Co 3+ is about 0.006 eV. The content of the low spin state Co III ion is predominant in LaCoO 3 and the content of the high spin state Co 3+ ion varies with temperature, reaching a maximum at about 350 K, then decreasing gradually with increasing temperature. At low temperature the contents of the Co II ion and the Co IV ion in LaCoO 3 are negligible, while above 200 K the contents of both the Co II ion and the Co IV ion increase with increasing temperature; however, the content of the Co II ion always is larger than that of the Co IV ion at any temperature. These calculated results are in good agreement with experimental results of the Mössbauer effect, magnetic susceptibility and electrical conductivity of LaCoO 3.