Structural phenomena associated with the spin-state transition in LaCoO₃

The structural properties of LaCoO₃ were studied by means of high-resolution neutron powder diffraction in the temperature range 5<~T<~1000K. Changes of the Co⁺³ spin states in this temperature interval are shown to affect not only the unit-cell volume, as previously known, but also internal parameters such as the metal-oxygen bond lengths. These data, as well as the temperature-dependent magnetic susceptibility, can be qualitatively modeled based on a three-state (low-spin, intermediate-spin, and high-spin) activated behavior, but correction terms are required for quantitative agreement. Our fits consistently indicate that the ionic radius of the intermediate-spin state (∼0.56 Å) is smaller than the low-spin/high-spin average (∼0.58 Å). We also present evidence of a third lattice anomaly, occurring around 800 K, which we attribute to the formation of oxygen vacancies.