  The structural properties of LaCoO3 were studied by means of high-resolution
neutron powder diffraction in the temperature range 5<T<1000 K. 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 3-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 Angstroms) is
smaller than the low-spin/high-spin average (~0.58 Angstroms). We also present
evidence of a third lattice, anomaly, occurring around 800 K, which we
attribute to the formation of oxygen vacancies.
