A powder X-ray diffraction study, combined with magnetic susceptibility and electric
transport measurements, was performed on a series of LnCoO3 perovskites (Ln = Y, Dy, Gd, Sm, Nd,
Pr and La) over a temperature range 100–1000 K. A non-standard temperature dependence of the
observed thermal expansion was modelled as a sum of three contributions: (1) weighted sum of
lattice expansions of the cobaltite in the diamagnetic low spin state and in the intermediate
(IS) or high (HS) spin state. (2) An anomalous expansion due to the increasing population of
excited (IS or HS) states of Co3+ ions over the course of the diamagnetic-paramagnetic
transition. (3) An anomalous expansion due to excitations of Co3+ ions to another
paramagnetic state accompanied by an insulator-metal transition.
The anomalous expansion is governed by parameters that are found to vary linearly with the Ln
ionic radius. In the case of the first magnetic transition it is the energy splitting E
between the ground low spin state and the excited state, presumably the intermediate spin state.
The energy splitting E, determined by a fit to magnetic susceptibility, decreases with
temperature. The values of E determined for LaCoO3 and YCoO3 at T=0 K are 164 K and
2875 K respectively, which fall to zero at T=230 K for LaCoO3 and 860 K for YCoO3. The second
anomalous expansion connected with a simultaneous magnetic and insulator-metal transition is
characterized by its center at T=535 K for LaCoO3 and 800 K for YCoO3. The change of the unit
cell volume during each transition is independent of the Ln cation and is about 1% in both
cases.
