  The structural and magnetic properties of two mixed-valence cobaltites with
formal population of 0.30 Co$^{4+}$ ions per f.u.,
(Pr$_{1-y}$Y$_{y}$)$_{0.7}$Ca$_{0.3}$CoO$_3$ ($y=0$ and 0.15), have been
studied down to very low temperatures by means of the high-resolution neutron
diffraction, SQUID magnetometry and heat capacity measurements. The results are
interpreted within the scenario of the spin-state crossover from a
room-temperature mixture of the intermediate spin Co$^{3+}$ and low spin
Co$^{4+}$ (IS/LS) at the to the LS/LS mixture in the sample ground states. In
contrast to the yttrium free $y=0$ that retains the metallic-like character and
exhibits ferromagnetic ordering below 55 K, the doped system $y=0.15$ undergoes
a first-order metal-insulator transition at 132 K, during which not only the
crossover to low spin states but also a partial electron transfer from
Pr$^{3+}$ 4f to cobalt 3d states take place simultaneously. Taking into account
the non-magnetic character of LS Co$^{3+}$, such valence shift electronic
transition causes a magnetic dilution, formally to 0.12 LS Co$^{4+}$ or 0.12
$t_{2g}$ hole spins per f.u., which is the reason for an insulating, highly
non-uniform magnetic ground state without long-range order. Nevertheless, even
in that case there exists a relatively strong molecular field distributed over
all the crystal lattice. It is argued that the spontaneous FM order in $y=0$
and the existence of strong FM correlations in $y=0.15$ apparently contradict
the single $t_{2g}$ band character of LS/LS phase. The explanation we suggest
relies on a model of the defect induced, itinerant hole mediated magnetism,
where the defects are identified with the magnetic high-spin Co$^{3+}$ species
stabilized near oxygen vacancies.
