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The structural and magnetic properties of two mixed-valence cobaltites withformal 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 beenstudied down to very low temperatures by means of the high-resolution neutrondiffraction, SQUID magnetometry and heat capacity measurements. The results areinterpreted within the scenario of the spin-state crossover from aroom-temperature mixture of the intermediate spin Co$^{3+}$ and low spinCo$^{4+}$ (IS/LS) at the to the LS/LS mixture in the sample ground states. Incontrast to the yttrium free $y=0$ that retains the metallic-like character andexhibits ferromagnetic ordering below 55 K, the doped system $y=0.15$ undergoesa first-order metal-insulator transition at 132 K, during which not only thecrossover to low spin states but also a partial electron transfer fromPr$^{3+}$ 4f to cobalt 3d states take place simultaneously. Taking into accountthe non-magnetic character of LS Co$^{3+}$, such valence shift electronictransition 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, highlynon-uniform magnetic ground state without long-range order. Nevertheless, evenin that case there exists a relatively strong molecular field distributed overall 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 contradictthe single $t_{2g}$ band character of LS/LS phase. The explanation we suggestrelies on a model of the defect induced, itinerant hole mediated magnetism,where the defects are identified with the magnetic high-spin Co$^{3+}$ speciesstabilized near oxygen vacancies.

The spin-state crossover and low-temperature magnetic state in yttrium doped Pr$_{0.7}$Ca$_{0.3}$CoO$_3$