Spin-state transition and metal-insulator transition in La 1 − x Eu x Co O 3

We present a study of the structure, the electric resistivity, the magnetic susceptibility, and the thermal expansion of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Eu}}_{x}\mathrm{Co}{\mathrm{O}}_{3}$. $\mathrm{La}\mathrm{Co}{\mathrm{O}}_{3}$ shows a temperature-induced spin-state transition around $100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and a metal-insulator transition around $500\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Partial substitution of ${\mathrm{La}}^{3+}$ by the smaller ${\mathrm{Eu}}^{3+}$ causes chemical pressure and leads to a drastic increase of the spin gap from about $190\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in $\mathrm{La}\mathrm{Co}{\mathrm{O}}_{3}$ to about $2000\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in $\mathrm{Eu}\mathrm{Co}{\mathrm{O}}_{3}$, so that the spin-state transition is shifted to much higher temperatures. A combined analysis of thermal expansion and susceptibility gives evidence that the spin-state transition has to be attributed to a population of an intermediate-spin state without orbital degeneracy for $xl0.5$ and with orbital degeneracy for larger $x$. In contrast to the spin-state transition, the metal-insulator transition is shifted only moderately to higher temperatures with increasing Eu content, showing that the metal-insulator transition occurs independently from the spin-state distribution of the ${\mathrm{Co}}^{3+}$ ions. Around the metal-insulator transition the magnetic susceptibility shows a similar increase for all $x$ and approaches a doping-independent value around $1000\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, indicating that well above the metal-insulator transition the same spin state is approached for all $x$.