Spin-state transition and metal-insulator transition in ${\mathrm{La}}_{1-x}{\mathrm{Eu}}_x\mathrm{Co}{\mathrm{O}}_3$

We present a study of the structure, the electric resistivity, the magnetic susceptibility, and the thermal expansion of ${\mathrm{La}}_{1-x}{\mathrm{Eu}}_x\mathrm{Co}{\mathrm{O}}_3$. $\mathrm{La}\mathrm{Co}{\mathrm{O}}_3$ shows a temperature-induced spin-state transition around $100\mathrm{K}$ and a metal-insulator transition around $500\mathrm{K}$. 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 $190\mathrm{K}$ in $\mathrm{La}\mathrm{Co}{\mathrm{O}}_3$ to about $2000\mathrm{K}$ 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 $x<0.5$ 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 $1000\mathrm{K}$, indicating that well above the metal-insulator transition the same spin state is approached for all $x$.

Figure 1

(Color online) Electrical resistivity of ${\mathrm{La}}_{1-x}{\mathrm{Eu}}_x\mathrm{Co}{\mathrm{O}}_3$ as a function of temperature for different $x$. The inset shows the same data on a reciprocal temperature scale.

Figure 2

(Color online) Magnetic susceptibility of $\mathrm{Eu}\mathrm{Co}{\mathrm{O}}_3$ and $\mathrm{La}\mathrm{Co}{\mathrm{O}}_3$ as a function of temperature (symbols) together with fits (solid lines) of the respective background contributions [Eq. (1)]. The inset shows an expanded view of the low-temperature susceptibility of $\mathrm{Eu}\mathrm{Co}{\mathrm{O}}_3$, which is almost entirely given by the ${\mathrm{Eu}}^{3+}$ van Vleck contribution shown by the dashed line.

Figure 3

(Color online) The upper panel presents the magnetic susceptibility of ${\mathrm{La}}_{1-x}{\mathrm{Eu}}_x\mathrm{Co}{\mathrm{O}}_3$ as a function of temperature. With increasing Eu content the susceptibility at low temperatures is more and more dominated by the van Vleck contribution of the ${\mathrm{Eu}}^{3+}$ ions. The lower panel shows the Curie susceptibility of the ${\mathrm{Co}}^{3+}$ ions after subtracting a background susceptibility according to Eq. (1) (see text).

Figure 4

(Color online) (Upper panel) Thermal expansion $\alpha$ of ${\mathrm{La}}_{1-x}{\mathrm{Eu}}_x\mathrm{Co}{\mathrm{O}}_3$ (solid lines) and ${\mathrm{La}}_{0.82}{\mathrm{Sr}}_{0.18}\mathrm{Co}{\mathrm{O}}_3$ (dashed line). (Lower panel) Anomalous thermal expansion $\Delta \alpha =\alpha -{\alpha }_{\mathrm{bg}}$ of ${\mathrm{La}}_{1-x}{\mathrm{Eu}}_x\mathrm{Co}{\mathrm{O}}_3$ for $0⩽x⩽0.25$ (solid lines), where the averaged $\alpha$ of ${\mathrm{La}}_{1-x}{\mathrm{Eu}}_x\mathrm{Co}{\mathrm{O}}_3$ with $0.5⩽x⩽1$ has been used as ${\alpha }_{\mathrm{bg}}$. The dashed line shows $\Delta \alpha$ of $\mathrm{La}\mathrm{Co}{\mathrm{O}}_3$ which was analyzed in Ref. 15 using $\alpha$ of ${\mathrm{La}}_{0.82}{\mathrm{Sr}}_{0.18}\mathrm{Co}{\mathrm{O}}_3$ as ${\alpha }_{\mathrm{bg}}$. The symbols (●) are obtained from the ${\mathrm{Co}}^{3+}$ susceptibility data (see the lower panel of Fig. 3) via the scaling relation $C\bullet \Delta \alpha \left(T\right)=\partial ({\chi }_{\mathrm{Co}}\bullet T)∕\partial T$ with scaling factors $C$ between 190 and $210\mathrm{emu}\mathrm{K}∕\text{mole}$ (see Table ). For clarity, the curves for different Eu contents have been shifted by $-0.5\times {10}^{-5}∕\mathrm{K}$ with respect to each other.

Figure 5

(Color online) Curie susceptibility of the ${\mathrm{Co}}^{3+}$ ions (open circles, right $y$ axis) in comparison with the temperature dependence of the activation energy ${\Delta }_{\mathrm{act}}\simeq d\ln \rho ∕d\left(T^{-1}\right)$ (filled circles, left $y$ axis). The peak of ${\Delta }_{\mathrm{act}}$ signals the metal-insulator transition, which slightly shifts towards higher temperatures as marked by the dashed vertical line. The solid (dashed) lines are fits of ${\chi }_{\mathrm{Co}}\left(T\right)$ sufficiently below $T_{\mathrm{MI}}$ within a LS/IS scenario with $\nu =1$ $(\nu =3)$ [see Eq. (3) and text].

Figure 6

(Color online) Temperature dependence of the energy gaps $\Delta$ between the LS and the IS states obtained from Eq. (4) using $\nu =1$ for $x⩽0.5$ and $\nu =3$ for $x⩾0.75$ and $g=2.4$ for all $x$ (see text).