Spin-state transition and metal-insulator transition in La1-xEuxCoO3 -: art. no. 014443

We present a study of the structure, the electric resistivity, the magnetic susceptibility, and the thermal expansion of La1-xEuxCoO3. LaCoO3 shows a temperature-induced spin-state transition around 100 K and a metal-insulator transition around 500 K. Partial substitution of La3+ by the smaller Eu3+ causes chemical pressure and leads to a drastic increase of the spin gap from about 190 K in LaCoO3 to about 2000 K in EuCoO3, 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 Co3+ ions. Around the metal-insulator transition the magnetic susceptibility shows a similar increase for all x and approaches a doping-independent value around 1000 K, indicating that well above the metal-insulator transition the same spin state is approached for all x.