Spin-state transition and metal-insulator transition inLa1−xEuxCoO3

Abstract

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 La 3+ by the smaller Eu 3+ 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 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 K indicating that well above the metal-insulator transition the same spin state is approached for all x.

Phase diagram and isotope effect in(Pr1−yEuy)0.7Kalinov1, Gorbenko2, Талденков3 et al. 2010Phys. Rev. BSelf Cite154200View full textAdd to dashboardCite

“…Above approximately 25 K, a higher-spin state, either IS or HS, becomes thermally populated affecting various physical properties, e.g., magnetic susceptibility χ or thermal expansion α, which both exhibit pronounced peaks in their temperature dependence. [15][16][17] The susceptibility is naturally affected because the excited spin state (IS or HS) induces a significant increase in the magnetization. The thermal expansion is affected due to the different ionic radii: the LS Co 3+ ions with empty e g orbitals are significantly smaller than the IS or HS Co 3+ ions with partially filled e g orbitals.…”

“…[21][22][23][24] This led to the conclusion that the thermally excited state in undoped LaCoO 3 is most probably the HS state 21,22,25 in contrast to many earlier claims. 10,11,15,17 This conclusion is still disputed.…”

“…For example, the energy gap between the LS and the excited spin state increases from about ∆E = 185 K for R = La to ∆E ≥ 2000 K for R = Eu. 17 In addition, with decreasing ionic radius in the lanthanide series, the structure changes from rhombohedral in LaCoO 3 to orthorhombic for R = Pr, Nd, and Eu. 16 The changes in the ionic radius can be also finetuned by mixing R elements with different ionic radii.…”

“…In this situation, the partial substitution of R 3+ by a smaller R ′3+ in (R 1−y R ′ y ) 1−x A x CoO 3 systematically increases the chemical pressure, which should enhance the crystal field splitting and therefore stabilize the low-spin state. 17,34 Thus, small changes in the lattice characteristics may critically determine the physical properties of cobaltites.…”

“…The chosen stoichiometries span the mean rare-earth radius r A in the range from 1.172Å to 1.164Å, for y = 0.12 − 0.26. The end-member of this series, Eu 0.7 Ca 0.3 CoO 3 , was also studied and some results obtained earlier 16,17 on RCoO 3 with R = La, Pr, Nd, Eu are referred below for comparison. The (Pr 1−y Eu y ) 0.7 Ca 0.3 CoO 3 samples were prepared by the "paper synthesis" technique:…”