Neutron investigation of the spin-state crossover in the distorted ErCoO3 cobaltite

Abstract

The spin-state nature across the two electronic transitions in LaCoO3 has been matter of controversy for many years. Two competing pictures in LaCoO3 involve the spin transitions either LS→(HS+LS)→IS or LS→IS→(HS+IS). The ground and thermally excited states of LnCoO3 perovskites (Ln:lanthanide or rare earth) with heavy rare-earths and more distorted structures are also attracting great interest. We present neutron powder diffraction (NPD) studies aimed to characterize the electronic phenomena associated with the spin-state of Co3+ ions in the highly distorted ErCoO3 cobaltite. NPD measurements carried out on the high-resolution D2B diffractometer up to 1000 K were combined with low temperature measurements using several ILL diffractometers (which confirmed the ordering of Er moments below 2 K). To our knowledge, ErCoO3 (prepared under moderate pressure conditions) is the smallest Co3+ simple perovskite investigated so far in a temperature range up to 1000 K. The orthorhombic Pbnm symmetry was confirmed in this range, but the lattice evolution proves a remarkable occupation of excited magnetic states on heating above TSS≈575 K. The spin-state crossover extends over a broad temperature interval and can be monitored by an atypical augmentation of the rthorhombic strain (s). The anomalous contributions to the lattice above that temperature are clearly visible in the evolution of the anisotropic or global linear thermal expansion coefficients. The occupation of excited magnetic states (S=1 or 2) is activated at TSS≈575 K but it develops progressively along the interval 575-780 K. Above ~800 K the progression decreases. An estimation of the anomalous volume expansion accumulated at 1000 K due to spin state excitations gives ~2 %. Several correlated structural modifications above TSS are driven by the anomalous expansion of the CoO6 octahedra, which is partially anisotropic. The activation of IS or HS spin states brings about the augmentation of the Co-O distances in the a-b plane respect to the distance along the c-axis. This fact produces a non monotonous evolution of the tilting between adjacent octahedra and the intensification of the orthorhombic distortion. In spite of this, some evidences suggest the stabilization of a metallic state in the thermally excited phase.