Abnormal thermal expansion, multiple transitions, magnetocaloric effect, and electronic structure of Gd₆Co_4.85

The structure of known Gd₄Co₃ compound is re-determined as Gd₆Co_4.85, adopting the Gd₆Co_1.67Si₃ structure type, which is characterized by two disorder Co sites filling the Gd octahedral and a short Gd-Gd distance within the octahedra. The compound shows uniaxial negative thermal expansion in paramagnetic state, significant negative expansion in ferromagnetic state, and positive expansion below ca. 140K. It also exhibits large magnetocaloric effect, with an entropy change of -6.4Jkg^-1K^-1 at 50kOe. In the lattice of the compound, Co atoms at different sites show different spin states. It was confirmed by the X-ray photoelectron spectra and calculation of electronic structure and shed lights on the abnormal thermal expansion. The stability of such compound and the origin of its magnetism are also discussed based on measured and calculated electronic structures.