Iridium Orbital Crossover at the Structural Phase Transition in   Ca10(Ir4As8)(Fe2-xIrxAs2)5
  We report a structural transition found in Ca10(Ir4As8)(Fe2-xIrxAs2)5, which exhibits superconductivity at 16 K, with a layer of divalent iridium coordinated by arsenic in between Fe2As2 layers. The c-axis parameter is doubled below a structural transition temperature of approximately 100 K, while the tetragonal symmetry with space group P4/n (No.85) is unchanged at all temperatures measured. Our synchrotron x-ray diffraction study clearly shows displacements along the z-direction occur in half of the iridium sites, resulting in a complex orbital ordering pattern. Combining our theoretical calculation of the 5d orbital energies with structural data, we propose the iridium orbital crossover transition between the dxy and dz2 orbitals. 
Kento Sugawara, Naoyuki Katayama, Yuki Sugiyama, Takafumi Higuchi,   Kazutaka Kudo, Daisuke Mitsuoka, Minoru Nohara, and Hir...
cond-mat.supr-con cond-mat.str-el