Theory of local distortion in ErO6 cluster: Spontaneous symmetry reduction in lanthanoid octahedrons

Molecular orbital calculations for an ErO6 cluster predicted a stable C4v pseudo-octahedron with an Er displacement of ∼0.1 Å from the center of the octahedron with an Er–O bond length of 2.27 Å. In this particular configuration, the hybridization balance of O 2p–Er 6s with O 2p–Er 5d minimizes electron transfer from the O anion to the Er cation, thereby strengthening the Er–O ionic bond. Excessive O 2p–Er 5d hybridization due to π-bond formation is found in the shorter Er–O bond range, while insufficient hybridization caused by a weak σ-bond is obtained in the longer Er–O bond range. Though spontaneous reduction of symmetry has also been confirmed in other LO6 (L=65Tb–71Lu) systems, the stablest pseudo-octahedron is obtained for ErO6.