The reaction of NaCl, EuCl2 and EuCl3 yields violet crystals of the new mixed valent chloride Na5Eu7Cl22 which crystallizes with a hitherto unknown structure (orthorhombic, Pmmn, Z = 2, a = 2525.9(4), b=846.91(9), c = 781.72(8) pm). The structure contains four crystallographically different europiumions,which can be attributed toEu2+ or Eu3+ with respect to their coordination numbers and Eu-Cl distances. The [EuClx] coordination polyhedra are connected to chains along the [001] direction. Absorption spectra show transitions which can be assigned either to Eu3+ or Eu2+ ions or to intervalence charge transfer transitions (IVCT). The latter are at lower energy compared to other mixed valent europium chlorides. The occurence of IVCT and the number of the crystal field levels of the Eu3+ ions reveal that the sites of the europium ions are occupied for a small amount by ions of the respective other valence.
Exploiting exciton coupling of ligand radical intervalence charge transfer transitions to tune NIR absorption