Mixed europium valence in Eu0.937Ba8[BN2]6 – Structure and spectroscopic behavior

Dierkes T., Seidel S., Benndorf C., Heletta L., de Oliveira Junior M., Holtkamp M., Karst U., Block T., Jüstel T., Eckert H., Pöttgen R.

Abstract

Polycrystalline samples of Sr0.95Eu0.05Ba8[BN2]6 and Eu0.937Ba8[BN2]6 were synthesized via conventional solid-state reaction from the binary precursor compounds Sr3N2, EuN, Ba3N2 and BN at 1000 °C. The structure of Eu0.937Ba8[BN2]6 was refined from single crystal X-ray diffractometer data: Eu0.937Ba8[BN2]6 type, Fd3¯m, a = 1594.54(9) pm, wR2 = 0.0654, 380 F2 values and 23 variables. The 8a europium site shows an occupancy of only 93.7(9) % suggesting partial oxidation of europium to fulfil an electron-precise description, i. e. EuII0.81EuIII0.13Ba8[BN2]6. The mixed europium valence was confirmed by magnetic susceptibility measurements (reduced magnetic moment of 7.28 μB per europium atom) and 151Eu Mössbauer spectroscopy (EuII: EuIII = 82: 18). The nitridoborate anions are coordinated by slightly distorted, mono-capped (europium) square prisms formed by the barium atoms. All metal cations are hexa-coordinated by nitrogen atoms. The EPR spectra of EuxSr1-xBa8[BN2]6 samples (0.001 ≤ x ≤ 0.01) suggest close to cubic local symmetry of the Eu2+ dopant ions and reveal some highly unusual features: Magnetic hyperfine splitting is only observed with one of the Eu nuclear isotopes, and the coupling constant of 243.6 MHz is extremely large compared to values (90–100 MHz) typically observed in the literature for Eu2+ doped crystalline materials.