Superstructure formation in SrBa8[BN2]6 and EuBa8[BN2]6

Abstract

X-ray pure samples of SrBa₈[BN₂]₆ and EuBa₈[BN₂]₆ were synthesized from appropriate amounts of binary nitrides (Sr₃N₂, Ba₃N₂ and BN in sealed niobium ampoules and EuN, Ba₃N₂ and BN in BN crucibles, respectively) at temperatures up to 1370 K. The structure of SrBa₈[BN₂]₆ was refined from single crystal X-ray diffractometer data: Fdm, a = 1595.1(1) pm, wR(F²) = 0.0515, 387 F² values and 21 variables. EuBa₈[BN₂]₆ has a lattice parameter of 1595.00(9) pm. Both nitridoborates adopt a new 2 × 2 × 2 superstructure variant of the LiCa₄[BN₂]₃ type, realized through ordering of vacancies and Sr²⁺ and Eu²⁺ cations, respectively. The structures of SrBa₈[BN₂]₆ and LiCa₄[BN₂]₃ are related by a group–subgroup scheme. The Sr²⁺/vacancy ordering leads to an asymmetric coordination (1 × Sr²⁺ and 8 × Ba²⁺ in a distorted, mono-capped square prism) for the [BN₂]³⁻ units with B–N distances of 132 and 136 pm. Vibrational spectra of SrBa₈[BN₂]₆ and EuBa₈[BN₂]₆ confirm the discrete linear [BN₂]³⁻ units and ¹¹B solid state MAS NMR spectra are compatible with single crystallographic sites for the boron atoms. In EuBa₈[BN₂]₆ the spectra are profoundly influenced by interactions of the ¹¹B nuclei with the unpaired electrons of the paramagnetic Eu²⁺ ions.