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