Ternary nitridoborates. 2. Synthesis, crystal structure, and vibrational spectra of new ternary compounds with the [n-b-n]3- anion

Abstract

The isotypic compounds LiM4[BN2]3 (M = Ca, Sr, Ba, Eu) and NaM4[BN2]3 (M = Sr, Ba) are formed as colorless to pale yellow prismatic crystals (black with Eu) by reaction of the binary components Li3N, M3N2, EuN and Na, NaN3, Ba3N2 and BN in sealed niobium ampoules at 1375 and 1275 K, respectively. The linear anions: [N-B-N]3- have bond lengths d(B-N) between 132.6 and 136.6 pm. Vibrational frequencies and force constants f(B-N) = 7.25-7.89 Ncm-1 reveal significant drifts related to bond length and_effective anionic charge. The cubic crystal structures (Im3m (No. 229), Z = 2; LiM4[BN2]3: a(Ca) = 711.5 pm; a(Sr) = 745.6 pm; a(Eu) = 742.5 pm, a(Ba) = 788.0 pm and NaM4[BN2]3: a(Sr) = 756.8 pm; a(Ba) = 791.7 pm)) are stuffed derivatives of the β-PtHg4 structure type, and the range of existence of this cubic structure is derived from the molar volume and the ionic radii. The cations form a partial structure of centered cubes E1(E2)8 which are condensed to a 3∞[E1(E2)8/2] network (E1 = Li, Na; E2 = Ca, Sr, Ba, Eu). The remaining open cubes are filled by the [BN2]3- anions yielding two interpenetrating 3∞[E1(BN2)6/2] networks. Periodic Nodal Surfaces (PNS) of Im3m symmetry show the regions of different interactions.