Li₇B₇Se₁₅: A novel selenoborate with a zeolite-like polymeric anion structure

Compared to the hitherto known polymeric seleno- and perselenoborates a new and remarkable structural principle was realized in the lithium selenoborate Li₇B₇Se₁₅. The three dimensional anion sublattice is formed by entities of formal composition [B₇Se₁₃]^5-. Additionally, isolated diselenide units and lithium cations are found in the structure. The connection pattern of the basic corner-sharing BSe₄-tetrahedra gives rise to this tetragonal structure hitherto unknown in both, oxidic and non-oxidic chalcogenoborate chemistry. A characteristic feature of the structure is an extensive system of channels facilitating ionic motion and providing different diffusion paths for the Li ions. This channel system offers several atom sites suitable for lithium cations. As expected, some Li positions are partially occupied and they show comparatively large anisotropic displacement ellipsoids which prove the mobility of the cations already at room temperature, indicating an above average ion conductivity in this chalcogenoborate. The new chalcogenoborate was prepared in a solid state reaction from lithium selenide, amorphous boron and selenium in evacuated carbon coated silica tubes at a temperature of 800 °C. Li₇B₇Se₁₅ crystallizes tetragonal, space group P 4₂ / n b c (no. 133) with a = 11.4107 (4)   Å, c = 16.4251 (5)   Å at 273 K, and Z = 4. JPDF-calculations based on the displacement parameters using a non harmonic approach and ⁶Li magic-angle spinning (MAS) NMR spectra at 210 K yield resolved resonances for the Li(1) site and a dynamically averaged Li(2,3) site. The site populations deduced from NMR and X-ray diffraction are found in excellent agreement with each other. Line broadening and coalescence phenomena observed within the temperature region 240   K ≤ T ≤ 300   K reveal thermally activated ion transfer between the Li(1) ↔ Li(2,3) sites, which can be simulated well by standard two-site exchange theory. From the temperature dependent motional correlation times we extract an activation energy of 59(1) kJ/mole for this process.