First-order phase transition in the Li2B12H12 system.

The thermal decomposition of anhydrous Pa3[combining macron] Li2B12H12 was studied in situ by high resolution synchrotron X-ray diffraction. A first-order phase transition can be observed at 355 °C where the unit cell volume expands by ca. 8.7%. The expanded β-Li2B12H12 polymorph simultaneously decomposes to a hydrogen poor γ-Li2B12H12-x phase. Expansion of the unit cell across the discontinuity is consistent with reorientational motion of B12H12(2-) anions, and the presence of a frustrated Li(+) lattice indicating Li ion conduction.