Phase Range of the Type-I Clathrate Sr8AlxSi46–x and Crystal Structure of Sr8Al10Si36

Samples of the type-I clathrate Sr8AlxSi46–x have been prepared by direct reaction of the elements. The type-I clathrate structure (cubic space group Pm3̅n) which has an Al–Si framework with Sr2+ guest atoms forms with a narrow composition range of 9.54(6) ≤ x ≤ 10.30(8). Single crystals with composition A8Al10Si36 (A = Sr, Ba) have been synthesized. Differential scanning calorimetry (DSC) measurements provide evidence for a peritectic reaction and melting point at ∼1268 and ∼1421 K for Sr8Al10Si36 and Ba8Al10Si36, respectively. Comparison of the structures reveals a strong correlation between the 24k-24k framework sites distances and the size of the guest cation. Electronic structure calculation and bonding analysis were carried out for the ordered models with the compositions A8Al6Si40 (6c site occupied completely by Al) and A8Al16Si30 (16i site occupied completely with Al). Analysis of the distribution of the electron localizability indicator (ELI) confirms that the Si–Si bonds are covalent, the Al–Si bonds are polar covalent, and the guest and the framework bonds are ionic in nature. The Sr8Al6Si40 phase has a very small band gap that is closed upon additional Al, as observed in Sr8Al16Si30. An explanation for the absence of a semiconducting “Sr8Al16Si30” phase is suggested in light of these findings.