Samples of the type-I clathrate Sr 8Al xSi 46-x have been prepared by direct reaction of the elements. The type-I clathrate structure (cubic space group Pm3̄n) which has an Al-Si framework with Sr 2+ guest atoms forms with a narrow composition range of 9.54(6) ≤ x ≤ 10.30(8). Single crystals with composition A 8Al 10Si 36 (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 Sr 8Al 10Si 36 and Ba 8Al 10Si 36, 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 A 8Al 6Si 40 (6c site occupied completely by Al) and A 8Al 16Si 30 (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 Sr 8Al 6Si 40 phase has a very small band gap that is closed upon additional Al, as observed in Sr 8Al 16Si 30. An explanation for the absence of a semiconducting "Sr 8Al 16Si 30" phase is suggested in light of these findings.
ASJC Scopus subject areas
- Inorganic Chemistry
- Physical and Theoretical Chemistry