Making and Breaking Bonds in Superconducting SrAl4–xSix (0 ≤ x ≤ 2)

Abstract

We explored the role of valence electron concentration in bond formation and superconductivity of mixed silicon–aluminum networks by using high-pressure synthesis to obtain the BaAl4-type structural pattern in solid solution samples SrAl4–xSix where 0 ≤ x ≤ 2. Local ordering of aluminum and silicon in SrAl4–xSix was evidenced by nuclear magnetic resonance experiments. Subsequent bonding analysis by quantum chemical techniques in real space demonstrated that the strong deviation of the lattice parameters in SrAl4–xSix from Vegard’s law can be attributed to the strengthening of interatomic Al–Al and Al–Si bonds within the layers (perpendicular to [001]) for 0 ≤ x ≤ 1.5, followed by the breaking of the interlayer bonds (parallel to [001]) for 1.5 < x ≤ 2 and leading to the structural transition from the BaAl4 structure type with three-dimensional anionic framework at lower x values to the two-dimensional anion of the BaZn2P2 structure type with increasing x values. Low-temperature measurements of the resisti...