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Zusammenfassung:
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 resistivity and heat capacity reveal that SrAl2.5Si1.5 and SrAl2Si2 prepared at high pressures exhibit superconductivity with critical temperatures of 2.1 and 2.6 K, respectively.
