High pressure synthesis and crystal structure of a ternary superconductor Ca2Al3Si4 containing layer structured calcium sub-network isomorphous with black phosphorus

The Zintl compound CaAl2Si2 is peritectically decomposed to a mixture of Ca2Al3Si4 and aluminum metal at temperatures above 600 °C under a pressure of 5 GPa. The new ternary compound Ca2Al3Sl4 crystalizes with the space group Cmc21 and the lattice parameters a=5.8846(8), b=14.973(1), and c=7.7966(5) Å. The structure is composed of aluminum silicide framework [Al3Si4] and layer structured [Ca2] network interpenetrating with each other. The electron probe microanalysis (EPMA) shows the formation of solid solutions Ca2Al3−xSi4+x (x<0.6). The layer structured [Ca2] sub-network is isomorphous with black phosphorus. The new ternary compound shows superconductivity with a transition temperature (Tc) of 6.4 K. The band structure calculation suggests that the superconductivity should occur through the conduction bands mainly composed of 3p orbitals of the aluminum silicide framework.