Phase diagram of the La–Si binary system under high pressure and the structures of superconducting LaSi5 and LaSi10

The La–Si binary phase diagram under a high pressure of 13.5 GPa was experimentally constructed. New superconducting silicides LaSi5 and LaSi10 were found, which have peritectic decomposition temperatures at 1000 and 750 °C, respectively. The single crystal X-ray structural analysis revealed that there are two polymorphs in LaSi5. The α-form obtained by heating a molar mixture of LaSi2 and 3 Si at about 700 °C or by a rapid cooling from 1000 °C under pressure crystallizes with the space group C 2/m and the lattice parameters a =15.11(3), b =4.032(6), c =8.26(1) Å, and β =109.11(1)°. The β-form obtained by a slow cooling from 800–950 °C to 600 °C under pressure has the same space group but with slightly different lattice parameters, a =14.922(7), b =3.906(2), c =8.807(4) Å, and β =107.19(1)°. The β-form is formed during the incomplete transformation of the α-form on cooling, and has always been obtained as a mixture with the α-form. The compound can be characterized as a Zintl phase with a polyanionic framework View the MathML source∞3[Si5]3- with large tunnels running along the b axis hosting lanthanum ions. In the β-form, three of the five Si sites are disordered. The two polymorphs contain one dimensional sila-polyacene ribbons, Si ladder polymer, running along the b axis. The α-form showed superconductivity with the transition temperature Tc of 11.5 K. LaSi10 crystallizes with the space group 63/mmc and the lattice parameters a=9.623(4), c=4.723(3) Å. It is composed of La containing Si18 polyhedra (La@Si18) of hexagonal beer-barrel shape, which form straight columns by stacking along the c-axis via face sharing. One-dimensional columns of La@Si18 barrels are edge-shared, and bundled with infinite Si trigonal bipyramid chains via corner sharing. The Si atoms in the straight chains have a five-fold coordination. LaSi10 became a superconductor with Tc=6.7 K. The ab initio calculation of the electric band structures showed that α-LaSi5 and LaSi10 are metallic, and the conduction electrons mainly come from Si-3p orbitals.