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 degrees C, respectively. The single crystal X-ray structural analysis revealed that there are two polymorphs in LaSi5. The alpha-form obtained by heating a molar mixture of LaSi2 and 3 Si at about 700 degrees C or by a rapid cooling from 1000 degrees C under pressure crystallizes with the space group C2/m and the lattice parameters a = 15.11(3), b = 4.032(6), c = 8.26(1) angstrom, and beta = 109.11(1)degrees. The beta-form obtained by a slow cooling from 800-950 degrees C to 600 degrees 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) A, and beta = 107.19(1)degrees. The beta-form is formed during the incomplete transformation of the alpha-form on cooling, and has always been obtained as a mixture with the alpha-form. The compound can be characterized as a Zintl phase with a polyanionic framework (3)(infinity)[Si-5](3-) with large tunnels running along the b axis hosting lanthanum ions. In the beta-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 alpha-form showed superconductivity with the transition temperature T-c of 11.5 K. LaSi10 crystallizes with the space group 6(3)/mmc and the lattice parameters a = 9.623(4), c = 4.723(3)angstrom. It is composed of La containing Si-18 polyhedra (La@Si-18) of hexagonal beer-barrel shape, which form straight columns by stacking along the c-axis via face sharing. One-dimensional columns of La@Si-18 barrels are edge-shared, and bundled with infinite Si trigonal bipyramid chains via corner sharing. The Si atoms in the straight chains have a fivefold coordination. LaSi10 became a superconductor with T-c = 6.7 K. The ab initio calculation of the electric band structures showed that alpha-LaSi5 and LaSi10 are metallic, and the conduction electrons mainly come from Si-3p orbitals. (C) 2009 Elsevier Inc. All rights reserved.