Phase diagram of the La-Si binary system under high pressure and the structures of superconducting LaSi{sub 5} and LaSi{sub 10}

Journal Article · Sat Aug 15 04:00:00 UTC 2009 · Journal of Solid State Chemistry

DOI:https://doi.org/10.1016/j.jssc.2009.05.010· OSTI ID:21370511

Yamanaka, Shoji ^[1]; Izumi, Satoshi; Maekawa, Shoichi; Umemoto, Keita ^[1]

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1. Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527 (Japan)

The La-Si binary phase diagram under a high pressure of 13.5 GPa was experimentally constructed. New superconducting silicides LaSi{sub 5} and LaSi{sub 10} were found, which have peritectic decomposition temperatures at 1000 and 750 deg. C, respectively. The single crystal X-ray structural analysis revealed that there are two polymorphs in LaSi{sub 5}. The alpha-form obtained by heating a molar mixture of LaSi{sub 2} and 3 Si at about 700 deg. C or by a rapid cooling from 1000 deg. 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) A, and beta=109.11(1){sup o}. The beta-form obtained by a slow cooling from 800-950 deg. C to 600 deg. 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){sup o}. 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 {sub i}nfinity{sup 3}[Si{sub 5}]{sup 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{sub c} of 11.5 K. LaSi{sub 10} crystallizes with the space group 6{sub 3}/mmc and the lattice parameters a=9.623(4), c=4.723(3) A. It is composed of La containing Si{sub 18} polyhedra (La-Si{sub 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{sub 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 five-fold coordination. LaSi{sub 10} became a superconductor with T{sub c}=6.7 K. The ab initio calculation of the electric band structures showed that alpha-LaSi{sub 5} and LaSi{sub 10} are metallic, and the conduction electrons mainly come from Si-3p orbitals. - Graphical abstract: In the study of the phase diagram of the La-Si binary system under high pressure and high temperature conditions, new Si-rich superconducting LaSi{sub 5} (T{sub c}=11.5 K) and LaSi{sub 10} (T{sub c}=6.7 K) have been found. LaSi{sub 5} contain unique one-dimensional sila-polyacene ribbons, and LaSi{sub 10} has a hexagonal columnar structure formed by face shared La-Si{sub 18} polyhedra.

• OSTI ID: 21370511
• Journal Information: Journal of Solid State Chemistry, Journal Name: Journal of Solid State Chemistry Journal Issue: 8 Vol. 182; ISSN 0022-4596; ISSN JSSCBI
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
CHARGED PARTICLES
COOLING
CRYSTAL LATTICES
CRYSTAL STRUCTURE
CRYSTALS
DIAGRAMS
ELECTRIC CONDUCTIVITY
ELECTRICAL PROPERTIES
HEXAGONAL LATTICES
INFORMATION
IONS
LANTHANUM COMPOUNDS
LANTHANUM IONS
LANTHANUM SILICIDES
LATTICE PARAMETERS
MONOCLINIC LATTICES
MONOCRYSTALS
PHASE DIAGRAMS
PHYSICAL PROPERTIES
PRESSURE RANGE
PRESSURE RANGE GIGA PA
RARE EARTH COMPOUNDS
SILICIDES
SILICON COMPOUNDS
SPACE GROUPS
SUPERCONDUCTIVITY
SUPERCONDUCTORS
SYMMETRY GROUPS
TEMPERATURE RANGE
TEMPERATURE RANGE 0000-0013 K
TEMPERATURE RANGE 0400-1000 K
TEMPERATURE RANGE 1000-4000 K
THERMODYNAMIC PROPERTIES
TRANSITION TEMPERATURE