Structure and lithium mobility of Li4Pt3Si

The lithium-rich silicide Li4Pt3Si was synthesised from the elements by high-temperature synthesis in a sealed niobium ampoule. Its structure was refined on the basis of single-crystal X-ray diffraction data: R32, a = 693.7(2), c = 1627.1(4) pm, wR2 = 0.0762, 525 F-2 values and 21 variables. The striking structural motifs of the Li4Pt3Si structure are silicon atoms with a slightly distorted trigonal prismatic platinum coordination with short Si-Pt distances (238-246 pm). Always two trigonal prisms are condensed via a common Pt-3 triangle, and these double units built up a three-dimensional network by condensation via common corners. The channels left by this prismatic network are filled by two crystallographically independent lithium sites in a 3:1 ratio. The single crystal X-ray data were fully confirmed by neutron powder diffraction and Li-7 magic-angle spinning (MAS)-nuclear magnetic resonance (NMR) results. The two distinct lithium sites are well differentiated by their Li-7 isotropic chemical shift and nuclear electric quadrupolar interaction parameters. MAS-NMR spectra reveal signal coalescence effects above 300 K, indicating chemical exchange between the lithium sites on the millisecond timescale. The spectra can be simulated with a simple two-site exchange model. From the resulting temperature-dependent correlation times, an activation energy of 50 kJ/mol is extracted.