7Li and 29Si solid state NMR and chemical bonding of La2Li2Si3
Résumé
The ternary silicide La2Li2Si3 was synthesized from the elements in a sealed niobium tube. La2Li2Si3 was characterized by powder and single crystal X-ray diffraction: Ce2Li2Ge3 type, Cmcm, a = 450.03(8), b = 1880.3(4), c = 689.6(1) pm, wR2 = 0.0178, 597 F2 values, and 26 parameters. The La2Li2Si3 structure contains two crystallographically independent silicon sites, both in slightly distorted trigonal prismatic coordination. The Si1 atoms are located in condensed La6 prisms and form cis-trans chains (two-bonded silicon) with Si1-Si1 distances at 238 and 239 pm, indicating single bond character. The Si2 atoms are isolated within La2Li4 prisms. La2Li2Si3 might be formally considered as an electron precise Zintl phase with an electron partition (2La3+)(2Li+)(2Si12-)(Si24-). Electronic structure calculations show a trend in this direction based on a charge density analysis with large electron localization around the Si1-Si1 chains. The compound is found weakly metallic with chemical bonding reminiscent of LaSi and additional features brought in by Li and Si2. High resolution solid state 7Li and 29Si MAS-NMR spectra are in agreement with the crystal structural information, however, the 29Si resonance shifts observed suggest strong Knight shift contributions, at variance with the Zintl concept. Variable temperature solid state 7Li spectra indicate the absence of motional narrowing on the kHz timescale within the temperature range 300K < T < 400 K.