~7Li and ~(29)Si solid state NMR and chemical bonding of La_2Li_2Si_3

摘要

The ternary silicide La_2Li_2Si_3 was synthesized from the elements in a sealed niobium tube. La_2Li_2Si_3 was characterized by powder and single crystal X-ray diffraction: Ce_2Li_2Ge_3 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 La_2Li_2Si_3 structure contains two crystallographically independent silicon sites, both in slightly distorted trigonal prismatic coordination. The Si1 atoms are located in condensed La_6 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 La_2Li_4 prisms. La_2Li_2Si_3 might be formally considered as an electron precise Zintl phase with an electron partition (2La~(3+))(2Li~+)(2Si1~(2-))(Si_2~(4-)). Electronic structure calculations show a trend in this direction based on a charge density analysis with large electron localization around the Sil-Sil chains. The compound is found weakly metallic with chemical bonding reminiscent of LaSi and additional features brought in by Li and Si_2. High resolution solid state ~7Li and ~(29)Si MAS-NMR spectra are in agreement with the crystal structural information, however, the ~(29)Si 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.