7Li and 29Si solid state NMR and chemical bonding of La2Li2Si3

Abstract

The ternary silicide 'LA IND.2''LI IND.2''SI IND.3' was synthesized from the elements in a sealed niobium tube. 'LA IND.2''LI IND.2''SI IND.3' was characterized by powder and single crystal X-ray diffraction: 'CE IND.2''LI IND.2''GE IND.3' type, Cmcm, a = 450.03(8), b = 1880.3(4), c = 689.6(1) pm, wR2 = 0.0178, 597 'F POT.2' values, and 26 parameters. The 'LA IND.2''LI IND.2''SI IND.3' structure contains two crystallographically independent silicon sites, both in slightly distorted trigonal prismatic coordination. The 'SI'1 atoms are located in condensed 'LA IND.6' prisms and form cisetrans chains (two-bonded silicon) with 'SI'1e'SI'1 distances at 238 and 239 pm, indicating single bond character. The 'SI'2 atoms are isolated within 'LA' IND.2''LI IND.4' prisms. 'LA IND.2''LI IND.2''SI IND.3' might be formally considered as an electron precise Zintl phase with an electron partition (2'LA POT.3+')(2'LI POT.+')(2SI'1 POT.2-')('SI''2 POT.4-'). Electronic structure calculations show a trend in this direction based on a charge density analysis with large electron localization around the 'SI'1e'SI'1 chains. The compound is found weakly metallic with chemical bonding reminiscent of 'LA''SI' and additional features brought in by 'LI' and 'SI'2. High resolution solid state 'ANTPOT.7 LI' and 'ANTPOT.29 SI' MAS-NMR spectra are in agreement with the crystal structural information, however, the 'ANTPOT.29 SI' resonance shifts observed suggest strong Knight shift contributions, at variance with the Zintl concept. Variable temperature solid state 'ANTPOT.7 LI' spectra indicate the absence of motional narrowing on the kHz timescale within the temperature range 300K < T < 400 K.