A systematic solid state NMR spectroscopic study of the equiatomic lithium half-Heusler phases LiTX (T = Mg, Zn, Cd; X = P, As, Sb, Bi)

Abstract

The equiatomic intermetallic lithium compounds Li TX ( T = Mg, Zn, Cd; X = P, As, Sb, Bi) were synthesized by reacting the elements in sealed niobium tubes in muffle furnaces. The compounds crystallize with the cubic MgAgAs type structure, space group F 4 ¯ 3 m . The T and X atoms build up blende-type tetrahedral substructures in which the lithium atoms fill octahedral voids formed by the X atoms. Basic structural characterization of the samples was performed by powder X-ray diffraction. The charge distribution in these intermetallic compounds was investigated using metal ion ( 7 Li, 25 Mg and 113 Cd) as well as pnictogen ( 31 P, 75 As, 121 Sb) static and MAS-NMR. The metal ion chemical shifts are uniformly correlated with the pnictogen Pauling electronegativities, reflecting a systematic trend in the bonding characteristics in these phases. Furthermore, static temperature dependent 7 Li wideline NMR spectra show motional narrowing effects on the NMR timescale, indicating lithium ionic mobility. Activation energies estimated from these data decrease systematically with decreasing difference of the electronegativities between the elements Li and X and between the elements T and X . These result can be rationalized in terms of shallower Coulomb potential wells in those compounds with lower ionic character.