63Cu NMR Study of Cu2 MTtCh 4 ( M = Zn or Cd; Tt = Si, Ge, or Sn; Ch = S or Se): A Tale of One or Two Sites

Abstract Quaternary chalcogenides Cu2MTtCh4 (M = Zn or Cd; Tt = Si, Ge, or Sn; Ch = S or Se) with diamond-like structures, derived from the cubic sphalerite and hexagonal wurtzite prototypes, have been extensively studied for their optoelectronic properties. The structural assignments for these compounds, which were sometimes unclear, can be addressed by probing the copper sites using 63Cu solid-state nuclear magnetic resonance (NMR) spectroscopy. NMR parameters were extracted through a combination of multifield magic-angle spinning (MAS) and nonspinning experiments to establish structural trends within these compounds. Substitution of S with Se led to lower frequencies for the 63Cu isotropic chemical shift. In contrast, substitution with heavier tetrels (Si to Ge to Sn) led to higher-frequency chemical shifts, which correlate with narrower band gaps. Ambiguities in structural models were resolved by arguments about the local symmetry of Cu sites through quadrupole coupling parameters. The experimental results were compared with density functional theory (DFT) calculations performed using the r2SCAN and PBE functionals. The predicted electric field gradient (EFG) parameters were accurate, although the calculated magnetic shieldings exhibited limitations in these narrow-gap semiconductors. This work provides an analytical 63Cu NMR spectroscopy method that can rapidly aid in assessing local structures in these diamond-like compounds and related chalcogenides.