Mercurial possibilities: determining site distributions in Cu₂HgSnS₄ using ^63/65Cu, ¹¹⁹Sn, and ¹⁹⁹Hg solid-state NMR...

Chalcogenides are an important class of materials that exhibit tailorable optoelectronic properties accessible through chemical modification. For example, the minerals kesterite, stannite, and velikite (Cu₂MSnS₄, where M = Zn, Cd, or Hg, respectively) are a series of Group 12 transition metal tin sulfides that readily exhibit optical bandgaps spanning the Shockley-Queisser limit; however, achieving consensus on their structure (space group I4̄ vs. I4̄2m) has been difficult. This study explores the average long-range and local structure of Cu₂HgSnS₄ and evaluates the parallels of M = Zn and Cd sister compounds using complementary X-ray diffraction and solid-state nuclear magnetic resonance (NMR) spectroscopy. The ^63/65Cu NMR spectra were acquired at multiple magnetic field strengths (B₀ = 7.05, 11.75, and 21.1 T) to assess the unique chemical shift anisotropy and quadrupolar coupling contributions. They reveal two inequivalent sets of Cu sites in Cu₂ZnSnS₄, in contrast to only one set of sites in Cu₂CdSnS₄ and Cu₂HgSnS₄, clarifying structural assignments previously proposed through X-ray diffraction methods. The presence of these Cu sites was further supported by DFT calculations. The ¹¹⁹Sn and ¹⁹⁹Hg NMR spectra suggest that an ordering phenomenon takes place in Cu₂HgSnS₄ when it undergoes annealing treatments. The trend in measured optical band gaps (1.5 eV for Cu₂ZnSnS₄, 1.2 eV for Cu₂CdSnS₄, and 0.9 eV for Cu₂HgSnS₄) was confirmed by electronic structure calculations, which show that the band gap narrows as the difference in electronegativity is diminished and that Hg-S bonds in Cu₂HgSnS₄ have greater covalent character.