Mercurial possibilities: determining site distributions in Cu 2 HgSnS 4 using 63/65 Cu, 119 Sn, and 199 Hg solid-state NMR spectroscopy.

Chalcogenides are an important class of materials that exhibit tailorable optoelectronic properties accessible through chemical modification. For example, the minerals kesterite, stannite, and velikite (Cu ₂ M SnS ₄ , 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 I 4̄ vs. I 4̄2 m ) has been difficult. This study explores the average long-range and local structure of Cu ₂ HgSnS ₄ 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 ^63/65 Cu NMR spectra were acquired at multiple magnetic field strengths ( B ₀ = 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 Cu ₂ ZnSnS ₄ , in contrast to only one set of sites in Cu ₂ CdSnS ₄ and Cu ₂ HgSnS ₄ , clarifying structural assignments previously proposed through X-ray diffraction methods. The presence of these Cu sites was further supported by DFT calculations. The ¹¹⁹ Sn and ¹⁹⁹ Hg NMR spectra suggest that an ordering phenomenon takes place in Cu ₂ HgSnS ₄ when it undergoes annealing treatments. The trend in measured optical band gaps (1.5 eV for Cu ₂ ZnSnS ₄ , 1.2 eV for Cu ₂ CdSnS ₄ , and 0.9 eV for Cu ₂ HgSnS ₄ ) 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 Cu ₂ HgSnS ₄ have greater covalent character.