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.