Accurate X-ray diffraction data required for proper evaluation of bond valence sums and global instability indexes: redetermination of the crystal structures of diamond-like Cu2CdSiS4 and Cu2HgSnS4 as a case study.

Our calculations of the global instability index (G) values for some diamond-like materials with the general formula I₂-II-IV-VI₄ have indicated that the structures may be unstable or incorrectly determined. To compute the G value of a given compound, the bond valence sums (BVSs) must first be calculated using a crystal structure. Two examples of compounds with high G values, based on data from the literature, are the wurtz-stannite-type dicopper cadmium silicon tetrasulfide (Cu₂CdSiS₄) and the stannite-type dicopper mercury tin tetrasulfide (Cu₂HgSnS₄), which were first reported in 1967 and 1965, respectively. In the present study, Cu₂CdSiS₄ and Cu₂HgSnS₄ were prepared by solid-state synthesis at 1000 and 900 °C, respectively. The phase purity was assessed by powder X-ray diffraction. Optical diffuse reflectance UV/Vis/NIR spectroscopy was used to estimate the optical bandgaps of 2.52 and 0.83 eV for Cu₂CdSiS₄ and Cu₂HgSnS₄, respectively. The structures were solved and refined using single-crystal X-ray diffraction data. The structure type of Cu₂CdSiS₄ was confirmed, where Cd²⁺, Si⁴⁺ and two of the three crystallographically unique S^2- ions lie on a mirror plane. The structure type of Cu₂HgSnS₄ was also verified, where all ions lie on special positions. The S^2- ion resides on a mirror plane, the Cu⁺ ion is situated on a fourfold rotary inversion axis and both the Hg²⁺ and the Sn⁴⁺ ions are located on the intersection of a fourfold rotary inversion axis, a mirror plane and a twofold rotation axis. Using the crystal structures solved and refined here, the G values were reassessed and found to be in the range that indicates reasonable strain for a stable crystal structure. This work, together with some examples gathered from the literature, shows that accurate data collected on modern instrumentation should be used to reliably calculate BVSs and G values.