The nickel chalcogenide family contains multiple phases, each with varying properties that can be applied to an expansive range of industrially relevant processes. Specifically, pyrite-type NiS2 and NiSe2 have been used as electrocatalysts for oxygen or hydrogen evolution reactions. These pyrites have also been used in batteries and solar cells due to their optoelectronic and transport properties. The phase evolution of pyrite NiS2 and polymorphism of NiSe2 have briefly been studied in the literature, but there has been limited work focusing on the phase transformations within each of these two systems. Using experiments and calculations, we detail how pyrite NiS2 nanocrystals decompose into hexagonal α-NiS, and how the synthesis of pyrite NiSe2 nanocrystals is affected by the presence of two polymorphs, a metastable orthorhombic marcasite phase and a more stable cubic pyrite phase. Each reaction can be controlled by fine-tuning the reaction parameters, including temperature, time, and the precursor identity and concentration. Interestingly, both NiS2 and NiSe2 nanopyrites are active catalysts in the selective reduction of nitrobenzene to aniline, in agreement with other catalysts containing an fcc (sub) lattice. Our results demonstrate a feasible, logical process for synthesizing nanocrystalline pyrites without common byproducts or impurities. This work can help in solving a major problem suspected in preventing pyrite FeS2 and similar materials from large-scale use: the presence of small amounts of secondary phases and impurities.

Seven acentric sulfides Ba6(Cu x Z y )­Sn4S16 (Z = Mg, Mn, Zn, Cd, In, Bi, Sn) were grown by a high-temperature salt flux method. The crystal structures of the Ba6(Cu x Z y )­Sn4S16 (Z = Mg, Mn, Zn, Cd, In, Bi, Sn) compounds were determined by single-crystal X-ray diffraction with the aid of solid-state NMR spectroscopy. The Ba6(Cu x Z y )­Sn4S16 (Z = Mg, Mn, Zn, Cd, In, Bi) compounds are isostructural and crystallize in the Ba6Ag4Sn4S16 structure type. The Sn-containing compound exhibits high structural similarity to Ba6(Cu x Z y )­Sn4S16 (Z = Mg, Mn, Zn, Cd, In, Bi) with the presence of an interstitial atomic position partially occupied by Sn atoms. The chemical bonding characteristics of Ba6(Cu2.9Sn0.4)­Sn4S16 were understood with electron localization function calculations coupled with crystal orbital Hamilton population calculations. The Ba–S and Cu–S interactions are dominantly ionic, but the Sn–S interactions consist of strong covalent bonding characteristics in Ba6(Cu2.9Sn0.4)­Sn4S16. The monovalent Cu atoms, mixed with certain metals with various oxidation states, significantly shift the optical properties of the Ba6(Cu x Z y )­Sn4S16 (Z = Mg, Mn, Zn, Cd, In, Bi) compounds. This results in a good balance between the second-harmonic-generation (SHG) response and laser damage threshold (LDT). Ba6(Cu1.9Zn1.1)­Sn4S16 possesses a high SHG response and a high LDT of 2.8 × AGS and 3 × AGS, respectively. A density functional theory calculation revealed that CuS4 and SnS4 tetrahedra significantly contribute to the SHG response in Ba6(Cu2Mg)­Sn4S16, which also confirmed that CuS4 tetrahedra are crucial for the stability and optical properties of the Ba6(Cu x Z y )­Sn4S16 (Z = Mg, Mn, Zn, Cd, In, Bi, Sn) compounds revealed by electronic structure analysis.

8 are mainly contributed by the chemical characteristics of Cd compared with Fe, which are elucidated by nonlinear optical property measurements, electronic structure calculations, and density functional theory calculations. The [CdS 4 ] tetrahedra within K 2 CdGe 3 S 8 exhibit a higher degree of distortion and larger volume compared to the [FeS 4 ] tetrahedra in K 2 FeGe 3 S 8 . This study possesses a good platform to investigate how d-block elements contribute to the SHG response. The fully occupied d 10elements are better for SHG susceptibility than d 6 -elements in this study. K 2 CdGe 3 S 8 is a good candidate as an infrared nonlinear optical material of high SHG response (2.1× AgGaS 2 , samples of particle size of 200−250 μm), type-I phase-matching capability, high laser damage threshold (6.2× AgGaS 2 ), and good stability.

A new compound Cu 4 TiTe 4 in the Cu−Ti−Te ternary system is prepared using high-temperature solid-state synthesis and characterized by single-crystal X-ray diffraction and energy-dispersive X-ray spectroscopy. The average structure of Cu 4 TiTe 4 crystallizes in the cubic space group P4̅ 3m (cP9; a = 5.9484(1) Å) and adopts the Cu 4 TiSe 4 structure type. Like Cu 4 TiSe 4 , it shows positional disorder in one of the two Cu sites. The three-dimensional structure of Cu 4 TiTe 4 is viewed as a cubic close-packed (ccp) array of Te, where half of the tetrahedral holes are orderly occupied by three Cu and one Ti and the disordered Cu atoms effectively occupied 1/4 of the octahedral holes. The calculated density of states (DOS) discerns that the compound is a narrow-bandgap semiconductor, and the crystal orbital Hamilton population (COHP) analysis shows that though the individual Cu−Te short contact is relatively weak compared to the Ti−Te contact, Cu−Te bonds largely contribute toward the overall stability. Due to the unique atomic arrangements, some Te atoms in the unit cell have unsaturated coordination, which presents 5s 2 lone pairs on the Te atoms. This has been confirmed by the density of states (DOS) and electron localization function (ELF) calculations.

Sodium transition metal silicates are potential candidate electrode materials to enable two-electron redox per transition metal ion center. Yet, the electrochemical reaction mechanism remains elusive despite the widely reported electrochemical activity for this class of materials as intercalation cathodes for Na-ion batteries. Adopting monoclinic Na 2 CoSiO 4 as a model compound, we used high-resolution synchrotron X-ray diffraction (XRD) and X-ray pair distribution function (PDF) analysis to elucidate the structure of the partially desodiated Na 2-x CoSiO 4 phases for the Co 3+ /Co 2+ redox couple. The appearance of satellite reflections in the intermediate Na 1.5 CoSiO 4 and NaCoSiO 4 phases manifests the formation of modulated structures, which are induced by Na + /vacancy and Co 2+ /Co 3+ charge orderings. Accounting for these structural orderings is important to understand the function and performance of sodium transition metal silicate electrodes.