Achintya Lakshan

The synthesis of phase-pure Cu-based multinary tellurides remains challenging due to the competitive formation of stable Cu–Te binary phases, which frequently intercept the reaction pathway as secondary products. In the Cu ₄ TiTe ₄ system, rapid synthesis temperature mapping reveals the synthesis window, whereas precise compositional control suppresses secondary Cu–Te binaries to unlock non-equilibrium phase selection. We demonstrate that addition of 2 wt% excess Ti shifts the reaction chemistry toward Cu ₄ TiTe ₄ , suppressing detectable Cu–Te secondary phases and yielding phase-pure Cu ₄ TiTe ₄ over the identified 623–873 K synthesis window. Cu ₄ TiTe ₄ crystallises in a ‘stuffed sulvanite’ type structure, similar to Cu ₄ TiSe ₄ , and exhibits an order–disorder phase transition, in which partial ordering of the Cu sublattice induces structural transformation primarily governed by Cu sublattice ordering. Bulk Cu ₄ TiTe ₄ is a p-type narrow bandgap semiconductor that achieves a low lattice thermal conductivity of 0.63 to 0.68 W m ^-1 K ^-1 between 300 and 423 K. Transport calculations attribute the low observed thermal conductivity to Cu-sublattice disorder, local symmetry breaking, and localized Ti-dominated vibrations that collectively suppress phonon transport.

In this study, we adopted a quasi-isostructural solid-solution strategy involving selenotitanate Cu4TiSe4 and sulvanite-type Cu3TMQ4 (TM: V, Nb, Ta; Q: S, Se). At room temperature, both systems crystallize in the cubic P4̅3m space group, where Cu4TiSe4 additionally contains positionally disordered Cu+ cations occupying two Wyckoff sites (1b and 4e) inside the sulvanite framework. The crystal structures of the solid-solution phases were determined by X-ray diffraction studies. Substituting Ti4+ for TM5+ in Cu3TMQ4 allows additional Cu+ to dissolve, maintaining the overall charge neutrality. The additional “interstitial” Cu+ ions occupy positionally disordered sites (1b and 4e) without affecting the three-dimensional (3D) sulvanite frameworks, with their occupancy controlled by the Ti4+/TM5+ ratio. Moreover, an ordering tendency of the interstitial Cu+ ions is observed within the cubic unit cell of the S-substituted solid solutions. This solid-solution formation leads to a substantial reduction in effective thermal conductivity relative to the parent sulvanite-type compounds, reaching the ultralow regime, while the optical band gaps vary systematically between 0.9 and 2.62 eV. The combination of this low thermal conductivity and tunable optical bandgap makes these materials highly attractive for diverse applications.

This investigation focuses on the high temperature synthesis and characterization using the single crystal and powder X-ray diffraction analysis of a partly disordered ternary intermetallic phase Cr _1−???? Ni ₂ Ga ₄ , 𝛿 ≈0.2 within the CsCl-related 𝑇 ₃ Ga ₄ family. With increase in annealing time during synthesis, the vacancy-disordered cubic (𝐹𝑚3̅𝑚, 𝑎 = 5.77 Å) phase transforms into a vacancy-ordered tetragonal phase (𝐼4 ₁ /𝑎𝑚𝑑, 𝑎 = 5.77 Å, 𝑐 = 11.54 Å) with three perpendicular twin domains. The superstructure reflections corresponding to vacancy ordering can be observed with both the single crystal and powder X-ray diffraction. Although the phase could not be separated in its pure form, the DFT simulations on the tetragonal model suggest the phase is suspectable to antiferromagnetic ordering. The partial occupancy of the Cr is rationalized by the antibonding populations near the fermi level. Furthermore, the variations in atomic sizes and electronegativities among the first-row transition metals provide additional insight to rationalize the structural distinctions of Co, and Fe containing analogues with the Cr-containing phase in the broader CsCl type/related 𝑇 ₃ Ga ₄ family and providing a rational pathway to discover novel intermetallic phases.

In this study, we adopted a quasi-isostructural alloying strategy between selenotitanate Cu ₄ TiSe ₄ and sulvanite-type Cu ₃ TMQ ₄ ( TM : V, Nb, Ta; Q : S, Se). At room temperature, both systems crystallize in the cubic 𝑃4̅3𝑚 space group, where Cu ₄ TiSe ₄ additionally contains positionally disordered Cu ⁺ cations, occupying two Wyckoff sites (1 b and 4 e ) inside the sulvanite framework. Crystal structures of the alloys were determined by X-ray diffraction studies. Substituting Ti ⁴⁺ for TM ⁵⁺ in Cu ₃ TMQ ₄ allows additional Cu ⁺ to dissolve, maintaining overall charge neutrality. The additional (stuffed) Cu occupy positionally disordered sites (1 b and 4 e ) without effecting the 3D sulvanite frameworks, with their occupancy controlled by Ti ⁴⁺ / TM ⁵⁺ ratio. Moreover, an ordering tendency of the stuffed Cu ⁺ ions is observed within the cubic unit cell of the S-substituted alloys. This alloying leads to a substantial reduction in thermal conductivity relative to the parent sulvanite-type compounds, reaching the ultralow regime, while the optical band gaps vary systematically between 0.9 and 2.62 eV. The combination of this low thermal conductivity and a tunable optical bandgap makes these materials highly attractive for diverse applications.

Unraveling the relationship between thermodynamic factors, interatomic interactions, and electronic structure remains a crucial yet elusive challenge in the discovery of novel materials in solid‐state (SS) chemistry. In the quest for new thermoelectric (TE) materials, we overcame that fundamental problem for the case of CuGe ₂ Se ₃ , whose synthesis, unique crystal structure, and transport properties are reported herein. The giant two‐dimensional (2D) structure of CuGe ₂ Se ₃ , consisting of Se/(Cu─Ge)/Se and Se/(Ge─Ge)/Se slabs stacked along the c ‐axis, exhibits short Cu─Ge and Ge─Ge interactions, as evidenced by single‐crystal X‐ray diffraction (SCXRD) and SS NMR spectroscopic studies. These homopolar bonds might be surprising, as such interactions are rarely observed in group IV‐chalcogenides. The compound is thermally stable up to ∼823 K. Transport properties measurements revealed a high Seebeck coefficient (∼373.6 µV·K ⁻¹ ) and ultralow thermal conductivity (∼0.35 W·m ⁻¹ K ⁻¹ ) at 755 K, ascribed to its weak bonding interactions. We followed up with a theoretical analysis to gain insight into its structural peculiarities, focusing on vibrational properties and the nature of chemical bonding. The formation of Ge─Ge bonds is favored in light of the presence of multicenter bonds, which receive contributions from stereochemically non‐active Ge lone pairs.

Unraveling the relationship between thermodynamic factors, interatomic interactions, and electronic structure remains a crucial yet elusive challenge in the discovery of novel materials in solid‐state (SS) chemistry. In the quest for new thermoelectric (TE) materials, we overcame that fundamental problem for the case of CuGe ₂ Se ₃ , whose synthesis, unique crystal structure, and transport properties are reported herein. The giant two‐dimensional (2D) structure of CuGe ₂ Se ₃ , consisting of Se/(Cu─Ge)/Se and Se/(Ge─Ge)/Se slabs stacked along the c ‐axis, exhibits short Cu─Ge and Ge─Ge interactions, as evidenced by single‐crystal X‐ray diffraction (SCXRD) and SS NMR spectroscopic studies. These homopolar bonds might be surprising, as such interactions are rarely observed in group IV‐chalcogenides. The compound is thermally stable up to ∼823 K. Transport properties measurements revealed a high Seebeck coefficient (∼373.6 µV·K ⁻¹ ) and ultralow thermal conductivity (∼0.35 W·m ⁻¹ K ⁻¹ ) at 755 K, ascribed to its weak bonding interactions. We followed up with a theoretical analysis to gain insight into its structural peculiarities, focusing on vibrational properties and the nature of chemical bonding. The formation of Ge─Ge bonds is favored in light of the presence of multicenter bonds, which receive contributions from stereochemically non‐active Ge lone pairs.

This study explores the structure and stability of partly disordered sulfur-substituted Ni_5.74InSe₂ (I4/mmm, a = 3.6766(1) Å, c = 18.8178(10) Å, Z = 2). The structure of Ni_7-δInSe_2-xS_x (x = 0.2, 0.36, 0.66, 0.80, 0.94) compounds is isotypic to their parent Ni_5.74InSe₂ and can be viewed as alternating heterometallic Cu₃Au-type _∞²[Ni₃In] slabs and defective Cu₂Sb-type _∞²[Ni_4-δ(Se/S)₂] slabs along the [001]-axis. Similar to the parent Se-compound, the Ni-Ch (Ch = chalcogen) fragment is non-stoichiometric and possesses a partially occupied Ni-site. It was observed that with sulfur insertion at the selenium site of Ni_5.74InSe₂, the interatomic distance between the partially occupied nickel and mixed (S/Se) sites decreases from ∼2.24 to ∼1.95 Å, and the occupancy of the disordered nickel site simultaneously increases. The limiting composition Ni_6.06InSe_0.67S_1.33 (x = 1.33, δ = 0.94) is formed in the sulfur-rich region. Its average structure resembles the Ni₆SnS₂-type and has a similar motif to Ni_5.74InSe₂; the only difference is that Cu₃Au-type _∞²[Ni₃In] alternates with two types of Ni-Ch fragments (Cu₂Sb or Li₂O type units). By using first-principles electronic structure calculations, we explained the presence of partially disordered nickel sites in the Ni-Ch fragment and rationalized why the nickel site occupancy increases with sulfur insertion.