Synthesis, Structure, and Properties of CuBiSeCl2: A Chalcohalide Material with Low Thermal Conductivity.

Mixed anion halide-chalcogenide materials have recently attracted attention for a variety of applications, owing to their desirable optoelectronic properties. We report the synthesis of a previously unreported mixed-metal chalcohalide material, CuBiSeCl2 (Pnma), accessed through a simple, low-temperature solid-state route. The physical structure is characterized through single-crystal X-ray diffraction and reveals significant Cu displacement within the CuSe2Cl4 octahedra. The electronic structure of CuBiSeCl2 is investigated computationally, which indicates highly anisotropic charge carrier effective masses, and by experimental verification using X-ray photoelectron spectroscopy, which reveals a valence band dominated by Cu orbitals. The band gap is measured to be 1.33(2) eV, a suitable value for solar absorption applications. The electronic and thermal properties, including resistivity, Seebeck coefficient, thermal conductivity, and heat capacity, are also measured, and it is found that CuBiSeCl2 exhibits a low room temperature thermal conductivity of 0.27(4) W K-1 m-1, realized through modifications to the phonon landscape through increased bonding anisotropy.

Designing Visible-Light Photoactive Thioapatites Using Thiovanadate Groups: The Ba5(VO4-αSα)3X (X = F, Cl, I) Phases.

The new thioapatite Ba5(VO4-αSα)3X (X = F, Cl, I) series of compounds was prepared and characterized. Compared to known apatite phases built from unconnected vanadate VO4 groups separated by Ba2+ cations delimiting halide-filled channels, their crystal structure is built from mixed anion thiovanadate VO4-αSα, where V5+ is surrounded by both O and S, therefore exhibiting a triple anion lattice. Here, the strategy consisting in incorporating a chalcogenide anion aims at raising the valence band to bring the band gap to the visible range in order to reach photoactive materials under visible light. Both the halide anion nature and the S/O ratio impact the materials' photoconductivity. While the photocurrent response is comparable to that found in the recently investigated apatite phase Pb5(VO4)3I, a short carrier lifetime is detected as well as a shift of the activity toward the visible light. This apatite series combining thiovanadate and halide-filled channels opens new perspectives in the extended field of apatites and their applications.

Versatile Interplay of Chalcogenide and Dichalcogenide Anions in the Thiovanadate Ba7S(VS3O)2(S2)3 and Its Selenide Derivatives: Elaboration and DFT Meta-GGA Study.

Oxychalcogenides are emerging as promising alternative candidates for a variety of applications including for energy. Only few phases among them show the presence of Q-Q bonds (Q = chalcogenide anion) while they drastically alter the electronic structure and allow further structural flexibility. Four original oxy(poly)chalcogenide compounds in the system Ba-V-Q-O (Q = S, Se) were synthesized, characterized, and studied using density functional theory (DFT). The new structure type found for Ba7V2O2S13, which can be written as Ba7S(VS3O)2(S2)3, was substituted to yield three selenide derivatives Ba7V2O2S9.304Se3.696, Ba7V2O2S7.15Se5.85, and Ba7V2O2S6.85Se6.15. They represent original multiple-anion lattices and first members in the system Ba-V-Se-S-O. They exhibit in the first layer heteroleptic tetrahedra V5+S3O and isolated Q2- anions and in the second layer dichalcogenide pairs (Q2)2- with Q = S or Se. Selenide derivatives were attempted by targeting the selective substitution of isolated Q2- or (Q2)2- (in distinct layers) or both by selenide, but it systematically led to concomitant and partial substitution of both sites. A DFT meta-GGA study showed that selective substitution yields local constraints due to rigid VO3S and pairs. Experimentally, incorporation of selenide in both layers avoids geometrical mismatch and constraints. In such systems, we show that the interplay between the O/S anionic ratio around V5+, together with the presence/nature of the dichalcogenides (Q2)2- and isolated Q2-, impacts in unique manners the band gap and provides a rich background to tune the band gap and the symmetry.

Preparation, characterization and DFT+U study of the polar Fe3+-based phase Ba5Fe2ZnIn4S15 containing S = 5/2 zigzag chains.

The polar magnetic chalcogenide phase Ba5Fe2ZnIn4S15 was synthesized and its structure was solved by single crystal XRD. It is the first member with a 3d magnetic metal (Fe3+) in the Pb5ZnGa6S15-type structure family of wide bandgap materials with non-linear optical properties. The three-dimensional framework possesses a low dimensional magnetic character through the presence of weakly interacting zig-zag chains made of corner-sharing FeS4 tetrahedra forming chain 1, [FeS2]-∞. The latter chains are separated by InS4 tetrahedra providing weak magnetic super-super exchanges between them. The framework is also constituted by chain 2, [In3Zn1S9]7-∞ (chain of T2-supertetrahedra) extended similarly to chain 1 along the direction c and connected through InS4 tetrahedra. Symmetry analysis shows that the intrinsic polarization observed in this class of materials is mostly due to the anionic framework. Preliminary magnetic measurements and density functional theory calculations suggest dominating antiferromagnetic interactions with strong super-exchange coupling within the Fe-chains.

A high dimensional oxysulfide built from large iron-based clusters with partial charge-ordering.

Herein we report the original Ba10Fe7.75Zn5.25S18Si3O12 oxysulfide which crystallizes in a new structural type. Contrary to the usual oxychalcogenides, it crystallizes with a non-centrosymmetric 3D spatial network structure built from large magnetic clusters consisting of twelve (Fe2+/3+/Zn)S3O tetrahedra decorating a central Fe2+S6 octahedron and exhibiting a spin glass state.

Oxysulfide Ba5(VO2S2)2(S2)2 Combining Disulfide Channels and Mixed-Anion Tetrahedra and Its Third-Harmonic-Generation Properties.

Mixed-anion compounds are among the most promising systems to design functional materials with enhanced properties. In particular, heteroleptic environments around transition metals allow tuning of the polarity or band-gap engineering for instance. We present the original oxysulfide Ba5(VO2S2)2(S2)2, the fifth member in the quaternary system Ba-V-S-O. It exhibits the mixed-anion building units V5+O2S2 and isolated disulfide pairs (S2)2-. The structure is solved by combining single-crystal and powder X-ray diffraction and transmission electron microscopy. First-principles calculations were combined in order to highlight the anion roles. In particular, our density functional theory study shows that the 3p states of the disulfide pairs dictate the band gap. In this study, we point out anionic tools for band-gap engineering that can be useful for the design of phases for numerous applications. Finally, third harmonic generation (THG) was measured and compared to the large THG observed for Cu2O, which reveals the potential for nonlinear-optical properties that should be further investigated.