Add a Pinch of Tetrel: The Transformation of a Centrosymmetric Metal into a Nonsymmorphic and Chiral Semiconductor.

Centrosymmetric skutterudite RhP3 was converted to a nonsymmorphic and chiral compound RhSi0.3 P2.7 (space group P21 21 21 ) by means of partial replacement of Si for P. The structure, determined by a combination of X-ray crystallography and solid state 31 P NMR, exhibits branched polyanionic P/Si chains that are unique among metal phosphides. A driving force to stabilize the locally noncentrosymmetric cis-RhSi2 P4 and fac-RhSi3 P3 fragments is π-electron back-donation between the Rh t2g -type orbitals and the unoccupied antibonding Si/P orbitals, which is more effective for Si than for P. In situ studies and total energy calculations revealed the metastable nature of RhSi0.3 P2.7 . Electronic structure calculations predicted centrosymmetric cubic RhP3 to be metallic which was confirmed by transport properties measurements. In contrast, the electronic structure for chiral orthorhombic RhSi0.3 P2.7 contained a bandgap, and this compound was shown to be a narrow gap semiconductor.

Tuning of Cr-Cr Magnetic Exchange through Chalcogenide Linkers in Cr2 Molecular Dimers.

A set of three Cr-dimer compounds, Cr2Q2(en)4X2 (Q: S, Se; X: Br, Cl; en: ethylenediamine), with monoatomic chalcogenide bridges have been synthesized via a single-step solvothermal route. Chalcogenide linkers mediate magnetic exchange between Cr3+ centers, while bidentate ethylenediamine ligands complete the distorted octahedral coordination of Cr centers. Unlike the compounds previously reported, none of the chalcogenide atoms are connected to extra ligands. Magnetic susceptibility studies indicate antiferromagnetic coupling between Cr3+ centers, which are moderate in Cr2Se2(en)4X2 and stronger in Cr2S2(en)4Cl2. Fitting the magnetic data requires a biquadratic exchange term. High-frequency EPR spectra showing characteristic signals due to coupled S = 1 spin states could be interpreted in terms of the "giant spin" Hamiltonian. A fourth compound, Cr2Se8(en)4, has a single diatomic Se bridge connecting the two Cr3+ centers and shows weak ferromagnetic exchange interactions. This work demonstrates the tunability in strength and type of exchange interactions between metal centers by manipulating the interatomic distances and number of bridging chalcogenide linkers.

New Noncentrosymmetric Tetrel Pnictides Composed of Square-Planar Gold(I) with Peculiar Bonding.

Three novel isostructural equiatomic gold tetrel pnictides, AuSiAs, AuGeP, and AuGeAs, were synthesized and characterized. These phases crystallize in the noncentrosymmetric (NCS) monoclinic space group Cc (no. 9), featuring square-planar Au within cis-[AuTt2 Pn2 ] units (Tt=tetrel, Si, Ge; Pn=pnictogen, P, As). This is in drastic contrast to the structure of previously reported AuSiP, which exhibits typical linear coordination of Au with Si and P. Chemical bonding analysis through the electron localization function suggests covalent two-center two-electron Tt-Pn bonds, and three-center Au-Tt-Au and Au-Pn-Au bonds with 1.6 e- per bond. X-ray photoelectron spectroscopy studies support the covalent and nonionic nature of Au-Pn and Au-Tt bonds. The title materials were found to be n-type narrow-gap semiconductors or semimetals, with nearly temperature-independent electrical resistivities and low thermal conductivities. A combination of the semimetallic properties with tunable NCS structure provides opportunities for the development of materials based on gold tetrel pnictides.

Centrosymmetric or Noncentrosymmetric? Transition Metals Talking in K2TGe3S8(T = Co, Fe).

Two new quaternary sulfides K2TGe3S8(T = Co, Fe) have been synthesized by a high-temperature solid-state routine and flux growth method. The crystal growth process of K2TGe3S8(T = Co, Fe) was elucidated by in situ powder X-ray diffraction and DSC thermal analysis. The millimeter-sized crystals of K2TGe3S8(T = Co, Fe) were grown. K2CoGe3S8 crystallizes in a new structure type in centrosymmetric space group P1 (no. 2) with unit cell parameters of a = 7.016(1) Å, b= 7.770(1) Å, c = 14.342(1) Å, α = 93.80(1)°, ß = 92.65(1)°, γ = 114.04(1)°. K2FeGe3S8 crystallizes in the K2FeGe3Se8 structure type and the noncentrosymmetric space group P21 (no. 4) with unit cell parameters of a = 7.1089(5)Å, b = 11.8823(8) Å, c = 16.7588(11) Å, ß = 96.604(2)°. There is a high structural similarity between K2CoGe3S8 and K2FeGe3S8. The larger volume coupled with higher degrees of distortion of the [FeS4] tetrahedra compared to the [CoS4] tetrahedra accounts for the structure's shift from centrosymmetric to noncentrosymmetric. The theory simulation confirms that [TS4]T= Co or Fe tetrahedra play a crucial role in controlling the structure and properties of K2TGe3S8(T = Co, Fe). The measured optical bandgaps of K2CoGe3S8 and K2FeGe3S8 are 2.1(1) eV and 2.6(1) eV, respectively. K2FeGe3S8 shows antiferromagnetic ordering at 24 K while no magnetic ordering was detected in K2CoGe3S8. The magnetic measurements also demonstrate the divalent nature of transition metals in K2TGe3S8(T = Co, Fe).

Tuning Fe-Se Tetrahedral Frameworks by a Combination of [Fe(en)3]2+ Cations and Cl- Anions.

A one-dimensional (1D) chain compound [Fe(en)3]3(FeSe2)4Cl2 (en = ethylenediamine), featuring tetrahedral FeSe2 chains separated by [Fe(en)3]2+ cations and Cl- anions, has been synthesized by a low temperature solvothermal method using simple starting materials. The degree of distortion in the Fe-Se backbone is similar to previously reported compounds with isolated 1D FeSe2 chains. 57Fe Mössbauer spectroscopy reveals the mixed-valent nature of [Fe(en)3]3(FeSe2)4Cl2 with Fe3+ centers in the [FeSe2]- chains and Fe2+ centers in the [Fe(en)3]2+ complexes. SQUID magnetometry indicates that [Fe(en)3]3(FeSe2)4Cl2 is paramagnetic with a reduced average effective magnetic moment, µeff = 9.51 µB per formula unit, and a negative Weiss constant, θ = -10.9(4) K, indicating antiferromagnetic (AFM) nearest neighbor interactions within the [FeSe2]- chains. Weak antiferromagnetic coupling between chains, combined with rather strong intrachain AFM coupling, leads to spin-glass behavior at low temperatures, as indicated by a frequency shift of the peak observed at 3 K in AC magnetic measurements. A combination of [Fe(en)3]2+ and Cl- ions is also capable of stabilizing mixed-valent 2D Fe-Se puckered layers in the crystal structure of [Fe(en)3]4(Fe14Se21)Cl2, where Fe14Se21 layers have a unique topology with large open pores. Property measurements of [Fe(en)3]4(Fe14Se21)Cl2 could not be performed due to the inability to either grow large crystals or synthesize this material in single-phase form.

Unraveling the Origin of Photocatalytic Deactivation in CeO2/Nb2O5 Heterostructure Systems during Methanol Oxidation: Insight into the Role of Cerium Species.

The study provides deep insight into the origin of photocatalytic deactivation of Nb2O5 after modification with ceria. Of particular interest was to fully understand the role of ceria species in diminishing the photocatalytic performance of CeO2/Nb2O5 heterostructures. For this purpose, ceria was loaded on niobia surfaces by wet impregnation. The as-prepared materials were characterized by powder X-ray diffraction, nitrogen physisorption, UV-visible spectroscopy, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and photoluminescence measurements. Photocatalytic activity of parent metal oxides (i.e., Nb2O5 and CeO2) and as-prepared CeO2/Nb2O5 heterostructures with different ceria loadings were tested in methanol photooxidation, a model gas-phase reaction. Deep insight into the photocatalytic process provided by operando-IR techniques combined with results of photoluminescence studies revealed that deactivation of CeO2/Nb2O5 heterostructures resulted from increased recombination of photo-excited electrons and holes. The main factor contributing to more efficient recombination of the charge carriers in the heterostructures was the ultrafine size of the ceria species. The presence of such highly dispersed ceria species on the niobia surface provided a strong interface between these two semiconductors, enabling efficient charge transfer from Nb2O5 to CeO2. However, the ceria species supported on niobia exhibited a high defect site concentration, which acted as highly active recombination centers for the photo-induced charge carriers.