Rb2BaNb2Se11: A new quaternary niobium polyselenide with infinite anionic chains composed of Nb2Se11 building block

Ba3TM2Se9 (TM = Nb, Ta): synthesis and characterization of new polyselenides

New ternary polyselenides Ba3TM2Se9 (TM = Nb, Ta) were synthesized through a solid-state reaction, and their structures were characterized using single-crystal X-ray diffraction. These compounds crystallized into a new structural type with a monoclinic space group P21/c. The structures were constructed from distorted, close-packed layers of ∞3[BaSe3]3.33– that incorporated TM atoms at octahedral sites and contained [(TM5+)2(Se2–)7(Se22–)] units. Diffuse reflectance spectra and electronic resistivity measurements indicated semiconducting properties and optical band gaps of 1.3 eV for Ba3Nb2Se9 and 1.6 eV for Ba3Ta2Se9. Raman spectra were used to investigate the interatomic interactions. Calculations of the electronic structure verified the semiconducting behavior and bonding interaction of short Se–Se contacts.

Cs6Nb4Se22 and K12Nb6Se35.3: two new compounds containing the M4Q22 building block

The title compounds, namely hexacaesium tetraniobium docosaselenide and dodecapotassium hexaniobium pentatriacontaselenide, were formed from their respective alkali chalcogenide reactive flux and niobium metal. Both compounds fall into the larger family of solid-state compounds that contain the M(2)Q(11) building block (M = Nb, Ta; Q = Se, S), where the metal chalcogenide forms dimers of face-shared pentagonal bipyramids. Cs(6)Nb(4)Se(22) contains two Nb(2)Se(11) building blocks linked by an Se-Se bond to form isolated Nb(4)Se(22) tetrameric building blocks surrounded by caesium ions. K(12)Nb(6)Se(35.3) contains similar Nb(4)Se(22) tetramers that are linked by an Se-Se-Se unit to an Nb(2)Se(11) dimer to form one-dimensional anionic chains surrounded by potassium ions. Further crystallographic studies of K(12)Nb(6)Se(35.3) demonstrate a new M(2)Se(12) building block because of disorder between an Se(2-) site (85%) and an (Se-Se)(2-) unit (15%). The subtle differences between the structures are discussed.