The noncentrosymmetric chain compounds, A 3M 2AsSe 11 (A = K, Rb, Cs; M = Nb, Ta)

Abstract

The noncentrosymmetric niobium and tantalum selenoarsenates, A 3Nb 2AsSe 11 (A=K, Rb, Cs) and K 3Ta 2AsSe 11, were synthesized in a polyselenoarsenate flux. All compounds crystallize in the polar monoclinic space group Cc. The structures are comprised of the same type of infinite chain anions, [M 2Se 2(Se 2) 3(AsSe 3)] 3− (M=Nb, Ta) separated by alkali metal cations. The As 3+ centers with nonbonded electron pairs play an important role in stabilizing the noncentrosymmetric structures. UV-Vis spectroscopy, Raman spectroscopy and differential thermal analysis data are reported. The energy gaps of these compounds vary between 1.35 and 1.53 eV.