Electronic structures of ANb2PS10 (A=Ag, Na) and AuNb4P2S20

Abstract

New quaternary compounds ANb2PS10 (A = Na, Ag) and AuNb4P2S20 were synthesized and characterized. The structures of three compounds consist of one-dimensional infinite chains built by [Nb2S12] and [PS4] units. Cation atoms are occupied within the van der Waals gap of sulfur atoms between infinite chains to make -S⋯M+⋯S- contacts. There is only one Au atom site and so crystallographically a unit cell contains four equivalent Au atoms in AuNb4P2S20. This is only the half of the numbers of Na or Ag atoms in NaNb2PS10 or AgNb2PS10. The ratio between Nb2PS10 matrix vs the cation is, therefore, 1: 1 for Ag and Na, but it is 2: 1 for Au. Mixed valency in Au or Nb was expected to balance the charge in the latter compound. The electronic structures calculated based on the extended Huckel tight-binding method show that ANb2PS10 (A = Ag, Na) are semiconducting, while AuNb4P2S20 is metallic, which is not consistent with the experimental results of these three compounds that all exhibit semiconducting property. The result of calculation suggests that AuNb4P2S20 might be a magnetic insulator. Magnetic measurement experiment exactly proved that the compound is a Slater antiferromagnetic material with the Neels' temperature of 45 K. It is recognized, therefore, that electronic structure analysis is very useful to understand the properties of compounds.