Crystal and electronic structures of one-dimensional transition metal thiophosphates: ANB₂PS₁₀ (A=Na, Ag)

A new quaternary compound NaNb₂PS₁₀ was prepared by reacting the mixture of Nb, P₂S₅, S and Na₂S at 750°C. AgNb₂PS₁₀ was also synthesized by the direct solid-state reaction of Ag, Nb, P and S at 900°C. Their structures were determined by single-crystal X-ray diffraction method. The needle-shape crystals of the two compounds crystallize in the common monoclinic space group C2/c. NaNb₂PS₁₀ crystallizes into purple crystals with cell dimensions of a = 24.7634(10) Å, b = 7.8407(11) Å, c = 12.947(3) Å, β = 90.83(2)° and Z = 8. AgNb₂PS₁₀ had dark-gray crystals with cell dimensions of a = 23.9609(10) Å, b = 7.7692(11) Å, c = 12.910(3) Å, β = 94.49(2)° and Z = 8. These structures consist of one-dimensional infinite chains built by [Nb₂S₁₂] and [PS₄] units. The Nb atoms are centered in distorted bicapped trigonal prismatic polyhedra and the neighboring polyhedra share square faces and edges to make Nb-Nb pairs. [PS₄] units are tetrahedra composed of one S atom at the prism corner and two other capping S atoms and an additional S atom. Ag⁺ and Na⁺ cations in NaNb₂PS₁₀ and AgNb₂PS₁₀ reside in the van der Waals gap of sulfur atoms between infinite chains. Calculation of the electronic structure shows that the two compounds are semiconducting materials. Optically measured band gaps were 1.72 and 1.77 eV for NaNb₂PS₁₀ and AgNb₂PS₁₀, respectively.