The Triple Salt Sr₁₄[Ta₄N₁₃][TaN₄]O-A Nitridotantalate Oxide with 19-fold Rock Salt Superstructure

A new structure motif in nitridometalate chemistry is the tetracatena-nitridotantalate anion [Ta₄N₁₃]^19-. It occurs in the crystal structure of the triple salt Sr₁₄[Ta₄N₁₃][TaN₄]O (monoclinic, space group P2₁/c with a = 15.062(2) Å, b = 7.2484(6) Å, c = 24.266(3) Å, and β = 97.280(10)^o) together with ortho-tantalate and isolated oxide anions. Synthesis followed a new approach with employment of Sr surplus and reductive conditions aimed at the preparation of subvalent compounds. The new structure type was established on the basis of single-crystal X-ray diffraction data and also Rietveld refinement. It is a complex superstructure of the rock salt structure type with Ta and Sr atoms forming the face-centered cubic packing and N and O atoms occupying 18/19 of the octahedral voids. We discuss structure and stability of the triple salt with respect to other known nitridometalates and the use of this triple salt for preparative access toward new metal-rich compounds in this field.