Synthesis, Structure, and Physical Properties of (Nb₁₈P_2.5O₅₀)

The crystal structure of a new niobium phosphate bronze, Nb₁₈P_2.5O₅₀, was determined by single-crystal X-ray diffraction, and its physical properties were studied. Nb₁₈P_2.5O₅₀ crystallizes in the tetragonal system with space group 14/m (No. 87) and Z = 1. The unit cell parameters are a = 15.593(1) Å, c = 3.8282(3) Å, and V = 930.8(1) ų. The structural refinement by a full-matrix least-squares technique led to R = 0.055 (R_w = 0.052). The structure of Nb₁₈P_2.5O₅₀ is built up from 3 × 3 slabs of ReO₃-type NbO₆ octahedra. These slabs are stacked alternatively along the crystallographic c axis and joined together by edge-sharing. In the ab plane, the slabs of NbO₆ octahedra are interconnected through corner-sharing PO₄ tetrahedra. Single-crystal resistivity measurement of Nb₁₈P_2.5O₅₀ along the crystallographic c axis showed a semiconducting behavior with an activation energy of 0.13(1) eV. The magnetic susceptibility data indicate a modified Curie-Weiss behavior with the Curie-Weiss temperature θ = −3.70 K and μ_eff/Nb⁴⁺ = 0.8 ¼B.