Synthesis, structure, and physical properties of niobium phosphate bronze (Nb18P2.5O50)

The crystal structure of a new niobium phosphate bronze, Nb[sub 18]P[sub 2.5]O[sub 50], was determined by single-crystal X-ray diffraction, and its physical properties were studied. Nb[sub 18]P[sub 2.5]O[sub 50] crystallizes in the tetragonal system with space group I4/m (No. 87) and Z = 1. The unit cell parameters are a = 15.593(1) [angstrom], c = 3.8282(3) [angstrom], and V = 930.8(1) [angstrom][sup 3]. The structural refinement by a full-matrix least-squares technique led to R = 0.055 (R[sub w] = 0.052). The structure of Nb[sub 18]P[sub 2.5]O[sub 50] is built up from 3 [times] 3 slabs of ReO[sub 3]-type NbO[sub 6] octahedra. These slabs are stacked alternatively along the crystallographic c axis and joined together by edge-sharing. In the ab plane, the slabs of NbO[sub 6] octahedra are interconnected through corner-sharing PO[sub 4] tetrahedra. Single-crystal resistivity measurement of Nb[sub 18]P[sub 2.5]O[sub 50] 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 [theta] = [minus]3.70 K and [mu][sub eff]/Nb[sup 4+] = 0.8 [mu]g.