PREPARATION, CRYSTAL-STRUCTURE, AND PROPERTIES OF THE MIXED-VALENCE COMPOUND NB3SE5CL7

The new compound Nb3Se5Cl7 was prepared by heating 2NbSe2Cl2 + 1NbCl4 at 530°C for 2-3 weeks. The compound is monoclinic with a = 7.599, b = 12.675, c = 8.051Å; β = 106.27°; space group P21 m. The corresponding bromide, Nb3Se5Br7 (obtained by decomposition of NbSe2Br2 under NbSeBr3), is isotypic with a = 7.621, b = 12.833, c = 8.069Å; β = 106.21°. from the crystal structure and XPS spectra it follows that Nb3Se5Cl7 can be formulated as: [Nb4+2Nb5+1(Se2)2-2Se2-1Cl-7]. The structure consists of chains of composition [Nb4+2(Se2)2-2Cl-5], to which side chains [Nb5+Se2-Cl-2] are attached. The Nb4+ atoms form pairs (NbNb = 2.94 Å) which explains that Nb3Se5Cl7 is a diamagnetic semiconductor with a band gap (1.59 eV at 5°K, 1.49 eV at 300°K) very similar to that of NbSe2Cl2. © 1979.