Preparation and Electrical Conductivities of Layer Structured Crystals ZrNX (X=Br, I)

Abstract

Layer structured crystals ZrNX (X=Br and I) were prepared by the reaction of ZrH2 with NH4X (X=Br and I) vapors at temperatures ranging from 550° to 750°C. The reaction proceeded in a manner very similar to that reported previously on the preparation of β-ZrNCl from ZrH2 and NH4Cl. ZrNBr was obtained as the α polymorph. It was chemically transported to the higher temperature zone in a glass tube sealed with NH4Br. In a temperature gradient of 450° to 550°C, α-ZrNBr was transported as the α-form, whereas in a temperature gradient of 750° to 850°C, it was transported as the β-form with the 3R type stacking. Only the α type layered structure was known for ZrNI, and the β type layered structure was not obtained even in the chemical transport in the temperature gradient of 750° to 850°C. Electrical properties of β-ZrNBr and β-ZrNCl are very similar in the sense that both of them are n-type semiconductors with relatively high electrical conductivities for their large optical band gaps. The conductivities at room temperature and the activation energies for the conduction are -0.3Scm-1 and 0.10eV for β-ZrNBr, and -1Scm-1 and 0.060eV for β-ZrNCl. The band gap energies are estimated to be 3.2 and 3.4eV, respectively from the measurement of optical absorption spectra. In contrast to the high conductivities of the β-forms, α-ZrNBr and ZrNI showed conductivities smaller than 10-9Scm-1. A schematic band structural model was proposed for the valence electrons of ZrNX (X=Cl, Br and I).