New T₃B₇ Compounds

Abstract

IN the course of a study on transition-element compounds of composition TB₂ (T = transition element, B = phosphorus-group element) two compounds with a body-centred cubic structure were found, a semiconducting one in the system Re–As and a metallic one in the system Mo–Sb. Both compounds formed on reacting, at 600°–900° C, the powdered elements in various ratios. A crude analysis of ReAsx by thermal dissociation at 1,000° C gave x ≈ 2.4. Our arsenide, therefore, must be identical to the phase Re As_2.3 reported by Wiechmann et al.¹. The non-metallic behaviour as well as the diamagnetism of rhenium arsenide confirmed the suspected stoichiometry Re₃As₇ which corresponds to the normal valencies of Re and As. From density measurements, the unit cell [a = 8.722 ± 2 Å] was found to contain four formula units of Re₃As₇. Since in the X-ray diagram only the general extinctions for body-centred cubic symmetry are observed, seven space groups are possible. However, the striking resemblance between the diagram of Re₃As₇ and that of Ir₃Ge₇ led me to postulate a D8_f structure for Re₃As₇. Best agreement between calculated and observed intensities was obtained for the following parameter values: Assuming the D8_f type to be correct^* for Re₃As₇, the coordination is as follows: This proximity is, in fact, reasonable since it makes allowance for saturated covalent bonds as can readily be seen by invoking a charge transfer Re₃⁺As₃⁻As₄ and by allowing half-bonds to form between Re and As_I. The non-metallic character of Re₃As₇ was established by resistivity measurements on sintered samples at temperatures above 300° C. Assuming ρ ≈ exp (δE/2kT), an energy gap δE ≈ 0.7 eV was derived. At room temperature the samples were n-type with a thermoelectric power of about 100 μV/°C. The magnetic susceptibility χM was found to be −210 and −250×10⁻⁶ cgs units/mole at liquid-nitrogen and room temperature, respectively.