Exploratory Study, by Low-Temperature X-Ray Diffraction Techniques, of Diborane and the Products of a Microwave Discharge in Diborane

Abstract

It has been suggested that atomic hydrogen may form a loose, one‐electron bond with stable electron‐deficient molecules such as diborane (B₂H₆). Such bonding might make possible the stabilization of quasi‐atomic hydrogen at higher temperatures or in higher concentrations than have been attained previously. To test this theory, diborane and the products of a microwave discharge in diborane have been studied by x‐ray diffraction in the temperature range 4.2° to 100°K. Two structurally related phases have been distinguished in ordinary diborane. The low temperature or α phase is formed by deposition from the gas at 4.2°K. It transforms slowly to β diborane above 60°K. The β phase is also obtained by depositing at 77°K and annealing above 90°K to eliminate traces of the α phase. An additional phase is found in specimens formed by freezing at 4.2°K the products of a microwave discharge in diborane. It is probably a boron‐hydrogen compound with a triple‐point temperature of about 60°K. It may be BH₃. This compound is not known definitely, but some evidence for its existence has been reported. In each experiment with ``discharged'' diborane, warm‐up began with a sudden increase in temperature from 4.2° to about 33°K while there was still helium in the Dewar. The source of heat has been traced to hydrogen frozen out with diborane and other products of the microwave discharge. The temperature rise, which may have been initiated by a process such as recombination of atomic hydrogen trapped in the solid, was sustained by heat transferred from the Dewar walls by hydrogen gas released from the sample. Since the heat release occurred spontaneously at a temperature near 4.2°K it cannot be concluded that diborane helped appreciably to stabilize atomic hydrogen in the solid.