KINETICS OF FORMATION OF MOLYBDENUM SELENIDES FROM MODULATED REACTANTS AND STRUCTURE OF THE NEW COMPOUND MO3SE

The kinetics of compound formation in the molybdenum-selenium system h as been investigated using elementally modulated reactants to control overall composition and diffusion length. We observed the facile forma tion of MoSe2 at low temperatures when the composition was above 50 at om % selenium. No evidence was found for the low-temperature formation of the other known stable molybdenum selenide, the cluster compound M o6Se8. When the composition of the initially modulated reactant was cl ose to 25% selenium, a previously unreported compound was observed to form. This new compound, Mo3Se, has the A-25 crystal structure. The su perconducting transition temperature appears to be very sensitive to c omposition, with a sharp resistive transition at 7 K in one sample and a sharp diamagnetic transition observed in a second sample at 2.2 K. The kinetics of phase formation in this system is analyzed in terms of nucleation kinetics.