The kinetics of compound formation in the
molybdenum−selenium system has been investigated using
elementally
modulated reactants to control overall composition and diffusion
length. We observed the facile formation of
MoSe2 at low temperatures when the composition was above 50
atom % selenium. No evidence was found for
the low-temperature formation of the other known stable molybdenum
selenide, the cluster compound
Mo6Se8.
When the composition of the initially modulated reactant was close
to 25% selenium, a previously unreported
compound was observed to form. This new compound,
Mo3Se, has the A-15 crystal structure. The
superconducting
transition temperature appears to be very sensitive to composition,
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.
In this article we compare and contrast the evolution of ternary modulated reactants of the form M-Mo-Se (M ) Ni, Zn, Sn, In, and Cu) with each other and with the binary Mo-Se system. The binary elementally modulated reactants interfacially nucleate MoSe 2 over a broad composition range surrounding that of the binary compound Mo 6 Se 8 . Increasing the concentration of any of the studied ternary elements except nickel above a critical value in the initial reactant suppressed interfacial nucleation of the diselenide. The nickel-containing reactants all interfacially nucleated Ni x MoSe 2 at low temperatures. The subsequent nucleation and growth of crystalline compounds from the amorphous intermediates obtained in the other four systems was found to depend on both the identity and concentration of the ternary element. In the Sn-Mo-Se system, a layered dichalcogenide was the first compound nucleated. In the Zn-Mo-Se system, the dichalcogenide and the ternary compound Zn x Mo 6 Se 8 were observed to nucleate at approximately the same temperature. In the copper-and indium-containing systems, the cluster-containing compounds, Cu x Mo 6 Se 8 and In 3.33(δ Mo 15 Se 19 , were observed to be the first crystalline compounds formed. Annealing all four of these systems at temperatures greater than 1100 °C resulted in the growth of Chevrel phase compounds at the expense of molybdenum diselenide.
Partially due to their lack of periodic structure, quasicrystals have inherently low thermal conductivity on the order of 1 - 3 W/m-K. AlPdMn quasicrystals exhibit favorable room temperature values of electrical conductivity, 500–800 (Ω-cm)-1, and thermopower, 80 μV/K, with respect to thermoelectric applications. In an effort to further increase the thermopower and hopefully minimize the thermal conductivity via phonon scattering, quartenary Al71Pd21Mn8-XReX quasicrystals were grown. X-ray data confirms that the addition of a fourth element does not alter the quasiperiodicity of the sample. Al71Pd21Mn8-XReX quasicrystals of varying Re concentration were synthesized where x had values of 0, 0.08, 0.25, 0.4, 0.8, 2, 5, 6, and 8. Both thermal and electrical transport property measurements have been performed and are reported.
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