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Tetrides and Pnictides for Fast-Ion Conductors, Phosphor-Hosts, Structural Materials and Improved Thermoelectrics


New routes to solid states materials are needed for discovery and the realization of improved reactions. By utilizing reactive approaches such as solid-state metathesis, pyrolysis, and nitride fluxes new routes to hard materials, phosphor hosts, and fast ion conductors were developed.

The fast ion conductor lithium silicon nitride, Li2SiN2, was produced in a metathesis reaction between silicon chloride, SiCl4 and lithium nitride, Li3N, initiated in a conventional microwave oven. The Li2SiN2 produced had a conductivity of 2.70 x 10-3 S/cm at 500 °C.

Colorless millimeter-sized crystals of Ca16Si17N34 were synthesized at high temperatures from a flux generated in situ from reaction intermediates. The compound was found to crystallize in the cubic space group F-43m (a = 14.8882 Å). The powder X-ray diffraction pattern of Ca16Si17N34 matches that of a phase identified as cubic CaSiN2 in the 1960s but never structurally characterized. In contrast to the orthorhombic phase of CaSiN2, in which Ca2+ sits in octahedral sites, this cubic phase has Ca2+ in cubic sites that makes it an interesting host for new phosphors and gives rise to unique crystal field splitting.

A carbon-free silazane was formed by reacting metallic silicon in a melt of sodium amide; pyrolysis of this material offers a convenient route to Si3N4. The use of oxides for solid-state metathesis reactions was investigated for the formation of ternary silicon nitrides for phosphor hosts. Red, green, and blue phosphors were rapidly produced using rare earth oxides dopants. Solid-state metathesis reactions were investigated for the production of nano-scale B4C and ZrC. The addition of a refractory, hard material significantly improves the mechanical properties of existing thermoelectric materials.

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