The superionic conductors lithium nitride and lithium phosphide and the semi-metallic conductor lithium arsenide were synthesized through elemental reactions of lithium melt with nitrogen gas and phosphorus or arsenic powders. A high mobility of the lithium ion was found in this class of compounds. These compounds are hard and brittle and have a dark brown color. The structures of these compounds were studied using x-ray diffraction techniques. All three compounds crystallize in a hexagonal structure with a P6/mm space group. The lattice dimensions expand anisotropically as the size of the anion increases.
The ionic and electronic conductivities of the compounds were studied using reversible lithium electrodes and an ion blocking molybdenum electrode. The temperature dependent conductivity measurements (Arrhenius plot) show a high ionic and a negligible electronic conductivity for both lithium nitride and lithium phosphide. However, lithium arsenide shows semi-metallic behavior with a high electronic conductivity. A higher degree of covalency in the ct direction and a more ionic character in x-y plane were observed. The anisotropic nature of the chemical bonds in these compounds provides a shallow potential well in x-y plane and allows the correlated motion of lithium ions.
Lithium phosphide and nitride are superionic conductors and useful materials for application in batteries and sensors. Lithium nitride decomposes below 0.5 volts and can be used in low voltage and zero current devices. Lithium phosphide is stable up to 2.2 volts and can be used for higher voltage devices. These compounds decompose in contact with water and are air sensitive.